23. - 25. 5. 2012, Brno, Czech Republic, EU

OPTIMIZATION OF MAGNETIC PROPERTIES OF THE PURE FERRUM - BEHANIT, MATERIAL FOR CONSTRUCTION OF THE MAGNETIC LENSES OF THE ELECTRON MICROSCOPES, INNOVATIVE RESEARCH OF ELECTRON MICROSCOPES Martin MAREK a, Daniel PETLAK a, Zbyněk RICHTER a, Regina HOLČÁKOVÁ a, Jiří REŽNAR b a) VŠB-Technical University of Ostrava, FEI, KAT410, Laboratory of Magnetic Measurements and Applications, 17. Listopadu 15, 708 33 Ostrava - Poruba, Czech Republic, mail: [email protected], , mobil: +420-724-033-727

Abstract: The paper deals with the basic goals and partial results of the project, which focuses on innovative research of the magnetic chambers of electron microscopes. This project and paper were created by financial support of state budget through the Ministry of Industry and Trade MPO-CR, project No. FR-TI1/334. This part contains the results of the optimization analysis of technology production process of the type’s soft ferromagnetic material which is determined for the building of the magnetic lenses of the electron microscopes. Solved material is pure ferrum with trade mark Behanit. Self optimization process is focused on the annealing process by vacuum technology and analysis of the influence of annealing temperature on the resulting magnetic properties. Keywords: magnetic properties, magnetic measuring, electron microscope, magnetic lens, pure ferrum, Behanit 1.

INTRODUCTION

Development and production of electron microscopes currently achieves a high degree that we can use them to observe and define the structure of atomic and subatomic materials, including specific local analysis at the level of micro and nanostructures. These options and the growing mass production of these devices set, after the age of development of these devices, the age of their significant use and practical application [1]. High requirements for target parameters of these systems lead to increasing demands on all components of the system and especially on optical systems and parts. An important part of the optical system consists of electromagnetic lenses and soft ferromagnetic materials from which the magnetic circuits of the lenses are produced. Requirements for these materials and the resulting components are numerous, ranging from mechanical, chemical and metallurgical over to surface roughness and accuracy. Key role is played by resulting magnetic properties defined primarily by stationary BH hysteresis characteristics and derived parameters. One of the materials that very well correspond with these requirements is technically pure iron prepared in a special procedure under the trademark Behanit. 2.

PURE FERRUM - BEHANIT

Behanit is technically pure iron prepared by vacuum melting and casting technologies, the basic material standard 19991. The purity of the material is around 99.5% Fe. Behanit is currently being prepared in the form of rods of various diameters. More detailed information was given in paper [6]. Product designs of rods of this material are shown in Fig.1 together with basic BH characteristics of this material Fig.2. One partial goal of further development of this material was to optimize the resulting magnetic properties by of the annealing.

23. - 25. 5. 2012, Brno, Czech Republic, EU

Fig. 1 Example of Behanit Ingots

1.1

Fig. 2 Behanit, Stationary BH Hysteresis Characteristic, for Hmax= 8.000 (A/m)

Annealing

Annealing process belongs to the known and standardized procedures for modification of properties of materials. The fundamental role of annealing process in area of the mechanical properties of structural materials is primarily homogenization of material structures and removal of internal stresses. According to the requirements on the target material the annealing process is divided to the following basic types, without pre-crystallization, with pre-crystallization, annealing for stress relief, anti-flake, homogenization, standardization, isothermal, etc. A more detailed description can be found for example in references [4], [5]. A special type of annealing is the magnetization annealing whose purpose is the basic requirement to eliminate the basic requirement internal tension and homogenization and especially the magnetic homogenization of the material and removal of the remanent magnetization and forming of the resulting magnetic properties. Thermal annealing cycle is here defined similarly to the annealing to for removal of internal stresses and is defined by controlled rapid-heating, heating time, and slow cooling. The basic critical points of this temperature annealing cycle are the maximum temperature, and then gradient of the cooling of the material. The general effect of the annealing process on magnetic properties of the type’s material is known from various experimental and theoretical works. For the general expression of the magnetic properties of ferromagnetic materials by using a general mathematical model it is necessary to take into account a wide range of material constants of materials ranging from simple mechanical parameters through the domain structure up to the quantum constants of the individual elements. The exact expression of the magnetic properties of types and specific materials or technological effects based on the theoretical calculation is therefore very complicated and mostly the error destination is often unbearable by both the value and the characteristics of the course. The only possible way for detection and accurate identification of the individual effects is then the performance of the challenging experimental sophisticated tests and measurements. The example of such analysis is shortly presented in this post. The aim of this experimental analysis was to determine the effect of annealing temperature cycle on the resulting magnetic properties of the developed material Behanit. 1.1

Description of the Analysis of the Impact of Annealing on the Magnetic Properties

The main objective of this analysis was to describe the influence of annealing in a wide range of maximum annealing temperature, both below and above the critical Tc value defined for pure ferrum. Range of annealing temperatures and the steps are based on the phase diagram of Fe and binary Fe-C diagram, which are listed on Fig.3. and Fig 4. Values of the specific annealing temperatures are then listed in Table 1.

23. - 25. 5. 2012, Brno, Czech Republic, EU

Fig. 3 Phase Diagram Fe

Fig. 4 Binary Diagram Fe-C

Table 1 Degrees of annealing temperature for each sample and experimental analysis Annealing Temperature TA[°C]

600

700

750

800

850

900

950

1000

1050

1100

For the own analysis and to determine the effect of the annealing there were prepared comprehensive series of toroidal samples (20 pieces) from one type of melting of material and prior the annealing process there was carried out a winding of these samples and also there were carried out measurements of magnetic properties in the form of a stationary BH hysteresis characteristics for different levels of magnetic saturation. This process has provided detailed data on the magnetic properties of individual samples of materials designated for self annealing cycles, as well as information about the homogeneity of the source material itself. After the magnetic measurements there was the measuring winding removed and the samples were subjected to the process of self annealing at particular stages of the temperature by vacuum heating. Afterwards, all samples were winded up again and there were carried out the measurements of resulting magnetic properties of individual samples annealed on particular temperature stages. The procedure is schematically illustrated in Fig 5., which also presents an independent method for determining the influence of annealing on magnetic properties of ferromagnetic materials with high demands on accuracy by measuring the BH hysteresis characteristics on the toroidal samples. Toroidal samples Before annealing

Winding

Winding removal

Toroidal samples after annealing

Measuring of magnetic properties before annealing

Self process of annealing

Winding

Measuring of magnetic properties after annealing

Fig. 5 Diagram of Method for Accurate Determination of Annealing Effect on the Resulting Magnetic Properties of Soft Ferromagnetic Materials with Demand on High Accuracy

23. - 25. 5. 2012, Brno, Czech Republic, EU

1.1

Measurement of Magnetic Properties

The measurement of magnetic properties was carried out in the Laboratory of magnetic measurements and applications at VSB-TU Ostrava, FEI. Measuring was carried out on the measuring system Remagraph intended to detect quasistationary BH hysteresis characteristics of ferromagnetic soft materials. Implementation of the system and basic block diagram is shown in figure Fig.6. and more detailed description can be found in references [2], [3], [4]

Fig. 6 Performing the Measurement System Remagraph 3.

ANNEALING EFFECT ON THE MAGNETIC PROPERTIES

The following section presents the basic results of the analysis of the influence of annealing on resulting magnetic properties of material Behanit. Magnetic properties of the material were evaluated at level of the DC quasistationary BH hysteresis characteristics for different sizes of magnetic field and then key parameters of remanent magnetization which means the value of coercive force, remanent induction, maximum permeability, the size of hysteresis losses. The following table provides a brief overview of comparision of the resulting BH hysteresis characteristics observed on samples for selected annealing temperatures in comparison with properties and BH characteristic of the material prior the annealing process. Table 2 Annealing Effect on the Magnetic Properties of the Material Behanit

Behanit, Basic Quasistationary BH Hysteresis and Magnetizing Characteristics, Material Before the Annealing Process

23. - 25. 5. 2012, Brno, Czech Republic, EU

Behanit, DC-BH Characteristics, Material After Process Annealing, Ta = 700, 800, 900, 1000 °C

23. - 25. 5. 2012, Brno, Czech Republic, EU

Table 3 Behanit, Remanent Parameters for Various Annealing Temperature CONCLUSION The paper presents the basic results of challenging analysis focused on the development and optimization of magnetic properties of type material for the construction of optical elements of electron microscopes. From these results it is clear that to significantly reduce the remanent parameters of this material it is not sufficient to make annealing with temperature on level Tc = 790 ° C but it is essential to increase the annealing temperature at least up to Ta =850 ° C, where further increasing of the annealing temperature has a positive effect on the resulting magnetic properties. For higher annealing stage it is necessary to take into account the metallographic structure and evolution of the grain structure of material. The contribution of this work is also in the preparation and verification of the method itself, which has been developed for determining the influence of annealing on the resulting magnetic properties of ferromagnetic soft materials, with high demand for accuracy. The results of analysis were used directly in the development of type optical system of electron microscopes and 2D and 3D layout simulation of magnetic field of the type’s magnetic lens of electron microscope [7]. ACKNOWLEDGEMENT This project and paper was created by financial support of state budget through the Ministry of Industry and Trade of the Czech Republic, MPO-CR, project No. FR-TI1/334. LITERATURA [1]

DELONG, A., FRANK, L., KNOR, Z., KOLAŘIK, V.,: Metody analýzy povrchů, elektronová mikroskopie a difrakce. Academia, Praha, 1996

[2]

MAREK. M.: Recognition methods of magnetic properties of the construction materials, specific examples of using and applications of the magnetic measuring., dissertation thesis, VŠB-TU Ostrava, 2005

[3]

VŠB-TU Ostrava, FEI, Laboratory of Magnetic Measurements and Applications: http://fei1.vsb.cz/kat453/www453/soubory/MAG_LAB/index.htm

[4]

DRAXLER, K., KAŠPAR, P, RIPKA, P.: Magnetické prvky a měření. CVUT-Praha, 1999

[5]

FREMUNT, P,. PODRABSKÝ, T.: Konstrukční oceli, CERM, Brno 1996

[6]

MAREK, M., PETLÁK, D., HOLČÁKOVÁ, R., JELEN, L., REŽNAR, J.,: Innovative research of electron microscopes – analyze of magnetic properties of technically pure ferrum - Behanit, 20th conference METAL2011, 18-20.5.2011, Hotel-Voronež, Brno, http://www.metal2011.com,

[7]

HOLCAKOVA R., MAREK, M.,: Innovative Research in Electron Microscopes, Analysis of Magnetic Field Distribution of Some Types of Magnetic Lenses by FEM, 10th EEEIC Conference, 8-11 May, Roma, 2011