Some aspects of processing, microstructure and properties of the various types of silicon nitride are discussed. Special emphasis is placed on the relationships between powder properties, process conditions, densification and microstructure, as well as the interdependence between microstructure and properties. After summarizing the areas of crystal structure and thermodynamic properties, and processing of the different types of Si3N4, the state-of-the-art of dense and reaction-bonded silicon nitride is given. For both types the formation mechanisms and microstructure, relationships between powder properties, additives (in the case of dense Si3N4), process conditions, and densification and microstructure, as well as data and microstructural effects of various mechanical, thermal and thermo-mechanical properties, are outlined. Advanced processing techniques, such as sintering, gas-pressure sintering, post-sintering, and the different routes of hot-isostatic pressing (starting with powder compacts, reaction-bonded Si3N4 or pre-sintered Si3N4 and the resulting properties, are discussed.

Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride

Photoacoustic and related photothermal techniques

Calculations based on a simple modified Wiener's model for thermal conductivity of a composite material predict that the thermal conductivity of β-Si 3 N 4 decreases quickly as the grain-boundary film thickness increases within a range of a few tenths of a nanometer and also that it initially increases steeply with increased grain size, then reaches almost constant values. Because of the faceted nature of the β-Si 3 N 4 crystal, the average grain-boundary film thickness is much greater than that in equilibrium. The present study demonstrates both theoretically and experimentally that grain growth alone cannot improve the thermal conductivity of β-Si 3 N 4 .

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Thermal Conductivity of ß-Si3N4: I, Effects of Various Microstructural Factors

Silicon nitride was fabricated by tape casting of {alpha}-Si{sub 3}N{sub 4} powder with 5 wt% Y{sub 2}O{sub 3} and 5 vol% rodlike {beta}-Si{sub 3}N{sub 4} seed particles, followed by tape stacking, hot pressing under 40 MPa, and annealing at 1,850 C for 2--66 h under a nitrogen pressure of 0.9 MPa. Silicon nitrides fabricated by this procedure exhibited a highly anisotropic microstructure with large elongated grains (developed from seed particles) uniaxially oriented parallel to the grain alignment were much higher than those measured in other directions and exhibited high values of up to 120 W/(m{center_dot}K). The anisotropic thermal conductivity of the specimen could be explained by the rule of mixture, considering that large elongated grains developed from seeds have higher thermal conductivity than a small-grained matrix.

High Thermal Conductivity in Silicon Nitride with Anisotropie Microstructure

Polycrystalline Si 3 N 4 samples with different grain-size distributions and a nearly constant volume content of grain-boundary phase (6.3 vol%) were fabricated by hot-pressing at 1800°C and subsequent HIP sintering at 2400°C. The HIP treatment of hot-pressed Si 3 N 4 resulted in the formation of a large amount of β-Si 3 N 4 grains ∼10 μm in diameter and -50 μm long, and the elimination of smaller matrix grains. The room-temperature thermal conductivities of the HIPed Si 3 N 4 materials were 80 and 102 W.m -1 .K -1 , respectively, in the directions parallel and perpendicular to the hot-pressing axis. These values are slightly higher than those obtained for hot-pressed samples (78 and 93 W.m -1 .K -1 ). The calculated phonon mean free path of sintered Si 3 N 4 was -20 nm at room temperature, which is very small as compared to the grain size. Experimental observations and theoretical calculations showed that the thermal conductivity of Si 3 N 4 at room temperature is independent of grain size, but is controlled by the internal defect structure of the grains such as point defects and dislocations.

Effect of Grain Size on the Thermal Conductivity of Si3N4

The effects of compositional and microstructural variables and processing conditions on the room temperature thermal diffusivity of hot-pressed and reaction-sintered silicon nitride were determined. The thermal diffusivity for hot-pressed silicon nitride increases withβ-content. Maximum thermal diffusivity is reached at about 3 wt % MgO. The higher thermal diffusivity of the β-phase is attributed to its higher purity level and the less distorted crystal structure compared to theα-phase. In reaction-sintered nitride the thermal diffusivity is strongly influenced by the relative amount and needle-like morphology of theα-phase. Correlations of the thermal diffusivity with mechanical properties are discussed.

G. Ziegler

Papers

Effect of phase composition and microstructure on the thermal diffusivity of silicon nitride