3 – Synthesis of solids from the gas phase Synthesis of inorganic materials / U. Schubert, N. Hüsing – chap. 3 inorganic solids prepared by reaction of gaseous precursors or intermediates

almost every conceivable solid compound can be made by chemical reactions with gases a.

chemical vapor transport

b.

chemical vapor deposition

c.

CVD-related techniques

d.

non-CVD deposition techniques

1

3a – Chemical Vapor Transport transport reaction: non-volatile solid A reversibly reacts with gas B to form gaseous product AB equilibrium constants are T dependent (van’t Hoff eq.)  equilibrium concentration of AB changes with T temperature gradient in growth system  concentration gradient of AB driving force for mass transport by diffusion from site at T1 (source material + AB) to another site at T2 (sink, growth site) where chemical equilibrium is shifted towards A and B T1

transport reaction in open systems with a continuous stream of transport agent

closed system

T2

A + B  AB exothermic reaction: decomposition of AB within the region at higher T (T1 < T2) endothermic reaction: decomposition of AB within the region at lower T (T1 > T2) formation of intermediate gaseous compounds distinguishes chemical transport from sublimation chemical transport: purification of solids, growth of single crystals, reactive sintering, preparation of new compounds when transport reaction is coupled with a subsequent reaction at the sink CVT faster than s.-s. reactions (better mass transport)

2

Example: Fe2O3 + volcanic gases at hot sites Fe2O3 reacts with HCl(g) (transport agent)

FeCl3(g) + H2O(g)

FeCl3 is transported with the gases and deposited at cooler sites by backreaction:

Fe2O3(s) + HCl(g)  FeCl3(g) + H2O(g) Example: Mond process for high purity Ni (99.9 - 99.99%) exothermic reversible reaction of Ni with CO:

Ni(s) + 4CO(g)



Ni(CO)4(g)

continuous stream of transport agent Ni(CO)4 is formed 50-80°C at 1 bar, transported out of the solid mixture and decomposed at 230 °C Example: incandescent halogen lamp

visible light yield at 2400 °C (Wien law) for incandescent W lamps: 1.4% (electrical power) major improvements: replacement of C by W, inert-gas filling, coiling of filament, addition of halogens W vapor pressure is very low  slow loss of filament material by evaporation (W m.p. = 3410 °C) improving yield of visible light with higher T of W is uneffective due to filament evaporation (+70% efficacy  filament T from 2500 to 2900°C  shortened lifetime by a factor of ~100 higher T  higher vapor pressure of filament  vaporized metal condenses at the colder regions of the bulb  a) W wire becomes thinner  filament blows b) bulb blackens  reduced yield of light 3

bulb size of traditional lamps are large to reduce blackening of the bulb inert-gas filling reduces rate of W evaporation by inhibiting the transport of tungsten (diffusion rate 1/P) halogen lamps: addition of small amounts of I2 (~0.1 mg/cm3 bulb volume) as transport agent original hypothesis: evaporated W diffuses through the inert gas towards the bulb wall, reacts in the cooler zones with the transport medium to form a volatile compound products diffuse backwards to the filament  dissociation at the filament to release W + I2  vaporized W is transported back to the hot filament

Deposition of tungsten crystals on the filament of a halogen lamp (130x magnification).

Halogen lamp: the W filament can be kept at a very high T because evaporating W is returned to the filament: at lower T, near the bulb wall the evaporated atoms do not deposit on the bulb wall but react with a volatile component that decomposes to metal W on the filament.

backreaction of deposited W(s) with I2(g) requires high T of glass wall  smaller size of halogen bulbs to heat the wall to ~600 °C

lower volume & increased mechanical strength of bulbs for use of high P filling

expensive Xe or Kr gas

high P & high molecular mass hamper transport of W  higher filament T true chemical process: low concentrations of O2 necessary to form WO2I2 at ~600 °C at the glass wall by reaction of deposited W with I2 + O2 (or H2O) in an exothermic reaction: W + O2 + I2  WO2I2

WO2I2 migrates towards the hot filament (~3000 °C) and dissociates sequentially into WO2, WO, and eventually W atoms (close to the filament) WO2I2  WO2  WO  W + ½O2 increasing T

W atoms are then deposited at the filament surface  condensation process, not a chemical reaction W atoms deposited at the coldest (already/still thickest) parts of the filament  transport reaction not self-healing process

W evaporates from "hot spots" (small regions with inhomogeneities) at a faster rate until the lamp fails through localized melting or fracture of the filament 5

Transport reactions lab setup: reactions in closed tubes (length ~10-20 cm, diameter ~ 1-2 cm) transport agent reacts at the source, is liberated by the backreaction and diffuses back to the source T1

T2 sample is placed in an ampoule with transport gas, tube is evacuated at P depending on transport agent, sealed and placed in a tubular furnace with a T gradient small amounts of the agent can transport large amounts of the solid transport reactions in closed systems exhibit four steps: 1.

chemical reaction at the source, leading to equilibrium between condensed and gas phase

2. mass transport by diffusion of gaseous compounds from source to sink 3. deposition of the condensed phase at the sink

4. back diffusion of transport gas to the source

6

overall reaction in most cases is diffusion limited mass flux between source & sink given by different partial P of the intermediate gaseous compound fast transport with large

P

high total P slows down balancing of partial pressures  total P in the ampoules kept at ~1 bar at the reaction T by adding the appropriate amount of transport agent true chemical transport reactions not simple: very often several different gaseous species involved

7

chemical synthesis

8

halogens are common transport agents for elemental metals (halogen lamp) Example: van Arkel-de Boer process 1st application of transport reactions for purification of metals (Ti, Cr, V) exothermic reaction between metal and I2 to form a volatile iodide (e.g. CrI2) metal is redeposited at a hot filament: extraction of metals from their oxides, nitrides, carbides, ... elemental halogens are less often used for transport of oxides (equilibrium concentration of binary metal halides is usually very low); more volatile oxo halides may be used metal halides can be transported by AlCl3 through volatile complexes:

CoCl2(s) + Al2Cl6 

use of transport as a preparative method: transport reaction coupled with a subsequent reaction at the sink Example: synthesis of Nb5Si3 metallic Nb do not react with silica at 1100 °C in a vacuum in the presence of small amounts of H2:

SiO2(s) + H2(g)  SiO(g) + H2O(g) 3SiO(g) + 8Nb(s)  Nb5Si3(s) + 3NbO(s) 9

3b – Chemical Vapor Deposition (CVD) one of the most important methods to prepare thin films / coatings of almost all inorganic materials synthetic technique from vapor phase by reacting gaseous precursors in a flow reactor one or more volatile precursors transported via the vapor phase to a reaction chamber where they decompose on a heated substrate  deposition of solid thin films or powders

CVD deposits: metals and multielement materials (oxides, sulfides, borides, carbides, silicides, nitrides, phosphides, arsenides, ...) with good control of the product microstructure suitable for very inert and high melting point materials (e.g. W, TaC, Si3N4, …) Example: TiB2, armor material; preparation of thin films by melting not feasible (m.p. = 3325 °C) CVD of TiB2 films at ~1000 °C: TiCl4 + 8BCl3 + 5H2

TiB2 + 10HCl

10

A few representative examples of CVD reactions: WARNING! The overall reaction equations are only mass balances, necessary for calculating equilibrium constants and driving forces for the processes but do not contain information on kinetics or mechanisms. The precursors always react through a complex set of reaction steps involving many reaction intermediates that usually remain unidentified. Deposition rates and reaction orders are mainly determined by the slowest of a set of consecutive reaction steps. ELEMENTS AND ALLOYS

OTHER CERAMICS:

(a) Pyrolysis: ZrI4(g) Zr(s) + 2I2(g) CrI2(g) Cr(s) + I2(g) Ni(CO)4(g) Ni(s) + 4CO(g) SiH4(g) Si(s) + 2H2(g) CH4(g) C(s) + 2H2(g) AlR3(g) Al(s) + nR'(g) (R, R’ organic groups) (C8H10)2Cr(g) Cr(s) + nR'(g)

BCl3(g) + NH3(g)

(b) Reduction of halide precursors by hydrogen:

Si(OEt)4(g) + 2H2O(g)

2AlCl3(g) + 3H2(g)

2AlCl3(g) + 3CO2(g) + 3H2(g)

SiCl4(g) + 2H2(g) 2TaCl5(g) + 5H2(g)

2Al(s) + 6HCl(g) 2Ta(s) + 10HCl(g) MoxW1-x(s) + 6HF(g)

TiI4(g) + 2Zn(s)

Ti(s) + 2MgCl2(g) Ti(s) + 2ZnCl2(g)

B4C(s) + 12HCl(g)

4BCl3(g) + CCl4(g) + 8H2(g)

B4C(S) + 16HCl(g)

3SiCl4(g) + 4NH3(g) SiCl4(g) + CH4(g) CH3SiCl3(g)

Si3N4(s) + 12HCl(g) SiC(s) + 4HCl(s)

SiC(s) + 3HCl(g)

AlCl3(g) + NH3(g)

SiO2(s) + 4C2H5OH(g)

GaAs(s) + 3C2H6(g)

BBr3(g) + PBr3(g) + 3H2(g)

BP(s) + 6HBr(g)

Al2O3(s) + 3CO(g) + 6HCl(g)

Al2O3(s) + 6HCl(g) AlN(s) + 3HCl(g)

Al(CH3)3(g) + NH3(g)

AlN(s) + 3CH4(g) AlN(s) + xR(g)

ZrCl4(g) + 2CO2(g) + 2H2(g)

ZrO2(s) + 2CO(g) + 4HCl(g)

TiCl4(g) + 2BCl3(g) + 5H2(g)

TiB2(s) + 10HCl(g) (also Zr, Hf, Ta)

Zr(OPr)4(g) + 2H2O(g)

ZrO2(s) + 4C3H7OH(g)

2ZrCl4(g) + N2(g) + 4H2(g)

SEMICONDUCTORS:

GaEt3(g) + AsH3(g)

B4C(s) + H2O(g) + 12HCl(g)

4BCl3(g) + CH4(g) + 4H2(g)

[(CH3)2AlNH2]3(g)

(c) Reduction by metals: TiCl4(g) + 2Mg(s)

4BCl3(g) + CO(g) + 7H2(g)

Al2Cl6(g) + 3H2O(g)

Si(s) + 4HCl(g)

xMoF6(g) + (1-x)WF6(g) + 3H2(g)

BN(s) + 3HCl(g)

2ZrN(s) + 8HCl(g) (also Ti and Hf)

WCl6(g) + CH4(g) + H2(g)

W(s) + W2C(s) + WC(s) + HCl(g)

TaCl5(g) + CH4(g) + H2(g)

TaC(s) + 5HCl(g) (also Nb)

2Mo(s) + CO(g) + H2(g)

Mo2C(s) + H2O(g)

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A few representative examples of CVD products:

•elements (B, C, Si, Al, W) •refractory and transition metals •fluorides of Li, Na, Ca •refractory carbides of B, Si, group 4-6 elements •borides of group 4-6 transition elements and rare earths •silicides of group 4-6 metals, in particular Ti, Mo, W •simple oxides of Al, Si, Sn, group 4-6 metals •mixed oxides such as perovskites •other chalcogenides such as the 12-16 semiconductors, TiS2, WS2 •nitrides of B, Al, Si, group 4-6 metals •other pnictides such as 13-15 semiconductors, BxAsy, Zn2P3

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CVD films modify chemical/physical properties of substrate materials: •hard coatings to improve life / performance of cutting tools & turbine components •intricate 3D structures on microelectronics substrates •coating large panels of glass with SiO2, SnO2, TiN •―synthetic gold" coatings (non-stoichiometric TiN) on personal jewelry •layers in solar cells •coatings for catalysis •optical layers in waveguides morphology of product material is controlled by ajusting nucleation and growth rates

steps of CVD:

•transport of reagents (e.g. TiCl4, BCl3, H2) in the gas phase (often in a carrier gas) to deposition zone •diffusion or convection of gaseous precursors through the boundary layer (hot layer of gas adjacent to the substrate)

•adsorption of film precursors onto the growth surface •surface diffusion of precursors to growth sites; probability that a precursor molecule reacts directly at the 1st site of contact with the surface should not approach unity  rough surface topologies; moderate surface mobility of precursors is desirable

•surface chemical reactions leading to deposition of solid film (e.g. TiB2) & formation of byproducts (e.g. HCl)

•desorption of byproducts •transport of gaseous byproducts out of the reactor

Steps in CVD processes

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gas-phase reaction of precursor (in the hot boundary layer) prior to adsorption on the surface is undesirable  particle formation, incomplete reaction of precursor, depletion of precursor at the surface (contrary example CVD of GaAs: gas-phase reactions beneficial for film quality)

total P in the reactor controls gas-phase reactions: low P  small probability for reactant collisions precursor is ad-sorbed onto the substrate, diffuses to growth sites, undergoes surface-initiated reactions nature of ligand & type of metal-ligand bonds determine the decomposition pathway ideal precursor:

•gaseous or liquid rather than solid, good volatility without early decomposition •good thermal stability in the delivery system and during evaporation & transport in the gas phase •clean decomposition in a controlled manner on the substrate, without incorporation of contaminants •gives stable byproducts readily removed from the reaction zone •high purity or readily purified •readily available with constant quality, low cost; non-toxic, non-pyrophoric criteria rarely met as a whole:

e.g. requirement of long-term thermal stability at RT (shelf-life) + high reactivity at higher T (for high deposition rates) gives a narrow thermal stability window

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choice of precursor determined by substrate, T permitted, side reactions, codeposition, resulting morphology popular reactants: halides, carbonyls, carbonyl halides, hydrides, organometallics (alkyls, arenes), alkanolates, substituted acetylacetonates, amides, boranes, … HALIDES: produce corrosive acid vapors CARBONYLS: of refractory transition metals and of a few noble metals. Easily decompose and usually are

rather volatile. Examples: Cr(CO)6, Ni(CO)4, Fe(CO)5, Mo(CO)6, W(CO)6, Ru(CO)5, Os(CO)5, Fe2(CO)9, Mn2(CO)10, Re2(CO)10, Ir2(CO)8, Ru3(CO)12, Os3(CO)12, Co4(CO)12, Rh4(CO)12, Ir4(CO)12. Carbonyls react at comparatively low T, usually in H2 atmosphere (can be very poisonous!) METAL CARBONYL HALIDES: preferred for noble metals because of their stability T range. Examples:

Ru(CO)2I2, Os(CO)3Cl2, Ir(CO)2Cl2, [Pt(CO)Cl2]2, Pt(CO)2Cl2

HYDRIDES: of nonmetals used extensively for semiconductors. Examples: B2H6, AsH3, PH3, SbH3, SiH4,

GeH4, H2Se. H2S is used to deposit sulfides at low T, use of NH3 instead of N2 + H2 increases the driving force for deposition of metal nitrides (several hydrides are very toxic) ORGANOMETALLIC COMPOUNDS: for OM-CVD with alkyl or aromatic groups (cyclopentadienyl, aryl).

Examples: Al(CH3)3, As(CH3)3, In(CH3)3, Mg(C2H5)2, Mg(C5H5)2, Pb(CH3)4, Si(CH3)4, Sn(CH3)4, Sn(C5H5)2, Zn(C2H5)2. Most metal-organics are very reactive and toxic ALKOXIDES/ALKANOLATES: for depositing oxides. Examples: Si(OC2H5)4 (TEOS), Ti(OC2H5)4, Zr(OC3H7)4 ACETYLACETONATES (acac): and their derivatives are very versatile. Many metals (including very

electropositive ones such as lanthanides) can be made volatile Commercially available volatile solids of Ba, Ca, Ce, Cr, Co, Cu, In, Fe, Pb, Li, Mg, Mn, Ni, Pd, Pt, rare earths, Rh, Sc, Sr, Ag, V, Y, Zn, Zr. Vapor pressure can be increased by fluorine substitution of the alkyl hydrogen in the acac ligand 16

C5H8O2 + H2O  C5H7O2- + H3O+

pKa = 9.0

acetylacetone

Zr(acac)4

17

industrial CVD uses simple precursors: hydrides (SiH4, AsH3), metal alkyls (Al(i-Bu)3, GaEt3), volatile metal halides (WF6, TiCl4) halides generally require a reducing agent and are corrosive or liberate corrosive byproducts (HX, X2) metal-organic/organometallic compounds more volatile; can decompose thermally at relatively low T possible contamination with C & other contaminants the substrate

enhancement of volatility: minimize all types of interactions between precursor molecules in the condensed state (e.g., H-bonds, dipole-dipole interactions, van der Waals interactions); fluorination of ligands/substituents reduces van der Waals interactions smaller non-polar compounds have higher vapor P  avoid oligomerization or aggregation of precursors -diketonate ligands often used in metal-containing precursors -diketonate generally bidentate & chelating  complexes of low coordination number metals often monomeric  volatile

18

volatility problems with high coordination numbers metals  oligomerization to satisfy metal's coordination (bridging ligands) strategies to prevent oligomerization:

1. employment of sterically demanding ligands to limit accessibility to metal center  two problems: a) metal generally very reactive due to low coordination number  small molecules (O2, H2O) can penetrate the ligand shell b) large ligands give high molecular weight precursor molecules  decreased volatility

2. introduction of chelating multidentate ligands to satisfy the high coordination number of the metal while preventing oligomerization  two problems: a) multidentate ligands may prefer to bridge two or more metal centers instead of chelating b) multidentate ligands may dissociate prior to/during transport leading to oligomerization

problem with organometallic precursors: C incorporation into the deposited film organometallic precursors must completely eliminate the organic ligands during the surface reactions volatile products must be removed rapidly from the deposition zone to prevent contamination 

possible when: ligand is a stable molecule (e.g. CO, alkenes) or a thermal reaction give a stable byproduct by a facile reaction (e.g. -elimination)

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CVD of multielement materials: two or more individual precursors (multiple source precursors) that decompose individually on the substrate and eventually give the multielement film (e.g. GaAs) GaMe3 + AsH3

GaAs + 3CH4

critical control of film stoichiometry  precursors/intermediates have different volatilities & reactivities (e.g. desorption of volatile PbO in deposition of lead-containing films) some problems circumvented by single-source precursor single-source precursors contain the desired elements in one component, preferably in the ratio required for the film (e.g. [Et2Ga-As(t-Bu)2]2 for GaAs)

GaAs + 4C2H4 + 4CH2=CMe2 + 4H2

single-source precursors: bonds between film-forming elements must be stronger than those to the supporting ligands simplification of precursor delivery system but precursors have higher molecular weights difficult deposit of films with non-integral stoichiometry, doped films, films with two or more metals stoichiometry of the precursor is not always retained in the film

20

Equipment 1. precursor(s) delivery system 2. reactor 3. exhaust system to remove byproducts

substrate T ~200-800 °C P in the reactor cell 10-3 - 1 bar

critical parameter: precursor delivery method  overall deposition rate can be limited by feed rate of precursor into the reactor liquid/solid precursors with low vapor P transported by bubbling a carrier gas through the precursor (heated if volatility at RT not sufficient) conventional delivery methods unsuitable with thermally unstable precursors  alternatives: liquid delivery, aerosol delivery, spray pyrolysis, supercritical fluid delivery

in situ synthesis of precursors is also practiced by reacting a metal with a halogen gas at high T in the CVD reactor shortly before deposition from the formed halide

21

Reactors types hot wall reactors: substrate and chamber walls maintained at the same T

advantages: •simple to operate •can accommodate several substrates •uniform substrate T easily obtained •can be operated under a range of P & T •different orientations of the substrate relative to the gas flow are possible major problems: •deposition also on reactor walls (spurious deposits can fall off the walls and contaminate the substrate) •large consumption of precursor makes control of gas composition difficult and can result in feed-rate limited deposition •gas-phase reactions in the heated gas can occur hot-wall used at the lab scale or industrial CVD of semiconductors and oxides with high vapor pressures precursors metal CVD in industry in cold wall reactors: substrate maintained at higher T than reactor walls

advantages: •P & T can be controlled •plasmas can be used •no deposition on the reactor walls •gas-phase reactions are suppressed •deposition occurs only on the heated substrate  higher deposition rates (higher precursor efficiency) disadvantage: steep T gradients near substrate surface may lead to severe convection  non-uniform coatings

Hot wall CVD reactor

Cold wall CVD reactor

large number of variables influencing CVD process & film properties: reactor geometry, reactant delivery, total P, gas and substrate T, substrate environment, gas composition & homogeneity, flow rate, substrate surface, time of deposition, gas flow behavior, deposition rate, nucleation density, thermodynamic & kinetic properties of all involved species (gas and surface chemistry), composition of products, reaction mechanism, ... process variables determine coating: composition, thickness, morphology, density, uniformity, adhesion to the substrate, crystallinity, stress, ...

23

Schematic of the gas line of a CVD reactor for the deposition of TiN/TiB2 composite coatings H2 purified by removing traces of O2 (Cu or Pd catalyst) and H2O (zeolites) purified carrier gas bubbled through liquid precursors TiCl4 and BCl3 concentration of saturated vapors reduced by a reflux condenser kept at the T of the wanted vapor P in the (hot- or cold-wall) reactor the gas mixture reacts at high T to form TiN and TiB2 on the substrate waste gas (HCl) is removed in the gas stream and neutralized in a scrubber a vacuum pump can lower the deposition P

24

Growth rates: > 0.1 m/min CVD in microelectronics, higher for large-area coatings (e.g. glass industry)

low T regime: growth rate is limited by surface reaction kinetics, e.g. feed rate sufficiently high  no diffusion limitations intermediate T regime: growth rate is diffusion or mass-transport limited, i.e. all reactants that reach the substrate decompose  reaction proceeds faster than supply of reactant to the surface by diffusion through the boundary layer

high T regime: growth rate tends to decrease because due to increased desorption rate of film precursors or film components, together with depletion of reactants by reaction at reactor walls 25

when both mass transport & diffusion are fast  deposition rate may be limited by the rate at which reactant(s) enter the chamber (feed-rate limited, e.g. precursors with low vapor pressures) pressure of CVD reactor determines the relative importance of each regime: e.g. at P 0.2 – 1 bar significant boundary layer  growth may be characterized by the intermediate and high-T regimes Growth in bimolecular systems (molecules A and B): Langmuir-Hinshelwood mechanism: both A and B are adsorbed onto the substrate and reaction takes place between adsorbed molecules; by increasing the AIB ratio the growth rate increases to a certain limit and then drops again  the species present in excess occupies the free adsorption sites on the substrate  not enough sites for species B rate of the heterogeneous reaction controlled by reaction of adsorbed molecules Eley-Rideal mechanism: only molecules of A are adsorbed (chemisorbed) and react directly with molecules of B from the gas phase  growth rate shows a saturation behavior for high A/B ratios  limiting growth rate determined by complete surface coverage by molecules of A B A

26

B

A

27

The three types of strong bonds between a solid and adsorbed species (atoms, molecules, radicals) are all relevant for surface chemistry. The two types of weak bonds between a solid surface and adsorbed species are also observed chemisorption: reaction between a surface and a molecule involving strong bonds (typical of ad-atoms, e.g. H or N) physisorption: reactions under low driving forces (typical of admolecules) after being physisorbed molecules may break open and resulting fragments can chemisorb

Chemisorption of H2 on an fcc metal (001) surface: a) potential energy of H2 adsorption on a metal surface b) front view of H2 adsorption on a Ni crystal face (large spheres Ni atoms, small circles H atoms) during the sorption process H2 molecule might be expected to straddle two neighboring metal atoms (case 1) modelling predicts an asymmetrical position for H2 molecule (case 2) where bonding is more favorable. This configuration corresponds to the activation state between physi- and chemisorption. When H2 is dissociated H atoms are bonded to Ni at surface interstitials (case 3) 28

Selective deposition: fabrication of multilevel electronic devices by patterning of surfaces: patterned films by covering the whole surface with the selected material (blanket deposition) followed by selective-area etching or

selective CVD: deposition of the material only on one substrate (growth surface) in the presence of another substrate (non-growth surface). Example: metals, Si = growth surface, SiO2 = non-growth surface selective growth of films is unique to CVD

29

strategies for selective deposition: inhibiting adsorption and reaction on the non-growth surface or promoting precursor’s reaction on the growth surface

• reaction rate of precursor on non-growth surface intrinsically slower than its reaction rate on growth surface

• growth surface acts as coreactant and is selectively consumed by a precursor while reaction at the non-growth surface is slower

• a chemical reaction of a gaseous coreactant occurs on the growth surface surface

but not on the non-growth

• rate is increased on the growth surface by radiation while thermal reaction at the non-irradiated nongrowth surface is slow

• selective passivation of the non-growth surface preventing adsorption and reaction of precursor while adsorption + reaction occur readily on the growth surface

• a free-energy barrier exists for nucleation on the non-growth surface assuming that the surface reaction is rapid

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3c – CVD-related techiques prefixes specify the kind of precursors or variations of the CVD technique:

•OM (OMCVD, OMVPE, ...) precursors = organometallic compounds •MO (MOCVD, MOVPE, ...) precursors = metal-organic compounds •low-pressure CVD (LPCVD) •ultrahigh vacuum CVD (UHVCVD) •plasma-enhanced / -assisted / -activated CVD (PECVD, PACVD) •remote-plasma CVD (RPCVD) •laser-assisted (laser-induced) CVD (LCVDO) •microwave plasma-assisted CVD (MPCVD)

alternatives to nonselective "heat it and beat it" method: activation of reactants with plasmas or light. Light generates active radicals or excited species

alternative methods of delivery: •aerosol-assisted CVD (AACVD) •direct liquid injection CVD (DLICVD) •chemical vapor infiltration (CVI) i.e. CVD on internal surfaces of porous preforms other CVD-related methods: •atomic layer deposition (ALD, also called atomic layer epitaxy, ALE) •chemical-beam epitaxy (CBE, also called metal-organic molecular-beam epitaxy MOMBE)

31

Plasma-enhanced CVD nonequilibrium method (kinetic control) used in synthesis of diamond and other non equilibrium compounds at low T and P plasma (ionized gas): electrical glow discharge by dc, rf, or microwave excitation at low P (0.1-10 mbar)  plasmas are not in thermodynamic equilibrium and kinetic energy of free electrons is much higher than the translational energy of the molecules

eV?

The temperatures in an air plasma. At low pressures there is no equipartition of the kinetic energy over the atoms and free electrons and they have different kinetic energies or different effective temperatures

32

inelastic collisions between high-energy electrons and gaseous precursors create more ions, reactive radicals and excited states than would correspond with equilibrium at the molecular gas T  generation of chemically reactive species (excited molecules, radicals, ions and free electrons)  high-T chemistry in a low-T gas

low-P plasmas are also used in sputtering & ion plating in plasma spray, coatings are made with high-P plasmas (at 1 bar very high gas T)  powder particles are molten and sprayed on a cooled substrate (similar to Verneuil method, see later) CVD processes resulting in the epitaxial growth of crystalline films: vapor phase epitaxy (VPE) Epitaxy: films share the specific crystallographic orientation with respect to the single-crystal substrate due to similar lattice constants VPE used to prepare semiconductor films

33

3d – non-CVD deposition physical vapor deposition (PVD): vacuum method with

•thermal evaporation (sublimation) •sputtering of a target material onto a substrate at low P using heat or gas discharge •molecular beam epitaxy (MBE) PVD steps:

•vaporization of a solid (metal, alloy, mixture of solids) •transport of the gaseous compound from source to substrate •condensation of the gaseous compound on the substrate surface

nucleation & growth of a new layer

low P very long mean free path of evaporated species (5 - 5000 m) evaporated material to substrate

collisionless transport of

CVD: films formed by reaction of precursor on the substrate (e.g. deposition of metallic Cu by thermal decomposition of metal-organic precursors) PVD: material volatilized & deposited as such on the substrate (e.g. elemental Cu itself) most metallizations for microelectronics are performed using PVD advantages of CVD vs PVD: possibility of epitaxial growth, selective deposition, large-scale production, metastable materials 34

PVD by thermal evaporation (e.g. deposition of metallic films for integrated circuits) simple method:

•electron or laser beam directed at the target material under high vacuum (10-5–10-8 mbar)

vaporization of material; laser ablation fast enough to give nonselective evaporation of solid mixtures

•crucible containing the material heated by resistive or induction heating

operating P for metals

Physical Vapor Transport

T of source controls rate of evaporation/sublimation: P of vapor high enough for sufficient mass trasport

evaporated material deposited on a substrate kept at lower T

supersaturated vapor

35

laser ablation

36

PVD by sputtering (e.g. deposition of films for integrated circuits or ultrafine powders) surface atoms of target material liberated by bombardment with energetic ions from a glow discharge or plasma sputtered atoms ballistically transported to the substrate surface where they condense applicable to all inorganic materials deposition of multielement materials feasible because composition of the target is the same as the composition of the film advantage: low substrate T

deposition of films on thermolabile substrates (e.g. organic polymers)

drawback: no coating of surfaces not facing the vapor source (step coverage) in CVD diffusion/convection can transport reactants to hidden areas

in PVD particle stream from evaporating or sputtering source is directional and deposited species are not sufficiently mobile on the substrate to migrate to areas not facing the source

deposition of ceramic films with reactive sputtering: reaction of metal vapor with a reactive gas (e.g. B2H6, CH4, NH3, N2 to deposit borides, carbides, nitrides) 37

Molecular·beam epitaxy (MBE): sources independently evaporate materials at a controlled rate substrate under UHV (10-10 mbar) low growth rates

molecular beams hit the (heated)

films with high purity & very complex layer structures (nanostructures) with precise control of doping of the deposited layers low processing T important for microelectronic production

38

diamond CVD outstanding properties (mostly insensitive to lattice defects): mechanical properties: extreme hardness (~90 Gpa) very high bulk modulus (1.2 x 1012 N/m2) very low compressibility (8.3 x 10-13 m2/N) acoustic properties: high sound velocity (18.2 km/s) thermal properties: very high thermal conductivity at RT (> 2 X 103 W m-1 K-1) low thermal expansion coefficient at RT (0.8 x 10-6 K-1) optical properties: transparency from deep UV to far IR electrical properties: good insulator (resistivity ~ 1016 cm at RT) semiconductor (10-106 cm) with a bandgap of 5.47 eV (Si 1.1 eV, GaN 3.44 eV) chemical properties: resistant to chemical corrosion and radiation

39

Estimated world production of synthetic diamond x106 ct (1 ct = 0.2 g ) Country

1995

1997

1999

Belarus

25.0

25.0

25.0

China

15.5

16.0

16.5

Czech Republic

5.0

5.0

3.0

France

3.0

3.0

3.0

Greece

1.0

0.75

0.75

Ireland

60.0

60.0

60.0

Japan

32.0

32.0

32.0

Poland

0.256

0.26

0.2

5.0

5.0

3.0

80.0

80.0

80.0

5.0

5.0

3.0

South Africa

60.0

60.0

60.0

Sweden

25.0

25.0

25.0

Ukraine

8.0

8.0

8.0

United States

115.0

125.0

208.0

Total

440.0

451.0

467.0

Romania Russia Slovakia

40

applications for diamond polycrystalline thin films: heat sinks in electronic devices and wear-resistant hard coatings for tools precision optical components for high-power lasers

radiation detectors surface-acoustic-wave devices

Diamond films deposited on a SIALON surface by microwave activation of 1.7% CH4 in H2. The different morphologies originate from different gas pressures and different microwave powers

41

common synthetic methods of polycrystalline diamond:

• activation of methane/hydrocarbons in an excess of H2 (1-2 vol% CH4 in H2 + some O2) by using

microwave, radio frequency or d.c. discharges (PE-CVD at low substrate T (800°C) and low P (10 mbar))

• decomposition of CH4 / H2 mixtures on hot W or Ta filaments (~2300 °C) at 1 bar • burning of hydrocarbons (acetylene, methane) with O2 in a welding torch (flame directed to the

water-cooled substrate at T = 700-1000 °C; diamond growth occurs at the intersection between substrate and primary combustion zone. Simple but not economical method: small yield and area of deposition, rough deposits with varying crystal quality)

(a) Hot·filament reactor; (b) Microwave plasma-enhanced CVD reactor

42

43

very complex surface chemistry H atoms + carbon radicals in the gas phase seem to be a necessary condition for deposition of diamond atomic hydrogen is produced from H2 near the surface during the decomposition process gas-phase hydrogen-abstraction reactions lead to hydrocarbon radicals (likely the main precursors for diamond) growth of diamond (metastable) thanks to atomic H that selectively etches codeposited graphitic nuclei and leaves diamond ―dissolution― rate of graphite & amorphous carbon by atomic H ~50 times that of diamond: yH + xCgraphite  CxHy

etching of graphite

mH + CxHy  (m+y)/2H2 + xCdiamond

diamond deposition

hydrogen prevents formation of sp2-hybridized C atoms at the growth front stabilizing the growing diamond surface H-terminated growth surface H and carbon radicals react with dangling bonds at the surface preventing formation of graphitic links between surface C atoms

44

Calculation of equilibrium concentrations with an initial gas mixture of N2/CH4/H2 of 0.1/1/100 mole ratios at P = 10 kPa, The shadowed region represent the substrate T for successful diamond growth. T = 2500 K is the highest T for a filament in hot-filament CVD.

45

diamond nucleation very sensitive to substrate material and surface composition, morphology, defects… pretreatment of substrate surface (polishing, abrading, …) enhance nucleation density nucleation on highly perfect surfaces (e.g. Si wafers) is sluggish with a long initiation period nucleation is also difficult if surface C concentration is lowered by diffusion/reaction into the substrate (e.g. Fe, Co, Ni) nucleation starts on substrates saturated with C-atoms chemical reactions between substrate and reaction-gas components (atomic H) also influence nucleation and adhesion of the films freestanding diamond sheets (several hundreds cm2) produced by deposition on dummy substrates dissolved after formation of the film

metal CVD

metal coatings on various substrates for: oxidation, corrosion or abrasion-protection films, reflective or conducting coatings, electrodes, microelectronics reaction problems for many metal-silicon contacts: Si may diffuse into the metal film to form solid solutions or silicides diffusion barrier layers (Ti, TiN, TiW, ZrN, RuO2)

47

Example: CVD of aluminum films aluminum films for metallization of polymers in food packaging (gas diffusion barrier), reflective films in mirrors, CDs, ... interconnects in microelectronics (resistivity of bulk Al only slightly greater than that of Cu or Ag) best precursor for depositing high-quality CVD films is tris(isobutyl)aluminum Al(i-Bu)3 pyrophoric liquid, RT vapor pressure 0.1 mbar, monomeric in the gas phase the first isobutyl ligand degraded by -hydride elimination already at T > 50 °C:

3Al(CH2CHMe2)3

diisobutylaluminum hydride trimeric

 3(CH2=CMe2)

+

lower vapor P (0.01 mbar at 40 °C)

formation of [HAl(i-Bu)2]3 suppressed by adding isobutene to the carrier gas (reversibility of -elimination) T = 200-300 °C in hot-wall reactors, growth rates of 20-80 nm/min overall reaction:

Al(CH2CHMe2)3

 Al + 3(CH2=CMe2) + 3/2H2 48

decomposition chemistry of Al(i-Bu)3: after adsorption on Al, all i-Bu groups diffuse over the surface and are no longer attached to one Al atom once a layer of Al is formed further decomposition of Al(i-Bu)3 occurs the rate-determining -hydride elimination

each i-Bu group participates in

Thermal decomposition of Al(i-Bu)3 on aluminum surfaces: low T (upper pathway) results in a clean deposition; higher T (> 330 °C, lower pathway) C impurities may be incorporated into the film

H atoms created by -elimination spread over the whole Al surface H2 and isobutene readily desorb from Al 49

-methyl elimination

propene + surface-bonded CH3 groups

higher activation energy

significant process only at higher T

surface-bonded CH3 groups give C only at T > 330 °C

trimethylaluminum (Al2(CH3)6) less suitable precursor: -hydrogen elimination not possible Al films deposited at T = 350 – 550 °C contain high levels of C. Al films with little C contamination in H2 atmosphere:

Al2(CH3)6 + 3H2  Al + 6CH4

calculations predict at 200 °C formation of AlH3 + CH4 which readily desorb

50

Chemical reactions of coordination compounds substitution reactions in square planar complexes e.g. d8 complexes Pt(II), Pd(II), Ni(II), Ir(I), Rh(I), Co(I), Au(III) several pathways available for ligand replacement:

nucleophilic substitution electrophilic substitution oxidative addition followed by reductive elimination

51

nucleophilic substitution (in square planar complexes): L | T—M—X | L

L | T—M—Y | L

+ Y

+

X

Y = entering nucleophilic ligand X = leaving ligand T = ligand trans to X classes based on relative importance of bond making / bond breaking in the rate-determining step: 1. associative: the M—Y bond is fully formed before M—X begins to break (SN2 of organic chemistry) 2. interchange associative: the M—X begins to break before the M—Y bond is fully formed, but bond making is more important than bond breaking 3. dissociative: the M—X is fully broken before the M—Y bond begins to form 4. interchange dissociative: the M—Y begins to form before the M—X bond is fully broken, but bond breaking is more important than bond making solvent molecules (S) are generally nucleophilic; reactions are generally faster in nucleophilic solvents associative pathways

ML2TX S

Y

ML2TS

ML2TY Y

52

nucleophilic substitutions in square planar complexes are believed to proceed by association rather than by dissociation key questions: what effect does the nature of the entering group have on the rate of reaction? what effect does the nature of the leaving group have on the rate of reaction? what effect does the nature of the ligand trans to the leaving group have on the rate of reaction? what effect does the nature of the ligand cis to the leaving group have on the rate of reaction? what effect does the nature of the central metal have on the rate of reaction? in all observed reactions the entering group occupies the site vacated by the leaving group

trans effect: studied exhaustively by varying the nature of the ligand trans to the leaving group it is possible to cause rate changes of orders of magnitude synthetic design can exploit the trans effect

53

e.g. synthesis of diamminedichloroplatinum(II)

  the observed isomer forms by substitution of a ligand trans to a Cl- ion

trans effect: kinetic labilization of ligands trans to certain other ligands (trans-directing ligands) comparison of the trans-directing capabilities of different ligands

trans-directing series (good -donors or good -acceptors):

CN-, CO, NO, C2H4 > PR3, H- > CH3-, C6H5-, SC(NH2)2, SR2 > SO3H- > NO2-, I-, SCN-, Br-, Cl-, py > RNH2, NH3 > OH-, H2O

cis effect: very small compared to trans effect relative trans andcis effect series based on rates of substitution of H2O for Cl- in [PtCl3L] complexes: trans ligand

trans effect

cis ligand

cis effect

H 2O

1

H 2O

1

NH3

200

NH3

1

Cl-

330

Cl-

0.4

Br-

3000

Br-

0.3

dmso

2 x 106

dmso

5

C2H4

1011

C2H4

0.05

Mechanism of nucleophilic substitution in square planar complexes:

nucleophile Y attacks the d8 complex from either side of the plane Y is attracted to the deficient metal center but is repelled by filled metal d orbitals and from bonding electrons Y may coordinate to the metal through an empty pz orbital

the trigonal bipyramidal species maybe either an activated complex or a true intermediate

55

ligand T can enhance the rate of reaction by:

• destabilizing the ground state by weakening the metal-ligand bond trans to itself • stabilizing the transition state both modes reduce Eatt

56

trans influence: extent to which a ligand affects the bond trans to itself in a complex trans influence can be assessed by looking at ground state properties (bond lengths, NMR coupling constants, stretching frequencies) trans influence can be viewed in terms of competition for metal orbitals common to T and X

strong M-T -bonds weaken the M-X bond destabilizing the starting complex series of X ligands ordered according to their ability as -donors similar to the trans effect series: H- > PR3 > SCN- > I-, CH3-, CO, CN- > Br- > Cl- > NH3 > OH-

CO and CN- are poor -donors but good -accepting ligands (effect on the transition state via withdrawal of electron density) series of X ligands ordered according to their ability as -accepting ligands : C2H2, CO > CN- > NO2- > SCN- > I- > Br- > Cl- > NH3 > OHgood -acceptors also favor an equatorial position on the trigonal bipyramidal activated complex/intermediate this forces X to be the ligand expelled in the formation of the square planar species

58

Metal carbonyls

Thermodynamic and kinetic stability: thermodynamics: stable or unstable

kinetics: inert or labile

[Ni(CN)4]2- + 414CN-

[Ni(14CN)4]2- + 4CN-

t1/2

30s labile;

Kstab = 1030.2

[Mn(CN)6]3- + 614CN-

[Mn(14CN)6]3- + 6CN-

t1/2

1h moderately labile;

Kstab = ??

[Cr(CN)6]3- + 614CN-

[Cr(14CN)6]3- + 6CN-

t1/2

24days inert;

Kstab = ??

[Co(NH3)6]3+ + 6H3O+

[Co(H2O)6]3+ + 6NH4+

t1/2

days

inert;

Kreaction = 1025

Kinetics of octahedral substitution:

Class I – exchange of water extremely fast (1st order K

108 s-1). Complexes are bound by electrostatic forces, with large and low charge metal ions (Z2/r up to 10 x 10-28 C2 m-1), group 1 metals and larger group 2 metals

Class II – exchange of water fast (1st order K 105 - 108 s-1). Complexes are more strongly bound than those of Class I, but LFSEs are relatively small, Z2/r 10-30 x 10-28 C2 m-1, 2+ transition metal ions and 3+ lanthanides

Class III – exchange of water fast on absolute scale but relatively slow compared to Class I and II (1st order K 1 - 104 s-1). Complexes are bound by electrostatic forces, with large and low charge metal ions (Z2/r up to 10 x 10-28 C2 m-1), most of the 3+ transition metal ions (stabilized by LFSE) and the very small Be2+ and Al3+ ions, Z2/r > 30 x 10-28 C2 m-1

Class IV – exchange of water slow (inert complexes, 1st order K 10-9 – 10-1 s-1). These ions are comparable in size to Class III ions and exhibit large LFSE (d3-Cr3+; low spin d5-Ru3+; low spin d8-Pt2+; Co3+)

61

Organometallic compounds

a 62

63

64

1u

65

Chemical reactions of organometallic compounds oxidative addition: a coordinatively unsaturated complex in a relatively low oxidation state undergoes a formal oxidation by +2 and at the same time increases its coordination number by two

Vaska’s complex

reverse process: reductive elimination

oxidative addition requires vacant coordination sites together with suitable orbitals for bond formation e.g. [Fe(CO)4]2- (18-electron complex) has only four ligands but addition of X-Y requires antibonding orbitals

66

mechanisms for oxidative additions vary according to the nature of X-Y nonpolar X-Y: concerted reaction leading to a three-centered transition state

with O2 the bond order is reduced from two to one but X-Y bond is not broken completely

electrophilic polar X-Y: oxidative additions tend to proceed by SN2 mechanism involving two-electron transfer or via radical, one-electron transfer mechanisms

other factors determining the tendency for a complex to undergo oxidative addition:

• ease of oxidation (electron-rich systems) • relative stability of coordination number 4 compared to 5 or 6 • strength of new bonds created (M-X and M-Y) relative to the bond broken (X-Y)

67

1,2-insertion and elimination: the term insertion is generally reserved for reactions which do not involve changes in metal oxidation state

e.g. olefin insertion into a metal-hydrogen bond

reverse process: -elimination

-elimination represents the main pathway for decomposition of transition metal alkyl complexes: deinsertion of the alkyl ligand to yield a metal hydrido complex and elimination of alkene

-elimination can be thwarted by using ligands with no the metal (e.g. C CH)

hydrogens or with

hydrogens too far from 68

69

70

71

Chemical vapor infiltration (CVI) source gases flow through a porous preform at high T material deposited as a matrix in the empty spaces (e.g. porous carbon infiltrated with gaseous SiO) when gaseous precursors pass through the preforms at high T the same chemical reactions as CVD result in the deposition of solid material in the empty spaces of the preform (e.g. TiN/C composites by CVI of highly porous carbon substrates with a gaseous mixture of TiCl4, N2, H2 at ~850 °C)

experimental conditions chosen to favor in-depth deposition and to fill up the pores totally clogging of the pore entrances must be avoided CVI widely used for fabricating fiber- or particle-reinforced composites (ceramic matrix composites (CMC) and carbon-carbon composites (C/C)) excellent mechanical properties (hardness, stiffness) at high or very high T C/C composites for rocket nozzles and aircraft brake-disks CMCs (e.g. C/SiC and SiC/SiC composites) for aircraft engines and related applications C/SiC composites with carbon·fiber reinforcement heat shield components for aerospace structures

high-T strength/toughness for structural and

macro- or mesoporous solids (e.g. solid foams or aerogels, particle agglomerates) can be infiltrated fibrous preforms may be cloths, felts or stacked fabric layers of ceramic or carbon fibers, carbonized wood, cotton, paper 72

standard CVI process proceeds at isothermal and isobaric conditions i.e. preforms are put in a hot·wall pressure CVD reactor fed with the gaseous precursor(s) relatively slow process obtained materials have some residual porosity and spatial density gradients highest density close to the surface compromise between deposit uniformity and infiltration rate

despite some drawbacks industry uses this process because of the following advantages:

•rather simple technology •reinforcing capability of fibers is retained owing to the relatively low processing temperatures •nature of the matrix easily modified by changing the precursors (e.g. first coating a fibrous preform with one kind of ceramic and then depositing a ceramic matrix of another material)

•different preforms (with different sizes and complex shapes) can be densified in the same run derived techniques to overcome the drawbacks of isothermal isobaric CVI: forced CVI: precursor gas injected at pressure P1 through one side of the preform, exhaust gas pumped off at a pressure P2 < P1 at the opposite side infiltration time lowered from several hundred hours to a few tens of hours deposit may have a density gradient technique only suited for simple shapes (disks, tubes) where a pressure gradient can be easily applied pulse CVI: a total pressure cycling is applied to periodically regenerate the entire gas phase

1) gaseous precursor is injected rapidly; 2) after holding at the desired P and residence time the gas is pumped off; this cycle is repeated several times 73

Aerosol Processes gas-phase reactions and particle formation in the gas phase during CVD are undesirable in gas-phase powder syntheses (aerosol processes) particles are produced in the gas phase by chemical or physical processes production of fine powders of inorganic compounds (titania, silica, carbon black) with nm-size particles several million tons of products per year produced worldwide advantages of aerosol processes:

•do not involve the large volumes of liquids as in wet processes •much shorter time scales than solid-solid reactions •can produce materials of high purity at high yields and with a high throughput •multicomponent or nanophase materials can be produced major routes for aerosol processes:

•gas-to-particle conversion route •spray pyrolysis

precursors used for aerosol processes are often the same as in CVD & PVD processes in CVD relatively low partial P can yield adequate film deposition rates; powder synthesis reactors require substantial precursor partial P to be economical

mainly commodities are produced by aerosol processes  precursors must be inexpensive & easy to use 74

75

terms associated with aerosol processes: agglomerates: assemblies of primary particles physically held together by weak interactions (soft agglomerates) aggregates: assemblies of primary particles connected by strong chemical bonds (hard agglomerates; powder sintering and consolidation more difficult) coagulation: attachment of two particles when they collide coalescence: fusion (sintering, condensation) of two particles Gas-to-particle conversion: mixtures of gaseous precursors are fed into the reactor reaction at high T to form molecular clusters and eventually ultra fine particles of the product particles form aggregates and agglomerates of solid powder powders are collected after the reaction zone additional postprocessing sometimes required to produce high-purity powders

76

steps for the formation of the powders: 1. homogeneous gas-phase reactions: between gaseous precursors  formation of molecular or cluster (oligomeric) compounds 2. nucleation: supersaturated vapors inherently unstable towards the condensed phase  formed molecules and clusters will form particles by homogeneous nucleation with thermodynamically stable clusters  nuclei formed by coagulation 3. particle growth: nuclei grow by condensation of precursor molecules on the particle surface and coagulation particle collision rate > particle coalescence rate  non-spherical agglomerates/aggregates particle collision rate < sintering rate  regularly shaped, monolithic (dense) particles (sintering rate depends on material, primary particle size, T) when particles grow by condensation of precursor molecules on the particles’ surface  similar to CVD: precursor adsorbtion to the surface and byproducts desorption after formation of the product reaction rate > vapor transport rate to particle surface  similar to diffusion or mass-transportlimited CVD reaction rate < vapor transport rate  similar to reaction-limited CVD powders by gas-to-particle conversion are usually aggregates or agglomerates of fine non-porous primary particles T, reactor residence time, chemical additives affect particle sizes, size distribution, extent of agglomeration or aggregation (powder morphology)

short reactor residence times (high flow rates)  small primary particles high reaction T  small primary particles (possibly increased nucleation rate & competition for reactant)

77

soft/hard agglomerates determined by aggregation T: if aggregation takes place at high T during synthesis  interparticle diffusion leads to strong bonds between primary particles; vapor deposition into the necks between aggregated primary particles may contribute to hard agglomerates in the high-T region of reactor if aggregation can be suppressed until T has decreased sufficiently, dispersed agglomerates are formed gas-to-particle conversion most suitable for single-component high-purity powders of small particle size, high specific surface area, controlled particle-size distribution

major disadvantage: gas-to-particle conversion results in aggregates/agglomerates with possible problems in consolidation & sintering of large ceramic parts multicomponent powders difficult to synthesize because of differences in vapor P, nucleation and growth rates of the various components  non-uniform product composition

78

Spray pyrolysis atomization of a solution / slurry /solid precursor powders suspended in a carrier gas (solid powders are actually not formed from the gas phase) atomization of liquid precursors can be carried out using a variety of atomizers (depending on rheological characteristics of the liquid or the required size of the droplets); atomization results in broad droplet-size distributions resulting aerosol is passed through a heated region where solvent evaporates and precursor particles or droplets are pyrolyzed or reacted with gaseous species to yield the product powder (synonyms: particle-to-particle conversion, evaporative decomposition, spray roasting, aerosol decomposition) size of product particles proportional to that of the aerosol droplets or particles

most work for aerosol synthesis of oxide powders with polymeric precursors for ceramic materials spray pyrolysis (with or without reactant gas) can become a viable means for non-oxide powders 79

advantages of spray pyrolysis:

•easy scaling up •particles with high purity, generally amorphous, unagglomerated, monolithic spherical morphology •multicomponent powders easily made  droplets contain precursors in the right stoichiometry limitations:

hollow and porous material easily formed when a solute concentration gradient is created during evaporation  solute precipitates at the more highly supersaturated surface if there is not enough time for solute diffusion in the droplet if the crust is impermeable to solvent  exploded particles when P within the particle builds upon further heating control of porosity by changing precursor concentration in the droplets and reactor T profile

80

Reactors for gas-phase powder synthesis: flame reactors: combustion of hydrocarbons or H2 (Aerosil process) or reaction of H2 and Cl2 easy to construct and operate: only need to bring fuel + oxidant into contact and make maximum use of heating energy

short residence time in the high·T region  flame-produced powders as aggregates of fine, non-porous primary particles with usually narrow size distribution agglomerates are often hard to break up and have a broad size distribution T profile of the flame, additives (to modify powder phase composition, morphology, size) and residence time  determine crystallinity, phase composition, extent of aggregation product powder may be contaminated because of direct contact with combustion reactants and gases furnace reactors: externally heated metallic or ceramic tubes through which precursor gases flow closed environment  excellent control over T and residence times, and hence particle characteristics key problems with externally heated reactors: •loss of product by deposition on the reactor walls •formation of hard agglomerates to prevent extensive agglomeration:

powders must be removed from the hot reaction zone (with possibly reactive gases) and quenched  •expose product particles to a large volume of inert gas •cool the gases containing the particles by sudden expansion •limit the length of the reaction zone with rapid withdrawal of particles from the reactor 81

laser reactors: improve the efficiency of energy transfer to gaseous reactants (high-T furnaces involves relatively slow heating of reactant gases by convection/radiation) premixed gases react in a small, well-defined zone where the laser beam intersects the reactant jet  rapid increase of T in the gas stream  no nucleation at the walls reactants are dilute  fine, loosely agglomerated powders steep T gradients at the reaction zone  precise control of particle nucleation and growth rates one of the reactants must absorb strongly the laser radiation (e.g. SiH4 or BCl3 with CO2 laser) when no reactants is absorbing  sensitizer added to the gas mixture (potential source of impurities) powders by laser reactions  high purity, controlled stoichiometry, uniform particle size laser-heated reactors used for non-oxide ceramics plasma reactors: highly ionized gases transfer energy to molecules participating in chemical reactions plasmas by ionizing a flowing gas (e.g. Ar) using DC or low-frequency AC with electrodes in direct contact with the gas (potential contamination), or by inductively coupling a radio·frequency source (no electrodes)

solid reactants in plasma syntheses: solid injected into the discharge zone where vaporization occurs reactants are heated quickly to high T and quenched rapidly  very finely divided materials with high specific surface areas

82

Products: large-scale aerosol processes: preparation of highly dispersed silica, titania, alumina powders from chlorides (SiCl4, TiCl4, AlCl3) by flame hydrolysis smaller scale: other oxides are produced by Aerosil process Bi2O3 (from BiCl3), Cr2O3 (CrO2Cl2), Fe2O3 (FeCl3 or Fe(CO)5), GeO2 (GeCl4), NiO (Ni(CO)4), MoO2 (MoCl5), SnO2 (SnCl4 or SnMe4), V2O5 (VOCl3), WO3 (WCl6 or WOCl4), ZrO2 (ZrCl4), AlBO3 (AlCl3 and BCl3), Al2TiO5 (AlCl3 and TiCl4), AlPO4 (AlCl3 and PCl5) spray pyrolysis for metal oxides containing two or more metallic species (high-T superconductors, spinels, magnetic oxides)

aerosol methods for non-oxide powders (borides, carbides, silicides, nitrides) chemistry of the gas-phase reactions is very complex: e.g. 119 separate reactions have been identified in the gas-phase synthesis of SiC from SiH4 and propane. The mechanism probably is still incomplete!

83

Examples of non-oxide powders prepared by aerosol methods Product

Method

Reactants

B4C

Plasma/Laser

BCl3 + CH4

B4C

Thermal

B2O3 + C

SiC

Laser/Plasma/Thermal

SiH4 + CH4

SiC

Plasma

SiCl4 or SiO2 or SiO + CH4

SiC

Plasma/Thermal

SiMe4

SiC

Laser/Plasma

SiO2 +C

SiC

Laser

H2SiCl2 + C2H4

TiC

Plasma/Thermal

TiCl4 + CH4

Mo2C

Thermal

MoCl5 or MoO3 + CH4

WC, W2C

Plasma/Thermal

WCl6 or W + CH4

BN

Laser/Thermal

BCl3 + NH3 + N2

AlN

Plasma/Thermai

Al + N2 [+ NH3]

AlN

Thermal

AlCl3 + NH3 + H2

AlN

Thermal

Al2Et6 + NH3

Si3N4

Laser/Plasma/Thermal

SiH4 + NH3

Si3N4

Laser/Plasma/Thermal

SiC!4 + NH3 + H2

Si3N4

Plasma

Si + NH3 or N2

SiAlON

Plasma

Si + Al + NH3 + O2

TiN

Plasma

Ti + N2 [+ H2]

TiN, ZrN

Thermal

TiCl4/ZrCl4 + NH3 + N2 + H2

VNx

Thermal

VCl5 + NH3 + N2 + H2

B4Si

Plasma

B2H6 + SiH4

TiSi2

Laser

TiCl4 + SiH4

TiB2

Laser

TiCl4 + B2H6

TiB2

Thermal

TiCl4 + BCl3 + Na or H2

WSi2

Plasma

WF6 + SiH4

84

Example: Aerosil process (1942 patent by Degussa) 2H2 + O2

2H2O

SiCl4 + 2H2O

SiO2 + 4HCl

SiCl4 + 2H2 + O2

SiO2 + 4HCl

(overall reaction: flame hydrolysis)

volatilized SiCl4 is fed into an oxygen-hydrogen flame:

water gives a very fast and quantitative hydrolysis of SiCl4 at ~1000 °C HC! recycled to produce SiCl4 fumed silica is cooled, collected by conventional means (e.g. cyclones, electrostatic precipitators, baghouse filters) and deacidified to remove adsorbed HCl surface properties of silica can be modified by post-treatment with silanes

85

primary particle size determined by reaction parameters

fumed silica consists of agglomerated spherical, amorphous primary particles of 7-40 nm diameter apparent density of silica & titania powders: 200 dm3 of fumed silica  ~10 kg (see aerogels) high specific (outer) surface area (50-400 m2/g)

Transmission electron micrograph of fumed silica (primary particle size 16 nm, specific surface area 130 ± 25 m2/g

uses for pyrogenic silica:

Scanning electron micrograph of fumed silica with primary particles of 40 nm and specific surface area of 50 ± 15 m2/g

•reinforcing filler for rubber, silicones, elastomers •thickening of liquids (thixotropy by 3D network of H-bonds) •retardant of sedimentation of solids dispersions •thermal insulation

86