Thermal Conductivity of Platelet-Filled Polymer Composites

Platelet-shaped particles of similar size and shape were investigated as fillers for improving the thermal conductivity of polymer–ceramic composite materials. The conductivities of composites filled with hard, stiff ceramic particles exceeded 3.5 W·(m·K)⁻¹, or >20 times the conductivity of the polymer matrix, and were shown to be almost independent of the intrinsic filler conductivity range of 33–300 W·(m·K)⁻¹. In contrast, the thermal conductivity of composites filled with soft, platelet-shaped BN fillers reached over 13 W·(m·K)⁻¹. A mechanism is proposed whereby deformation of the soft filler particles provides improved particle-to-particle connectivity and allows greater packing density, resulting in the ability to achieve much higher conductivity than is possible for hard and stiff particles of similar initial morphology. Experimental results are discussed in light of various thermal conductivity prediction models in the literature.     

Microstructural Characterization of High-Thermal-Conductivity Aluminum Nitride Ceramic

An aluminum nitride (AlN) ceramic with a thermal conductivity value of 272 W·(m·K)⁻¹, which is as high as the experimentally measured thermal conductivity of an AlN single crystal, was successfully fabricated by firing at 1900°C with a sintering aid of 1 mol% Y₂O₃ under a reducing N₂ atmosphere for 100 h. Oxygen concentrations were determined to be 0.02 and 0.03 mass% in the grains and in the grain-boundary phases, respectively. Neither stacking fault in the grains nor crystalline phase in the grain-boundary regions was found by transmission electron microscopy. An amorphous phase possessing yttrium and oxygen elements was detected between the grains as thin films with a thickness of <1 nm. Because the amount of grain-boundary phase was small, the high-thermal conductivity of the ceramic was attributable to the low oxygen concentration in the AlN grains.     

Comparison of Aqueous and Non-Aqueous Tape Casting of Aluminum Nitride Substrates

A direct comparison of aqueous and non-aqueous tape casting was investigated. Aqueous and non-aqueous formulations were developed for tape casting of aluminum nitride (AlN) powder. The oxygen content of the AlN powder in aqueous ball-milling media had a slight increase, but it did not almost influence the thermal conductivity of AlN substrate. The solid loading of aqueous AlN slurry was higher than that of the non-aqueous one, but its viscosity was lower than that of non-aqueous AlN slurry. Under the same burnout program, the residual carbon content in aqueous AlN green sheet was lower than that of the non-aqueous one. The thermal conductivity of the aqueous AlN substrate sintered at 1850°C for 3 h was 138 W·(m·K)⁻¹, which was close to 142 W·(m·K)⁻¹ of the non-aqueous AlN substrate.     

Boron Nitride–Aluminum Nitride Ceramic Composites Fabricated by Transient Plastic Phase Processing

BN–AlN ceramic composites have been successfully fabricated by a novel process referred to as transient plastic phase processing (TPPP). The process used BN as both the reactant phase and the matrix and Al as the transient plastic phase. The products AlN and AlB₁₂ were regarded as the reinforcing phases. With the addition of Al powder in BN, both the mechanical and thermal properties were improved. Relatively high green density (2.03 g/cm³, 82.0% of theoretical density (TD)) and as-sintered density (2.18 g/cm³, 92.6%TD), high bending strength (106 MPa), and high thermal conductivity (72 W/(m·K)) were attained for one kind of BN–AlN composite. A low thermal expansion coefficient of 2.0 × 10⁻⁶/K was also achieved.     

High Thermal Conductivity Aluminum Nitride Substrates Prepared by Aqueous Tape Casting

High thermal conductivity aluminum nitride (AlN) substrates were prepared by aqueous tape casting. The characteristics of surface-treated AlN powder were studied in aqueous ball-milling media. The oxygen content of AlN powder with the dispersant was lower than that of AlN powder without the dispersant at the same ball-milling time. The isoelectric points of the surface-treated AlN with and without a dispersant were, respectively, at pH∼3.35 and pH∼3.90. The atmosphere had considerable effects on organic additive burnout of aqueous AlN green sheets. The composition of grain boundaries changed with increasing holding time at 1850°C. A translucent AlN substrate with a uniform microstructure and a thermal conductivity of 263 W·(m·K)⁻¹ was obtained by pressureless sintering at 1850°C for 6 h in a nitrogen atmosphere.     

In Situ Reaction Synthesis, Electrical and Thermal, and Mechanical Properties of Nb4AlC3

In this work, a bulk Nb₄AlC₃ ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb₄AlC₃ were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10⁻⁶ K⁻¹ in the temperature range of 200°–1100°C. The thermal conductivity of Nb₄AlC₃ increased from 13.5 W·(m·K)⁻¹ at room temperature to 21.2 W·(m·K)⁻¹ at 1227°C, and the electrical conductivity decreased from 3.35 × 10⁶ to 1.13 × 10⁶Ω⁻¹·m⁻¹ in a temperature range of 5–300 K. Nb₄AlC₃ possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m^1/2. Most significantly, Nb₄AlC₃ could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C.     

Mechanical and Thermal Properties of Antiperovskite Ti3AlC Prepared by an In Situ Reaction/Hot-Pressing Route

Bulk Ti₃AlC ceramic containing 2.68 wt% TiC was prepared by an in situ reaction/hot-pressing route. The reaction path, microstructure, mechanical and thermal properties were systematically investigated. At room temperature Vickers hardness of Ti₃AlC ceramic is 7.8 GPa. The flexural strength, compressive strength, and fracture toughness are 182, 708 MPa, and 2.6 MPa·m^1/2, respectively. Its apparent Young's modulus, shear modulus, bulk modulus and Possion's ratio are 208.9, 83.4, 140.4 GPa, and 0.25 at room temperature. Apparent Young's modulus decreases slowly with the increasing temperature, and at 1210°C the modulus is 170 GPa. The average coefficient of thermal expansion of Ti₃AlC ceramic is about 10.1 × 10⁻⁶ K⁻¹ in the temperature range of 150°–1200°C. Both the molar heat capacity and thermal conductivity increase with an increase in the temperature. At 300 and 1373 K, the molar heat capacities are 87 and 143·J·(mol·K)⁻¹, while the thermal conductivities are 8.19 and 15.6 W·(m·K)⁻¹, respectively.     

Reactive Hot Pressing and Properties of Nb2AlC

Dense Nb₂AlC ceramic was synthesized from NbC, Nb, and Al powder mixture at 1650°C and a pressure of 30 MPa for 90 min using an in situ reaction/hot-pressing method. The reaction kinetics, microstructure, physical, and mechanical properties of the fabricated material were investigated. A thermal expansion coefficient of ∼8.1 × 10⁻⁶ K⁻¹ was measured in the temperature range of 30°–1050°C. At room temperature a thermal conductivity of ∼20 W·(m·K)⁻¹ and a Vickers hardness of ∼4.5 GPa were determined. The material attained Young's modulus, four-point bending strength and fracture toughness of ∼294 GPa, ∼443 MPa, and ∼5.9 MPa·m^1/2, respectively. The nanolayered grains with a mean grain size of 17 μm contributed to the damage tolerance of this ceramic. Quenching from 600°, 800°, and 1000°C into water at room temperature resulted in decrease in bending strength from 443 MPa for the as-synthesized Nb₂AlC to 391, 156, and 149 MPa, respectively.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

Thermal and Electric Properties in Hot-Pressed ZrB2–MoSi2–SiC Composites

The thermal and electrical properties of MoSi₂ and/or SiC-containing ZrB₂-based composites and the effects of MoSi₂ and SiC contents were examined in hot-pressed ZrB₂–MoSi₂–SiC composites. The thermal conductivity and electrical conductivity of the ZrB₂–MoSi₂–SiC composites were measured at room temperature by a nanoflash technique and a current–voltage method, respectively. The results indicate that the thermal and electrical conductivities of ZrB₂–MoSi₂–SiC composites are dependent on the amount of MoSi₂ and SiC. The thermal conductivities observed for all of the compositions were more than 75 W·(m·K)⁻¹. A maximum conductivity of 97.55 W·(m·K)⁻¹ was measured for the 20 vol% MoSi₂-30 vol% SiC-containing ZrB₂ composite. On the other hand, the electrical conductivities observed for all of the compositions were in the range from 4.07 × 10–8.11 × 10 Ω⁻¹·cm⁻¹.     

Microstructural and Thermoelectric Characteristics of Zinc Oxide-Based Thermoelectric Materials Fabricated Using a Spark Plasma Sintering Process

M-doped zinc oxide (ZnO) (M=Al and/or Ni) thermoelectric materials were fully densified at a temperature lower than 1000°C using a spark plasma sintering technique and their microstructural evolution and thermoelectric characteristics were investigated. The addition of Al₂O₃ reduced the surface evaporation of pure ZnO and suppressed grain growth by the formation of a secondary phase. The addition of NiO promoted the formation of a solid solution with the ZnO crystal structure and caused severe grain growth. The co-addition of Al₂O₃ and NiO produced a homogeneous microstructure with a good grain boundary distribution. The microstructural characteristics induced by the co-addition of Al₂O₃ and NiO have a major role in increasing the electrical conductivity and decreasing the thermal conductivity, resulting from an increase in carrier concentration and the phonon scattering effect, respectively, and therefore improving the thermoelectric properties. The ZnO specimen, which was sintered at 1000°C with the co-addition of Al₂O₃ and NiO, exhibited a ZT value of 0.6 × 10⁻³ K⁻¹, electrical conductivity of 1.7 × 10⁻⁴Ω⁻¹·m⁻¹, the thermal conductivity of 5.16 W·(m·K)⁻¹, and Seebeck coefficient of −425.4 μV/K at 900°C. The ZT value obtained respects the 30% increase compared with the previously reported value, 0.4 × 10⁻³ K⁻¹, in the literature.     

Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering

The precursor powders of Ca₃Co₄O₉ were synthesized by a sol–gel method. The results of X-ray diffraction and thermogravimetric and differential thermal analyses patterns indicate that pure Ca₃Co₄O₉ powders could be obtained by calcining the precursor at 800°C for 2 h. High dense Ca₃Co₄O₉ ceramic samples (∼99% of theoretical density) were prepared by the spark plasma sintering (SPS) method. Compared with the conventional sintering (CS), the SPS samples exhibit much higher electrical conductivity and power factor which are respectively about 118 S/cm and 3.51 × 10⁻⁴ W·(m·K²)⁻¹. The SPS method is greatly effective for improving the thermoelectric properties of Ca₃Co₄O₉ oxide ceramics.     

Thermal Shock Resistance of an AlN–BN–SiC Ceramic

Mechanical and thermal properties of AlN–BN–SiC (ABS) ceramics were used to calculate the R, R ', and R "" thermal shock parameters. The R parameter values ranged from ∼400° to 450°C. Specimens were thermal shocked by water quenching and the critical quench temperatures (Δ T _C) were compared with those of a baseline SiC composition. The behavior of the ABS was predicted by R parameter calculations while the behavior of the baseline material was predicted by the R ' calculations due to its higher thermal conductivity (87 W·(m·K)^-1) as compared with the ABS materials (∼30 W·(m·K)⁻¹). The highest critical quench temperature for ABS was ∼415°C with the lowest at 360°C, while the critical quench temperature for the baseline material was 450°C. Using temperature dependent data over an appropriate temperature range (room temperature to the predicted Δ T _C), the R parameters of the ABS materials were within 15°C of predictions. The baseline material was ∼1.7 times higher than predicted and this was attributed to the high-thermal conductivity of the material resulting in soft thermal shock during quench testing.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.     

Spherical Aluminum Nitride Fillers for Heat-Conducting Plastic Packages

It is necessary for encapsulants to have not only a suitable coefficient of thermal expansion (CTE) compatible to IC devices and a low dielectric constant to reduce the device propagation delay, but also a high thermal conductivity to dissipate large amounts of heat from power-hungry, high-speed IC and high-density packages. Fillers such as silica have been mixed with polymers to improve their properties. Aluminum nitride (AlN) is considered as an alternative one, because it has a higher theoretical thermal conductivity of ∼320 W/mK¹, a compatible CTE with silicon chips and a low dielectric constant. Commercial AlN fillers are angular in shape, because they are prepared via grinding coarse AlN powders synthesized by direct nitridation of aluminum metal and classification. The angular AlN are not expected to have high fluidity when mixed with polymers and hence low packing density. Recently, we successfully obtained single-crystalline spherical AlN fillers. Furthermore, polymer composites filled with the spherical AlN showed excellent thermal conductivity (>8 W/mK) as encapsulants for dissipating the heat generated in electronic devices.     

Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride. I: Effect of Si Powder Characteristics

This paper will present sintered reaction-bonded silicon nitride (SRBSN) material with a high thermal conductivity of 121 W·(m·K)⁻¹, which has been successfully prepared from a coarse Si powder with lower levels of oxygen and aluminum impurities, using a mixture of Y₂O₃ and MgSiN₂ as sintering additives, by nitriding at 1400°C for 8 h and subsequent post-sintering at 1900°C for 12 h at a nitrogen pressure of 1 MPa N₂. This thermal conductivity value is higher than that of the materials prepared from high-purity α-Si₃N₄ powder (UBE SN-E10) with the same additive composition under the same sintering conditions. In order to study the effects of Si powder characteristics on the processing, microstructure, and thermal conductivity of SRBSN, the other type of fine powder with higher native oxygen and metallic impurity (typically Al and Fe) contents was also used. The effects of Si particle size, native oxygen, and metallic impurities on the nitriding process, post-sintering process, and thermal conductivity of the resultant SRBSN materials were discussed in detail. This work demonstrates that the improvement in thermal conductivity of SRBSN could be achieved by using higher purity coarse Si powder with lower levels of oxygen and aluminum impurities. In addition, this work also shows that the nitriding temperature has no significant effect on the microstructure and thermal conductivity of SRBSN during post-sintering, although it does affect the characteristics of RBSN formed during nitridation.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).     

MgSiN2 Addition as a Means of Increasing the Thermal Conductivity of β-Silicon Nitride

Si₃N₄ powders with the concurrent addition of Yb₂O₃ and MgSiN₂ were sintered at 1900°C for 2–48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content, and thermal conductivity of the sintered specimens were evaluated and compared with Si₃N₄, Yb₂O₃, and MgO addition. MgSiN₂ addition was effective for improving the thermal conductivity of Si₃N₄ ceramics, and a material with high thermal conductivity over 140 W·(m·K)⁻¹ could be obtained. For both specimens, lattice oxygen content was decreased with sintering time. However, the thermal conductivity of the MgSiN₂-doped specimen was slightly higher than the MgO-doped specimen with the same oxygen content.     

Enhancement of Densification and Thermal Conductivity in AlN Ceramics by Addition of Nano-Sized Particles

The effect of addition of nano-sized particles on densification and thermal conductivity of AlN ceramics was investigated. The commercially available AlN powder (∼0.9 μm) was mixed with 1.89 mass% nano-sized AlN particles (<0.1 μm), 3.53 mass% Y₂O₃, and 2.0 mass% CaO as sintering aid. The mixture was fired at 1500° and 1600°C in a tungsten resistance furnace under flowing N₂ atmosphere. The results showed that a fully densified specimen was obtained at the lower temperature of 1600°C by addition of nano-sized particles. The thermal conductivity of the resulting product was 133 W/m°C. The value is much higher than the 52 W/m°C for the sample prepared without adding the nano-sized AlN powder. This study indicates a strong potential for the use of nano-sized particles as additives in the densification of AlN ceramics.