Recent advanced thermal interfacial materials: A review of conducting mechanisms and parameters of carbon materials

http://research.physics.berkeley.edu/hellman/papers/science129.pdf

Specific heat and thermal conductivity of low-stress amorphous Si–N membranes

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

The fabrication of dense Si3N4/SiC nanocomposite materials that contained 2.5-30 wt% SiC via gas-pressure sintering and hot pressing was investigated. The SiC particles originated from admixed commercial SiC powders, SiCN powders produced by plasma synthesis, in situ reaction pyrolysis of carbon-coated Si3N4 particles, and pyrolysis of a polycarbosilazane-based SiCN precursor. Based on thermodynamic calculations, criteria for minimum liquid-phase decomposition during sintering were developed. The best sintering results were obtained for sintering cycles that observed this criteria. Materials that contained plasma-synthesized SiCN exhibited high strengths (835-995 MPa) and fracture toughness values (7.4-7.8 MPam1/2) at room temperature. Post-sintering thermal treatments led to a strength reduction.

Silicon Nitride/Silicon Carbide Nanocomposite Materials: I, Fabrication and Mechanical Properties at Room Temperature

Chemical reactions to increase thermal conductivity by decreasing oxygen contents during AlN sintering with an Y2O3 additive in a reducing nitrogen atmosphere with carbon were investigated. They were: Al2O3 + N2 + 3CO ⇋ 2AlN + 3CO2, Al2Y4O9 + N2 + 3CO ⇋ 2AlN + 2Y2O3 + 3CO2 and Y2O3 + N2 + 3CO ⇋ 2YN + 3CO2. Some of the CO2 gas reduced to CO gas in the presence of carbon by a chemical reaction: CO2 + C ⇋ 2CO. These reactions were confirmed by examining oxygen contents, the grain boundary phases of the sintered AlN, and the trapped CO and CO2 gases in the sintered bodies. These reducing reactions proceed with increasing sintering temperature and periods, and hence the thermal conductivity is increased.

Sintering chemical reactions to increase thermal conductivity of aluminium nitride

Extremely large grain size AIN ceramics were produced by HIP sintering at an ultra-high temperature of 2773 K without reducing the oxygen content in order to determine experimentally whether the factor controlling thermal conductivity is either grain boundaries or the internal structure of the grains. The room-temperature thermal conductivity of the HIPed AIN with a grain size of ∼40 μm was 155 Wm−1 K−1, and was almost equal to that of the normally sintered AIN with a grain size of 4 μm. Therefore, thermal conductivity at room temperature is independent of AIN grain size, or the number and amount of grain-boundary phase for reasonably well-sintered AIN ceramics. The calculated phonon mean free path of sintered bodies was 10–30 nm at room temperature, which is too small to compare with the AIN grain size. Consequently, it is shown that the thermal conductivity of sintered AIN is controlled by the internal structure of the grains, such as oxygen solute atoms.

Thermal conduction mechanism of aluminium nitride ceramics

Thermal diffusivity and conductivity as well as specific heat of capsule-HIPped Si3N4 with Y2O3 and Al2O3 6mol% in total as additives were measured at different temperatures ranging from 100 to 1300K. The isobaric specific heat CP agreed with the isochoric specific heat CV calculated by the Debye's theory, assuming that silicon and nitrogen atoms are lattice dynamical ly equivalent. The Dulong-Petit's rule applied well to sintered Si3N4 at high temperatures and CV•à25J/(K•Emol). The glassy phase quantity could be estimated by the difference of the measured specific heat and the calculated specific heat at low temperatures. The specimens with higher thermal diffusivity and conductivity at low temperatures were those with higher ƒÀphase fraction, larger particle size of raw Si3N4 and richer Y2O3 additive. For these specimens the thermal diffusivity and conductivity showed a larger temperature depencence. [Received September 16, 1988; Accepted November 18, 1988]

Temperature Dependence of Thermal Coefficients for HIPped Silicon Nitride

Thermal diffusivity and conductivity as well as specific heat were studied for capsule-HIP sintered Si3N4. Yttria and alumina additives with a total amount of 6mol% were used. For a given additive percentage, specimens of higher β phase fraction had higher thermal diffusivity and conductivity, as expected However, samples with larger amounts of Y2O3 additive had higher thermal diffusivity and conductivity in spite of a lower β phase fraction than those with more Al2O3 additive. The highest thermal conductivity was 70W/(m·K) with 6mol% Y2O3 additive alone. The reason is that aluminium and oxygen ions are soluble in β-Si3N4 particles, in the case for Al2O3 addition.Oxygen, presented as an impurity, also had a profound effect on the thermal diffusivity and conductivity. Higher oxygen contents reduced the thermal diffusivity and conductivity. This may be due to the amount of grain boundary glassy phase.The specific heat of sintered Si3N4 was 0.67±0.02kJ/(K·g). The crystalline structure (α or β-Si3N4), additive composition, and raw material particle size did not show any effect on the specific heat.

Koji Watari

Papers

Sintering chemical reactions to increase thermal conductivity of aluminium nitride

Thermal conduction mechanism of aluminium nitride ceramics

Temperature Dependence of Thermal Coefficients for HIPped Silicon Nitride

Thermal Properties of HIP Sintered Silicon Nitride

Oxygen behavior of normal and HIP sintered A1N