Thermal Conductivity Of Aluminum Nitride Research Articles

AlSi10Mg/AlN Interface Grain Structure after Laser Powder Bed Fusion

Details and features of the grain structure near the interface region between the AlN ceramic phase and AlSi10Mg matrix after the laser powder bed fusion (LPBF) were investigated. Aluminum nitride particles were obtained through self-propagating high-temperature synthesis and mechanically mixed with aluminum matrix powder. Optimization of the LPBF process parameters resulted in synthesized material free of pores and other defects. Optical microscopy analysis of etched cross-section and more detailed EBSD analysis revealed regions with relatively coarse grains at melting pool boundaries and fine grains in the melt pool core and near the AlN particles. Moreover, a pronounced orientation of fine elongated matrix grains towards the center of the ceramic particle was obtained. The such formed microstructure is attributed to directional heat sink during crystallization due to the higher thermal conductivity of aluminum nitride compared to the AlSi10Mg matrix. On the contrary, poor wettability of AlN by melt partly prevented the formation of such features, thus a combination of these factors determines the final microstructure of the interface in the resulting material.

A High Temporal-Spatial Resolution Temperature Sensor for Simultaneous Measurement of Anisotropic Heat Flow.

The thermal measurement sensor applied to hypersonic vehicles requires characteristic size in the order of micrometers and characteristic time in the order of microseconds. The measurement criteria of localized detection, high temporal-spatial precision, and long-term stability cannot all be reached by current thermal measuring techniques. This work presents a temperature sensor with excellent temporal-spatial resolution that can measure both in-plane and out-of-plane heat flow. The sensor was made of thin platinum nano-film and an aluminum nitride ceramic sheet. The sensor was calibrated using a thermostatic vacuum chamber and used for in-plane and out-of-plane heat flow measurements. The temperature measurement accuracy of the sensor was estimated to be 0.01 K. The sensor’s resolution for measuring heat flow density is more than 500 W/m2 and its measurement uncertainty is roughly 3%. To ensure the structural stability of the device, the aluminum nitride ceramic sheet was chosen as the substrate of the thermal sensing unit, and the response time became longer due to the high thermal conductivity of aluminum nitride. The suspension platinum nano-film sensor can reach a sub-microsecond response time according to the theoretical derivation. Experimental results of pneumatic thermal effects of high-temperature flames using the sensor prove that the designed sensor has good sensitivity and accuracy.

2D-Modeling for the temperature-composition dependent thermal-conductivity of AlmNn compounded semiconductor materials

The quantitative effect on the thermal conductivity of Aluminum Nitride(AlN) regarding to Al mole fraction and temperature has not been reported computationally. Therefore, the goal of this study is to investigate the impact of Al mole fraction and temperature on the thermal conductivity of AlmNn numerically based on Boltzmann Transport equation by considering phonon scattering mechanisms. The theoretical thermal conductivity of AlmNn versus Al mole fraction and temperature is showed in this study. It is found that Point-defect and Umklapp scatterings are dominant scattering mechanisms for the thermal conductivity of AlmNn. The calculated thermal conductivity of AlmNn is verified through the comparison of experimental data versus Al mole fraction and temperature.

Measuring anisotropic thermal conductivity of aluminum nitride films with the ultra-fast hot strip technique

In this work, we report the implementation of the ultra-fast transient hot strip (THS) technique for measuring the anisotropic thermal conductivity of aluminum nitride (AlN) thin-films. AlN films were produced by reactive DC magnetron sputtering grown at low temperature (>250 °C) on silicon oxide (SiO2) thin-film produced on a silicon substrate. Precise measurement of thermal conductivity was performed with an experimental device generating ultra-short electrical pulses, and subsequent temperature increases were electrically measured on nanosecond and microsecond time scales. The electrical pulses were applied within metalized strips patterned on SiO2 before AlN elaboration and the temperature increases were analyzed within time periods selected in the range [0.1–10 μs]. AlN thermal conductivity of cross-plane (in-plane) increased from 60 to 90 W m−1K−1 (33–44 W m−1K−1) when thickness was raised from 1 to 2 μm, respectively. This shows clearly the anisotropy in thermal conductivity of AlN films. In addition, the volumetric heat capacity of AlN was estimated to be ~2.5 × 106 J K−1 m−3.

Enhancement of thermoelectric power of a Si nanowire micro thermoelectric generator by improving the thermal conductivity of AlN thermally conductive film

We report the enhancement of thermoelectric power of Si nanowire micro thermoelectric generator (SiNW-μTEG) by improving the thermal conductivity of aluminum nitride (AlN) thermally conductive film. This work builds on the previous results that have been reported at SSDM 2017. [1] This work shows the measured thermal conductivities of the two different AlN films to understand the underlying mechanisms of the enhancement of the thermoelectric power.

Comparison of Two Computing Method on Thermal Conductivity of Aluminum Nitride

Thermal conductivity of aluminum nitride (AlN) has been calculated by density functional perturbation theory (DFPT) and quasi-harmonic approximation (QHA) combined with Debye theory in the paper. Debye temperature is evaluated respectively from sound velocity and heat capacity. From 300K up to 1000K, the predicted thermal properties in pure crystal AlN based on these two Debye temperatures are compared with each other and the latter shows excellent agreement with Slack’s experimental data. The relative difference based on Debye temperature from heat capacity is within the limits of ±5.5%. This agreement with experiment is due to the Debye temperature derived from capacity contains the temperature effect while describe the three phonon process.

Synthesis of an aluminum nitride–yttria (AlN–Y2O3) composite from nano-sized porous AlN and YCl3

In order to enhance the thermal conductivity of aluminum nitride (AlN) with sintering additives including yttria (Y2O3), it is necessary to form yttrium aluminate garnet (YAG) and secondary phases both within and around the boundaries of AIN drains. Nano-sized porous AlN particles were produced to form YAG and secondary phases within AlN grains, after which a AlN–Y2O3 nano–nano composite was formed from AlN and amorphous Y2O3. Porous AlN powders were first successfully synthesized by the chemical vapor synthesis (CVS) method. Highly crystalline and nano-sized porous AlN powders were synthesized at 1,200 °C. Brunauer–Emmett–Teller (BET) analysis showed that these powders had very large surface areas, suggesting that the particles approached nano-scale sizes with very small pores. To form composites of Y2O3 and AlN, we prepared a yttrium source solution that infiltrated the nano-sized pores of the AlN particles. Such an infiltration of AlN with amorphous Y2O3 was expected to effectively reduce the residual oxygen content by facilitating the formation of YAG and secondary phases during the sintering process. We characterized the composite powders of AlN–Y2O3 and the sintered bodies using BET, XRD, SEM, TEM, and thermal conductivity analyses.

Investigation on the thermal conductivity of AlN thin films by using Molecular Dynamics simulation

In order to investigate high thermal property of AlN ceramic, the Equilibrium Molecular Dynamics (EMD) was used to simulate the thermal conductivity of Aluminum Nitride (AlN) films. The average temperature was selected within 20-4000 K in the simulation. The Stillinger?Weber potential and Green?Kubo's formula were introduced to calculate thermal conductivity. The results show that the thermal conductivity of the thin films is much lower than that of its bulk counterpart, and the size-effect on thermal conductivity is significant.

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.

Effect of Metal Impurities on Shading and Thermal Conductivity of Aluminum Nitride

Because aluminum nitride (AlN) has a high thermal conductivity and a high electrical resistivity, it is used for substrates and packaging for high power or high density circuits. Silicon chips need to be protected from faults caused by exposure to ultra-violet rays. AlN packaging and substrates have to be shaded for UV protection. The characteristics of AlN are influenced by a number of factors. Specifically, metallic impurities affect the shading and thermal conductivity, and are particularly easily mixed with AlN as minerals or solutions during processing. It is very important to understand how metallic impurities influence the characteristics of AlN, and either refine processing to eliminate such effects, or add metal to achieve the desired industrial characteristics. Iron (Fe), silicon (Si), magnesium (Mg), and titanium (Ti) are metals that are easily mixed, so it is important to understand their influence. The metallic impurities Fe, Si, Mg, and Ti were added to raw powder AlN and uniformly distributed at a molecular level. The powder was sintered and tested for any effects on shading or reduction in thermal conductivity. Some Ti compounds were found at the grain boundaries, but these did not reduce the AlN thermal conductivity and provided efficient shading. In contrast, Si and Mg compounds were not found at the grain boundaries but formed solid solutions in the AlN grains, so they significantly reduced the AlN thermal conductivity and provided no shading. The effect of Fe was intermediate between that of Ti and Si and was characterized by its tendency to form solid metal ion solutions in the AlN grains.

Properties and microstructure of aluminum nitride sintered by millimeter-wave heating

Aluminum nitride is not so extensively applied because of low productivity and high cost due to necessity of high sintering temperature over 1900 °C and long sintering time around 10 h. In the present study, high thermal conductivity over 200 W/(m K) was attained by sintering with 28 GHz millimeter-wave heating at 1700 °C for 2 h under nitrogen/hydrogen mixed gas atmosphere. Attainment of such a high thermal conductivity of aluminum nitride sintered by millimeter-wave at low temperature for a short time is attributed to a characteristic microstructure induced by millimeter-wave heating. From the results of the observation by high resolution TEM, the intergranular film layer between aluminum nitride grains in the sintered body by millimeter-wave heating was as thin as difficult to be observed, resulting in remarkable enhancement of heat transfer at the thinner intergranular phase. Therefore, high thermal conductivity was attained in the millimeter-wave-sintered aluminum nitride in spite of short sintering time and low sintering temperature, compared with the conventional sintering method.

Improving thermal conductivity of Sm2O3-doped AlN ceramics by changing sintering conditions

The effects of sintering conditions including soaking time and packing powder on the thermal conductivity of Aluminum nitride (AlN) ceramics with Sm2O3 additives were investigated. The thermal conductivity values of AlN ceramics sintered were improved after increasing soaking time, but not considerably due to the interconnected secondary phases. While the specimen sintered without packing powder showed much higher thermal conductivity by 58 W·m−1·K−1 than the specimen sintered with packing powder. It is due to the removal of the secondary phase with poor thermal conductivity to a fairly low level, and the decreasing of the oxygen content via carbothermal reduction.

On the effects of secondary phase on thermal conductivity of AlN ceramic substrates using a microstructural modeling approach

This paper reports on mathematical and finite element modeling of the effects of secondary phase (Y 2O 3) volume and location on the thermal conductivity of aluminum nitride (AlN) substrates. Finite element analysis results were also compared with experimental results. From finite element analysis, thermal conductivity of AlN(Y 2O 3) substrates with secondary phase located at the grain boundaries was in agreement with Buhr's model. Maxwell's equation was found to give a good estimate of the thermal conductivity of AlN(Y 2O 3) substrates with the secondary phase located at the triple points, with a varying gradient depending on the thermal conductivity of the individual components. With increasing secondary phase volume, thermal conductivity was higher when the secondary phase was located at the triple points as compared with grain boundaries.

Microstructural characterization and microstructural effects on the thermal conductivity of AlN(Y2O3) ceramics

Abstract Two different high density aluminum nitride (AlN) ceramic materials with Y2O3 as a sintering additive have been sintered at 1880°C, and characterized using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The lattice dissolved oxygen content, grain size and amount of secondary phase were fairly similar for the two materials, however, the thermal conductivity was quite different. SEM and TEM studies show that the secondary phase distribution along the grain boundaries or at grain junctions may have a major influence on the thermal conductivity. Microstructural changes were found to disrupt the connection between the high thermal conductivity AlN grains, resulting in a decrease in the thermal conductivity of the material as a whole. No amorphous layer was observed by high resolution transmission electron microscopy (HRTEM) at the cleanest grain boundaries in any of the materials.

Microstructure and Thermal Conductivity of AlN(Y2O3) Ceramics

Aluminum nitride (AIN) with high thermal conductivity has been considered as a potential substrate material for microelectronics. However, the thermal conductivity of polycrystalline AIN ceramics, prepared by liquid-phase sintering, is significantly lower than that of AIN single crystal. It has been shown that the thermal conductivity of AIN is inversely proportional to the oxygen content dissolved in the AIN lattice. Furthermore, the microstructure of AIN can also influence the thermal conductivity considerably. In the present study, we investigated the effect of grain boundaries and secondary phase distributions on thermal conductivity of two selected high density samples which contained similar oxygen content but showed different thermal conductivities.Experimental details and selected properties of Samples A and B are shown in Table 1. Sample A with 0.8 wt% Y2O3 additive was embedded in an AIN powder bed in a BN crucible covered by a BN lid (not gas tight) during sintering, while Sample B with 3.9 wt% Y2O3 was embedded in an AIN powder bed in an open graphite crucible.

High‐Thermal‐Conductivity Aluminum Nitride Ceramics: The Effect of Thermodynamic, Kinetic, and Microstructural Factors

Improvement in the thermal conductivity of aluminum nitride (AlN) can be realized by additives that have a high thermodynamic affinity toward alumina (Al2O3), as is clearly demonstrated in the aluminum nitride‐yttria (AlN‐Y2O3) system. A wide variety of lanthanide dopants are compared at equimolar lanthanide oxide:alumina (Ln2O3: Al2O3, where Ln is a lanthanide element) ratios, with samaria (Sm2O3) and lutetia (Lu2O3) being the dopants that give the highest‐ and lowest‐thermal‐conductivity AlN composites, respectively. The choice of the sintering aid and the dopant level is much more important than the microstructure that evolves during sintering. A contiguous AlN phase provides rapid heat conduction paths, even at short sintering times. AlN contiguity decreases slightly as the annealing times increase in the range of 1–1000 min at 1850°C. However, a substantial increase in thermal conductivity results, because of purification of AlN grains by dissolution‐reprecipitation and bulk diffusion. Removal of grain‐boundary phases, with a concurrent increase in AlN contiguity, occurs at high annealing temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquidphase‐sintered AlN ceramics.

Ab-initio simulation of thermal properties of AlN ceramics

Aluminum nitride (AlN) ceramics are prospective materials for electronic parts, particularly LSI substrates, because of their excellent mechanical, dielectrical and thermal properties. Their high thermal conductivity is estimated to be about 320 W m-1 K-1 from some theoretical and experimental considerations. However, existence of crystalline imperfections such as impurities, lattice defects or a different phase (zincblende phase) practically reduces it to only two-thirds of the theoretically predicted one. In this paper an interatomic potential of AlN which is adequate for molecular dynamics MD calculation is established on the basis of ab initio molecular orbital MO analysis of an AlN cluster. Then the thermal conductivity of AlN is estimated by equilibrium MD simulation, and its dependence on crystallographic structure and lattice defects is examined. We find that the energy difference between two kinds of phase structure in AlN (the wurtzite and zincblende) is so small that they are able to coexist in the realistic material. The thermal conductivity of the zincblende is also found to be much lower than that of the wurtzite.

Effect of Yttria on the Thermal Conductivity of Aluminum Nitride

The effect of yttria additions up to 10 wt% on the thermal conductivity of pressureless‐sintered aluminum nitride was investigated. Additions of up to 3 wt% increased the thermal conductivity to values around 160 W/(m·K). Additions higher than 5 wt% slowly decreased the thermal conductivity because of the increasing volume fraction of the low‐conductivity yttrium aluminate phases. In all cases samples containing the same yttria content with the binder burned out in nitrogen had higher thermal conductivity than those with the binder burned out in air. Comparisons are made to previous studies containing additives of silica and calcia to aluminum nitride.

Effect of Silica on the Thermal Conductivity of Aluminum Nitride

The effect of SiO2 on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y2O3 as a sintering aid was investigated. SiO2 additions greatly decreased the thermal conductivity of the sintered parts from values of around 160 W/m·K on undoped samples to about 25 W/m·K with 5 wt% SiO2 added to the green body composition. Microstructural studies, combined with the temperature dependence of the thermal conductivity and lattice parameter measurements, indicated that defect phonon scattering was the mechanism responsible for the decrease in thermal conductivity. SiO2 can be incorporated in limited solid solution into the AIN lattice, generating AI vacancies for charge compensation in a process not unlike the solution of oxygen in AIN. The mass difference introduced by the vacancies is the main phonon scattering defect. Beyond a concentration threshold of 2%, the SiO2‐induced defects cluster to form SiAION polytypoids derived from the basic 2H AIN structure with stacking sequences that depend on the SiO2 levels in the sample.

Effect of CaO on the Thermal Conductivity of Aluminum Nitride

The effect of CaO on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y2O3 as a sintering aid was investigated. Over the composition range of 0 to 2.0 wt% additions, CaO decreased the thermal conductivity of the sintered parts by 10%. CaO doping rendered the secondary oxide phases more wetting and thus with a greater tendency to penetrate along the grain boundaries. Furthermore, CaO segregation to the grain boundaries was observed even on those grain boundaries apparently free of secondary phases. These microstructural changes disrupted the connectivity of the high thermal conductivity AIN grains and were the main factors contributing to the decrease in the thermal conductivity of the ceramic parts. CaO additions to samples doped with SiO2 had the opposite effect, increasing the thermal conductivity. CaO removed SiO2 from the AIN grains and incorporated it into the oxide second phases, most likely through charge‐compensating substitutions Ca2++ Si4+ for Y3+ and/or Al3+. Thus, AIN samples containing both SiO2 and CaO had higher thermal conductivity than those containing comparable amounts of SiO2 alone.