Precursors for Carbide, Nitride and Boride Synthesis

PRECURSORS FOR CARBIDE, NITRIDE AND BORIDE SYNTHESIS

N.T.Kuznetsov Institute of General and Inorganic Chemistry of Russian Academy of Sciences 117907 GSP-1 Moscow Leninsky prosp. 31 Russian Federation

Contents 1. Introduction 2. Classification of Ceramic Precursors 3. Ceramic Precursors for Metal Carbides of Elements 3.1. Precursors for Boron Carbide 3.2. Precursors for Silicon Carbide 3.3. Precursors for Transition Carbide 4. Ceramic Precursors for Nitrides of Elements 4.1. Precursors for Boron Nitride 4.2. Precursors for Aluminium Nitride 4.3. Precursors for Silicon Nitride 4.4. Precursors for Transition Metal Nitrides 5. Ceramic Precursors for Metal Borides 6. Conclusion References

1. Introduction The classical methods of preparation of non-oxide ceramic materials based on direct reaction of elements, carbothermal reduction of oxides and reduction of halides do not always provide required stoichiometry, homogenity, microstructural composition, high purity, desirable grain size and forms such as fibres, films and coating, foam and microporous membranes, submicron powders, as well as some multicomponent systems. Hence the development of chemical synthesis of ceramics through preceramic routes at moderate temperature and pressure is an essential current problem, determining progress in advanced ceramic thechnology. An optimal approach to the problem from the chemist's point of view is that of ceramic precursors. The ceramic precursors can be referred to as a chemical compounds containing all (or almost all) the elements to be present in the final

92 ceramics and that can be reprocessed into the end ceramic product. Practically all known advanced ceramic materials can be produced via ceramic precursors routes. Nevertheless, chemical compounds should satisfy a number of common and technological requirements to be ceramic precursors. Some of them were listed above. In addition one should mention the environment stability, solubility in common solvents, high ceramic yield, relatively inexpensive starting materials and exclusion of toxic substances from the process. Progress in preceramic routes has led to the development of non- classical methods for ceramics preparation such as gas phase reactions, thermal decomposition, non-aqueous liquid reactions, sol-gel processing, polymer pyrolysis, etc.

2.Ceramic precursors classification All known chemical compounds used as ceramic precursors can be divided into five basic groups: - ordinary inorganic compounds - coordination compounds - metal organic compounds - organometallic compounds - polymers The selected examples are listed in Table 1.

Table 1 Groups and types of General formula Selected examples compounds 1 2 3 Ordinary inorganic compounds

Halides MXn SiCl4, TiCl4, BCl3

Hydrides Mn,Hm, B10H14, NH3

Oxides Mn,0m Ti02, Nb2O5

Amides M(NR2)n T(NR2)4,Nb(NR2)5

Imides M(NH)n Si(NH)2

Azides R2MN3 R2A1N3, R3SuN3

93 1 2 3 Coordination compounds

Amine complexes [MLn]X [Ni Phen3] B12H12

MXnmNH3 H3AlNH3 B2H62NH3 n-Complexes (C5H5)2MXn (C5H5)2Fe, (C5H5)2TiCl2

Carboranes CxByHz; C2B10H12, C2B6H8

Tetrahydroborates M(BH4)n A1(BH4)3, Ti(BH4)3,

Zr(BH4)4

Polyhedral Boranes M(BnHn)x CaB6H6, (RxNH4-x)2B12H12

B10H14-xLx

Metalloboranes (C5H5)xMyBnHn R4N[(C5H5)3Ni3B6H6]

Metallocarboranes - [(C5H5)2Ni2CB7H8]

Metal organic compounds

Metal alkoxides M(OR)n Ti(OC3H7)4

Metal carboxylates M[OC(0)R] A1[OC(0)CH3]3 MOC with mixed functional groups - (C2Hs)2AlN(CH3)2

Organometallic compounds

Metal alkyls MRn A1(CH3)3, Pb(C2H5)4

Metal aryls MRn Ca(C6H5)2

Metal alkenyls - A1(CHCH2)3

Metal carbonyls M(CO)n Ni(CO)4, Co2 (C0)8 Mixed organometallic - (C2H5)3Ti(CH3) ligands

Polymers

Polysilanes -[Si(RR')]-n -[Si(CH3)(C2H5)]-n

Polycarbosilanes -[(RR')Si-CH2] -n -[(H)(CH3)SiCH2]-n

94 1 2 3

Polysiloxanes -[Si(RR')-0] -n -[Si(CH3)20]-n

Polysilazanes -[(RR')Si-NR]-n -[(CH3)2SiN(CH3)]-n

Borazines (RBNR')3 (HBNH)3 and

Polyborazines -[B(R)N(R)]-n [CH3BNH]6

Polyamides =[M-NR] -n =[AlN(CH3)] -n

=[Ti(NCH3)2]=

The preceramic route includes the following stages: synthesis of monomer units, their conversion into oligomer/polymer precursors and crosslinking into a preceramic network. The stages of ceramization and crystallization will be considered only schematically.

3. Ceramic Precursors for Carbides The chemistry of precursors for carbide ceramics has remained limited to boron carbide, silicon carbide and to a certain degree to transition metal carbides.

3.1. Boron carbide B12C3 Non-classical methods of boron carbide preparation based on precursor route include carboranes, boron hydrides and polyhedral boron hydride systems. Thus the polymer precursor [-CH-CH 2- ]n obtained by the reaction B5H8, Catl T

B5H9 + CH  CH B5H8CH = CH2 polymer on heating 250

at 1200°C results in the amorphous boron carbide B12C3 [1]. The most preferred boron carbide precursors are carboranes - cluster systems containing closo - frame - works of boron and carbon atoms (Fig.l).

95 Fig.l Closo-carboranes.

Such as carborane C2B10H12, due to its high chemical and thermal stability, , structural similarity and B/C ratio close to boron carbide (Fig2), molecular crystal lattice with high volatility at 100-

120°C and very low toxicity. Carborane C2B10H12 obtained via the reaction CHCH

B10H14+2CH3CN B10H12 (CH3CN)z C2B10H12 at 700°C

forms the polymeric precursor (C1,9B10H8,3)4 that yields boron carbide at 900-1000°C. The reasonably high temperature dependence of vapour pressure and deposition rate (0,6 mm/h at 1050°C and 3,0 mm/h at 1260°C) makes the carborane an outstanding precursor for CVD preparation of boron carbide films and coating [2].

96 Fig.2. Boron carbide (B4C) and related structures consist of icosahcd-ral B12 ; building blocks with the equatorial (B I) and axial (B2) boron atoms and the three-atom chain inserted between them. The icosahedra are linked via B2 in a rhombohedral unit cell.

Many other monomeric and polymeric carborane derivatives can be used for boron carbide preparation:

[NC-C6H4-C-C]2=C6H4 \°/

B10H10

-[C6H4-C-C-C6H4-CC]-n \°/

B10H10 Similar precursors yield boron carbide on heating up to 1500°C in an argon atmosphere [3]. Boron carbide can also be obtained via pyrolysis of lewis base adducts of decarborane [4-6].

The polyhedral boron hydride compounds (RxNH 4-x)2BnHn represent novel types of precursors for boron carbide and boron carbonitride preparation [3]:

97 B12C3+/-x

(RxNH4-x)2BnHn

B12C2,12N1,6

= R CH3, C2H5, C4H9 n=10,12 x=0-4 The pyrolitic decomposition of these compounds at 400°C leads to polymer precursors; heating them up to 1500°C yields boron carbide or boron carbonitride, depending on C/N ratio. The saturation of organic anion exchange polymers based an 2- 2- 2- 3- polyacrylnitrile or cellulose with B10H10 , B12H12 , B20H18 and B20H19 anions in aqueous or organic solutions followed by heat treatment and pyrolysis of obtained compositions results in core-free boron carbide fibres.

3.2. Silicon Carbide SiC The organosilanes and organohalosilanes are ordinary ceramic precursors for silicon carbide preparation [8-11]: Selected examples are listed below:

CH3SiCl3 (CH3)2SiCl2 (CH3)3SiCl 1000-1700°C CH3SiHCl2 SiC (CH3)4Si H2 (C6H5)3SiH (C6H5)3SiCH=CH2 (C6H5)3Si(CH3)3 As a rule silicon carbide obtained in this way contain excess carbon in most cases. Organosilicon polymers are more convenient precursors for silicon carbide. These are polysilanes, polycarbosilanes and polysiloxanes [12-16]. Their suitability as precursors is determined by facility of synthesis in the pure form, processing conditions and silicon

98 carbide grade. These polymers are usually prepared from relatively inexpensive reagents. The polymer preparation routes are presented below[17]:

-Thermolysis: (CH3)4Si [(CH3)2Si] -n, -[(CH3)H Si CH2]-n T T

- Dehalocoupling: (CH3)R Si Cl2 -[(CH3 R Si]-n -[RH Si CH2 Si HRCH2]-n M,T T (M=Li-K,R-alkyl)

- Dehydrocoupling: RSiH3 -[RSiH]-n T cat

- Hydrosilylation: = = - Si R2 H H-[Si R2 CH2 CH2]-n T

-[SiR2CH(CH3)SiR2]-m= =

- Ring Opening Polymerisation: [R2 Si CH2]2 -[R2 Si CH2]-n T

- Redistribution: [R Si Cl2]2 R Cl2 Si - [R Cl Si]-n Si Cl2 R T

The thermal treatment of polymers up to 1000 - 1700°C in inert or reducing atmosphere leads to silicon carbide. The polysiloxanes countaining oxygen always require reducing media for the preparation of pure silicon carbide : 1500°C -[Si(CH3)2O]-n SiC H2 3.3. Carbides of transition metals

99 The preparation of transition metal carbides from ceramic precursor is essentially limited to compounds containing a direct metal- carbon bond as well as some polymer derived from metal alkoxides and coordination compounds [18-23]:

(C5H5)2Ti(CH3)2 TiC T

[(C5H5)(CO)2 W(CR)]2 W2C T

Ar+H2, [(C6H402)2Ti]n TiC T

[CoEn3]W04 WC+Co(23°/o) T

(NH4)8[H2Co2W11040] WC + Co T

(R - alkyi. En - ethylene diamine ).

Ni(CO)4+CO Ni3C T

Solid carbide nanorods of TiC, NbC and Fe3C have been prepared by reacting pure carbon nanotubes with volatile metal iodides [23].

4. Ceramic Precursors for Nitrides The last ten years have seen a great growth of research in the ceramic nitride precursor chemistry. At the same time this area of research is mostly limited to the nitrides of boron, aluminum and silicon and to a smaller extent to the preparation of nitrides of transition metals. It should be emphasized that most precursors for nitride preparation are air and/or moisture-sensetive compounds. Nevertheless, many compounds, particularly those with polymeric structure have been

100 suggested as suitable precursors for nitride ceramics preparation in the form of powders, bodies, fibres, coating, films and binders.

4.1. Boron nitride BN

The majority of precursors for boron nitride preparation are based on borazene, substituted borazene and their polymeric forms [10].

The basic compound borazene B3N3H6 is easily available according to the reaction [24]:

3NaBH4 + 3NH4Cl B3N3H6 + 3NaCl + 9H2

Numerous substituted borazenes obtained both by direct substitution of hydrogen atoms in borazene and by reaction between boron halides or boron alkyls and ammonia or amines make a large part of borazenes are presented below:

C13B3N3H3 (R3N)3B3N3R'3 etc. C13B3N3R3 R3B3N3R'3 R, R'-alkyl, aryl

Borazene and its derivatives have structures similar to benzene with alternating B-N bound in the ring. The pyrolysis of borazene and its monomeric derivatives proceeds with condensation through BN - analogues ofnaphtalenes and biphenyl derivatives and with further crosslinking up to boron nitride [25]. Substituted borazenes form polymers upon heating in the range 300-400°C. These are good precursors for the boron nitride in the form of powders, green bodies, fibres, films and coating and binders [26] A large variety ofbarazene polymers have been prepared by cross -linking according to the following scheme:

>B-C1+H-N< ——————> >B-N<

>B-C1+H2NR< ——————> >B-NR-B<

>B - Cl + (CH3)3 Si NRR'—————— >B-NRR'

101 Polymer pyrolysis usually needs to be carried out in ammonia atmosphere to obtain pure boron nitride. Some examples of polymerization are presented below:

The obtained macromolecules are soluble in common organic solvents thus providing a variety of boron nitride forms. Functional groups in these polymers are capable of substitution reactions [27]:

NMe2 H B H3B*THF B HN NH HN NH B B B B N N N N N N H H n n

102 Another approach to the borazene polymers involves ammonolysis ofaminosubstituted borazenes, such as [(CH3)2NBNH]3, [(C2H5)2NBNH]3 for the preparation of pure inorganic aminosubstituted borazenes with give high purity boron nitride at 1200°C with a good yield [28-34]. Boranes can be used as starting compounds for polymer precursors preparation according to scheme [4,5]:

B10H14 + H2NCH2 CH2NH2 [B10H12(H2NCH2 CH2 NH2)]n

Borane - derived polymers being deficient in nitrogen, their pyrolysis proceeds under ammonia atmosphere to produce BN [1]:

H H C -C H H H H C C + B B H 5NH3 H N N N H + 8H2 B H 5 9 HB B BH N N H H

-[B10H12(H2NCH2 CH2NH2)]-n BN 10000C

Thise polymer gives BN/B4C composite on pyrolysis under argon atmosphere at 1000-15000C. Such polymers are suitable binders and have been used to prepare fibres and monoliths [4,35]. Boron nitride can also be prepared by pyrolysis of some boron cluster compounds [7]

103 NH3

(RX NH4-X)2 BnHn —————— BN 1000°C

R — CH3, C2H5, C4H9 n= 10, 12 x = 0 - 4. Pyrolysis in argon results in boron carbonitrides similar to pyrolysis of some borane coordination compounds.

4.2. Aluminum nitride A1N Aluminium nitride can be prepared via a precursor mainly byammonolysis of organoalanes [36,37]:

R3Al+NH3 —— [R3AlNH3]—— [R3AlNH2]3 ——

NH3 [RAlNH]n A1N

R - alkyi CH3 – C10H21 Aluminium hydride can also be used [38-39]:

Scheme I -300C, THF 20-1000C 4300C

AlH3 + 3NH3 Al(NH2)3 Al(NH)NH2 3H2

AlN1,27Hx AlN1,13Hx

Scheme II -800C, THF -450C 200C

AlH3 + NH3 AlH3NH3 H2AlNH2

1000C 1500C

HalNH H0,27AlNH0,27 H0,23AlNH0,23

104 The reaction of methylsilylamine with aluminium culoride also leads to aluminium nitride [40-42]: 60°C >200°C

AlCl3 + [(CH3)3 Si]2NH —— [Cl2AlNHSi(CH3)3]2 ————

500°C 1200°C

— [ClAlNH]n ——— Al(amorphous) ———> AlN(crystal). Ar Aluminium nitride fibres were prepared by pyrolysis of (CH3)2AlN3 and (C2H5)2A1N3 [43]:

R2A1N3 ——>R2A1N ——A1NH2 ——— A1N Polymeric aluminium compounds were also used [44]:

[C2H5CH =NA1(C4H9)2]2 ———— [(C2H5NA1C4H9)x (C2H5NAl)y]n

NH3 1300-1600°C

4.3. Silicon nitride Si3N4 The first step to obtain silicon nitride is the Si - N bond formation leading to silazane. All precursors for silicon nitride production are based on the silazane chemistry. The reaction of silicon tetrochloride and ammonia proceeds stepwise forming silicon imide precursors that are converted to silicon nitride at 1400°C [45-47]:

105 Process scheme for the production of Si3N4 powder by precipitation reactions.

Ammonolysis and aminolysis ofmonomeric or polymeric chlorosilanes is a common synthetic route for silicon-nitrogen bond formation [48]: >Si-Cl+H-N<———— >Si-N<+>NH HC1 Hexamethyldisilazane can also be used to replace the Si-Cl bond by the Si-NH bond thus producing silicon-nitrogen precursor [49];

[(CH3)3Si]2NH + HsiCI3 ————— (CH3)3SiCI + + [-HSi(NH)1,5]n [HSiNH(NHSi(CH3)3-]m

The product of ammonolysis with primary amines give off amines under ambient conditions and then undergo a condensation reaction [23]:

______>Si(NHR)2  N-Si-N-Si-N

-RNH2 H | | H

The silicon-nitrogen bond can be produced by catalytic dehydrocoupling [51]:

106 Cat >Si-H+H-N< —————Si-N<

Extensive efforts have been made to develop the method silazane polymerization because direct methods ofpolysilazane synthesis are limited and often give low yields. The pyrolysis of polysilazanes is the most convenient way to prepare silicon nitride materials. Polymerization ofsilazanes is based on base-catalyzed dehydrocyclodimerization, hydrosilylation, transition metal-catalyzed dehydrocompling and acid-catalyzed rearangement [52,53]. Some examples:

The silazanes are bound to have Si-H bounds to possess polymerization ability. High molecular weight polysilanes can also be prepared directly from pyridine-modified halosilanes reacting with ammonia [54]:

RHSiCl2 + 2C5H5N ——— [RHSiCl2 2C5H5N] ————> T

107 ———— - [RHSiNH]-n

Aminosilane (C2H5NH)4Si polymerizes by refluxing at 120- 180°C during three days [55]: T

(C2H5NH)4Si ————polymer+C2H5NH2+(C2H5)2NH

The fragments of the structure are presented below:

The copolymers can also be prepared by the reaction of

CH2=CHSiCI3 and CH3SiHCl2 with ammonia on heating to 220°C [56]:

Rl,R2 = H, Alk(en)yl R3 = Alk(en)yl a > 0, b > 0, n = 2 – 12

The silicon nitride obtained from silazane and polysilazane

108 precursors containing organo-groups bouded to silicon include a quantity of silicon carbide or carbon. The more organo-groups there are on silicon the greater carbon content is found an silicon nitride. Pure silicon nitride Si3N4 can be prepared via pyrolysis ofpolysilazanes under ammonia atmosphere according to the reaction:

Si – CH3 + NH3 Si – NH2 + CH4

Amorphous silicon nitride forms -Si3N4 above 1300°C transforming to -Si3N4 above 1600°C

4.4. Precursors for transition metal carbides It is known that transition metal chlorides interact with ammonia to form in soluble, nonvolatile metal amides and on heating at high temperature under nitrogen then convert to transition metal nitrides [29]:

MClx + NH3 M(NH2)x MN

Transition metal amides containing organogroups can be converted to transition metal nitrides only under ammonia atmosphere [57-64]:

(R2N)4M —————— MN(M=Ti,V)

(R2N)4M —————— M 3N 4(M=Zr,Nf,Nb,Ta) T

(R2N)4M —————— M3N4 (M = Zr, Nf, Nb, Ta) T

(R2N)5Nb —————— Nb3N4 R = CH3, C2H5

(R2N)5Ta —————— Ta3N5 T

The high temperature ammonolysis of transition metal amides is followed by the formation of a wide voriety of polymeric forms.

5. Ceramic Precursors for Metal Borides Zittle information is available on preceramic routes for metal borides. They include a theiwal decomposition oftetrahydroboranes, some borane derivatives and cluster boron hydride compounds. Most metals form tetrahydroborates that can be transformer into boride phases on pyrolysis according to the following scheme [65-72]:

- - MClx + BH4 —————M(BH4)x + xCl

M(BH4)x —————— MBx + 2xH2 Temperature limits of transformation depend on the metal nature (Table 2).

Table 2 Tetrahydroborate Temperature of Boride transformation, °C 1 2 3

A1(BH4)3 300 A1B2

Cr(BH4)3 430-450 CrB2

Ni(BH4)2 320-450 NiB

Mo(BH4)3 160-170 MoB3 + Mo5B18

W(BH4)3 170-190 WB

110 Ti(BH4)3 400-420 TiB2

Zr(BH4)3 250 ZrB2

Hf(BH4)3 250 HfB2

The limitation of these routes consists in air and/or moisture- sensetivity of tetrahydroborates. The use of coordination compounds allowed this problem to be solve partically: T

[Ni(NH3)6](BH4)2—————NiB2 + (H3BNH3)n + NH3 +H2

[Cr(NH3)6](BH4)2 ——————> CrB2 + (H3BNH3)n + NH3a + H2

The cluster boron compounds consisting of a polyhedral closed cage or open polyhedral fragments can be used as perspective ceramic precursors for metal borides. The basic structures of clusters consist of polyhedrons (Fig 3) wich can involve either only boron atoms or one or a few carbon atoms. On the other hand the metal atoms can be incorporated into the boron cage, forming polyhedral metalloboranes and metallocarboranes (Table 3).

Table 3 Types of polyhedra in reactions of polyhedral expansioa Number Introduced metal atom of Anion Ni Co Ru Rh Pd Os Ir Pt Zr vertices in the initial poli- hedron

2- 6 [B6H6] [NiB6]

[Ni3B6] - [CB5H6]

2- 8 [B8H8] - [СB7H8] [IrCB7]

2- 9 [B9H9] [NiB9] [Ir2B4]

111 - [СB8H9] [IrCB8]

2- 10 [B10H10] [Ni2B10] [RuB10] [RhB10] [OsB10]

- [СB9H10] [Co2CB9]

2- 11 [B11H11] [NiB11] - [СB10H11] [NiCB10] [PdCB10] [PtCB10]

2- 12 [B12H12] [RuB12] - [СB11H12] [ZrCB11] Finally, boron clusters can play on outer sphere role in coordination compounds of transition metals. Pyrolysis of abovementioned boron clusters can lead to metal borides or metal carborides [73]: T + + + M12B12H12 —————— MB6 + H2 (M = K – Cs ) +2 +2 +2 +2 +2 MB6H6 ———————— MB6 + H2 (M = Ca – Ba -, Eu , Sm )

530-550°C 800

[NiPhen3] B12H12 ——————— [NiB12H12] ———>NiBx + B - phen -H2 (phen - phenantrolyne).

112 Fig.3 Idealized structure and numbering order of the atoms in the closo- polyhedral frameworks (number of vertices n =6- 12). The advantages of these routes consist in the presence of the required structure that provides stoichiometry in many cases.

113 Fig.4 CaB6 structure

Fig.4 shows that the boride sublattice in CaB6 contains the 2- octahydrones B6 wich form the cage of the starting precursor B6H6 . Another preceramic route to borides is based on the reaction of monomeric and polymeric decarbone derivatives with metal oxides or metal ponders [74-77]. The ceramic precursors were prepared by dispersing metal or metal oxide powders in decaborane derivative solutions. Then the solvent was vacuum evaporated and the resulting solid dispersion was ground into fine powder. The metal borides or metal boronitrides were prepared by pyrolysis of obtained precursors at 1300-1500°C.

B10H14+2RCN—————B10H12(RCN)2 (R=alkyl)

B10H12(RCN)2 + MO2 ————— MB2 (M = Ti, Zr)

T _____ B10H14+2A B10H12A2 ———— [B10H12A2]n

[B10H12A2]n + Ti—————TiB2/TiN (A=amine)

B10H14+NC(CH2)5CN —— -[B10H12NC(CH2)5CN]-n

114 Ar

[B10H14 + NC(CH2)5CN] + M205(M02)————— MB2 14500

(M = Ti, Zr, Hf, Nb, Ta).

The ferraboranes Fe2(CO)6B6H6, HFe3(CO)9BH4 and HFe4(CO)12BH2 served as valatile precursors for amorphous iron boride films having approximatelly the same Fe/B ratio as the precursors used. Annealing the films results in the formation of principally crystaline

FeB, Fe3B, Fe3B/B respectively [78].

The composite TiB2 / TiN was prepared by pyrolysis of the precursor involving the suspension of titanium powder in polymeric borazine [79]: T

[B3N3Hx]n + Ti ————>• TiB2 / TiN + H2

6. Conclusion The chemistry of ceramic precursors is a relatively new field of material science that grows intensively at the present time. Although preceramic techniques can not certainly replace the classical methods of advanced ceramics technology due to their relatively high cost. Nevertheless ceramic precursors are indispensable where ceramics with rigidly or other specific properties have to be manufactured.

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