AlN with high strength and high thermal conductivity based on an MCAS-Y2O3-YSZ multi-additive system

The additive composition of an AlN ceramic substrate material was optimized to achieve high strength and thermal conductivity. MgO-CaO-Al₂O₃-SiO₂ (MCAS) glass and Y₂O₃ were used as basic additives for improved sintering properties and thermal conductivity, thereby allowing for AlN to be sintered at a relatively low temperature of 1600 °C without pressurization. Yttria-stabilized zirconia (YSZ) was added (0–3 wt%) to further improve the strength of the AlN ceramic. YSZ and Y₂O₃ reacted with AlN to produce ZrN, Y₄Al₂O₉, and Y₃Al₅O₁₂ secondary phases. The formation of these yttrium aluminate phases improved the thermal conductivity by removing oxygen impurities, while ZrN formed at the AlN grain boundaries provided resistance to grain boundary fractures for improved strength. Overall, the AlN ceramic with 1 wt% MCAS, 3 wt% Y₂O₃, and 1 wt% YSZ exhibited excellent thermal and mechanical properties, including a thermal conductivity of 109 W/mK and flexural strength of 608 MPa.     

Effect of extended annealing cycles on the thermal conductivity of AlN/Y2O3 ceramics

The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y₂O₃, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y₂O₃ composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y₂O₃-phase and grain-growth of the bulk AlN grains.     

Effect of YF3 on densification behavior and thermal conductivity of AlN ceramics with Y2O3–YF3 additives under reducing atmosphere

The effect of yttrium fluoride (YF₃) on the densification behavior, microstructure, phase composition and thermal conductivity of aluminium nitride (AlN) ceramics with yttrium oxide (Y₂O₃) and YF₃ additives were studied. Since YF₃ provided liquid phases and promoted densification at a lower temperature, the sintering temperature required to reach the full density of AlN samples decreased with the increase in YF₃ content. Appropriate addition of YF₃ could improve the thermal conductivity of AlN ceramics, but the values of thermal conductivity decreased as YF₃ increased further. It is attributed to the ability of YF₃ to react with oxygen impurity was worse than that of Y₂O₃. Moreover, the reducing atmosphere significantly affected the phase composition, and the oxygen content in grain boundary phases decreased at 1750 °C and 1800 °C. Therefore, the proper proportion of Y₂O₃–YF₃ additives could simultaneously improve densification and the thermal conductivity of AlN samples at a low sintering temperature.     

Enhanced mechanical and thermal properties of AlN ceramics via a chemical precipitation process

A uniform dispersion of sintering additives is crucial to improve the thermal and mechanical properties of AlN ceramics. In this study, the Y₂O₃-coated AlN composite powder was successfully prepared by the chemical precipitation (CP) process, thereby improving the homogenization of Y₂O₃ in AlN green compacts. The precipitation coating behavior of Y₂O₃ precursor was investigated by FTIR and TG-DSC, and the corresponding reaction equation was proposed. The results of TEM, XRD, and XPS for the CP processed AlN powder indicated that a uniform amorphous Y₂O₃ layer was fully wrapped on the surface of AlN powder. The microstructures and phases of the sintered AlN samples prepared via the CP and conventional ball-milling (BM) processes, respectively, were compared. The CP process can result in decreasing oxygen content in AlN grains, facilitating the formation of the desirable isolated second phases, and strengthening the grain and grain boundary of AlN ceramic. As a result, the thermal conductivity, bending strength and fracture toughness of the CP processed AlN ceramic are 9.43%, 10.56%, and 18.50% higher than those of the BM processed sample, respectively, illustrating the CP process is a pretty effective way to simultaneously improve the thermal and mechanical properties of AlN ceramics.     

Microstructure and properties of aluminum nitride ceramics prepared by aqueous gelcasting method with nontoxic curdlan as gelling agent

Aluminum nitride ceramics were prepared by aqueous gelcasting method and pressureless sintering technique in N₂ atmosphere using Y₂O₃ as sintering additives with nontoxic curdlan as gel system. The solidification mechanism of curdlan was studied. The effects of curdlan content and solid content on the microstructure, relative density and flexural strength of green bodies were investigated. The influences of Y₂O₃ content and sintering soaking time on the microstructure and properties of sintered bodies were also studied. The results show that, as the temperature increases to 80 °C, the ceramic powders solidify through three-dimensional gel networks of curdlan during gelling process. The green bodies can be successfully fabricated through aqueous gelcasting method with modified powder as original materials. Suitable curdlan content and solid content contribute to fabricating green body with uniform microstructures and high flexural strength. The relative density and flexural strength of sintered bodies enhance as the Y₂O₃ content and soaking time increase. The flexural strength and thermal conductivity are about 107.5∼172.3 MPa and 75.2∼112.5 W/(m·K), respectively. The sintered body with 4 wt% Y₂O₃ soaking for 3 h exhibits the highest thermal conductivity because of appropriate relative density, uniform microstructure and reasonable intergranular phase distribution. The mechanical property and thermal conductivity of sintered bodies can be improved by optimizing the gelcasting process parameter, Y₂O₃ content, and soaking time. The nontoxic gelling system will have wide application for aqueous gelcasting ceramic with complex shape.     

Reticulated open-cellular aluminum nitride ceramic foams: Effect of sintering aids on microstructural,thermal, and mechanical properties

Open-celled aluminum nitride ceramic foams were prepared by the polymer sponge replication technique involving aqueous dispersions of passivated AlN. The amount of the Y₂O₃ and Dy₂O₃ as sintering aid was varied, and the effects on the densification, microstructure formation, phase composition, and finally, the thermal conductivity were investigated. A typical thermal conductivity of 1.1 W m⁻¹ K⁻¹ was determined for foams at a porosity level of 94.3 vol.%, on average. This measured foam thermal conductivity was subsequently modeled using different porosity ↔ thermal conductivity relations considering the different hierarchical levels of porosity in these foams. From these models, the thermal conductivity of the bulk AlN strut material was determined, correlated with the strut microstructure and the phase composition, and compared to literature data.     

Phase and microstructure optimization of grain boundary oxides and its effect on the thermal conductivity of Y2O3-doped AlN ceramics

AlN green bodies with variable O and C contents were employed to fabricate Y₂O₃-doped AlN ceramics of different grain-boundary phase compositions and microstructures via debinding in air and N₂, respectively. The microstructural evolution and grain-boundary oxide migration and their effects on the properties of the ceramics were explained. Finally, modified models were built to predict the thermal conductivity of these AlN ceramics with complex microstructures. During sintering, the oxide melt migrates to the ceramic surface driven by the differences between the surface energy and solid/liquid interface energy of the melt. The phase compositions and distributions of the grain-boundary oxides vary with sintering temperature. In addition, the amorphous layers were detected experimentally. All of these factors have great effects on ceramics properties. AlN ceramics were shown to have a thermal conductivity as high as 221.64 W/(m·K), which agrees with the value predicted via modified models.     

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.     

Secondary phases,microstructures and properties of AlN ceramics sintered by adding nitrate sintering additives

AlN ceramics were sintered at a temperature range from 1650 to 1800°C through adding the Ca and Y nitrate sintering additives. Secondary phases, microstructures and properties of the AlN ceramics were studied. When the AlN ceramics are sintered at 1650 or 1675°C, CaO and Y₂O₃ from the sintering additives react with Al₂O₃ in the AlN powder to generate CaAl₄O₇ and Y₃Al₅O₁₂. Part of Y₃Al₅O₁₂ reacts with CaO and Al₂O₃ to form CaYAl₃O₇ at 1700°C. At 1800°C, CaYAl₃O₇ decomposes into CaAl₄O₇ and Y₃Al₅O₁₂. Finally, CaAl₄O₇ volatilises and only Y₃Al₅O₁₂ remains. As the sintering temperature increases, the AlN grains grow continuously and the bending strength and thermal conductivity of the AlN ceramics increase first and then decrease. The AlN ceramics sintered at 1700°C are fully dense and have the highest bending strength and thermal conductivity of 373·7 MPa and 136·7 W m^?1 K^?1 in this work.     

Low-temperature hot-press sintering of AlN ceramics with MgO–CaO–Al2O3–SiO2 glass additives for ceramic heater applications

Aluminum nitride (AlN) is used a ceramic heater material for the semiconductor industry. Because extremely high temperatures are required to achieve dense AlN components, sintering aids such as Y₂O₃ are typically added to reduce the sintering temperature and time. To further reduce the sintering temperature, in this study, a low-melting-temperature glass (MgO–CaO–Al₂O₃–SiO₂; MCAS) was used as a sintering additive for AlN. With MCAS addition, fully dense AlN was obtained by hot-press sintering at 1500 °C for 3 h at 30 MPa. The mechanical properties, thermal conductivity, and volume resistance of the sintered AlN–MCAS sample were evaluated and compared with those of a reference sample (AlN prepared with 5 wt% Y2O3 sintering aid sintered at 1750 °C for 8 h at 10 MPa). The thermal conductivity of AlN prepared with 0.5 wt% MCAS was 91.2 W/m?K, which was 84.8 W/m?K lower than that of the reference sample at 25 °C; however, the difference in thermal conductivity between the samples was only 14.2 W/m?K at the ceramic-heater operating temperature of 500 °C. The flexural strength of AlN–MCAS was 550 MPa, which was higher than that of the reference sample (425 MPa); this was attributed to the smaller grain size achieved by low-temperature sintering. The volume resistance of AlN–MCAS was lower than that of the reference sample in the range of 200–400 °C. However, the resistivity of the proposed AlN–MCAS sample was higher than that of the reference sample (500 °C) owing to grain-boundary scattering of phonons. In summary, the proposed sintering strategy produces AlN materials for heater applications with low production cost, while achieving the properties required by the semiconductor industry.     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering

Aluminum nitride/boron nitride (AlN/BN) ceramics with 15–30 vol.% BN as secondary phase were fabricated by spark plasma sintering (SPS), using Yttrium oxide (Y₂O₃) as sintering aid. Effects of Y₂O₃ content and the SPS temperature on the density, phase composition, microstructure and thermal conductivity of the ceramics were investigated. The results revealed that with increasing the amount of starting Y₂O₃ in AlN/BN, Yttrium-contained compounds were significantly removed after SPS process, which caused decreasing of the residual grain boundary phase in the sintered samples. As a result, thermal conductivity of AlN/BN ceramics was remarkably improved. By addition of Y₂O₃ content from 3 wt.% to 8 wt.% into AlN/15 vol.% BN ceramics, the thermal conductivity increased from 110 W/m K to 141 W/m K.     

Stability of intergranular phases in hot-pressed Si3N4 studied with mechanical spectroscopy and in-situ high-temperature XRD

The impulse excitation technique (IET) and high temperature X-ray diffraction (HTXRD) were used to investigate the intergranular glass phase and its crystallisation behaviour in four hot-pressed silicon nitrides. The internal friction or damping peak height measured with IET near the glass transition temperature, T_g, is used as a qualitative indicator for the amount of residual intergranular amorphous phase after sintering. Silicon nitride powder was hot-pressed with different sintering additives. The silicon nitride containing 4 wt.% Al₂O₃ does not reveal an internal friction peak at T_g, i.e. it does not contain a significant amount of intergranular glass phase. Three other silicon nitrides, containing either 8 wt.% Y₂O₃, 6 wt.% Y₂O₃+2 wt.% Al₂O₃, or 2 wt.% Y₂O₃+4 wt.% Al₂O₃+2 wt.% TiN, do show an internal friction peak near T_g. This “T_g-peak” is nearly unaffected by heating up to 1400 °C in the silicon nitride with Y₂O₃+Al₂O₃+TiN sintering aids, whereas the amount of intergranular glass in the ceramics containing either Y₂O₃+Al₂O₃ or Y₂O₃ as a sintering aid is strongly reduced by subsequent heating. As observed from HTXRD, the onset temperature of crystallisation of the intergranular glass in the ceramic containing Y₂O₃+Al₂O₃ sintering aids is about 1100 °C, with the formation of Y–N-apatite (Y₂₀N₄Si₁₂O₄₈) and O-sialon (Al₀.₀₄Si₁.₉₆N₁.₉₆O₁.₀₄). The O-sialon phase in the yttria and alumina containing ceramics, formed either during sintering or during heat treatment, is not stable at elevated temperatures and dissolves in the intergranular glass phase between 1300 and 1400 °C. The O-sialon phase in the ceramic without Y₂O₃ sintering additive, however, is thermally stable. The presence of Ti⁴⁺ ions in the intergranular glass phase is suggested to inhibit its crystallisation, resulting in a stable high temperature damping behaviour.     

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.     

Effect of Y doping on microstructure and thermophysical properties of yttria stabilized hafnia ceramics

A series of Y₂O₃-doped HfO₂ ceramics (Hf_1-xY_xO_2-0.5×, x?=?0, 0.04, 0.08, 0.12, 0.16 and 0.2) were synthesized by solid-state reaction at 1600?°C. The microstructure, thermophysical properties and phase stability were investigated. Hf_1-xY_xO_2–0.5x ceramics were comprised of monoclinic (M) phase and cubic (C) phase when Y³⁺ ion concentration ranged from 0.04 to 0.16. The thermal conductivity of Hf_1-xY_xO_2–0.5x ceramic decreased as Y³⁺ ion concentration increased and Hf_0.8Y_0.2O_1.9 ceramic revealed the lowest thermal conductivity of ~?1.8?W/m*K at 1200?°C. The average thermal expansion coefficient (TEC) of Hf_1-xY_xO_2–0.5x between 200?°C and 1300?°C increased with the Y³⁺ ion concentration. Hf_0.8Y_0.2O_1.9 yielded the highest TEC of ~?10.4?×?10^?6 K^?1 while keeping good phase stability between room temperature and 1600?°C.     

Compositional identification of the intergranular phase in liquid phase sintered SiC

A model system consisting of coarse SiC (32–160 μm) as starting powder and Y₂O₃ and AlN as sintering additives was liquid phase sintered. Coarse-grained starting powder led to large intergranular phase regions which allowed an accurate determination of the chemical composition by wavelength-dispersive X-ray microanalysis (WDS). When N₂ was used as sintering atmosphere, a N-rich amorphous phase (about 44 at.% N) was identified by WDS to be the main triple-junction phase in the sintered SiC ceramics, while three further crystalline intergranular phases were AlN, Y₂SiN₄O₃ and an O-rich phase (Y₁₀Al₂Si₃O₁₈N₄). The overall O content was found to be reduced in comparison to the initial powder composition. The incorporation of N from the sintering atmosphere into the intergranular phase and a subsequent carbothermal reduction are believed to be responsible for the removal of O and the formation of the N-rich amorphous phase.     

Hydrolysis protection and sintering of aluminum nitride powders with yttria nanofilms

Aluminum nitride (AlN) is a promising material for electronic substrates and heat sinks. However, AlN powders react with water that adversely affects final part properties and necessitates processing in organic solvents, increasing the cost of AlN parts. Small quantities of yttrium oxide (Y₂O₃) are commonly added to AlN particles to enable liquid phase sintering. To mitigate the reaction of AlN particles with water, particle atomic layer deposition (ALD) was used to coat AlN powders with conformal films of Y₂O₃ prior to densification and powder processing. When AlN particles were coated with 6 nm thick films of amorphous Y₂O₃, the hydrolysis reaction was significantly suppressed over 48 h, demonstrating that Y₂O₃ nanofilms on AlN powders act as a barrier coating in an aqueous solution. AlN powders with Y₂O₃ addition by particle ALD sintered to high relative densities (≥90% theoretical) after sintering at 1800°C for 50 min.     

Eutectic bonding of copper to ceramics for thermal dissipation applications – A review

Metallic copper, which has low electrical resistivity and high thermal conductivity, is widely used as an interconnector or substrate within microelectronic packages. If a small amount of oxygen is introduced to the surface of the copper, a eutectic liquid forms above 1065 °C. The eutectic liquid wets many ceramics well; it is thus possible to bond slightly oxidized copper to many ceramics directly. The present report summarizes previous results on three systems, Al₂O₃/Cu, AlN/Cu, and Si₃N₄/Cu laminates, prepared by the eutectic bonding process. The reported data demonstrate that ceramic/copper interfaces prepared with this technique are strong. Though little attention has been paid to the thermal characteristics of ceramic/copper laminates, the limited data suggest that the thermal conductivity of the laminates is high, the potential for using the laminates for thermal dissipation is thus high. In the present report, the current status for the technique is summarized; critical topics for further improvement are also proposed.     

Mechanical performances of AlN/Al metallized ceramic substrates fabricated by transient liquid phase bonding and pre-oxidation treatment

For the miniaturization of high-power electronic components, AlN/Al is a promising metallized ceramic substrate due to its superior mechanical and thermal performances. Numerous bonding processes have been proposed for fabricating the metallized ceramic substrate. Unfortunately, the influences of various bonding techniques on the mechanical performance of AlN/Al metallized ceramic substrate remain undetermined to date. The objective of this study was thus to investigate the effects of the transient liquid phase (TLP) technique and pre-oxidation treatment on the bonding, microstructure, and mechanical strength of the AlN/Al metallized ceramic substrate.The results indicated that the three-layered AlN/Al/AlN specimen could be effectively bonded by the TLP process and pre-oxidation treatment. However, the bending strengths of the specimens fabricated by the two techniques were obviously divergent. The bending strength of raw AlN substrate was 333 MPa. In contrast, the bending strengths of the three-layered specimens with AlN substrates pre-oxidized at 1050 °C, 1150 °C, and 1250 °C were 292 MPa, 250 MPa, and 224 MPa, respectively. Raising the pre-oxidation temperature of the AlN substrate from 1050 °C to 1250 °C obviously increased the thickness of the Al₂O₃ layer and deteriorated the bending strength, for the fracture propagated along the Al₂O₃ layer and the Al₂O₃/AlN interface. For the TLP bonding, the Cu film deposited on the AlN substrate contributed to the generation of Al–Cu transient liquid and to bonding. The bending strength of the three-layered specimens fabricated by TLP at 650 °C was 417 MPa, which was 25% and 43% better than those of the raw AlN substrate and the three-layered specimens prepared by the pre-oxidation treatment, respectively.