Process induced alignment of carbon nanotube decreases longitudinal thermal conductivity of Al2O3 based porous composites

Alumina (Al₂O₃) based porous composites, reinforced with zirconia (ZrO₂), 3 and 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) and 4 wt% carbon nanotube (CNT) are processed via spark plasma sintering. The normalized linear shrinkage during sintering process of Al₂O₃-based composite shows minimum value (19.2–20.4%) for CNT reinforced composites at the temperature between 1650 °C and 575 °C. Further, the combined effect of porosity, phase-content and its crystallite size in sintered Al₂O₃-based porous composite have elicited lowest thermal conductivity of 1.2 Wm⁻¹K⁻¹ (Al₂O₃-8YSZ composite) at 900 °C. Despite high thermal conductivity of CNT (∼3000 Wm⁻¹K⁻¹), only a marginal thermal conductivity increase (∼1.4 times) to 7.3–13.4 Wm⁻¹K⁻¹ was observed for CNT reinforced composite along the longitudinal direction at 25 °C. The conventional models overestimated the thermal conductivity of CNT reinforced composites by up to ∼6.7 times, which include the crystallite size, porosity, and interfacial thermal resistance of Al₂O₃, YSZ and, CNT. But, incorporation of a new process induced CNT-alignment factor, the estimated thermal conductivity (of <6.6 Wm⁻¹K⁻¹) closely matched with the experimental values. Moreover, the high thermal conductivity (<76.1 Wm⁻¹K⁻¹) of the CNT reinforced porous composites along transverse direction confirms the process induced alignment of CNT in the spark plasma sintered composites.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Effect of lattice defects on the thermal conductivity of β-Si3N4

Two types of β-Si₃N₄ were sintered at 1900 °C one for 8 h and the other for 36 h by using Yb₂O₃ and ZrO₂ as sintering additives. The latter specimen was further annealed at 1700 °C for 100 h to promote grain growth. The microstructures of the sintered materials were investigated by SEM, TEM, and EDS. The thermal conductivities of the specimens were 110 and 150 Wm⁻¹K⁻¹, respectively. The sintered material which possessed 110 Wm⁻¹K⁻¹ had numerous small precipitates that consisted of Yb, O and N elements and internal dislocations in the β-Si₃N₄ grains. In the sintered material with 150 Wm⁻¹K⁻¹ neither precipitates nor dislocations were observed in the grains. The microscopic evidence indicates that the improvement in the thermal conductivity of the β-Si₃N₄ was attributable to the reduction of internal defects of the β-Si₃N₄ grains with sintering and annealing time as the grains grew.     

Microstructure and thermal conductivity of gas-pressure-sintered Si3N4 ceramic: the effects of Y2O3 additive content

Si₃N₄ ceramics were sintered at 1900 °C under a nitrogen pressure of 1 MPa using Y₂O₃-MgO additives. The effects of Y₂O₃ content (0.5-4 mol%) on microstructure and thermal conductivity were systematically investigated. The increasing Y₂O₃ content led to increases in amount and viscosity of liquid phase during sintering, which induced a “bimodal to normal” transition in distribution of grain size, decreased Si₃N₄/Si₃N₄ contiguity and enhanced devitrification degree of intergranular phase in sintered bulks. Moreover, the decreasing Y₂O₃ content was found to improve the elimination efficiency of SiO₂ impurity during sintering, resulting in lower lattice oxygen content in densified specimens. The microstructure had a strong effect on thermal conductivity. The samples sintered for 3 h gained an increase of thermal conductivity from 65 to 73 W·m^-1 K^-1 with increasing Y₂O₃ content, while the samples sintered for 12 h obtained a substantial increase of thermal conductivity from 87 to 132 W·m^-1 K^-1 with decreasing Y₂O₃ content.     

Effect of lattice impurities on the thermal conductivity of β-Si3N4

Effect of impurities in the crystal lattice and microstructure on the thermal conductivity of sintered Si₃N₄ was investigated by the use of high-purity β-Si₃N₄ powder. The sintered materials were fabricated by gas pressure sintering at 1900 °C for 8 and 48 h with addition of 8 wt.% Y₂O₃ and 1 wt.% HFO₂. A chemical analysis was performed on the loose Si₃N₄ grains taken from sintered materials after the chemical treatment. Aluminum was not removed from Si₃N₄ grains, which originated from the raw powder of Si₃N₄. The coarse grains had fewer impurities than the fine grains. Oxygen was the major impurity in the grains, and gradually decreased during grain growth. The thermal conductivity increased from 88 Wm⁻¹ K⁻¹ (8 h) to 120 Wm⁻¹ K⁻¹ (48 h) as the impurities in the crystal lattice decreased. Purification by grain growth thus improved the thermal conductivity, but changing grain boundary phases might also influence the thermal conductivity.     

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y₂O₃–Sc₂O₃ sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm⁻¹. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m⁻¹ K⁻¹ to 70 W·m⁻¹ K⁻¹, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m⁻¹ K⁻¹ up to 238 W·m⁻¹ K⁻¹ for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.     

High-strength AlN ceramics by low-temperature sintering with CaZrO3–Y2O3 co-additives

Aluminum nitride (AlN) ceramics with the concurrent addition of CaZrO₃ and Y₂O₃ were sintered at 1450-1700 °C. The degree of densification, microstructure, flexural strength, and thermal conductivity of the resulting ceramics were evaluated with respect to their composition and sintering temperature. Specimens prepared using both additives could be sintered to almost full density at relatively low temperature (3 h at 1550 °C under nitrogen at ambient pressure); grain growth was suppressed by grain-boundary pinning, and high flexural strength over 630 MPa could be obtained. With two-step sintering process, the morphology of second phase was changed from interconnected structure to isolated structure; this two-step process limited grain growth and increased thermal conductivity. The highest thermal conductivity (156 Wm⁻¹ K⁻¹) was achieved by two-step sintering, and the ceramic showed moderate flexural strength (560 MPa).     

Effects of microstructure and intergranular glassy phases on thermal conductivity of silicon nitride

In this study, the binary sintering additives Y₂O₃-Sc₂O₃, were first applied to the Si₃N₄ system to investigate their effects on microstructure and thermal conductivity. The microstructure and thermal conductivity of both sintered silicon nitride (SSN) and sintered reaction-bonded silicon nitride (SRBSN) were found to be significantly dependent on the additive composition. Among various combinations of Y₂O₃ and Sc₂O₃, 1 mol% Y₂O₃−3 mol% Sc₂O₃ prominently enhanced thermal conductivity, and the enhancement could not be attributed to any difference in microstructure or lattice defects. TEM observation revealed that this composition was more liable to devitrify the glassy phase with a lower degree of stress accumulation, and to possibly produce a grain boundary that was cleaner or with a higher order of atomic arrangement. A microstructure model for thermal conductivity was proposed which took the thermal resistance of the grain boundaries into account. The grain boundary state exerted a remarkable influence on the thermal conductivity of fine microstructures, and the experimentally measured thermal conductivity values were consistent with those given by the proposed model.     

Effect of a new nonoxide additive,Y3Si2C2, on the thermal conductivity and mechanical properties of Si3N4 ceramics

Enhancement of the thermal conductivity of silicon nitride is usually achieved by sacrificing its mechanical properties (bending strength). In this study, β-Si₃N₄ ceramics were prepared using self-synthesized Y₃Si₂C₂ and MgO as sintering additives. It was found that the thermal conductivity of the Si₃N₄ ceramics was remarkably improved without sacrificing their mechanical properties. The microstructure and properties of the Si₃N₄ ceramics were analyzed and compared with those of the Y₂O₃-MgO additives. The addition of Y₃Si₂C₂ eliminated the inherent SiO₂ and introduced nitrogen to increase the N/O ratio of the grain-boundary phase, inducing Si₃N₄ grain growth, increasing Si₃N₄ grain contiguity, and reducing lattice oxygen content in Si₃N₄. Therefore, by replacing Y₂O₃ with Y₃Si₂C₂, the thermal conductivity of the Si₃N₄ ceramics was significantly increased by 31.5% from 85 to 111.8Wm⁻¹K⁻¹, but the bending strength only slightly decreased from 704 ± 63MPa to 669 ± 33MPa.     

Additive-free Y2O3:Eu3+ red-emitting transparent ceramic with superior thermal conductivity for high-power UV LEDs and UV LDs

To obtain red-emitting luminescent material for high-power UV LED and UV LD applications, an additive-free Y₂O₃:Eu³⁺ phosphor ceramic was successfully prepared in this work. The nitrate pyrogenation method is applied to obtain raw nanopowders with high reactivity, and a hybrid sintering method combining low-temperature presintering and subsequent hot isostatic pressing (HIP) is then applied to realize full densification of the final ceramic products. The effects of the presintering temperature on the density, microstructural, and optical properties are investigated in detail. The HIP-treated Y₂O₃:Eu³⁺ ceramic presintered at 1450 °C exhibits a high transmittance near 80 % at 600 nm. Due to the nonuse of sintering additives, the thermal conductivity of Y₂O₃:Eu³⁺ ceramic product reaches 10.9 Wm⁻¹ K⁻¹ at room temperature. The achieved Y₂O₃:Eu³⁺ ceramic also exhibits good applicability under the excitation of a UV LED chip and UV laser light, showing promise as a color converter for high-power UV LED and UV LD applications.     

Thermal conductivity and mechanical properties of Si3N4 ceramics with binary fluoride sintering additives

Silicon nitride (Si₃N₄) ceramics were fabricated by gas pressure sintering (GPS) using four sintering additives: Y₂O₃–MgO, Y₂O₃–MgF₂, YF₃–MgO, and YF₃–MgF₂. The phase composition, grain growth kinetics, mechanical properties, and thermal conductivities of the Si₃N₄ ceramics were compared. The results indicated that the reduction of YF₃ on SiO₂, induced a high Y₂O₃/SiO₂ secondary phase ratio, which improved the thermal conductivity of the Si₃N₄ ceramics. The depolymerization of F atom reduces the diffusion energy barrier of solute atom and weakens the viscous resistance of anion group, which was beneficial to grain boundary migration. Besides exhibiting a lower grain growth exponent(n = 2.5)and growth activation energy (Q = 587.94 ± 15.35 kJ/mol), samples doped with binary fluorides showed excellent properties, including appreciable thermal conductivity (69 W m⁻¹ K⁻¹), hardness (14.63 ± 0.12 GPa), and fracture toughness (8.75 ± 0.18 MPa m^1/2), as well as desirable bending strength (751 ± 14 MPa).     

Effects of Y2O3–RE2O3 (RE = Sm,Gd, Lu) Additives on Electrical and Thermal Properties of Silicon Carbide Ceramics

In this study, we investigated the electrical and thermal properties of SiC ceramics with 2 vol% equimolar Y₂O₃–RE₂O₃ (RE = Sm, Gd, Lu) additives. The three SiC ceramics with 2 vol% equimolar Y₂O₃–RE₂O₃ additives showed electrical conductivities on the order of ~10³ (Ω·m)^?1, which is one order of magnitude higher than that of the SiC ceramics sintered with 2 vol% Y₂O₃ only. The increase in electrical conductivity is attributed to the growth of heavily nitrogen‐doped SiC grains during sintering and the confinement of oxide additives in the junction area. The thermal conductivities of the SiC ceramics were in the 176–198 W·(m·K)^?1 range at room temperature. The new additive systems, equimolar Y₂O₃–RE₂O₃, are beneficial for achieving both high electrical conductivity and high thermal conductivity in SiC ceramics.     

Enhanced thermal conductivity in Si3N4 ceramic with the addition of Y2Si4N6C

Si₃N₄ ceramic was densified at 1900°C for 12 hours under 1 MPa nitrogen pressure, using MgO and self‐synthesized Y₂Si₄N₆C as sintering aids. The microstructures and thermal conductivity of as‐sintered bulk were systematically investigated, in comparison to the counterpart doped with Y₂O₃‐MgO additives. Y₂Si₄N₆C addition induced a higher nitrogen/oxygen atomic ratio in the secondary phase by introducing nitrogen and promoting the elimination of SiO₂, resulting in enlarged grains, reduced lattice oxygen content, increased Si₃N₄‐Si₃N₄ contiguity and more crystallized intergranular phase in the densified Si₃N₄ specimen. Consequently, the substitution of Y₂O₃ by Y₂Si₄N₆C led to a great increase in ~30.4% in thermal conductivity from 92 to 120 W m^?1 K^?1 for Si₃N₄ ceramic.     

Effects of starting powder on microstructure and thermal conductivity of pressureless-sintered fully ceramic microencapsulated fuels

The effects of the starting SiC powder (α or β) with the addition of 5.67 wt% AlN–Y₂O₃–CeO₂–MgO additives on the residual porosity and thermal conductivity of fully ceramic microencapsulated (FCM) fuels were investigated. FCM fuels containing ～41 vol% and ～37 vol% tristructural isotropic (TRISO) particles could be sintered at 1870 °C using α-SiC and β-SiC powders, respectively, via a pressureless sintering route. The residual porosities of the SiC matrices in the FCM fuels prepared using the α-SiC and β-SiC powders were 1.1% and 2.3%, respectively. The thermal conductivities of FCM pellets with ～41 vol% and ～37 vol% TRISO particles (prepared using the α-SiC and β-SiC powders, respectively) were 59 and 41 Wm^?1K^?1, respectively. The lower porosity and higher thermal conductivity of FCM fuels prepared using the α-SiC powder were attributed to the higher sinterability of the α-SiC powder than that of the β-SiC powder.     

Mechanical properties and thermal conductivity of dense β-SiAlON ceramics fabricated by two-stage spark plasma sintering with Al2O3-AlN-Y2O3 additives

Fully dense β-SiAlON ceramics with excellent mechanical properties and good thermal conductivity were fabricated by two-stage spark plasma sintering (SPS) processes without and with applying pressure respectively, using α-Si₃N₄ powder and 6 Al₂O₃-3 AlN-6 Y₂O₃ (in wt.%, label with 636), 424 and 422 additives. In the first stage SPS process without pressure, the relative dense β-SiAlON ceramics with interlock microstructures of elongated grains and density of 3.14˜3.18 g cm⁻³, hardness of 14.00˜14.82 GPa and fracture toughness of 6.00˜6.63 MPa m^1/2 were obtained by sintering at about 1600 °C for 20 min. In the second stage SPS process at about 1425 °C for 5 min under pressure of 24 MPa, the fully dese β-SiAlON ceramics with density of 3.22˜3.24 g cm⁻³, high hardness of 15.68˜15.95 GPa, high fracture toughness of 6.38˜7.03 MPa m^1/2 and thermal conductivity of 13.5˜19.6 Wm^-1K^-1 were obtained. The reaction between the samples and the graphite mold can be avoided in this fabrication method.     

Improved thermal conductivity of β-Si3N4 ceramics by lowering SiO2/Y2O3 ratio using YH2 as sintering additive

A two-step sintering process was conducted to produce β-Si₃N₄ ceramics with high thermal conductivity. During the first step, native SiO₂ was eliminated, and Y₂O₃ was in situ generated by a metal hydride reduction process, resulting in a high Y₂O₃/SiO₂ ratio. The substitution YH₂ for Y₂O₃ endow Si₃N₄ ceramics with an increase of 29% in thermal conductivity from 95.3 to 123 W m⁻¹ K⁻¹ after sintered at 1900°C for 12 hours despite an inferior sinterability. This was primarily attributed to the purified enlarged grains, devitrified grain boundary phase, and reduced lattice oxygen content in the YH₂-MgO-doped material.     

Enhanced thermal conductivity and flexural strength of sintered reaction-bonded silicon nitride with addition of (Y0.96Eu0.04)2O3

Sintered reaction-bonded silicon nitride (SRBSN) with high thermal conductivity was obtained using (Y_0.96Eu_0.04)₂O₃ and MgO as sintering additives. Green compacts were nitrided at 1400°C for 4 h. Post-sintering was carried out at 1850 and 1900°C for 4 h, respectively. In reaction-bonded silicon nitride (RBSN) doped with Y₂O₃ and MgO, the β-Si₃N₄ content and nitridation degree were 51.1% and 93.8%, respectively. However, the β-Si₃N₄ content and nitridation degree were 72.6% and 96.7% in a nitrided compact doped with (Y_0.96Eu_0.04)₂O₃ and MgO. After post-sintering, the phase composition, microstructure, mechanical properties, and thermal conductivity were investigated. After sintering at 1900°C for 4 h, the thermal conductivity of SRBSN doped with (Y_0.96Eu_0.04)₂O₃ and MgO was increased by 16.5% compared to that of the samples doped with Y₂O₃ and MgO. The highest hardness of 1639 HV and the good flexural strength of 776.4 MPa were also achieved in the sample doped with 2-mol.% (Y_0.96Eu_0.04)₂O₃ and 5-mol.% MgO.