Silicon Nitride Based Hard Materials

Review on the physicochemical treatments of rice husk for production of advanced materials

“…Because of the importance of silicon nitride (Si 3 N 4 ) in engineering materials, their properties and potential applications at high temperature, in microelectronics and optoelectronics, in recent years there has been a growing interest in this material from different perspectives.Silicon nitride (Si 3 N 4 ) is one of the most promising structural materials for hightemperature and high mechanical stress applications because of its excellent properties such as high strength retention at elevated temperature, low thermal expansion coefficient, good thermal shock resistance, and high corrosion resistance. It has a much higher creep resistance than metals and its thermal shock resistance is much better than other ceramics [246,247].There are basically four different synthetic routes for this powder, which differ in purity, morphology and sintering activity:(i) a direct reaction of elemental silicon with nitrogen that gives relatively coarse products and have to be milled prior to further processing [248]; (ii) carbothermal reduction utilizes a fine mixture of carbon and SiO 2 powders in a flowing of N 2 at 1500°C [249]; (iii) the so-called diimide-process is a liquid or gas-phase route to Si 3 N 4 powders [250]; (iv) CVD techniques using volatile silicon compounds such as SiCl 4 , SiH 4 or related molecular compounds yield Si 3 N 4 powders upon gas phase reaction with ammonia [251].Becauseof the highpurityof fine silica and carbon and their commercial availability, the carbothermal reduction of silica is themain method for manufacturing silicon nitride powder.However, this method has a high requirementon raw materials;high reactivity and agood distribution of the silica and carbon are importantfor the reaction.The better the distribution, the morehelpful for nitridation. Therefore, RH, which naturally contains both silica and carbon, is a potential material for producing silicon niride [2,81].…”

Mechanism and Parameters Controlling the Decomposition Kinetics of Na2SiF6 Powder to SiF4

“…While for the tests in N 2 :NH 3 , the activation energies for formation of the gaseous species from Na 2 SiF 6 are 140, 120, and 115 kJ/mol, in N 2 the values of E a are 121, 109, and 94 kJ/mol for flow rates of 20, 60, and 100 cm 3 /min, respectively. It is thus clear that atmosphere plays an important role in the decomposition kinetics of Na 2 SiF 6 and that interestingly, in N 2 , a lower activation energy is required than in N 2 :NH 3 (see Fig.…”

Leaching behaviour of silicon nitride materials in sulphuric acid containing KF

“…Furthermore the thin films with a thickness of about 1 to 10 nm between two adjacent grain faces show no evidence for a significant corrosive attack. 10,11 This causes even fully corroded Si 3 N 4 materials to have a residual strength of 400-500 MPa. It was described that silicon nitride materials under certain corrosion conditions exhibit a passivization reaction which blocks the corrosive attack nearly completely.…”

“…In the acids mentioned first, corrosion of silicon nitride ceramics occurs mainly along the triple junctions. 10,11 The glass network modifying ions like Mg 2+ , Al 3+ and Y 3+ are leached by the acid, the network building ions like silicon remain partially inside the corroded triple junctions. Furthermore the thin films with a thickness of about 1 to 10 nm between two adjacent grain faces show no evidence for a significant corrosive attack.…”