AlN Ceramics Research Articles

Dielectric characteristics of pressureless sintered AlN-based composites in the 3–37 GHz frequency range

In a system of dielectric matrix–conducting particles, the dielectric characteristics are analyzed over a wide frequency range (3–37 GHz). Experimental values of the dielectric constant e′ and dielectric loss tangent tanδ are presented for pressureless sintered AlN-based composites with different contents of the conducting particles (Mo, W, and TiN) within the interval from 0% to the percolation threshold. Both the real e′ and imaginary e″ parts of the dielectric constant of the investigated composites monotonically increased, reaching maximum values (e′ = 15 – 26.5, e″ = 0.14–0.28) when the content of the conductive particles approached the percolation threshold. The dielectric loss tangent of the composites, depending on the conducting particle content, reached values of 0.0085 for AlN–16.6%Mo, 0.0095 for AlN–16%W, and 0.0105 for AlN–20.4%TiN. The dielectric losses e″ in the AlN-based composites, as long as they remain nonconductive for the direct current, are low compared to losses of e″ = 0.04 in polycrystalline AlN ceramics and exceed them by only 4–7 times. A relationship between the dielectric losses and the level of microwave absorption has been established. The dielectric characteristics, electrical resistance, and thermal conductivity of the produced AlN-based composites and their achievement of a high absorption of the microwave radiation (L = 23–32 dB/cm) make these materials promising bulk absorbers in microwave devices (TWTs, klystrons).

Study of Changes in Optical and Heat-Conducting Properties of AlN Ceramics under Irradiation with Kr15+ and Xe22+ Heavy Ions.

AlN-based ceramics have great prospects for use in the field of structural materials for reactors of the new generation of GenIV, as well as dosimetric and optical devices. Interest in them is due to their unique physical and chemical properties, high resistance to degradation and excellent insulating properties. This work is devoted to the study of changes in the optical and heat-conducting properties of AlN ceramics as a result of irradiation with Kr15+ and Xe22+ heavy ions with energies close to those of fission fragments of uranium nuclei, and fluences 1014–1015 ion/cm2. During the study, dose relationships of changes in the optical properties of ceramics were established, as well as the effect of the type of ions on the degree of radiation damage and deterioration of optical characteristics. It has been found that an increase in the irradiation dose for Kr15+ ions leads to a slight increase in the depth of electron traps, while for samples irradiated with Xe22+ ions there is a sharp increase in the depth of occurrence from 5 to 20%, depending on the irradiation dose. For samples irradiated with Xe22+ ions, the greatest decrease in thermal conductivity was 19%, while for ceramics irradiated with Kr15+ ions, the maximum decrease was not more than 10%. The results show a significant resistance of ceramics to radiation damage by Kr15+ ions and negative effects, leading to a decrease in the resistance of optical and conductive properties of ceramics when irradiated with Xe22+ ions with doses higher than 5 × 1014 ion/cm2. Using the X-ray diffraction method, the dependences of structural distortions and changes in dislocation density in the structure of ceramics on the radiation dose were established. It has been determined that the main structural changes are associated with the fragmentation of grains, which result in an increase in the dislocation density, as well as deformation and distortion of the crystal lattice as a result of the formation of complex defects in the structure.

Development and preliminary process design of beta-SiAlONs by the spark plasma sintering process

There is a need for improvement in the existing ceramic-based products such as the cutting tools due to the structural failures often experienced and the increasing operational costs. The development of refractory material (β-SiAlON) with excellent properties, such as high strength and wear resistance, is vital in order to meet the service requirements. In this study, the spark plasma sintering (SPS) technique was used for the sintering of ceramic powders, namely, β-Si3N4, AlN, Al2O3, and Y2O3, during the production of β-SiAlON. The density, wear behavior, hardness, and microstructure of the β-SiAlON produced were evaluated and optimized in relation to the effect of the sintering parameters in the following range: sintering temperature (1450–1750 °C), holding time (5–10 min), heating rate (50–100 °C/min), and the z-value (0–1.5). The approach is based on the Taguchi method which uses the signal-to-noise ratio and the analysis of variance to predict the optimum sintering parameters for maximum densification, superior wear resistance, and hardness of the resulting products. Thus, the properties of the sintered materials produced via the physical experimentations were measured as the products’ quality characteristics and statistically analyzed to determine the contribution of each process parameter to the measured properties. The following are combination of optimum parameters which produced excellent mechanical properties: maximum density (1600 °C, 10 min, 75 °C/min, and 1.5), superior wear resistance (1600 °C, 7.5 min, 100 °C/min, and 0.75), and hardness (1600 °C, 5 min, 75 °C/min, and 1.5). Hence, work provides an insight into the process design and determination of the optimal combination of the process parameters for the development of β-SiAlON with excellent mechanical properties via the SPS technique.

The influence of the energy of incident protons on the defect formation and radiation resistance of AlN ceramics

The main goal of this work is to study the effect of irradiation with protons with an energy of 0.5–1.5 MeV at a fluence of 1х1017 proton/cm2 on the mechanical and structural properties of AlN ceramics. The choice of AlN ceramics as objects of study is due to their high resistance to radiation damage, their great potential for using them as structural materials, dosimetric materials, etc. During the tests, it was found that at low incident particle energies the degree of deformation of the crystal structure is greatest, due to the fact that all radiation defects are closed in a small volume of the surface layer. An increase in proton energy leads to an increase in the path length of protons in ceramics, while a decrease in damage to the surface layer is observed, however, the damage depth becomes larger with an increase in proton energy. The obtained results confirm a high degree of resistance to hydrogenation processes when irradiated with protons with energies of 1.0–1.5 MeV, however, at energies of incident protons of 0.5 MeV, a decrease in strength properties is observed due to the presence of a large number of dislocation and point defects in the near-surface layer of small volume, which leads to partial disordering and degradation of structural and optical properties.

New approach of friction AlN ceramics metallization with the initial FEM verification

Although, the friction method is well known for metals surface modification, the novelty of the article is based on the new idea of ceramics surface treatment with metal. The paper describes AlN ceramic metallization process by titanium coating deposition, obtained in friction surfacing method, which has been developed by the authors. The friction energy is directly transformed into heat and delivered in a specified amount precisely to the joint being formed between the metallic layer and the ceramics substrate material. The stress and temperature fields (as factors promoting the formation of diffusion joints) induced in the joint during the metallization process were qualitatively determined with the finite element method analysis and these results were verified experimentally. Finally, obtained structures of the metallic coatings were investigated and the results are discussed in the paper. As a novelty it was found, that the conditions of frictional metallization can favour the formation of a coating-substrate bond based on diffusion phenomena and atomic bonds of the coating components with the components of the substrate, despite the fact that it happens for metal–ceramics pairs. This type of connection is usually associated with long-term heating/annealing in chamber furnaces, because for diffusion in a solid state the most effective factor is time and temperature. It was shown that other components of the chemical potential gradient, such as temperature gradient, gradient and stress level, load periodicity and configuration of pairs of elements with high chemical affinity may play an important role in friction metallization. As a result, the relatively short time of operation (friction) is compensated.

Dynamics of Radiation Damage in AlN Ceramics under High-Dose Irradiation, Typical for the Processes of Swelling and Hydrogenation

The use of nitride ceramics, in particular AlN, as structural materials for nuclear power is primarily limited by their resistance to swelling and hydrogenation processes due to the accumulation of poorly soluble helium and hydrogen ions in the structure of the surface layer. In this regard, research in this area is of great importance not only from a fundamental point of view, but also practical, since any new data on radiation resistance can make a great contribution to the development of the theory of resistance to radiation influences of structural materials of a new generation. This work is devoted to a systematic study and comparative analysis of the dynamics of radiation damage during high-dose irradiation with protons and helium ions in nitride ceramics, which have great potential for use as structural materials for GenIV reactors. The choice of irradiation doses of 1 × 1017–5 × 1017 ion/cm2 is due to the possibility of modeling the processes of radiation damage characteristic of displacements of 10–50 dpa. During the study, the dependences of the change in the dielectric and conductive characteristics of nitride ceramics depending on the radiation dose, as well as on the type of ions, were established. The kinetics of degradation and accelerated aging was determined depending on the type of exposure. The mechanical and strength properties of ceramics were determined.

Preparation of AlN powder of low oxygen content via carbothermal reduction and nitridation by active gas exchange technique

By generating a periodic impulse-like pressure (2.0–4.4 kPa, 84 s) to actively exchange the gas in synthesis furnace, pure AlN powder of low oxygen content was synthesized via additive free carbothermal reduction and nitridation (CRN) of Al2O3 powder. Compared with the conventional CRN method, the proposed extra gas exhaust process can more effectively remove the side-produced CO from the reaction sites to accelerate nitridation process and decrease the residual oxygen content in the obtained AlN powder. For example, with 39 wt% activated carbon loaded in the raw material at 1650 °C for 4 h, the prepared AlN powder by the proposed synthesis scheme has only 0.68 wt% residual oxygen. The effects of carbon content, synthesis temperature and holding time on the residual oxygen content in AlN powder by the proposed synthesis scheme were also studied. The ball-milled as-prepared AlN powder was pressureless sintered at 1880 °C for 2.5 h to obtain a translucent AlN ceramics (37.6% at ~5700 nm), which demonstrates the excellent sinterability of the as-prepared AlN powder.

Electroinsulated Heat Sinks for Concentrated Photovoltaic Solar Cells

The results of studies of characteristics of heat-removing electrical insulating plates of different designs based on heat-conducting AlN and Al2O3 ceramics and aluminum-oxide plates based on aluminum plates are presented. Thermophysical parameters of the heat-removing plates were assessed by measuring the temperature of p–n junctions of the photoelectric converters mounted on the plates with thermal load applied to them. It was found that the minimum overheating temperature of p–n junctions, 45°C, was observed at the supplied thermal power of 13 W, when the photoelectric converters were installed on the AlN ceramics. In photoelectric converters installed on the Al2O3 ceramics, the overheating of p–n junctions was 49°C, while it reached 70°C on the aluminum-oxide substrate.

Mechanical, thermal, and electrical properties of pressureless sintered SiC–AlN ceramics

SiC–AlN ceramics were prepared by pressureless sintering and the effects of the polytype of the starting SiC powder and the sintering atmosphere on the mechanical, thermal, and electrical properties of the ceramics were investigated. Almost fully densified SiC–AlN ceramics with a high SiC–AlN solid solution (2Hss) content (75–85%) were fabricated through pressureless sintering using 5 mol% Y2O3 as a sintering additive. Higher flexural strength, hardness, and thermal conductivity of the SiC–AlN ceramic were achieved under nitrogen atmosphere. In contrast, under argon atmosphere, a higher 2Hss content, toughness, and electrical resistivity could be achieved. The use of α-SiC as the starting powder provided higher hardness, electrical resistivity, and thermal conductivity, whereas the use of β-SiC as the starting powder led to higher 2Hss content and toughness. The mechanical, thermal, and electrical properties of the SiC–AlN ceramics prepared by pressureless sintering were significantly affected by the 2Hss content and nitrogen concentration in the liquid phase. The 2Hss content and nitrogen concentration in the liquid phase were influenced by the sintering atmosphere and starting polytype of the SiC powder. Thus, the mechanical, thermal, and electrical properties of the SiC–AlN ceramics could be adjusted, to some extent, through judicious selection of the sintering atmosphere and polytype of the starting SiC powder.

Thermal conductivities and mechanical properties of AlN ceramics fabricated by three dimensional printing

AlN ceramics were successfully fabricated through a joint process of digital light processing (DLP) 3D printing technology and heat treatment at 1780 °C∼1845 °C. DLP is an addictive manufacturing process, enabling the near net shape fabrication. The AlN grains in this work developed well and there were small amounts of grain-boundary phases at the three-grain junctions. The particle size of AlN became larger and the densification increased with increasing sintering temperature. The pores of AlN ceramics also decreased, which led to the increase of thermal conductivity and flexural strength. The optimal thermal conductivity and flexural strength of AlN ceramic reached 155 W/(m·K) and 265 ± 20 MPa when sintered at 1845 °C.

AlN nanoparticles prepared through a gelation-polymerization process

A gelation-polymerization process is developed to fabricate AlN nanoparticles using boehmite sol, phenol, and formaldehyde as the starting materials. This process is characterized by simultaneously gelation and polymerization reactions to form two interactive three-dimensional networks in the same reactor. The gelation forms boehmite gel from the sol, while polymerization yields phenol-formaldehyde resin (PF). The interactive networks enable the formation of AlN nanoparticles entirely at 1300 °C from alumina and carbon. Meanwhile, the PF network prevents the crystalline of boehmite, increases the temperature for the dehydration reaction from boehmite to γ-alumina, and keeps the γ-alumina phase stable at temperature up to 1200 °C. The AlN nanoparticles are sintered to AlN ceramics without any sintering additives, and the thermal conductivity is 105 W m−1 k−1 for the ceramic with 92% relative density.

Titanium metallization coating deposited on AlN ceramics substrate by means friction surfacing process

The article characterizes a titanium metallization coating frictional deposited on the surface of AlN ceramics. The method of preparing materials for metallization and the idea of the process were described. The results of stereometric tests of coating surfaces, metallographic tests and phase analysis are presented. Tests were carried out on the adhesion of the coating to the substrate and brazing tests of metallized ceramics with an FeNi42 alloy in an argon sheath using AgCu28 eutectic solder.

Enhanced bonding strength of Al2O3/AlN ceramics joined via glass frit with gradient thermal expansion coefficient

Ceramics joining has been widely applied in MEMS sensors, microwave and imaging devices as well as other ceramic packaging applications to fabricate multifunctional ceramic components. In this study, a glass frit with gradient thermal expansion coefficient (TEC) has been applied as the bonding material to join the ceramic of Al2O3 and AlN. This glass frit consists of two types of glasses with different TEC. The Al2O3 and AlN ceramics have been successfully joined with this glass frit in muffle furnace at 850 °C for 1 h. The bonding strength of Al2O3/AlN ceramic joined via this glass frit exceeds 40 MPa without any cracks in the joining interface, which is much superior compared with that of conventional glass frit bonding. Our work demonstrates that the gradient-TEC glass frit bonding is an effective method to join different types of ceramics.

Теплоотводящие электроизолирующие платы для фотоэлектрических преобразователей концентрированного солнечного излучения

In the article the results of investigation of characterics of electro insulated heatsinks of different design are presented. For the material of the heatsinks AlN and Al2O3 ceramics along with alumina were chosen. Thermal characteristics of the heatsinks has been studied by measuring temperature of p-n junctions of solar cells mounted on heatsinks under thermal loading. The minimum overheating temperature of p-n junction was 45°C with input thermal power equal 13 W, when solar cells were mounted on AlN ceramic. In case of using Al2O3 ceramic and alumina heatsinks the temperatures were 49°C, and 70°C respectively.

Strengthening mechanism of Si, V and Ti atoms to the Ag/AlN interface: a study from first-principles calculations.

Active elements play key roles in improving the interfacial bonding between Ag-based fillers and AlN ceramics. An understanding of the influence mechanism of active elements to interfacial adhesion can help us optimize the composition of active filler metals. In this paper, Ag(111)/AlN(0001) interfaces with different terminations and stacking sequences were constructed first. The N-terminated A-site interface was found to have the largest work of adhesion (Wad). Then, the effects of Si, Ti and V dopants on the Ag/AlN interfacial bonding were investigated via first-principles calculations. The results reveal that the Ti and V dopants can increase the values of Wad significantly. Electronic structure analysis reveals that the Si-N, Ti-N and V-N bonds formed at the interface are mainly ionic, and with some composition of covalent. Ti and V atoms can form strong bonds with not only the AlN slab, but also the neighboring Ag layers. It can be concluded that Ag-Ti and Ag-V active fillers are more suited to braze AlN. Ti-N and V-N compounds formed at the interface can greatly improve the interfacial bonding strength.

Retraction notice to "Effect of swift heavy ions irradiation on AlN ceramics properties" [Ceramics International 44 (2018) 19787-19793

Retraction notice to "Effect of swift heavy ions irradiation on AlN ceramics properties" [Ceramics International 44 (2018) 19787-19793

Experimental study on the cycling stability and corrosive property of Al-Si alloys as phase change materials in high-temperature heat storage

Cycling stability and corrosive property of Al-Si alloys with different silicon mass ratios are experimentally investigated for high-temperature latent heat storage purpose, respectively. Melting temperatures, latent heat, thermal diffusivities of Al-Si alloys after 0, 50 and 100 melting-solidification cycles are measured and their thermal conductivities are calculated. Microstructures of Al-Si alloys after 0, 50 and 100 melting-solidification cycles are observed and the relations between thermophysical parameters and microstructures are discussed. Six kinds of common engineering ceramics are adopted to examine the corrosive property of molten Al-Si alloys. The microstructures and element distributions at the interfaces are detected. It is concluded that the microstructures of Al-Si alloys all changes after cycles; the melting temperatures of Al-Si alloys remain relatively stable after cycles, but the variation of latent heat is only found small for AlSi12 alloy and AlSi20 alloy; due to the microstructure variation, the heat transfer performance of Al-Si alloys changes significantly after cycles except AlSi12 alloy; Al2O3 ceramics and AlN ceramics show excellent corrosion resistance in molten Al-Si alloys; the corrosion resistance of SiC ceramics increases with the silicon mass ratio of Al-Si alloys; corrosion exists between Si3N4/ BN/ ZrO2 and Al-Si alloys, which however remains to be studied further.

Effect of hot-pressing sintering on thermal and electrical properties of AlN ceramics with impedance spectroscopy and dielectric relaxations analysis

The effects of hot-pressing sintering on the phase composition, microstructure, thermal and electrical properties of AlN ceramics with CeO2–CeF3 additives were studied. During hot-pressing sintering, high pressure reduced the grain boundary phase CeAlO3 and decreased the concentration of oxygen in AlN ceramics. The hot-pressing sintered AlN samples had a much higher thermal conductivity of 191.9 W/m·K than pressureless sintered ones because of the great reduction of grain boundary phases and oxygen impurities in AlN ceramic. As the carbon content in hot-pressing sintered sample was very high, carbon contamination led to the decrease in electrical resistivity and changes in polarization mechanisms for AlN ceramics. The relaxation peak in the dielectric temperature spectrum with an activation energy of 0.64 eV for hot-pressing sintered samples was caused by electrons from free carbon at low temperature. Overall, hot-pressing sintering can effectively increase the thermal conductivity and change the electrical properties of AlN ceramics.

A Low-Temperature Micro Hotplate Gas Sensor Based on AlN Ceramic for Effective Detection of Low Concentration NO2.

Air pollution is one of the major threats to human health. The monitoring of toxic NO2 gas in urban air emission pollution is becoming increasingly important. Thus, the development of an NO2 sensor with low power consumption, low cost, and high performance is urgent. In this paper, a planar structural micro hotplate gas sensor based on an AlN ceramic substrate with an annular Pt film heater was designed and prepared by micro-electro-mechanical system (MEMS) technology, in which Pt/Nb/In2O3 composite semiconductor oxide was used as the sensitive material with a molar ratio of In:Nb = 9:1. The annular thermal isolation groove was designed around the heater to reduce the power consumption and improve the thermal response rate. Furthermore, the finite element simulation analysis of the thermal isolation structure of the sensor was carried out by using ANSYS software. The results show that a low temperature of 94 °C, low power consumption of 150 mW, and low concentration detection of 1 to 10 ppm NO2 were simultaneously realized for the Nb-doped In2O3-based gas sensor. Our findings provide a promising strategy for the application of In2O3-based sensors in highly effective and low concentration NO2 detection.

Corrosion resistance of ceramic candidates for tritium permeation barriers exposed to molten lithium

For the design of liquid lithium first wall in future fusion reactor, chemical compatibility between the tritium permeation barrier (TPB) and liquid lithium should be studied preferentially. The redox reaction is studied from the perspective of thermodynamics at different temperature, which indicates that metallic nitrides and rare earth oxides are predictable to provide higher stability in lithium corrosion situation. Then static compatibilities of Al2O3, AlN, SiC and Er2O3 ceramics with liquid lithium are tested. The surface analysis of TPB and lithium droplets shows that Al2O3 ceramic could react with molten Li, though it is considered as the preferred candidate of TPB for its easy fabrication, low cost, and self-repair ability. However, the other three ceramics did not show any reduction product peak, indicating the high stability with liquid lithium. These results provide meaningful compatibility with liquid Li and valuable reference in engineering design of structural and TPB materials for future fusion device.