Thermal conductivity of calcium-doped aluminium nitride ceramics

Aluminium nitride ceramics were prepared with the addition of up to 12wt% of calcium oxide as a sintering aid. Both the oxygen and the calcium content of the samples decreased during sintering with increasing sintering temperature and soaking time. Higher amounts of calcium oxide resulted in higher thermal conductivities, with values up to 142 W m^–1 K^–1. Moderate sintering temperatures, short temperature soaking times and the use of inexpensive Ca-based sintering additives should enable the production of aluminium nitride ceramics with sufficiently high thermal conductivity at relatively low cost.     

The influence of microstructure on the thermal conductivity of aluminium nitride

Starting from three different commercial powders, AIN materials were densified by pressureless sintering under various temperature and time values in order to investigate the influence of microstructure on thermal conductivity. The influence of the sintering aids (3 wt% Y₂O₃ and 2 wt% CaC₂) and of the forming processes (cold isostatic pressing and thermocompression of tape cast pieces) were also been evaluated. Thermal conductivity increased with the purity level of the starting powder and with an increasing the sintering temperature and soaking time. The highest thermal conductivity values (196 Wm^–1 K^–1) were obtained with the purest powder and high temperature (1800 °C) sintering over long periods (6 h). No influence on thermal conductivity was detected from the forming technique.     

温度对高压烧结氮化铝陶瓷热导率的影响

以直接氮化法合成的AlN微米粉为原料,添加3%(质量分数)的CaC2为烧结助剂,在5GPa的压力下烧结30min,考察不同烧结温度对AlN陶瓷热导率的影响。用阿基米德排水法、XRD、SEM等技术手段对AlN烧结体进行性能检测。研究表明,在1500～1800℃范围内,温度的升高能促使AlN陶瓷内部晶粒长大,晶型饱满,尺寸均一,晶界相减少,实现烧结致密化,利于热导率的提高。     

Low-temperature sintering of aluminium nitride substrate with high thermal conductivity

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering chemical reactions to increase thermal conductivity of aluminium nitride

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.     

Phonon scattering and thermal conduction mechanisms of sintered aluminium nitride ceramics

From thermal diffusivity measurements of sintered AIN at temperatures ranging from 100 to 1000 K, the phonon mean free path of AIN was calculated in order to investigate phonon scattering mechanisms. The calculated mean phonon scattering distance was increased with decreasing temperature. The mean phonon-defect scattering distances were respectively limited to about 50 nm at temperatures ranging from 100 to 270 K and about 30 nm at temperatures ranging from 100 to 700 K, for AIN specimens with a room-temperature thermal conductivity of 220 and 121 Wm^–1 K^–1 containing 0.1 and 1.4 wt % oxygen, respectively. These short phonon-defect scattering distances were considered to correspond to the separation of oxygen-related internal defects in AIN grains. Calculation of the mean phonon scattering frequencies indicated that the phonon scattering is dominated by phonon-defect scattering at temperatures below 270 K for an AIN specimen with an oxygen content of 0.1 wt %, and at temperatures below 350 K for an AIN specimen with an oxygen content of 1.4 wt %.     

Effect of thermal mismatch induced residual stresses on grain boundary microcracking of titanium diboride ceramics

The effect of thermal mismatch induced residual stresses on grain boundary microcracking in titanium diboride (TiB₂) ceramics has been studied by finite element method. A cohesive zone model was used to simulate the microcracking initiation in four-point bending specimens. In particular, the microcracking was assumed to occur at a grain boundary which is located in the center of the specimen, surrounded by a thermally anisotropic area. The predicted failure strength appears to be significantly reduced by the presence of residual stresses when the cohesive energy of the microstructure is small. The failure load from experiments has been used to determine the critical damage parameters for microcracking initiation in both pristine and aluminum-infiltrated TiB₂. A viscous regularization technique is employed in the simulations to improve the rate of convergence of the solution and the effect of the value of the viscosity parameter on the simulation results, has been investigated. The effect of grain size, grain orientation, and number of employed thermally anisotropic grains, on the microcracking is also discussed.     

Synthesis of oxygen-free aluminium nitride ceramics

The aluminum nitride raw material in the form of powder was synthesized using the Self propagating High temperature Synthesis (SHS) method which provides no oxygen impurities. Then AIN powder was sintered to the full density without sintering additives and under a high pressure in a belt apparatus. For the AIN ceramics obtained the temperature dependences of the thermal diffusivity were measured with the laser-flash method. Finally we produced oxygen-free aluminium nitride ceramics with parameters comparable with theoretical data.     

High-temperature electrical conductivity of aluminium nitride

The electrical conductivity of hot-pressed polycrystalline aluminium nitride doped with oxygen and beryllium was measured as a function of temperature from 800 to 1200° C and over a range of nitrogen partial pressure from 10² to 10⁵ Pa. The effect of beryllium dopant, the independence of conductivity from nitrogen partial pressure, and the observed activation energy suggested extrinsic electronic species or aluminium vacancies as charge carriers. Polarization measurements made with one electrode blocking to ionic species indicated that the aluminium nitride with oxygen impurity was an extrinsic electronic conductor.     

Development of as-fired aluminium nitride substrates with smooth surface and high thermal conductivity

As-fired aluminium nitride (AIN) substrates with smooth and uniform surface have been developed by green sheet and firing technology. The effect of setting for firing on surface roughness was investigated. AIN substrates were fabricated by pressureless sintering of green sheets piled up and sandwiched between AIN plates in an AIN crucible. The thermal conductivity, surface roughness and bending strength of the substrate sintered at 1770 °C for 2 h under a pressure of 1 MPa nitrogen were 194 Wm^–1 K^–1, 0.15 (m and 353 MPa, respectively.     

Thermal conduction mechanism of aluminium nitride ceramics

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 m was 155 Wm^–1 K^–1, and was almost equal to that of the normally sintered AIN with a grain size of 4 m. 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.     

Effect of grain boundary characteristics on mechanical behaviour of quenched aluminium

Abstract

Tests concerning the mechanical properties of quenched aluminium were carried out on samples having different grain boundary characteristics. In comparison with aluminium of equilibrium structure, the quenched aluminium was found to be characterised by: a greater value of yield point in samples having a low fraction of special grain boundaries, and a lower value of yield point and a reduction of the Lüders strain in samples having a high fraction of special grain boundaries. The effects can be ascribed to the activity of dislocation loops as sources of dislocations or to a change in grain boundary characteristics as a result of annihilation of non-equilibrium vacancies on the grain boundaries.

MST/965     

Effect of grain boundary scattering on the electrical conductivity of antimony films

By starting from the theory of Mayadas and Shatzkes, a model for the effect of grain boundaries on the electrical conductivity of an intrinsic semimetal is proposed. The influence of the size of crystallites on the electrical conductivity of polycrystalline antimony films is studied theoretically and experimentally.

Excellent agreement is obtained over a large temperature range (10–500 K) for various sizes of crystallites (100–2000 Å).     

The effect of grain boundary phase on contact damage resistance of alumina ceramics

The effect of grain boundary phase on contact damage behavior is investigated in alumina ceramics. Four types of aluminas doped with MgO, anorthite (CaO·Al₂O₃·2SiO₂), silica, and with both MgO and anorthite are prepared such that they have similar average grain size by adjusting sintering conditions. MgO-doped alumina composed of equiaxed grains shows brittle fracture behavior, and anorthite-doped alumina composed of elongated grains shows a quasi-plastic response under Hertzian sphere indentation. The co-doped alumina with MgO and anorthite, however, is damage tolerant even with its rounded grains, while silica-doped alumina with similar grain size and shape to anorthite-doped alumina shows abrupt strength degradation with low critical load for cone cracking. The damage behavior is discussed from the viewpoint of residual stress induced by thermal expansion mismatch between the grains and grain boundary phases. The damage tolerant behavior of alumina ceramics is significantly affected by the composition of grain boundary phase.     

Effect of grain growth and measurement on fracture toughness of silicon nitride ceramics

Two kinds of -Si₃N₄ powders with different properties (high purity and low purity) were pressureless sintered using MgAl₂O₄-ZrO₂ as sintering additives. The grain growth was controlled by sintering time and temperature. The fracture toughness was determined using indentation microfracture (IM) and single-edge-precracked-beam (SEPB) methods. A discrepancy of fracture toughness was found between the values obtained by these two methods, and the SEPB method provided higher fracture toughness than the IM method. Materials with more large elongated Si₃N₄ grains gave higher fracture toughness, R-curve behavior and larger discrepancy. This is contributed to the effects of grain bridging, pull-out and deflection by the large elongated Si₃N₄ grains. Comparing the specimens from two kinds of -Si₃N₄ powders with different purity level, both gave high fracture toughness of over 9 MPa·m^1/2 by the SEPB method, while the E10 samples have a little higher flexural strength.     

The grain size effect on the yttria stabilized zirconia grain boundary conductivity

Conductivity of yttria stabilized zirconia produced from weakly agglomerated nanopowder was investigated by ac impedance spectroscopy. Dense ceramic samples with the grain size from 90 to 800 nm were prepared at the variation of both pressing and sintering conditions. It was found that the bulk conductivity is not affected by grain size, while grain boundary conductivity is dependent on this factor. Observed grain boundary resistance increases with grain size. This relationship is contrary to the previous results obtained for the range from 1 to 18 microm where grain boundary resistance decreased with grain size. Maximum of grain boundary resistance versus ceramics grain size is observed at 450 degrees C for the grain size about 270 nm.     

Sintering and characterization of fully dense aluminium nitride ceramics

Aluminium nitride ceramics with no sintering additives could be densified to close to theoretical density (99.6% theoretical) by pressureless sintering of tape-cast green sheets at 1900 °C for 8 h. The thermal conductivity and bending strength of the specimens were 114 Wm^–1 K^–1 and 240 MPa, respectively. The effect of Y₂O₃ additive on sinterability, thermal conductivity and microstructure of aluminium nitride ceramics was investigated. Thermal conductivity increased with increasing amount of Y₂O₃ additive, sintering temperature and holding time at the sintering temperature. Samples with a thermal conductivity up to 258 Wm^–1 K^–1 were fabricated by elimination of the grain-boundary phase.