Stability and transformations

Silicon carbide is a crystalline compound in which carbon and silicon are bounded with covalent bounds whose energy is around 318 kJ/mol.¹¹

The Si-C phase diagram is shown in Fig. 3.3. Silicon carbide is the only solid intermediate in the Si-C system that crystallizes both in the cubic and in the hexagonal form with a lot of polytypes. Silicon carbide doesn’t show any solid solution with Si or C but it has a certain non-stoichiometry in the composition due to carbon vacancies which has been reported by Prochazka.¹² SiC decomposes in a peritettic point forming liquid silicon and solid carbon at around 2545°C with an atomic percentage of liquid silicon of 27%.¹³

Since SiC has been firstly synthesised it has always interested crystallographers for its wide variety of crystallographic structures in which it can be present without changing the stoichiometry. The phenomena by which a material shows different dispositions of one-dimensional layers of identical planes of atoms is called polytypism and the deriving structures are called polytypes. It is important to know the conditions of stability in that way to forecast and prevent undesired transformations during processing. Some important electrical properties depend on the specific polytype and in the case of structural applications polytypic transformations, which occur during sintering, can determine variations in mechanical properties of the material.¹⁴

Fig. 3.3: Silicon carbide phase diagram.

A great number of experiments were conducted in order to identify the stability regions of SiC polytypes. The performed studies and the developed theories are based on kinetic and thermodynamical considerations. These studies had a big impulse especially because of the possibility to exploit the interesting electronic properties of silicon carbide, which changes depending on the polytype, and bounded to the necessity to obtain crystals made of one single polytype. In any cases the stability of a polytype depends on the temperature and on the grain growth rate. Kippenberg suggested a stability diagram in Fig. 3.4 of the different polytypes drawn out from experiments on single crystals growth.^15,16

Fig. 3.4: SiC polytype stability diagram as a function of the temperature.

It is usually observed that the 3C (β-SiC) cubic polytype is formed at mild temperature and at high temperature turns into α-SiC. It could be thought that these two polytypes are the two stable phases at low and high temperature respectively, nevertheless β-SiC has been observed even at high temperature and quantum-mechanical calculations evidenced that other polytypes should be more stable at lower temperature, this indicating that β-SiC is a metastable phase.^17-19

β-SiC is the polytype which forms first in the processes of crystal growth, even if the cubic phase is not the lowest energy structure at temperature above 1873°C.¹⁶ The reasons for this metastable form consist in the high symmetry and multiplicity of the 3C lattice which allows the rearrangement of the tetrahedrons in a lower local energy configuration, further lowered by electron-donor impurities.²⁰ These factors kinetically favour the nucleation and growth of the 3C form. Once the 3C phase is formed other polytypes can be observed (α-phases): 2H, 4H, 6H, 15R. Many of the hexagonal polytypic structures derive from this transformation. In the temperature range 1673-1873°C none of these transformations takes place.

The most common transformation is 3C → 6H,

but others are possible: 3C→4H, 3C → 2H, 3C → 15R, 4H → 6H, etc.

Among the suggested mechanisms for the solid state transformations, periodic slip and diffusional rearrangement are supposed to occur. The first case hypothesizes that the transformation is the result of the partial sliding of the dislocations on the basal plane with periodicity in the stacking direction, caused by the rotation of these dislocations on other dislocations with Bourger’s vector parallel to the c axes.²¹ In the case of diffusional rearrangement, nucleation and stacking fault expansion occur on the basal plane. The defects expand for thermal effects, actived by defect/matrix interface.

Heuer et al.^22-25 demonstrated that transformation processes and α-phase growth during annealing treatment of polycrystalline SiC occur through a rapid growth of composite grains constituted by a layer of α-SiC in a β-SiC matrix. This growth is faster than the development of the grains at the interface with β-grain. The study on the energy evidenced a high anisotropy of the interface energy between α and β-SiC:

the interface {111}β║(0001)α energy is much lower than any interface and this favors the development in this preferential direction.

The β→α transformation during sintering or during annealing processes is often associated to the formation of plate and elongated grains, that is an anisotropic growth, which offers the possibility to a in situ reinforcement and, as a consequence, to an increased fracture toughness.^26-30

A crucial point is the purity of the sample. Some transformation can be associated to the presence of specific impurities either in the material or in the atmosphere. The reverse transformation 6H→3C occurs by heating 6H SiC in nitrogen atmosphere.

This transformation is the opposite of what is commonly observed during high temperature sintering of silicon carbide in N2. In nitrogen atmosphere the transformation β → α is hindered or slowed down.

The presence of aluminium or boron in the starting materials or in the liquid phase favors a series of polytypic transformations, such as 6H→4H→2H or 3C→4H and the microstructure shows elongated and plate grains characterized by a high aspect ratio, where aspect ratio is the ratio between length and width of the grains.³¹ The degree of transformation is more evident when the amount of Al or B is high.

Electron-donor species, as nitrogen or phosphorus, stabilize tetrahedral layer in the cubic structure, whilst electron-acceptor species, as aluminium or boron, stabilize the hexagonal structure.

Also the pressure plays an important role. During the sintering of β-SiC, the 3C→6H transformation occurs at temperatures increaseing with the pressure. At temperature above 2500°C and with applied pressure higher than 4.5 GPa the reverse transformation (6H→3C) occurs.³²

During sintering at temperature below 1900°C and at pressure of 30 MPa, the transformation β → α is inhibited.

One of the aspect to evaluate is that we often deal with starting powder containing traces of different polytypes, so experimental systems show a notable complexity and during sintering much more interactions can occur. Although extensive studies on phase transformation were conducted, the mechanisms which take place during the polytypic transformation are still unclear because the achieved results are often contradictory. These systems are very complex because of the sintering conditions and the various parameters which in turn take part to the transformation.

3.3 SYNTHESIS

Silicon carbide is most frequently synthesized on industrial scale by the Acheson method.³³ In this technique, two solid electrodes are connected with graphite powder, a mixture of silica and coke is packed in the surrounding area, and the whole assembly is electrically heated at 2700°C for producing SiC mainly by the reaction (1):

SiO2+3C→SiC+2CO (1)

SiC crystal blocks from the reaction are then ground, refined and classified to produce SiC powders. The SiC grades thus prepared are characterised as α-SiC having a relatively coarse grained structure with a mean particle size of 5 μm. This requires a further refining process to produce submicron grain powders suitable for sintering.³⁴ β-SiC is also produced by the Acheson method at lower temperature (1500-1800°C) or by vapour-phase reactions.^35-37

This last method foresees the reaction of SiH4 35 or SiCl4 with hydrocarbons such as CH⁴ and C³H^{8 31} or the thermal decomposition of CH³SiCl³, (CH³)⁴Si,³² or polycarbosilane.³⁷ The steps leading to the production of SiC by vapour-phase method can be summarised as follows (2-5):

SiH4 + CH4 → SiC + 4 H2 (2)

CH3 SiCl3 → SiC + 3 HCl (3)

Si(CH3)4 → SiC + 3 CH4 (4)

(CH3)2SiCl2 → SiC + CH4+ 2HCl (5) Particle size and stoichiometry are controlled by varying reaction temperature, gas concentration and gas flow rate.

Other methods include the direct reaction of silicon and carbon³⁴ and the gas evaporation method,³⁸ in which the raw material surface is heated and melted using an arc discharge in a mixed gas (consisting of an inert gas and H² or N²) to form ultrafine SiC particles.

A new synthesis process has been recently developed to produce small, high purity, non-agglomerated powders of SiC.^39,40 In this process, the ceramic powder is synthesised by rapid heating (plasma heating) a reactant gas (silane, SiH4, mixed with methane, CH4, or ethylene,C2H4). The characteristics of the powders are controlled by the cell pressure, the reactants, their flow rates and flow ratios and the temperature

1400° C 950-1400°C

plasma

1200°C

distribution within the reaction zone. Luce et al.⁴¹ further studied the same process by using laser heating and by varying the reaction conditions (laser power, flow rates and ratio of reacting gases). The powder synthesised by this technique is a mixture of amorphous and crystalline powder.

3.4 SINTERING

The high degree of covalence implies a difficult sintering of SiC-bodies by simple heating of powder compacts. As a result, many different sintering techniques have been developed. Among these, pressureless sintering, hot-pressing and reaction sintering are the most important ones and, depending on the impurities or the sintering additives, a solid state or a liquid phase sintering is activated.