Technology Today H ighlighting R aytheon ’ s T echnology

2012 ISSUE 1

Raytheon’s Materials Technology Shaping the future

A Message From

Mark E. Russell Vice President of Engineering, Technology and Mission Assurance

Materials advances have created new industries and have redefined how we live. Demand is constant for new materials that deliver greater performance, improved cost effectiveness, superior reliability and better safety, while having minimal environmental impact. Raytheon has a long history of materials discovery and innovation. We recognize that the proper choices and applications of materials technology are vital to the quality and performance of our products. This issue of Technology Today features articles that discuss the engineering of materials from atomic building blocks, the development of composite structures that exhibit unique materials properties, the engineering of materials that imitate characteristics observed in nature and the application of new materials to meet the demands for higher performance and new capabilities. Articles on materials restrictions and counterfeit parts underscore the importance of sustainability and reliability. In our Leaders Corner, Jim Wade, vice president of Mission Assurance, builds upon this issue’s theme by focusing on the disciplines that ensure quality, reliability and dependability in the materials we use and processes we follow in the production and deployment of our products and systems. In addition, he shares his Mission Assurance vision and his views on leadership, Raytheon Six Sigma™ and Mission Assurance competencies and careers. Materials advances cannot be realized without accompanying innovations in manufacturing. Among the articles in our Eye on Technology section, we introduce our Manufacturing Technology Network, a companywide association of experts enabling the rapid transition of advanced material technologies from the laboratory into our products. We complete this issue with a summary of recent events including Raytheon’s prestigious Excellence in Engineering and Technology Awards, a focus on our graduates of the Massachusetts Institute of Technology Leaders for Global Operations program, and a discussion of PRISM — our enterprise manufacturing process. On the cover: Metamaterials enable tunable antenna and radome capabilities. Raytheon Engineer, Dr.Jacquelyn Vitaz, inspects a tunable metamaterial sample.

Best regards,

Mark E. Russell 2

2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

View Technology Today online at: www.raytheon.com/technology_today

inside this issue Feature: Raytheon’s Materials Technology Overview: Materials Innovations Enable System Performance

Technology Today is published by the Office of Engineering, Technology and Mission Assurance. Vice President Mark E. Russell Chief Technology Officer Bill Kiczuk Managing Editor Cliff Drubin Feature Editor Randal Tustison

Nanoscale Colloidal Quantum Dots for Imaging System Applications

10

Carbon-Based Nanotechnology

14

Improving Thermal Performance of DoD Systems

18

Raytheon’s Diamond Technology

20

Bio-inspired Shutters and Apertures for Infrared Imaging Applications

22

Realizing the Potential of Metamaterials

24

Detection and Identification of Radiological Sources

28

Advanced Sonar Projector Materials

31

Materials Solutions to Meet the Needs for Large-Scale Energy Storage

34

Zinc-Bromine Flow Battery Technology for Energy Security Liquid Metal Battery

35 36

Material Restrictions and Reporting

38

Responding to the Counterfeit Threat

41

Raytheon Leaders Leaders Corner: Q&A With Vice President of Mission Assurance Jim Wade

44

EYE on Technology

Senior Editors Corey Daniels Tom Georgon Eve Hofert

Raytheon Manufacturing Technology Network

Art Director Debra Graham

Focus Center Research Program and Raytheon

Photography Fran Brophy Rob Carlson Charlie Riniker

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46

Crew Comm 46 48

Events MathMovesU® Day at the University of Arizona

51

Raytheon Excellence in Engineering and Technology Awards

52

Website Design Nick Miller

Mechanical, Materials and Structures Technology Network Symposium

54

2011 Raytheon Power and Energy Technology Symposium

54

Publication Distribution Dolores Priest

2011 Raytheon Energy Summit

55

Contributors Kate Emerson Lindley Specht Frances Vandal

People Leaders for Global Operations Program

56

Resources Unleashing the Power of Raytheon Manufacturing Through PRISM

Editor’s note: Correction: Technology Today 2011 Issue 2, page 41. The photo is copyright AIRBUS S.A.S. 2010, photo by exm Company, N. Fonade.

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Patents 60

RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1

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Feature

Materials Innovations Enable System Performance …

T

he importance of materials in the chronicle of human

materials scientists to develop the Siemens Process, an economical

development cannot be overemphasized. Mankind has been

method of refining metallurgical grade silicon, which enabled the

exploiting materials since prehistoric times. In fact, the three

emergence of the semiconductor industry. Today, silicon integrated

epochs of prehistory, the Stone Age, Bronze Age and Iron Age, are

circuits are ubiquitous, being directly or indirectly involved with

named after the materials and the related tool-making technologies

virtually everything that we touch.

that define them. Perhaps the earliest quantitative study of materials appeared in Galileo Galilei’s Discourses and Mathematical

Material development and adoption challenges

Demonstrations Relating to Two New Sciences (1638). The two

The demand for stronger, lighter, greener yet less expensive materi-

sciences were the “strength of materials” and the “motion of

als continues to outstrip availability, presenting many challenges

objects” (kinematics). Materials science as a discipline has its roots

as well as opportunities to the materials engineer. Unfortunately,

in the study of metallurgy; only recently has it become the truly

the development of a new material and the related manufactur-

interdisciplinary science that it is today, merging metallurgy,

ing technology can be a lengthy process, taking much longer than

ceramics and polymer science, and including aspects of chemistry

we would like. In Technology Review, Thomas Eagar2 pointed out

and solid state physics.

that it typically takes 20 years from the discovery of a new material to its commercialization. This proposition is underscored by many

History has shown us that a new material technology can change the

world.1

As a result, innumerable technological advances owe

examples ranging from the vulcanization of rubber to diamond-like coatings. The reasons for this lag in materials technology insertion

their success to materials science. To illustrate this point, consider

are many and varied. For example, too frequently product engineers

silicon. Silicon is the second most abundant element in the earth’s

want to use the new material in the same way that they used the

crust, but it rarely occurs in the elemental form. With the inven-

previous material, rarely exploiting all of its favorable properties;

tion of the transistor, the need for high-purity forms of silicon led

hence, the promise of the new material is not immediately realized.

System Challenge: Third-generation infrared sensing systems based on advanced mercury cadmium telluride (HgCdTe) focal plane arrays (FPAs) require large area lattice-matched cadmium zinc telluride (CdZnTe) substrates for the epitaxial growth of multilayer HgCdTe devices by molecular beam epitaxy.Existing second-generation infrared systems utilize HgCdTe FPAs grown by liquid phase epitaxy (LPE) on CdZnTe substrates that are cut in the (111) crystal orientation. MBE-grown HgCdTe thirdgeneration FPAs require larger CdZnTe substrates that are cut in a different crystal orientation (211) with additional requirements for surface polishing and preparation.

Materials Innovation: Raytheon is a leader in the development of techniques for growing and processing these crystals for the production of affordable high-performance third-generation FPAs.

< Large-diameter CdZnTe boule used as the starting substrate material for the production of advanced HgCdTe focal plane arrays. 4

2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

Feature

Shaping the Future of Materials Technology Structure

luctant to specify a new material that might not be available in the For aerospace applications, qualification of a new material can be a very costly and time-consuming endeavor, which can also be a significant barrier to adoption. For these reasons, a new material with superior performance, even at a lower cost, is frequently slow to be accepted. Nevertheless, as Eagar points out, industries like the

Processing Process

fraction/density

required quantities or may only be available from a single source.

Defect Distributions

Properties Statistical variations in property values

Performance Life Predictions percent failed

Control of Statistical Properties

the demand develops. An unfortunate result is that designers are re-

cycles/time to failure

Initially, new materials are produced in small quantities, at least until

load/time stress

size

Figure 1. The Accelerated Insertion of Materials program materials development methodology (source: QuesTek Innovation LLC).

aerospace industry are most likely to lead in the introduction of new material technologies. Here the value associated with a high level of performance (for example in advanced composites, the cost savings associated with a pound of saved weight) can be significantly larger than for many commercial, less demanding applications.

Cycle time acceleration initiatives

kit for use by designers. The purpose of the tool kit was to predict and control the statistical material properties through microstructure control (structure-property relations), thereby reducing insertion risk (Figure 1).

Several concerted efforts have been made to reduce this 20-year

continued on page 6

cycle time. For example, the Defense Advanced Research Projects Agency’s (DARPA) Accelerated Insertion of Materials program attempted to do this by creating a more rigorous materials development and qualification methodology, including a computational tool

System Challenge: Domes and infrared windows with increased transmission and greater durability are needed to extend the performance envelope of DoD weapons and sensors.

Raytheon Materials Innovation: Raytheon’s NanoComposite Optical Ceramic (NCOC) material, developed under a four-year DARPA (Defense Advanced Research Projects Agency) project, provides a revolutionary improvement in infrared missile seeker performance. NCOC domes enhance the lethality of air-to-air missiles, increasing the seeker’s signal-to-noise ratio (sensitivity). NCOC also extends the transmission bandpass beyond the mid-wave infrared (MWIR) spectrum and provides durability improvements.

Raytheon’s new NCOC infrared > transparent dome (left) and as viewed through an infrared camera. RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1

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Materials Innovations Enable System Performance … continued from page 5

Discovery

Development

Optimization

System Design & Integration

Certification

Manufacturing

Deployment/ Sustainment

Figure 2. Materials Development Continuum Since then, computational materials engineering (CME) has emerged as a powerful tool in contemporary materials science, and as the central theme in efforts to accelerate materials discovery and insertion of new materials technologies. Still, the lag between new material discovery and insertion continues to be a challenge. Figure 2 illustrates the traditional materials development continuum. In June 2011, President Obama announced the launching of the Materials Genome Initiative, with the objective “to help business develop, discover and deploy new materials twice as fast.”3 In the words of John Holdren,4 Assistant to the President for Science and Technology and Director of the White House Office of Science

and Technology Policy, “In much the same way that silicon in the 1970s led to the modern information technology industry, the development of advanced materials will fuel many of the emerging industries that will address challenges in energy, national security, healthcare and other areas. Yet the time it takes to move a newly discovered advanced material from the laboratory to the commercial market place remains far too long. Accelerating this process could significantly improve U.S. global competitiveness and ensure that the nation remains at the forefront of the advanced materials marketplace. This Materials Genome Initiative for Global Competitiveness aims to reduce development time by providing the infrastructure and training that American innovators need to discover, develop, manufacture and deploy advanced materials in a more expeditious and economical way.”

System Challenge: Evolving threats and mission requirements dictate a need for higher performance radio frequency (RF) sensors and electronic warfare (EW) systems.

Materials Innovation: The evolution of existing RF device technology was not meeting new system cost, size, weight and performance goals. Therefore, in the late 1990s Raytheon identified wide bandgap gallium nitride (GaN) as an enabling technology with the potential to meet evolving performance goals. Over the last 10 years we have developed and proven a GaN technology baseline, which is now released to production. Raytheon’s GaN technology is the cornerstone of multiple system pursuits. This decade-long development, a partnership among material technologists, device physicists, process engineers, component engineers and system engineers, serves as a model for developing other enabling technologies. < A four-inch gallium nitride wafer being processed into individual RF components.

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2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

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Shaping the Future of Materials Technology

The Materials Genome Initiative will rely on CME advancements in conjunction with new experimental tools, and in particular, with coordinated and open data management systems that allow researchers to access and compare data; thereby facilitating far more collaboration than is currently available. The Initiative also considers the whole lifecycle of the material, including issues of recyclability and sustainability, which we will touch upon in this issue.

Raytheon’s past material innovations

transmit/receive modules and panel arrays; each of which includes a multitude of underlying and critical supporting materials technologies. Chemically vapor deposited (CVD) zinc sulfide has become the standard long-wavelength transparent electro-optical material for passive imaging at 8–12 micrometers wavelength in the infrared. Raytheon’s materials process innovation supplanted the competing hot-pressed material, Irtran-2. Raytheon produced thousands of electro-optic windows and domes, beginning in 1972 with the first CVD dome ever made.

Raytheon has a long and successful history of materials discovery and innovation going back to the earliest days of the company. The Klixon Disk, a simple bi-metallic device invented by Al Spencer, launched the Spencer Thermostat Company and established the company’s founding fathers as successful entrepreneurs.5 Numerous successes followed, including the development of gallium arsenidebased microwave integrated circuits, then gallium nitride-based

continued on page 8

System Challenge: Windows and domes on infrared imaging systems that operate in the long wavelength infrared (LWIR) portion of the spectrum are subject to environmental degradation when exposed to high-velocity raindrop and sand particle impact during mission execution. Unfortunately, materials that are transparent at these wavelengths are generally soft and weak, or in the worst case, water soluble. What is needed is a truly durable, multispectral window/dome material.

Materials Innovation: Raytheon invented a rain erosion protective and durable antireflective coating for LWIR transparent materials like zinc sulfide. This coating increases the abrasion resistance of this benchmark material while also improving its resistance to high-velocity raindrop impact damage by more than a factor of two.

Zinc sulfide radomes treated with Raytheon’s abrasion-resistant optical coating. > RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1

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Feature

Materials Innovations Enable System Performance … continued from page 7

Raytheon’s materials innovations today Today, Raytheon continues to be actively involved in materials discovery, innovation and insertion at all points along the materials development continuum. With the advent of CME, in tandem with the ability to manipulate materials at the atomic level, Raytheon engineers are creating materials with properties that were simply not attainable before. In this edition of Technology Today, our first three articles discuss engineering of materials from the ground (nanoscale) up, one of which includes leveraging the remarkable properties of the carbon nanotube (CNT). Parenthetically, it has been 20 years since CNTs were first observed. One could conclude, based on Eagar’s thesis that wide-scale acceptance of CNTs is just around the corner. Sometimes, materials discovery can be as straightforward as observing the world around us, as is the case with the bio-inspired optical shutters being developed at Raytheon for infrared imaging. Nature’s

perfect material, diamond, is formed over millions of years within the earth’s crust at high temperatures and pressures. Since the 1960s, low-temperature processes have been developed to produce diamond in the laboratory environment. We will discuss Raytheon’s pioneering efforts in the chemical vapor deposition of diamond and its use as a superior conductor of heat, a critical characteristic for thermal management in high-power device applications. Finally, we will review progress in developing an exciting new form of “material” referred to as metamaterial. Metamaterials are engineered materials in the truest sense of the term, deriving their properties not from their constituent materials but from the periodic arrangement of these materials. As we have seen, the processes of designing, optimizing and integrating materials into a product, component or subsystem occupy the center of the materials development continuum. Several articles on materials optimization and integration are included. New sensors that derive their unique capabilities from materials engineering are becoming important elements for ensuring our national defense

System Challenge: Provide an air-supported radome to protect the world’s largest X-Band radar system with a probability of survival of 99.9 percent over the course of 20 years.

Materials Innovation: The requirement for high reliability and long-term survivability under extreme environmental conditions, while having minimal impact on radar performance, drove development of a novel composite radome material and structure to protect the 10-story X-Band Radar (XBR). Built around a urethane-coated Vectran® fabric with bias ply laminate construction, the resulting spheroid shaped radome was constructed by joining 81 sections (gores) together and clamping them at the bottom. The result was a first-of-its-kind radome that can withstand 150 mile per hour winds with less than 2 feet deflection.

< The XBR radome measures 120 feet at the equator, is 103 feet tall and weighs approximately 18,000 pounds. Inset is a view from the inside showing the antenna array face underneath the radome’s protective shell. 8

2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

Feature

Shaping the Future of Materials Technology and homeland security in the face of evolving threats. Examples include novel sonar transducer materials and a class of materials that give off light in the presence of nuclear radiation. Solutions to address energy security are discussed in a series of articles on materials technologies for large-scale energy storage.

Technology section, we introduce our latest technology network — the Manufacturing Technology Network, a companywide association of experts established to better integrate manufacturing technologies and strategies with new materials development in order to accelerate deployment. • Randal Tustison

Beyond development Once a new material technology is integrated and deployed in a new system, the focus turns to sustainment. This is particularly true in aerospace and defense systems where very high importance is placed on reliability. Our remaining feature articles address the sustainment phase. This includes a discussion of regulations that control a material’s impact on the environment and Raytheon’s responsiveness to those regulations, as well as Raytheon’s response to the growing industry threat from counterfeit parts. Our discussion of materials technology at Raytheon would not be complete without mentioning the role that manufacturing plays in the materials development continuum. In our Eye on

References 1. Robert Friedel, Materials That Changed History, Nova (2010). http://www. pbs.org/wgbh/nova/tech/materials-changed-history.html 2. Thomas W. Eagar, Bringing New Materials to Market, Technology Review, February/March (1995). Pg. 43. 3. Ceramic Tech Today, ACerS Ceramic Materials, Applications & Business Blog, Eileen De Guire, editor, June 30, 2011. http://ceramics.org/ ceramictechtoday/2011/06/30/materials-genome-initiative/. 4. Materials Genome Initiative for Global Competitiveness, National Science and Technology Council, June 24, 2011. http://www.whitehouse.gov/ sites/default/files/microsites/ostp/materials_genome_initiative-final.pdf 5. Alan R. Earls and Robert E. Edwards, Raytheon Company, The First Sixty Years, Arcadia Publishing (2005).

ENGINEERING PROFILE involved in all aspects of materials and mechanical engineering science.

Principal Engineering Fellow, IDS

A former research associate with MIT, Tustison began his Raytheon career 32 years ago. He became materials engineering manager and ultimately managed the research facility until 2002. He has been engaged in both independent research and development and contract research and development (CRAD) throughout his career. He acknowledges the importance of being able to provide the best solutions at the lowest cost, particularly in our dynamic environment. “We must constantly innovate. Through CRAD projects we have the opportunity to shape solutions to difficult problems while working directly with the customer.”

Dr. Randy Tustison is a principal engineering fellow on staff in Integrated Defense Systems’ Mechanical Engineering Directorate where he is capture manager for Advanced Technology. He is also Mechanical, Materials and Structures Technology Area Champion for Raytheon,

Since joining Raytheon, Tustison has been active in the area of optical materials and coatings development. He was a member of DoD Militarily Critical Technology Working Group − Optical Materials and a member of the U.S. delegation to the 4th NATO Conference on

Randal Tustison

Infrared Materials. He is a member of the American Vacuum Society and past chair of the Vacuum Technology Division. He was chair of SPIE’s Windows and Dome Technologies and Materials Conference 2011. Tustison also serves on the industrial advisory board of the National Science Foundation’s Center for High-Rate Nanomanufacturing. Tustison notes that the thing that excites him most about his work is being able to contribute to the creation of something that hasn’t existed or been done before. “The creative part of research and development can be very rewarding.” He holds a bachelor’s degree in physics from Purdue University and master’s and doctoral degrees in materials science and metallurgical engineering from the University of Illinois at Urbana-Champaign. He is a member of Sigma Pi Sigma and a qualified Raytheon Six SigmaTM Specialist.

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Feature

Nanoscale Colloidal Quantum Dots Providing Innovative Solutions for Evolving Imaging System Applications Imaging systems for defense and homeland security are evolving from expensive, single-color, planar components into cost-effective, multi-color, conformal configurations. Nanotechnology is enabling these changes.

A

focal plane array (FPA) is the modern day “film” of an imaging system. Photons emitted or reflected by a scene are collected by the camera optics and imaged onto the FPA. The FPA is composed of two components: the detector array and the readout integrated circuit (ROIC). The detector array contains thousands to millions of detector elements. Through a hybrid circuit manufacturing process, each of the detector elements in the detector array is connected electrically and mechanically to a companion unit cell circuit on the ROIC by an indium bump interconnection. The detector elements produce photocurrents that travel through the indium interconnects into ROIC unit cell circuitry, where the photocurrent is integrated and stored for subsequent readout via a multiplexer. Detector characteristics usually limit operation to a certain band of photon wavelengths or spectral region. For example, to detect two widely separated bands (such as infrared [IR] and ultraviolet [UV]) two sets of FPAs are typically required, more than doubling the cost to the enduser. The FPA manufacturing process is also expensive, requiring extensive capital equipment not only to fabricate the small indium interconnects on each array, but also to carefully align and press the hybrid circuit layers together. Quantum dots offer a potential technology solution to mitigate both of these cost drivers.

Quantum Dots Quantum dots are minute semiconductor crystals, typically a few nanometers (nm) in size. At these small dimensions, the physical extent of the quantum dot becomes smaller than the natural size of an electron-hole pair, and an effect called quantum confinement occurs. This can favorably change the optical properties that are governed by the size of the quantum dot. For example, by adjusting only the size of the quantum dot one can fine tune the photon absorption or emission spectra without requiring a complicated change of semiconducting material composition or stoichiometry. In addition, the photon absorption can be greater in quantum-confined nanocrystals, thus making for a more efficient detector. Quantum dots are typically produced using one of two processes. They are either grown with molecular beam epitaxy onto a crystal substrate, or they are synthesized with solution-based chemistry, producing a nanocrystal colloidal suspension. The synthesis and application of colloidal quantum dots (CQDs) are discussed in the remainder of this article. Figure1 shows the structure of a CQD; however, they need not be only spherical. Disks, rods and other structures have been produced by means of colloidal quantum dot processing. This example uses lead sulfide (PbS) CQDs that are made by

injecting lead and sulfur organo-metallic precursors into a flask held within an inert atmosphere. Precise temperature and solvent combinations control the size of the CQDs. The CQDs are often encased in a semiconducting shell to protect and passivate the surface. Surface groups, such as ligands*, are attached to the shell and allow the CQDs to be suspended in solution. This solution-suspended state permits convenient deposition onto a variety of surfaces. Deposition techniques include drop casting, jet printing, stamping and spin casting to produce novel devices on a variety of substrates. As shown in Figure 2, varying the CQD size modifies its physical properties, such as the bandgap, while using the same CQD

Core: Active material 1−20 nm size spheres, rods, disks, etc.

Shells: Protective or complementary layers

X Functional Groups: Chemically, electrically, or optically active groups

Surface Groups: Passivating, protective and chemically active layer

Figure 1. Colloidal quantum dot composition. (Courtesy of the Bawendi Group, MIT.)

* Ligands – Ions or molecule that bind to transition metal ions to form complex ions. 10 2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

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Quantum Dot Applications Raytheon is developing quantum dot applications in collaboration with Prof. Moungi Bawendi’s Group at the Massachusetts Institute of Technology (MIT). The Institute of Soldier Nanotechnology (ISN), an Army University Affiliated Research Center (UARC), provides funding for this joint activity.1,2 These applications include not only short wavelength infrared (SWIR, nominally defined as the 1–2 micron spectral range) sensitive surfaces for use in FPAs, but also emitting surfaces for use in microdisplays, communications and dual-band detection devices. Since CQDs can downshift the frequency of photons from the UV (nominally 200–400 nm) to the visible or SWIR region, a layer of CQDs on the detector transforms a SWIR FPA into a combination UV/SWIR FPA. So in addition to the usual SWIR imaging, the camera now has the ability to image in the UV for applications in UV optical communications (line-of-sight and

CdSe

2 nm

8 nm

Conduction Band

(arb. unit

)

Energy Gap Valence Band

ty PL Intensi

material. In contrast, significantly changing the bandgap of a bulk semiconducting material typically requires a different chemical composition or stoichiometry. The left side of the figure shows that as the CQDs get smaller, the energy gap becomes larger, with discrete conduction and valence band levels. This means that photon absorption versus photon energy is no longer continuous, as in the bulk semiconductor case. Instead, the spectra are divided into a discrete set of levels that have enhanced absorption at each resonance. Similarly, photon emission depends on CQD size, which is demonstrated by using ultraviolet light to excite different size CQDs contained in a set of vials (right side of figure). The CQDs fluoresce with different colors and the smaller the CQD, the more blue-shifted is the emission. Figure 2 demonstrates this effect for CdSe dots ranging in size from 2 to 8 nm in diameter.

Bulk semiconductor: Energy bands are continuous

Nanosemiconductor: Energy bands are discrete

8 nm Diameter 500

600 700 Wavelength (nm)

800

2 nm

Photoluminescence (PL) Spectra of CdSe and CdTe Quantum Dots

Figure 2. Quantum confinement effects in quantum dots modify the semiconductor energy level structure.

non-line-of sight), in detection of covertly placed UV tags, in UV biometrics and in UV muzzle flash detection. In this way the technology increases the spectral range for broadband imaging applications.

Quantum Dot Imaging Technology CQD enhanced imagers can take advantage of both direct detection photocurrent generation and the frequency downshifting properties of CQDs. Figure 3 illustrates how Raytheon employs the photocurrent properties of CQDs. The upper illustration depicts conventional technology that uses a hybridization technique. The lower illustration (cross section and top views) shows the CQDs deposited on the top over-glass layer of a CMOS (complementary metal oxide semiconductor) ROIC that has been post-processed with a gold grid structure. The CQDs are photo-active in the annular sections of the grid and are surrounded by the gold electrodes. The CQDs absorb photons and produce photocurrents,

Conventional Technology Detector Substrate Detector Array Pixels Readout Integrated Circuit (Indium bumps between detector and ROIC substrates)

Colloidal Quantum Dot Technology QCDs detecting film − no substrate

Readout Integrated Circuit (QCDs lay in between metallic grid deposited on top surface)

Figure 3. Comparison of colloidal quantum dot technology with conventional hybrid technology for an imager employing the photocurrent properties of CQDs. Due to their inherent simplicity, CQD imagers promise lower fabrication costs and higher reliability.

which are injected via the gold electrodes into the underlying CMOS circuitry for further processing. continued on page 12

RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1 11

ENGINEERING PROFILE Lauren Crews Manager, IRAD IDS Dr. Lauren Crews manages Integrated Defense Systems’ portfolio of independent research and development (IRAD) projects. She is also chair of Raytheon’s Mechanical, Materials and Structures technology network, a companywide community of more than 1,000 engineers and technologists. “Much of our new technology that advances the state of the art and creates solutions to customers’ unmet needs is developed through our IRAD program,” says Crews. As IRAD manager, she ensures that all projects are clearly defined, well planned, aligned with customer needs and achieve their intended results. Prior to her current roles, Crews served as the Mechanical, Materials and Structures Technology Area director for Raytheon Corporate Research and Technology. Before this, she was a section manager within IDS’ Mechanical Engineering Directorate. She was also the mechanical engineering lead for the Advanced Spectroscopic Portal program and the deputy lead of the Ship Integrated Product team for the Cobra Judy Replacement program. Crews started her career with Raytheon as a mechanical design engineer and thermal analyst for ship- and ground-based radar systems. Crews is a certified Raytheon Six Sigma™ Expert and a graduate of the IDS Program Management College, the Raytheon Leadership Excellence Program and the IDS Strategic Development Program. She received a bachelor’s degree in aerospace engineering from the University of Maryland, and she has both a master’s and Ph.D. in aeronautics and astronautics from MIT. Crews speaks to her experience with new technology and IRAD innovations: “It’s exciting to watch technologies mature and see first-of-akind demonstrations, knowing that the success of these projects will enable great things for our customers and support the future success of our business.” 12 2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

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Quantum Dots

continued from page 11 The CQD detecting film is applied by spin coating onto a packaged ROIC. The ROIC itself forms the detector substrate, so no hybridization is required. The conventional hybrid technology shown above requires several layers of processing with many more interfaces and interconnections, which increase the fabrication cost. Figure 4 shows how CQD technology can significantly reduce complexity, cycle time, size, weight, power and cost by employing its downshifting properties. CQDs are deposited onto the detector surface of a conventional hybrid focal plane array. The CQDs once again absorb photons; but instead of producing a photocurrent, they emit longer-wavelength (red-shifted) photons that are subsequently detected by the FPA. This extends the detection band of the imager without the need for a second FPA. The middle section of Figure 4 shows the conventional technology, and the lower section depicts the reduction in complexity through the use of CQDs.

Experimental Results Figure 5 illustrates an imaging setup for evaluating the performance of directly deposited CQDs onto ROICs. Separate quad structures with circular depositions of CQDs on three of the four quadrants of the ROIC allow testing of different CQD materials. Also shown is the image formed by an infrared light beam focused by the camera onto the CQD focal plane. The image shown in Figure 5 demonstrates that an IR focal plane can be produced by simply drop casting CQD material onto an appropriately modified silicon ROIC. Visible focal planes can be made in a similar fashion. It is also possible to extend the spectral response of the resultant FPA device beyond the visible range into the SWIR spectral band consistent with the spectral characteristics of the CQDs. While this nascent CQD FPA technology does not

CQD Downshifting Property

Detector Array Readout Integrated Circuit

Dual Band CQD Focal Plane Array (FPA) Conventional Technology Two Optics

Band 1 Band 1 FPA Image

Band 1 Band 2 Scene

Band 2 Band 2 FPA Image

Combined Image

CQD Enhancement Colloidal Quantum Dot Technology Band 1

Band 2 Scene

One Optic

Single FPA

CQD converts Band 1 to Band 2

Combined Image

Figure 4. The downshifting property of CQDs can considerably simplify the design of a multi-band focal plane array. currently match the performance of stateof-the-art SWIR FPAs, it does significantly reduce complexity and, therefore, cost. This has the potential to address more costsensitive applications in which the higher level of performance is not required. For example, one application may be the formation of non-planar conformal focal plane arrays as opposed to the current rigid, rectangular structures. Another example is to replace a few expensive systems that monitor a field of regard, with numerous, less expensive networked systems that can monitor and expand the field of regard with system redundancy. We have also demonstrated the downshifting capability by depositing CQDs on a quartz plate that is displaced off-focal plane. The CQDs absorb in the UV and

Feature

ENGINEERING PROFILE Christopher Solecki Technology Area Director, SAS

Readout Integrated Circuit

Infrared Camera

CQD Film

Blow up section of CQD Film with Gold Grid on Top Surface 4.0 µm

Gold Pad

Image of a SWIR light beam on the CQD film

Gold Ground Plane 8.3 µm

CQD Annular Region

Figure 5. CQDs are deposited on a readout integrated circuit and mounted in a test assembly. An image is formed by the readout integrated circuit upon application of a bias voltage and exposure of the CQDs to infrared radiation. visible bands and emit at ~1,300 nm in the SWIR region. Using a re-imaging technique that focuses the CQD emissions from the downshifting plate onto an InGaAs (indium gallium arsenide) FPA, it was possible to extend the spectral response of this FPA into the UV region. Raytheon is working with the MIT Bawendi Group on several CQD applications, including biomedical imaging research. By functionalizing** the CQD surface groups, they can be designed to stick to stem cells, tumors and other biological structures.3 This allows for direct imaging and monitoring of medical and biological processes using Raytheon SWIR cameras set to detect certain tissue transparency bands. Recently this was demonstrated at MIT by imaging a mouse liver. The liver collects the functionalized CQDs that were injected into the

blood stream. These CQDs were irradiated, and the SWIR downshifted emissions were imaged with a SWIR camera using one of the SWIR transparency bands of the epidermal tissue. CQDs can be used to modify the capabilities of conventional FPA imaging devices. As the technology matures, CQDs will not only enable more cost-effective defense products, but they will also help drive new products and innovations in both adjacent sciences and commercial marketplaces. • Frank Jaworski 1Geyer,

Scott, Ph.D. Dissertation: Science and Applications of Infrared Semiconductor Nanocrystals, MIT Chemistry DeptBawendi Group, 2010. 2Efficient Luminescent Down-shifting Detectors Based on Colloidal Quantum Dots for Dual-Band Detection Applications, May 2011, ACS Nano (American Chemical Society). 3W. Liu, et al. Compact Biocompatible Quantum Dots Functionalized for Cellular Imaging, JACS, Jan. 5, 2008.

Christopher Solecki is the Mechanical, Materials and Structures Technology Area director for Raytheon Corporate Research and Technology. In this role, he is responsible for coordinating this technology development across the company. He has a total of 17 years of unique engineering experience — designing, testing, manufacturing and implementing materials and design solutions for challenging applications. As technology area director, Solecki interfaces with Raytheon’s customers on strategies for applying our technologies to address their mission requirements. Internally, he works closely with engineers, technologists and engineering leadership in developing technology plans and road maps to align with our customers’ needs. Solecki has always had a keen interest in composites and structures, radomes, and high-temperature materials. He is a subject matter expert in these areas and, as a result, has held lead radome development roles on numerous programs, including Future Naval Components – Phase 3, Volume Search Radar (VSR) DDG 1000, Advanced Medium Range Air-to-Air Missile (AMRAAM), proprietary radome programs and the Hypersonic Wideband Radome (HWR). Before joining Raytheon in 2004, Solecki worked at Lockheed Astronautics, Ball Aerospace, Cytec Engineered Materials and Spectrum Astro. “From early in my career, I have always looked to do more; make an impact and make a difference,” Solecki states. “Being the technology area director for Mechanical, Materials and Structures affords me this opportunity.”

** Functionalizing – Adding new features by altering the surface chemistry. RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1 13

Feature

Carbon-Based Nanotechnology Realizing the Promise and Overcoming the Challenges

A

s early as 1970, there was speculation that carbon fullerenes* existed in addition to the well known allotropes** of carbon found in the forms of coal, soot, diamond and graphite.1,2 The existence of C60 fullerenes, or “buckyballs,” was first demonstrated by Kroto, Curl and Smalley of Rice University in 1985. For their work, they were awarded the 1996 Nobel Prize in Chemistry. With the discovery of this new class of carbon allotropes, research interest in this family of materials exploded. In addition to buckyballs, fullerene structures include other spherical, ellipsoidal and tubular shapes, all of which display a hollow, cage-like structure formed by each carbon atom being covalently bonded to three others. The first carbon nanotubes (CNTs) were synthesized in 1991, and they have attracted increased attention since then as a result of their unique and tailorable properties.3 The structure of a CNT is shown in Figure 1. The figure depicts a single-wall carbon nanotube (SWNT). Note that CNTs are composed entirely of sp2 bonds, which are stronger than the sp3 bonds found in the diamond form of carbon.4 Multiwalled nanotubes

Figure 1. The structure of a carbon nanotube. Image generated by Ninithi Software for Nanotechnology.

(MWNT) also exist, and they are essentially the equivalent of concentric SWNTs. A number of CNT-based nanomaterials are under development based on their unique properties, which make them attractive alternatives to traditional materials. CNTs can be formed into a thin sheet. “Buckypaper” is a particular type of CNT sheet. Figure 2 shows a scanning electron microscopy image of buckypaper on the left, as well as a large sheet of CNT paper on the right (produced at Nanocomp Technologies, Inc.). Due to advances in the manufacturability of buckypaper, it is becoming increasingly common to find this form of CNTs being used in structural, electromagnetic interference (EMI) shielding and thermal applications. Graphene is another carbon-based nanomaterial that is receiving a lot of attention since Geim and Novoselov were awarded the 2010 Nobel Prize in Physics for its discovery. Graphene is a single atomic layer of carbon, equivalent to a CNT that has been “unrolled” into a two-dimensional structure as shown in Figure 3. Graphene is highly transparent yet conductive, making it an

excellent candidate for photovoltaic applications, liquid crystal displays (LCDs) and light-emitting diodes (LEDs).5 CNTs can behave as semiconductors or metals, depending on their structure, and they can support high current densities. The thermal conductivity of CNTs is elevated along the nanotube axis and is approximately ten times that of copper. Yet, CNTs are excellent thermal insulators along their radial axis. In addition to their electrical and thermal characteristics, CNTs are promising for structural applications due to their high strength and stiffness. With diameters on the order on 1−10 nanometers (nm), and lengths ranging from the submicron scale to several millimeters or more, CNTs exhibit tensile strengths along their axes approximately ten times that of Kevlar®. At Raytheon, CNTs are being developed for use in EMI shielding and high-strength applications where lightweight materials are required. They are also being developed for use as thermally conducting interfaces in high-power devices. Additional uses for

Figure 2. Scanning electron microscopy image of buckypaper (left); 400 foot roll of CNT (right). Source: Nanocomp.

* Fullerine – Any of various cage-like, hollow molecules composed of hexagonal and pentagonal groups of carbon atoms (adapted from the American Heritage Dictionary, 2009). ** Allotrope – Any of two or more physical forms in which an element can exist (adapted from the Collins English Dictionary 2009). 14 2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

Feature 10 ~60

Tensile Strength (GPa)

CNTs

8

Steel

5

CNTs are being investigated through collaborations with university laboratories. However, challenges still exist in the development of CNT technologies for real-world applications. The behavior of bulk CNT materials often falls short for that of a single nanotube. Novel CNT growth techniques, as well as CNT alignment and integration into bulk materials, are ongoing and critical research areas. In fact, twenty years after they were first synthesized, the potential for CNTs is just now being realized.

Properties of Carbon-Based Nanomaterials In applications where strength or weight is an issue, CNTs and buckypaper can have a significant advantage over traditional materials. While individual CNTs have measured tensile strengths ranging from 10–150 gigapascals (GPa) in laboratory demonstrations, the tensile strength of mass-produced yarns of CNTs and buckypaper has fallen far short of those values. State-of-the-art CNT yarns and sheets have tensile strengths of 1.5–3.0 GPa and 0.4–1.2 GPa, respectively.6

Figure 3. The structure of graphene (image generated by Ninithi Software for Nanotechnology).

Copper

Density (g/cm3)

6

4

Kevlar 3

CNT yarns

2 1

4

Buckypaper (non-aligned) Buckypaper (aligned)

Graphite Steel

2

Aluminum

Copper

CNT yarns Buckypaper

Aluminum

Kevlar

0

0

Figure 4. Plots showing the relatively high tensile strength and low density of carbon-based materials. While still lagging behind Kevlar (3.7 GPa), these values meet or exceed those of copper, aluminum and steel. Moreover, when the density of the materials is considered (typical densities < 1 g/cm3), the strengthto-weight ratio of CNT-based materials holds a dramatic advantage over these other materials, including Kevlar. The tensile strength and density of carbon-based and other common materials are shown in Figure 4.6

CNT Fabrication Techniques Synthesizing CNTs remains a primary challenge because low yields, and often diverse and unpredictable properties, are obtained7 due to the assortment of material diameters, lengths and chiralities*** produced; yielding a mix of semiconducting and metallic material. CNTs can be fabricated using a number of techniques, including chemical vapor deposition (CVD), electric arc discharge and pulsed laser ablation. Each technique has numerous process variables that affect uniformity, defects and purity, thereby affecting ultimate quality and utilization. Because of its potential for large-volume CNT production, CVD synthesis is the method most commonly employed in industry. In the CVD process, catalysts such

as cobalt, molybdenum, iron and nickel are nucleated using thermal or chemical methods. Gaseous carbon compounds are introduced at high temperatures, typically on the order of 700–1,000 degrees Celsius (lower for plasma-enhanced CVD). Carbon migrates to the nucleation sites and nanotubes are grown on the catalysts. Electric arc discharge is probably the most common synthesis method used in research due to its relatively low equipment cost. Using this method, high current discharge between graphite electrodes generates individual carbon atoms that migrate to a cathode and crystallize, forming nanotubes. Laser ablation techniques similarly use high-power lasers (pulsed or continuous wave) to vaporize graphite, forming nanotubes as the carbon condenses onto a cool substrate. Table 1 summarizes some advantages and disadvantages of these techniques. While optimal CNT synthesis remains a topic of much investigation, manufacturing processes for CNTs have matured significantly. CNTs are produced at many small and large companies throughout the industry. Annual global production is projected to reach ~1,000 tons in the coming years.9 Additionally, improvements in manufacturing continued on page 16

Table 1. Common methods for CNT synthesis.8 Method

Typical Yield

Pros

30−90%

Inexpensive, simple equipment

Short, random-length tubes

Chemical Vapor Deposition ~30%

Long tubes, good purity, scalable

High density of defects

Laser Ablation

High purity, good diameter control Capital equipment cost

Arc Discharge

< ~ 70%

Cons

*** Chirality – The configuration (or handedness) of an asymmetric structure (adapted from the Collins English Dictionary 2003).

RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1 15

ENGINEERING PROFILE

Feature

Nanotechnology

Mary Herndon Principal Engineer, IDS Since joining the materials engineering group in January 2006, Dr. Mary Herndon has implemented novel optical techniques for characterizing scatter and absorbance properties in laser materials. She currently leads nano- and metamaterials independent research and development for Raytheon Integrated Defense Systems and the corporate-level Nano Science and Engineering Technology Interest Group. In her position, Herndon has had the opportunity to improve upon existing products as well as identify new solutions. As a materials engineer, she gets to look at both incremental, near-term solutions and potentially disruptive materials and technologies. Herndon served as the materials engineering lead for various Zumwalt, Patriot and Terminal High Altitude Area Defense (THAAD) efforts. She participated in Raytheon-sponsored projects involving metamaterials as well as chemical, biological and explosive (CBE) sensing technologies. Before joining Raytheon, she spent six years working at startup companies, where she managed a wafer fabrication lab and specialized in transitioning engineering processes to volume manufacturing. Herndon has experience in optical characterization techniques, semiconductor processing, and the transfer of processes from prototype to production manufacturing. She received her Ph.D. in applied physics with an emphasis in materials science from the Colorado School of Mines, while working within the Center for Solar and Electronic Materials and collaboratively with partners at the National Renewable Energy Lab. “I went into physics because I enjoy handson experiments and figuring out how things work,” Herndon comments. “I am also somewhat of an obsessive planner and organizer. My work allows me to be in the lab and enjoy the creative and applied aspects of research while also organizing the details and logistics of project management.”

16 2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

continued from page 15 methods have led to about a 75 percent drop in the cost of CNTs over the last ten years.10 Now that reliable sources of CNTs are available, the largest barrier to commercialization of CNT-based products is the integration of CNTs into composites and other material systems. By themselves, CNTs are typically sold suspended in a solvent; however, agglomeration often prevents uniform dispersion and inhibits the potential benefits of CNT use. In addition, lack of material standards and metrology capabilities, as well as environmental, health and safety (EH&S) concerns, have limited the rate at which CNT technologies have been accepted and therefore can mature.

Nanocomp Technologies produces CNT sheets. They use the CVD-based nanotube growth process combined with a drum uptake to allow batch sheets to be fabricated in sizes up to 4.5 by 9 feet. These pieces can then be seamed into indefinitely sized roll stock (see Figure 2). Nanocomp also produces CNT yarns at a rate of ~15 km/week. Depending on the processing, buckypaper can be malleable or brittle. Typical sheets are on the order of 25 microns thick and less than 2 grams/ft2.12 Some typical properties of CNT sheets are listed in Table 2.

Table 2. CNT sheet properties compared to copper and aluminum. Property

CNT Sheet

Copper

Aluminum

Thermal Conductivity (W/m-K)

22 random/100 aligned*

395

237

17

23.6

Coefficient of Thermal Expansion 2 DC Resistivity (Ohm-cm)

2 x 10-4

1.56 x 10-6

2.45 x 10-6

Resistivity at 1MHz (Ohm-cm)

1 x 10-5

1.69 x 10-7

2.82 x 10-6

*Individual single-wall carbon nanotubes can exhibit thermal conductivities of up to 3,000 W/m-K. Sources: Nanocomp Technologies and Solid State Physics, Ashcroft & Mermin, 1976

Buckypaper Compared to composites loaded with CNTs, the use of buckypaper circumvents many of the agglomeration and EH&S issues. CNT sheets or film (buckypaper) was first produced by Smalley et al in 1998, when suspensions of functionalized CNTs were vacuum dried on membranes.11 A similar technique is being used at the High Performance Materials Institute (HPMI) at Florida State University, where sonicated CNTs are being filtered through membranes to produce buckypaper in both batch and continuous processes. Aligned buckypaper can be fabricated from carbon nanotube “forests” by essentially flattening the CNTs in one direction against the substrate. The strong Van der Waals (intermolecular) forces between nanotubes create a structure that is then easily removed from the substrate. Alignment can also be achieved by postprocessing randomly oriented buckypaper.

CNT sheets and yarns can be used in cables to replace conventional EMI shielding and conductor materials. Nanocomp Technologies has demonstrated a weight reduction of 40–50 percent for coax cables, and as much as 70 percent for USB cables. In addition to cable applications, buckypaper is well suited for use as a pre-impregnated material. Composites that incorporate CNT sheet can provide EMI shielding, integrated de-icing heaters and lightning protection solutions for aircraft. A Nanocomp CNT sheet was recently deployed on the Materials International Space Station Experiment-8, and it was implemented on the Juno mission, launched in August 2011, to provide electrostatic discharge protection for the spacecraft. Completion of stringent qualification criteria for NASA, combined with Nanocomp’s high-volume capabilities, has proven that these technologies are poised to realize the potential of CNTs in real-world applications.

Feature Graphene Graphene was recognized for years as a contaminant, which often formed on the surface of semiconductors or metals and interfered with electronic transport experiments.5 Mechanical exfoliation of graphene was first demonstrated in 2004, promoting the explosive growth of graphene research. With intrinsic charge carrier mobilities higher than any other known material,13 and better thermal conductivity at room temperature than diamond,14 graphene’s extraordinary properties are being investigated to provide next-generation solutions for complementary metal oxide semiconductors (CMOS), among possible applications.

as nickel, copper and platinum has been demonstrated.16,17,18 Epitaxial growth on SiC is achieved by heating the substrate to temperatures of 1,200–1,800 degrees Celsius, where silicon desorbs and promotes the formation of graphene through the rearrangement of the remaining carbon atoms.19 Compared to exfoliation techniques, similarly high mobilities at room temperature have been achieved with epitaxial methods.20

While mechanical exfoliation continues to be the most common method for graphene synthesis, epitaxial growth is also being optimized to allow for oriented, large-scale production. As the name implies, mechanical exfoliation of graphene is achieved by rubbing graphite on a smooth surface.15 The resulting film has good electrical properties and can reliably be produced at the millimeter scale.5

Graphene is a remarkable electronic conductor, capable of achieving intrinsic charge carrier mobilities of 200,000 cm2/Volt-sec at room temperature and capable of sustaining current densities5 of 5 x 108 A/cm2. Its thermal conductivity (at room temperature) is as high as 5,000 watts/meter-Kelvin, more than double that of high-quality diamond and more than an order of magnitude greater than copper. Additionally, graphene has a high intrinsic strength of 130 GPa, a Young’s Modulus of 1 terapascal (TPa), and it can support strains in excess of 20 percent without breaking.21

Epitaxial growth is currently performed on silicon carbide (SiC) substrates, but growth on large-area polycrystalline materials such

Because of its single atomic layer thickness, the surface area to volume ratio of graphene is very high, making it potentially

interesting for sensor applications and energy storage. Quantum confinement of a charge in graphene allows bandgap tuning, yielding essentially unlimited design possibilities for nanoscale transistors.22,23 Graphene applications for LCDs, organic LEDs and transparent conducting electrodes for solar cells are enabled by its unique combination of electronic and optical properties. Raytheon continues to explore these promising allotropes of carbon to enable new functionalities, as well as to improve the performance and reduce the weight of our products. The use of CNT-loaded composites has been explored for light-weight body armor applications. We are currently partnering with Purdue and Georgia Tech to develop CNT-based interface materials to enable efficient thermal contact between electronic devices and heat spreaders. These projects and many others are fueling the evolution and ultimately the realization of advanced carbon-based materials solutions for the warfighter. • Mary K. Herndon, Stephanie Fernandez The authors would like to thank John Dorr and David Lashmore of Nanocomp Technologies, Inc., for contributing data and graphics used in this article.

References: 1. E. Osawa, Kagaku, 25, 854-863 (1970). 2. Henson, R.W., The History of Carbon 60 or Buckminsterfullerene, website revision March 2010, http://www.solina.demon.co.uk/c60.htm. 3. Iijima, Sumio, Nature, 354, 56–58 (1991). 4. R. Saito, G. Dresselhaus, and M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, World Scientific Publishing Co, 1998. 5. Michael Fuhrer, Chun Ning Lau and Allan H. MacDonald, Graphene: Materially Better Carbon, MRS Bulletin, 35 (4), April 2010, pg 289. 6. Source: Nanocomp Data, April 2011. 7. Alvarenga, J., Carbon Nanotube Fabrication and Characterization, Faculty Presentation, Department of Physics, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623-5603, USA, February 2008. 8. Laura Young, Trevor Seidel, Pradip Rijal, Jason Savatsky, Carbon Nanotubes, Presentation for Texas A&M Dept of Chemical Engineering, March, 2010. 9. Science and Technology Policy Institute, White Papers on Advanced Manufacturing Questions, 2010 10. Jon Evans, Manufacturing the Carbon Nanotube Market, Chemistry World, November 2007, Royal Society of Chemistry. 11. Rinzler, Liu, Dai, Niolaev, Huffman, Rodriguez-Macia, Boul, Lu, Heymann, Colbert, Lee, Fischer, Rao, Eklund and Smalley. Large-Scale Purification of Single-Wall Carbon Nanotubes: Process, Product, and Characterization, Applied Physics A, 67, 29–37 (1998). 12. Source: High Performance Materials Institute, Florida State University, Tallahassee, Fla. 13. S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak and A. K. Gein, Phys Rev Letters, Volume 100, 026602 (2008). 14. A. A. Balandin, S. Ghosh, W. Bao, I Calizo, D. Teweldebrhan, F. Miao, C. N. Lau, Nano Letters, 8, 902 (2008). 15. K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, A. K. Geim, PNAS V.102, 10451 (2005). 16. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, J. H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 457, 706 (2009). 17. X. S. Li, W. W. Cai, J. H. An, S. Kim, J. Nah, D. X. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, R. S. Ruoff, Science, 324, 1312 (2009). 18. A. Reina, X. T. Jia, J. Ho, D. Nezich, H. B. Son, V. Bulovic, M. S. Dresselhaus, J. Kong, Nano Letters, 9, 30 (2009). 19. Philip N. First, Walt A. de Heer, Thomas Seyller, Claire Berger, Joseph A. Stroscio, Jeong-Sun Moon, MRS Bulletin, 35, 296 (April 2010). 20. Y. Zhang, Y.-W. Tan, H. L. Stormer, P. Kim, Nature, 438, 201 (2005). 21. C. Lee, X. D. Wei, J. W. Kysar, J. Hone, Science, 321, 385 (2008). 22. K. Nakata, M. Fujita, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B, 54, 17954 (1996). 23. Yuanbo Zhang, Tsung-Ta Tang, Caglar Girit, Zhao Hao, Michael C. Martin, Alex Zettl, Michael F. Crommie, Y. Ron Shen, and Feng Wang, Nature, 459, 820–823 (11 June 2009). RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1 17

Feature

R

aytheon is an industry leader in high-power radio frequency (RF) semiconductor device development and integration. Raytheon’s wide bandgap gallium nitride (GaN) device technology for radar, electronic warfare and communications applications offers significant cost, size, weight and power advantages over conventional devices employing gallium arsenide (GaAs) technology. For comparably sized devices, GaN produces five to 10 times more RF power than GaAs. As shown in Figure 1, this significant increase in output power and die-level dissipation within the same footprint presents a challenge for controlling device junction temperatures. The extreme heat fluxes present in GaN semiconductor devices are similar to those experienced within a rocket nozzle, or upon ballistic entry. If thermal resistance is not effectively addressed, device junction temperatures can easily exceed levels that affect reliability. To realize GaN’s full potential, Raytheon is collaborating with leading researchers from academia and industry to investigate micro- and nanoscale technology-enabled approaches for improved thermal management.

These investigations are motivated by the fundamental and potentially beneficial characteristics unique to engineered micro and nanomaterials. For example, individual carbon nanotubes (CNTs) have been reported to exhibit extraordinary on-axis thermal conductivities of greater than 3,000 watts/ meter Kelvin (W/mK), which is nearly eight times that of good thermal conducting metals such as copper. By utilizing micro and nanomaterials, designers can take full advantage of physical scaling laws to increase the effectiveness of thermal management components. This approach has been studied extensively for cold plates and heat exchangers, where reducing the size of channels results in improved heat transfer. Today, microchannel cold plates are used in many commercial products and in fielded military hardware.

Raytheon’s comprehensive technology development strategy, which addresses heat transport, heat spreading, thermal interfaces and chip-scale thermal management, is shown in Figure 2 for a transmit/ receive (T/R) module in an active electronically scanned phased array radar. Advances are required in all aspects of the thermal management system to avoid thermal bottlenecks that can limit performance. Chip-Scale Thermal Management: In collaboration with Group4 Labs, Stanford University and the Georgia Institute of Technology, Raytheon is pursuing approaches to integrate synthetic diamond directly into high-power devices. Integration of polycrystalline diamond, with conductivity greater than three times that of silicon carbide (SiC), has the potential to improve device power handling by a factor of three over the current GaN-on-SiC technology. This enables more powerful and affordable devices.

Radar Transmit/Receive Module

Chip-Scale Thermal Management Goal: Reduce intra-device temperature rises Approach: Highly Integrated Diamond

Heat Flux (W/cm2)

103 102 10

GaN

Ballistic Entry Rocket Nozzle Throat Nuclear Blast

GaAs Si

Re-entry from Earth Orbit

Goal: Reduce interface temperature rise with improved compliance and reworkability Approach: Nano Thermal Interface Materials

Heat Spreading

Rocket Motor Case

Goal: Achieve low temperature rise spreading with minimized thickness Approach: Thermal Ground Plane

1 10-1

Thermal Interfaces

Solar Heating

Chip/Device Interface Chip Carrier Interface T/R Module Package Base Interface TRIMM Coldplate

Coldplate

Cooling Fluid

Interface 1000

2000

3000

Temperature (K)

4000

Figure 1. Next-generation GaN device technologies present formidable thermal challenges, driving the development of advanced thermal management solutions. (Adapted from Bar-Cohen, ca 1985.) 18 2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

Heat Transport Goal: Achieve low-resistance, high-efficiency heat rejection in limited volume Approach: Micro Technology Enabled Heat Sinks/Cold Plates

Cooling Rib

Figure 2. Raytheon, in collaboration with its team members, is taking a multifaceted approach to eliminating thermal bottlenecks to allow maximum device performance. Illustrated are representative thermal interfaces for a radar transmit/receive module.

Feature Thermal Interfaces: In collaboration with Georgia Institute of Technology and Purdue University, Raytheon is developing nano thermal interface materials (nTIMs) based upon metallically bonded, vertically aligned carbon nanotubes (VACNTs), as shown in Figure 3. These nTIMs provide the thermal performance of a solder joint while maintaining the compliance typical of a low-conductivity filled epoxy or grease. Results to date have achieved a factor of three improvement in interfacial resistance relative to state-of-the-art commercial materials. Heat Spreading: In collaboration with Georgia Institute of Technology Research Institute, Purdue University and Thermacore Incorporated, Raytheon is developing radio frequency thermal ground plane (RFTGP) heat spreader technology. RFTGPs use capillarydriven two-phase (liquid and vapor) flow to achieve highly efficient heat spreading from high-power devices in a low-profile, semiconductor thermal expansion-matched chip carrier. RFTGPs are intended to replace solid conductor substrates currently used in device packages, while providing greater than three times the conductivity of copper. This type of heat spreader can potentially reduce device operating temperatures by tens of degrees Celsius in a typical GaN-based radar, improving system performance and reliability. To achieve these goals, various engineered micro and nanostructured thermal wicking materials have been investigated for use in the TGP, including copper-functionalized CNTs (Figure 4). RFTGPs have demonstrated effective thermal conductivities of greater than 1,000 W/mK in form/fit/function interchangeable RF packaging geometries, paving the way for technology insertion. Heat Transport: Advanced heat transport technology development efforts have focused on implementing micro-scale features to enhance heat transfer via a variety of cooling mediums. High efficiency and performance air cooling was developed in Raytheon’s Integrated Microchannel and Jet Impingement Cooler (IMJC) program. Raytheon has demonstrated robust, distributed two-phase microchannel cooling suitable for servicing future naval sensors and effectors on the U.S. Navy’s Advanced Naval Cooling System (ANCS) program. Both efforts seek three-fold improvements in efficiency over current stateof-the-art air and liquid cooling approaches with a two to four times improvement in

200 nm Heat

Heat

Semiconductor Chip/Device

Cu Foil

Vertically Aligned CNT Film

Chip Carrier Heat

Heat

Figure 3. Nano thermal interface materials (nTIMs) employ metallic bonding of vertically aligned carbon nanotubes grown on metallic foils to reduce interfacial thermal resistance by greater than three times relative to state-of-the-art epoxy TIMs. Low CTE Composite Construction

CNTs

Nano-functionalized Grid Patterned Evaporator heat sink

Condenser condensed liquid

Vapor Space Evaporator with CNT Functionalized Patterned Wick

chip

Figure 4. Radio frequency thermal ground planes (RFTGPs) are constructed with nanomaterial-enhanced thermal wicking structures that have low thermal coefficients of expansion, close to that of the semiconductors that are mounted on them.

thermal performance. The improved heat transport afforded by IMJC technology enables the implementation of air-cooling, which was previously not feasible. Twophase microchannel cooling promises to substantially reduce the size, weight and power consumption associated with cooling next-generation high-power electronics systems. As the demand for system performance grows, it stresses the thermal limits of semiconductor devices. Raytheon and its partners are leaders in the development

and integration of new materials (synthetic diamond and carbon nanotubes) and new structures (2-phase microchannel heat spreaders and heat transport devices) that will extend device performance through improved thermal management. • David Altman The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. This is in accordance with DoDI 5230.29, January 8, 2009. Distribution Statement “A” (Approved for Public Release, Distribution Unlimited as per DISTAR case 19086).

RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1

19

Feature

Providing Thermal Management for Power Semiconductors

N

Diamond Manufacturing Process

1800 1600 1400 1200

Raytheon Optical Quality Diamond (OQD)

2000

Thermal Management Grade CVD Diamond

1000 800 600 400 200

(W/mK)

0.2 17 29.3

223 237 149 170

395 428

nd mo Dia r ve Sil er pp Co inum e m xid Alu m O ride liu Nit ryl Be um n mi Alu n ide Ox ico Sil num mi Alu r va Ko ide m lyi Po

Raytheon is an acknowledged leader in the growth and fabrication of diamond in sheet form. At Raytheon, diamond is synthesized by the CVD process from methane and hydrogen gases in the presence of a microwave plasma. The microwave plasma CVD process (MPCVD) produces the highest quality diamond; other methods, such as hot filament CVD, tend to produce lower quality, less thermally conductive material due to the incorporation of impurities from the hot filament.

2000

Thermal Conductivity

Raytheon’s Diamond Technology

ext-generation radar, communications and electronic warfare systems, especially those employing high-power gallium nitride (GaN) based radio frequency (RF) devices, will benefit from advanced methods of thermal management to remove the large quantities of heat generated in these systems. Figure 1 compares the thermal conductivity of materials commonly used in the semiconductor industry. Diamond, especially the high optical quality material produced at Raytheon using the chemical vapor deposition (CVD) process, has considerably higher conductivity than the other conventional materials.

Figure 1. The thermal conductivity of commonly used materials in the semiconductor industry. Diamond’s thermal conductivity is more than three times greater than silver.

Figure 2 illustrates a microwave plasma CVD reactor employed at Raytheon to produce high quality diamond. The microwave plasma, created by exciting hydrogen gas with microwave radiation, decomposes the methane gas into several different carbon species that react/combine with hydrogen in the gas. Diamond growth takes place on a metallic substrate onto which diamond particles have been added. These particles or diamond “seeds” act as nucleating sites for the diamond wafer growth. The atomic hydrogen formed in the plasma plays a critical role by etching away any graphitic or non-diamond material that might deposit along with diamond. Rotation of the metal disk ensures diamond thickness uniformity. Raytheon’s diamond deposition reactors are capable of growing up to 5-inch diameter diamond parts. In Figure 3, one can see the various sizes and shapes of diamond material produced at Raytheon.

20 2012 ISSUE 1 RAYTHEON TECHNOLOGY TODAY

Feature

Figure 2. A schematic of the MPCVD diamond reactor is shown on the left. The substrate is rotated to ensure diamond uniformity. On the right are photos of Raytheon’s high-power diamond reactors in action.

Figure 3. Various shapes and sizes of diamond made at Raytheon. Diameters up to five inches have been demonstrated, with laser cutting used to fabricate the desired sizes and shapes.

Diamond Heat Transfer Technology The primary use for diamond at Raytheon is thermal management, specifically the dissipation of the large heat flux that is generated by high-power devices such as GaN HEMTs (high electron mobility transistors). High junction temperatures degrade RF performance and decrease reliability. A common approach to reduce temperatures is to space transistor gate fingers further apart and reduce the operating power density. However, this increases monolithic microwave integrated circuit (MMIC) size and cost and reduces output power. A novel solution that is being developed at Raytheon to remove heat from high-power devices and enable them to realibly achieve full performance, is to integrate diamond directly into the device structure as illustrated in Figure 4. Thermal modeling has

shown that this technology can improve thermal transport through the substrate so that power handling can be significantly increased relative to the current state-ofthe-art of GaN-on-SiC (silicon carbide) technology. Diamond attached to the bottom of the GaN efficiently spreads the extraordinarily high heat fluxes (0.1–1 kW/mm2) typical of GaN device junctions with minimal substrate temperature drop. Although additional heat spreading is expected by incorporating thin film diamond on top of the device, thermal simulations indicate that the majority of the benefit is achieved with the bottom diamond; hence there is little additional benefit to be gained by incorporating diamond coatings in these device structures. Raytheon’s CVD diamond technology produces very high thermal conductivity material. By combining the extraordinary

Diamond Coating AlGaN epi

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G D

GaN epi Bottom Diamond Substrate

Area of heat generation

Figure 4. High electron mobility transistor (HEMT) schematic showing incorporation of diamond both below and on top of the device structure. S, G and D are source, gate and drain. The red dot indicates approximate location of the source of heat generation.

thermal conductivity of this material with a fully integrated GaN-diamond device architecture, Raytheon seeks to advance our industry-leading GaN device technology to even higher power and performance levels. • Ralph Korenstein RAYTHEON TECHNOLOGY TODAY 2012 ISSUE 1 21

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urrently, applications for infrared (IR) imaging devices are limited by the need for costly, slow, bulky components that require cooling. These components include high-magnification sensors that require large lenses and telescope configurations, noisy mechanical shutters that limit the usefulness of many imaging systems in covert applications, electronics required to mitigate focal plane array (FPA) saturation effects (i.e., blooming) in all-weather applications, and filters that improve imaging contrast at dusk and in haze. The drive to reduce system size, weight and power consumption, while increasing the resolution and format of IR imaging systems, presents many new challenges. Current efforts to develop lightweight, fast components

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have been focused on established micro-electromechanical systems (MEMS) approaches; however, reliable MEMS-based technologies are processing intensive and still not cost effective. Raytheon, in collaboration with the University of California at Santa Barbara (UCSB), has developed new polymers to address these challenges. Polymers, with their infinite customizability and low production costs, offer a novel solution.

Biological Inspiration While investigating the use of polymers for IR imaging, Raytheon looked to nature and the world’s oceans for inspiration. Cephalopods (e.g., octopus, squid and cuttlefish) manipulate light for camouflage and inter-individual signaling using their ability to selectively scatter light with both

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protein platelet, 1.34