Thermal Conductivity Of AlN Ceramics Research Articles

Limiting the lattice oxygen impurities to obtain high thermal conductivity aluminum nitride ceramics

Currently, the lattice oxygen is the key factor affecting the thermal conductivity of AlN ceramics. In this paper, three methods are proposed to limit lattice oxygen, namely reducing the original powder oxygen content, changing the debinding atmosphere, and extending the sintering time. The results show that the low powder oxygen content and protective debinding atmosphere alter the carbon and oxygen content, and further promote the formation of the Y4Al2O9 (YAM) phase. The generated YAM phase can effectively reduce the lattice oxygen in terms of thermodynamics. Prolonged high temperatures can provide kinetic conditions to reduce the lattice oxygen to 0.0343 wt%. In addition, morphological changes of the phase at the grain edges and their influence on the mechanical properties were noted. Finally, AlN ceramics with a thermal conductivity of 220 W m−1 K−1 and a flexural strength of 340 MPa were obtained. This study can provide a valuable reference for the industrial production of AlN ceramics.

Fabrication of High Thermal Conductivity Aluminum Nitride Ceramics via Digital Light Processing 3D Printing.

The sintering of high-performance ceramics with complex shapes at low temperatures has a significant impact on the future application of ceramics. A joint process of digital light processing (DLP) 3D printing technology and a nitrogen-gas pressure-assisted sintering method were proposed to fabricate AlN ceramics in the present work. Printing parameters, including exposure energy and time, were optimized for the shaping of green bodies. The effects of sintering temperature, as well as nitrogen pressure, on the microstructure, density, and thermal conductivity of AlN ceramics were systematically discussed. A high thermal conductivity of 168 W·m-1·K-1 was achieved by sintering and holding at a significantly reduced temperature of 1720 °C with the assistance of a 0.6 MPa nitrogen-gas pressure. Further, a large-sized AlN ceramic plate and a heat sink with an internal mini-channel structure were designed and successfully fabricated by using the optimized printing and sintering parameters proposed in this study. The heat transfer performance of the ceramic heat sink was evaluated by infrared thermal imaging, showing excellent cooling abilities, which provides new opportunities for the development of ceramic heat dissipation modules with complex geometries and superior thermal management properties.

Electrical properties, microstructure, and thermal conductivity of hot-pressed CaO-doped AlN ceramics

AlN ceramics exhibit good physical and chemical properties and are ideal materials for the dielectric layer of electrostatic chucks. However, they cannot generate a strong J−R-type electrostatic adsorption force because of their high resistivity. Therefore, adding other substances is necessary to regulate the electrical properties and enable their application. In this study, AlN ceramics were prepared using hot-pressed sintering with 0.2 wt% CaO as an additive at a sintering temperature range of 1700–1900 °C. The effects of CaO doping on the phase composition, microstructure, electrical properties, and thermal conductivity of the AlN ceramics were systematically investigated. The addition of CaO not only enhanced the sintering process of the AlN ceramics, but also significantly reduced the electrical resistivity and increased the thermal conductivity. The relative density of CaO-doped AlN ceramics reached 98.88 % at 1700 °C, with a thermal conductivity of 81.51 W m−1•K−1. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and AC impedance spectroscopy were used to analyse the impurities and defects in the AlN ceramics. The lattice parameters and fitting grain resistance indicated that the CaO-doped AlN presented a higher lattice oxygen concentration, increasing the concentration of Al vacancies and electrons. Moreover, the level of dissolved oxygen could be controlled by altering the sintering temperature, resulting in AlN ceramics with electrical resistivity ranging from 8.1 × 106 to 1.7 × 1012 Ω cm. Further research was conducted to investigate the effect of the electrical resistivity of AlN ceramics on the J−R-type electrostatic adsorption force, and it was found that AlN ceramics with a specific resistivity of 109 Ω cm exhibited the highest electrostatic adsorption of 351.7 g/cm2.

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.

Bionic hierarchical porous aluminum nitride ceramic composite phase change material with excellent heat transfer and storage performance

In this paper, we synthesized bionic bone hierarchical porous AlN ceramics which encapsulate polyethylene glycol2000 (PEG2000) to form a new composite phase change material. Argon adsorption and SEM demonstrate that the AlN ceramic has hierarchical structure with most probable diameters of 2.4 nm and 27.3 nm. Raman spectroscopy show that no aluminum vacancies are formed inside the material, which effectively reduce the probability of phonon scattering and improve the thermal performance. EDS and XRD demonstrate that the main crystalline phase of the ceramics is AlN with a small amount of impurity peaks. The thermal conductivity of AlN ceramics with a porosity of 75.2% and 67.5% are 5.10 and 13.86 W/m·K, and that of the composite material is 17.16 W/m·K. The latent heat of melting of the composite material is 88.73 J/g and the melting point is 54.75 °C. The material breaks through the defects of slow dynamic response time and low thermal conductivity of traditional energy storage materials.

Effect of oxidation on flexural strength and thermal properties of AlN ceramics with residual stress and impedance spectroscopy analysis of defects and impurities

Abstract The effect of oxidation in air on the phase composition, microstructure, flexural strength and thermal property of AlN ceramics was investigated. Oxidation was found to produce a continuous oxide layer on the surface of AlN samples. The flexural strength and thermal conductivity of AlN ceramics significantly improved after oxidation at 1000 °C and 1100 °C. Residual stress in the AlN ceramic matrix was enhanced by oxidation. The enhanced residual compressive stresses inhibited crack propagation and reduced interfacial thermal resistance, thereby improving the flexural strength and thermal conductivity of AlN ceramics. Electrochemical impedance spectroscopy was further used to analyze the defects in AlN ceramics. The increase in fitting grain resistance revealed a decrease in aluminum vacancy concentration in oxidized AlN sample, which resulted in high thermal conductivity. Therefore, oxidation at a certain temperature is pretty effective to obtain excellent performance for AlN ceramics.

Effects of Y2O3-LaF3 on the low temperature sintering and thermal conductivity of AlN ceramics

PurposeAluminum nitride (AlN) ceramics are suitable substrate and package materials for high-power integrated circuits.Design/methodology/approachDense AlN ceramics with Y2O3 and LaF3 as sintering additives are prepared. The effects of these additives on the density, phase composition, microstructure and thermal conductivity of AlN ceramics are investigated.FindingsResults show that 2 Wt.% Y2O3-doped additive is insufficient for the samples to achieve the full densification sintered at 1,700°C. When LaF3 is added with Y2O3, the samples are perfectly densified at the same sintering condition. The relative density and thermal conductivity of the samples are 97.8-99.07 per cent and 169.104-200.010 W·m-1·K-1, respectively. The density of the samples and their microstructure, especially the content and distribution of secondary phases, is necessary to control the thermal conductivity of AlN ceramics.Originality/valueY2O3 and LaF3 additives can effectively promote densification and enhance the thermal conductivity of AlN ceramics in a low sintering temperature, and the AlN ceramics added with Y2O3-LaF3 might have potential applications in package materials for high-power integrated circuits.

Effects of isothermal annealing on the oxidation behavior, mechanical and thermal properties of AlN ceramics

Abstract To evaluate the oxidation behavior of AlN ceramics prepared using a pressureless sintering method, aluminum nitride bulk samples were annealed at temperatures ranging from 900 °C to 1300 °C for 1–5 h under an air atmosphere. The phase composition, microstructure and weight change of the oxidized samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis-mass spectrometry (TGA-MS), respectively. The results showed that certain of the gaseous oxidation products of AlN ceramics were nitrogen oxides (NO x ). The oxidation reaction started at approximately 800 °C to 900 °C. The thermal conductivity and flexural strength of AlN ceramics were enhanced slightly after the heat treatment in the 900–1100 °C temperature range. The entire surface of the AlN block was covered with a thin, compact alumina layer after oxidizing at 1100 °C for 3 h, which may enhance the metal adhesion to the AlN surface in a metallization process. At 1200 °C and higher, the AlN ceramics oxide layer became rough, porous, and cracked, leading to a sharp drop in the flexural strength and thermal conductivity of the AlN ceramics.

Effect of Additive Size on the Densification and Thermal Conductivity of AlN Ceramics with MgO–CaO–Al2O3–SiO2 Additives

In this study, we investigate the effect of additive size on the densification and thermal conductivity of AlN ceramics with MgO–CaO–Al₂O₃–SiO₂ (MCAS) additives. Micro-sized MCAS powder prepared via melting and nano-sized MCAS powder synthesized via the polymeric complex method are used as sintering additives. We analyze the densification behavior of AlN added with 5 wt.% of MCAS by dilatometry as well as by isothermal sintering in the temperature range of 1300 ~ 1700℃. AlN exhibits higher sinterability with nano-MCAS than with micro-MCAS, and both specimens approach their maximum densities when sintered at 1600℃ for 4 h. The thermal conductivities of AlN with 5 wt% of nano- and micro-MCAS additives sintered at 1600℃ are 82.6 and 32.0 W/mK, respectively. We find that nano-MCAS is more effective in sintering of AlN ceramics at lower temperatures, and thus for enhancing their thermal conductivities.

Processing and Characterization of Aluminum Nitride Ceramics for High Thermal Conductivity

The effects of sintering additives, microstructure, and morphology of second phase on the thermal conductivity of aluminum nitride ceramics are discussed in this review. The thermal conductivity of AlN is highly dependent on the types and amount of sintering additives, second phase morphology and microstructure. The morphology of second phase in AlN ceramics could be controlled by changing the cooling rate. The amount of second phase was able to be minimized by using the transient sintering additives. The thermal conductivity of AlN could be enhanced with the inclusion of the transient sintering additives. The thermal conductivity of AlN ceramics was evaluated by Raman spectroscopy. High temperature AC impedance studies were conducted to characterize the electrical conduction mechanism of AlN ceramics.

Analyses of Microstructure and Oxygen Content Effects on Thermal Conductivity of AlN Ceramics by Using Slack’s Plot

In this study, we analyzed the effect of microstructure and oxygen content on the thermal conductivity of dense AlN ceramics by Slack’s plot, which plots the oxygen content estimated from the lattice parameter c versus the thermal resistivity, which is inverse of the thermal conductivity. The analyses were carried out for the AlN ceramics sintered with oxide additives such as Y2O3, Y2O3-Al2O3, or Y2O3-CaO-B. The data of AlN ceramics sintered with Y2O3 or Y2O3-CaO-B located on the plot of the relation between oxygen content in AlN lattice and thermal resistivity of AlN presented by Slack (Slack’s line) or Harris et al (Harris’s line), respectively. In contrast, the data of the sample sintered with Y2O3-Al2O3, which contains excess grain boundary phases, located above the Harris’s line. These results suggest that the thermal conductivities of AlN ceramics are influenced by not only the oxygen contents in the AlN lattices but also the contents of grain boundary phases.

Study on Heat Treatment of Aluminum Nitride (Y<sub>2</sub>O<sub>3</sub>) Ceramics Sintered at High Pressure

Heat treatment is an effective means of structural adjustment and performance improvement of AlN ceramics. AlN ceramics prepared at high pressure with Y2O3 as a sintering aids were heat treated in a nitrogen flow atmosphere. The effects of heat treatment on microstructure and thermal conductivity of AlN ceramics were studied. The results show that the grain size of the AlN ceramics heat-treated at 970 °C for 2 h is significantly increased, the actual crystal morphology is realistic and the second phases are almost present at the grain boundaries or triple pockets compared with the samples without heat treatment, which thermal conductivity has reached 156.7 W/(m·K) from 77.3 W/(m·K). However, the pore size of AlN ceramics is increased and there is the phenomenon of anti-densification while the heat treatment time is extended to 4 h. The thermal conductivity of AlN ceramics heat-treated at 970 °C for 4 h is reduced to 92.6 W/(m·K).

Microstructure and thermal conductivity of spark plasma sintering AlN ceramics

Dense aluminium nitride ceramics were prepared by spark plasma sintering at a lower sintering temperature of 1700°C with Y2O3, Sm2O3 and Dy2O3 as sintering additives respectively. The effects of three kinds of sintering additives on the phase composition, microstructure and thermal conductivity of AlN ceramics were investigated. The results showed that those sintering additives not only facilitated the densification via the liquid phase sintering mechanism, but also improved thermal conductivity by decreasing oxygen impurity. Sm2O3 could effectively improve thermal conductivity of AlN ceramics compared with Y2O3 and Dy2O3. Observation by scanning electron microscopy showed that AlN ceramics prepared by spark plasma sintering method manifested quite homogeneous microstructures, but AlN grain sizes and shapes and location of secondary phases varied with the sintering additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of secondary phases.

Effect of High Pressure Annealing on Microstructure and Thermal Conductivity of Aluminum Nitride Ceramics

The thermal annealing is an effective means of structural adjustment and performance improvement for AlN ceramics.AlN ceramics prepared at high pressure with Y2O3 as sintering additive,were annealed at high-pressure(5.0GPa) in a chinese cubic anvil ultra high-pressure and high-temperature device.The effects of high pressure annealing on microstructure and thermal conductivity of aluminum nitride ceramics were studied.The results show that the grain size of the AlN ceramics annealed at 5.0GPa and 970℃ for 2h is significantly increased,the actual crystal morphology is realistic and the second phases are almost present at the grain boundaries or triple pockets compared with the samples before annealing at high pressure.Its thermal conductivity reaches 173.2W/(m·K),which is 2.2 times of the samples without heat annealing at high pressure.However,while the annealing time is extended to the 4h,the pore size of AlN ceramics is increased with anti-densification.And the thermal conductivity of AlN ceramics annealed at 5.0GPa and 970℃ for 4h is reduced to 80.9W/(m·K).

Thermal conductivity of Spark Plasma Sintered AlN ceramics with multiple components sintering additive

Dense aluminum nitride ceramics were prepared by Spark Plasma Sintering with Sm 2O 3-CaF 2 as multiple components sintering additives. The effect of sintering additives on the density, phase composition, microstructure and thermal conductivity of AlN ceramics was investigated. The results showed that Spark Plasma Sintering could fabricate dense AlN ceramics with superior thermal properties in a short time. The multiple components sintering additive not only purified the lattice of AlN ceramic by decreasing oxygen impurities but also reduced the content of grain boundaries by forming the evaporable second phases, thermal conductivity of AlN ceramics sintered with multiple components sintering additives was greatly improved. During the Spark Plasma Sintering process, the microstructures played a key role in controlling the thermal conductivity of AlN ceramics.

Effects of rare earth oxide additives on the thermal behaviors of aluminum nitride ceramics

The effects of Y2O3 and Er2O3 on the sintering behaviors, thermal properties and microstructure of AlN ceramics were investigated. The experimental results show that the sintering temperature can be decreased; the relative density and thermal behavior can be improved by adding rare earth oxide in AlN ceramics. For AlN ceramics with 3 wt.% Er2O3 additive, the relative density is 98.8%, and the thermal conductivity reaches 106 W·m−1·K−1. The microstructure research found that no obvious aluminum erbium oxide was found in AlN ceramics doped with 3 wt.% Er2O3, which favored the improvement of the thermal conductivity of AlN ceramics.

Effect of Sm 2O 3 content on microstructure and thermal conductivity of Spark Plasma Sintered AlN ceramics

Dense aluminum nitride ceramics were prepared by Spark Plasma Sintering at a lower sintering temperature of 1700 °C with Sm 2O 3 as sintering additives. The effect of Sm 2O 3 content on the density, phase composition, microstructure and thermal conductivity of AlN ceramics was investigated. The results showed that Spark Plasma Sintering could fabricate dense AlN ceramics with superior thermal properties in a very short time. Sm 2O 3 not only facilitated the densification via the liquid-phase sintering mechanism but also improved thermal conductivity by decreasing oxygen impurity. Thermal conductivity decreased with increasing amount of Sm 2O 3 and the highest thermal conductivity was obtained for the AlN ceramics with 2 wt.% Sm 2O 3 content. During Spark Plasma Sintering process, only 2–3 wt.% sintering additives was enough to fabricate dense AlN ceramics, and the microstructures played a key role in controlling the thermal conductivity of AlN ceramics.

Y2O3첨가가 AlN 세라믹스의 방전 플라즈마 소결 거동 및 열전도도에 미치는 영향

Spark plasma sintering (SPS) of AlN ceramics were carried out with Y₂O₃ as sintering additive at a sintering temperature 1,550~1,700℃. The effect of Y₂O₃ addition on sintering behavior and thermal conductivity of AlN ceramics was studied. Y₂O₃ added AlN showed higher densification rate than pure AlN noticeably, but the formation of yttrium aluminates phases by the solid-state reaction of Y₂O₃ and Al₂O₃ existed on AlN surface could delay the densification during the sintering process. The thermal conductivity of AlN specimens was promoted by the addition of Y₂O₃ up to 3 wt% in spite of the formation of YAG secondary phase in AlN grain boundaries because Y₂O₃ addition could reduced the oxygen contents in AlN lattice which is primary factor of thermal conductivity. However, the thermal conductivity rather decreased over 3 wt% addition because an immoderate formation of YAG phases in grain boundary could decrease thermal conductivity by a phonon scattering surpassing the contribution of Y₂O₃ addition.

Effect of Second Phase After-Heat Treatment on the Thermal Conductivity of AlN Ceramics

The high thermal conductivity of Aluminum nitride, coupled with its high electrical resistivity and nontoxic nature, makes it a very promising material for electronic substrate. In this study, microstructural characterization on the thermal conductivity of AlN ceramics was investigated. An AlN ceramic was prepared with a dopant Y2O3 under a reducing nitrogen atmosphere with carbon. In order to obtain high thermal conductivity, cooling rate control and after-heat treatment was carried out. Morphology of the second phase was characterized using scanning electron microscopy (SEM). SEM studies showed that the microstructural change caused by after-heat treatment have a major influence on the thermal conductivity.

Role of additives in enhancing the thermal conductivity of AlN ceramics

The thermal conductivity of AlN ceramics with different amounts of Y2O3 as sintering additive has been calculated at low, intermediate and high temperatures by applying the Callaway theory and using a detailed account of three-phonon scattering processes. By making a quantitative analysis of the results, it is found that the increase in the room-temperature conductivity due to the addition of Y2O3 is largely attributed to the decrease in the oxygen impurities level in the AlN crystal. In addition to matching the experimental measurements (available in a limited temperature range) our theoretical work predicts the resulting thermal conductivity over a large temperature range.