Sintering Of AlN Research Articles

Experimental phase diagram study for the AlN-Al2O3-CaO system and application of thermodynamic calculations to AlN sintering process

In this study, the phase diagram of the AlN-Al2O3-CaO system at 1600 °C was experimentally investigated using the classical equilibration and quenching technique. The liquidus of CaO and AlN in the AlN-Al2O3-CaO system at 1600 °C was accurately determined for the first time using the mixture of pre-melted CaO-Al2O3 glass, CaO and AlN powder as the starting materials under the N2-5 vol% H2 atmosphere. The accuracy of FactSage FTOXCN database was confirmed by the comparison of the calculated diagram with the present experimental results. Thermodynamic analysis of the AlN sintering process with CaO additive was carried out using the FTOXCN database to understand the influence of CaO additive to densification of AlN.

Self-Propagating High-Temperature Synthesis and Sintering of AlN–Y2O3–Al2O3 Composites

Self-Propagating High-Temperature Synthesis and Sintering of AlN–Y2O3–Al2O3 Composites

Annealing of AlN guided by estimation on oxygen content

In this paper, an estimation method revealing relationship between residual oxygen content and Y2O3 additive amount used in AlN sintering was presented. It is revealed that AlN sample added with 3 wt% Y2O3 showed better performance than the others while the oxygen content is 0.765 wt% in the initial powders. Nevertheless, short-time sintering does not make AlN to obtain ultra-high performance consistent with the estimation. By two-step sintering regimes, the distribution of second phase in AlN ceramic was changed from continuous distribution to island-like distribution significantly. The bending strength increases first and then decreases, reaching a higher value of 418 MPa after 2-h annealing. The residual oxygen content continues to decrease as more liquid second phase precipitated with long-time annealing. After reaching a state with lower residual oxygen content than the estimation, high-performance AlN is prepared with thermal conductivity of 230 W/m·K and lower bending strength of 358 MPa.

High-temperature plasticity of SiC sintered with Lu2O3-AlN additives

Abstract SiC ceramics containing Lu2O3–AlN sintering additives were gas-pressure sintered, and creep testing in four-point bending geometry was performed at temperatures between 1200 °C and 1500 °C in air. Compared with the more conventional additive system Y2O3 –AlN, the creep resistance of the Lu2O3-containing ceramics is higher. In particular, strain rates down to 6 ⨯ 10–10 s–1 were measured after 60 h at both 1300 °C/300 MPa and 1400 °C/100 MPa; in no case was steady-state creep reached. Annealing the ceramics at 1950 °C for extended periods of time led to significant changes in the amount and distribution of intergranular phases. However, in contrast to earlier work using Y2O3 –AlN additives, the influence of annealing on the creep resistance was negligible. Compliance testing as a function of temperature was conducted in order to monitor the softening behavior of materials with different additive compositions. In favorable cases, the drop in stiffness was only 12% up to a test temperature of 1450 °C.

Effects of AlN Content and Sintering Atmospheres on the Thermal Conductivity of Hot-Pressed SiC Ceramics

In pursuit of high thermal conductivity of SiC ceramics, this article reports the effects of AlN and sintering atmospheres on the thermal properties and microstructure evolution of SiC ceramics. Dense SiC ceramics with different contents of AlN additive were fabricated through hot-press sintering at 1950°C in Ar/N2 atmosphere under a pressure of 40 MPa for 3 h. The results showed that the thermal conductivity of samples without AlN addition was 53.5 W.m−1.K−1 when sintered in Ar and 50.3 W.m−1.K−1 when sintered in N2. The SiC ceramics with 4 wt% AlN sintered in Ar showed abnormally large grains and exhibited the highest thermal conductivity of 121.7 W.m−1.K−1 among all the samples. The sample with 2 wt% AlN sintered in N2 exhibited a thermal conductivity of 108.6 W.m−1.K−1; the thermal conductivity deceased with the increasing AlN content afterwards. Such a decreasing trend of thermal conductivity was attributed to the ascending 2Hss content and smaller grains of ceramics. There existed an optimal content of AlN and a proper sintering atmosphere for perfecting the microstructure and the thermal conductivity of SiC ceramics.

Про механізм формування зеренної структури і границь розділу в самоармованому нітриді алюмінію

Results of an investigation by scanning electron microscopy methods of the microstructure of self-reinforced aluminum nitride obtained on the basis of aluminum nitride powder and containing 3 mass.% oxygen by plasma-chemical synthesis in the temperature range 1700-2000 °C are presented. Initial aluminum nitride was represented by the wurtzite (2H) phase. Samples were obtained by free sintering in a nitrogen atmosphere. It was established that, during sintering of AlN in the indicated temperature range, three microstructural types of the material and six types of interfaces (three types of intergranular and three types of intragranular ones) formed. The features of the microstructure of the materials are fully determined by the development of intergranular crystal-oriented polytype transitions of 2H AlNmultilayer polytypes (MP) in sintering. The sequence of successive structural transformations that determine the development of polytype transitions was established. 1. Formation of initial 2H AlN grains of the solid solution 2H AlN-O. The substitution of nitrogen by oxygen takes place. 2. The development of isomorphous delamination of the solid solution in every grain with the precipitation of an interlayer enriched in oxygen. 3. In interlayers, polytypes consisting of a series of polytypes with different number of layers (MP) form. Such an interlayer has developed base surfaces and propagates from one boundary to another in the grain, which determines the formation of a special structural state of fragments of boundaries, that border interlayers in the direction <hk0>. 4. The high mobility of the indicated fragments of boundaries determines their break-off from the common boundary and formation of a grain nucleus of anisometric (plate-like) shape. This process begins already at a sintering temperature Tsint. = 1800 °C, and, at Tsint. = 2000 °C, polycrystals practically entirely consist of grains of plate-like shape. In this case, with increase in the sintering temperature, the aspect ratio (the length-to-width ratio) of such grains rises. Keywords: aluminum nitride, polytype transformations, grains, microstructure, boundaries, self-reinforcement.

Mechanical and Thermal Conductivity Properties of Yttrium Nitrate Added AlN Sintering Body

Mechanical and Thermal Conductivity Properties of Yttrium Nitrate Added AlN Sintering Body

Study on Grain Fragmentation of Aluminum Nitride Microcrystal under High Pressure

Grain fragmentation of AlN micron powder under high pressure(2.0–6.0 GPa) was studied on cubic high pressure apparatus.Phase composition,grain size and microstructure of the samples were characterized by X-ray diffraction(XRD) and scanning electron microscope(SEM).The results show that relative density of AlN microcrystal increases and open pore decreases with the rise of high pressure."Cold-sintering" occurs in the AlN sample pressed at 6.0 GPa because its relative density reaches 88.72%.Obvious pressure-induced grain fragmentation is observed during the compacting of AlN powder under high pressure.The average particle diameter drops to 1.47 μm from2.10 μm when high pressure is increased to 6.0 GPa from atmospheric pressure.Pressure-induced grain fragmentation can improve the surface free energy of AlN powder and increase sintering driving force.Meanwhile,it can activate sintering to improve AlN sintering rate because of the presence of defects(such as dislocation,crack,etc.) caused by high pressure.

Fluorescence of Er^3+:AlN polycrystalline ceramic

We report what we believe to be the first preparation and optical spectroscopy of Er3+ doped into bulk AlN ceramic. The material was prepared via hot press sintering of AlN with Er2O3 and [NH4][ErF4], which yielded fully dense, translucent, hexagonal AlN. The Er3+ concentration is a small fraction of a percent, and resides in multiple sites, with one type of center dominant. A number of the energy levels of Er3+ are identified for this center. The temperature dependent fluorescence lifetime is probably radiative, and on that basis the stimulated emission and absorption cross section spectra are inferred for the 4I13/2 ↔ 4I15/2 transitions.

Joining of Graphite and Aluminum Nitride Ceramics by using CBC Interlayer

Graphite material and AlN ceramic were joined successfully using an interlayer tape of Ceramic Bonded Carbon (CBC) by spark plasma sintering. The CBC tape was prepared from the slurry of graphite and AlN powders by tape casting method. The CBC interlayer sintered consists of graphite particles bonded with the three-dimensional network of AlN thin layer. The graphite and AlN joints were analyzed by SEM observation, XRD phase identification, and EDX elements distribution. There were no cracks and voids at interfaces of AlN ceramic layer, CBC interlayer and graphite materials. It was suggested that the joining was performed mainly by sintering of AlN through the AlN ceramics layer and the CBC interlayer, and physical bonding between the CBC layer and graphite. The thickness of AlN layer and the CBC interlayer was about 3 % and 4 %, respectively, against the entire thickness of graphite and AlN joint. Nevertheless, the joints showed much higher strength of 82 MPa than the graphite's strength of 41 MPa. Graphite-AlN joints would be expected to use as light, insulative and high heat-conductive substrates or components.

Roles of rare earth oxide additives in millimeter-wave sintering of AlN

Roles of rare earth oxide (RE 2O 3) additives in millimeter-wave(MM) sintering of AlN were investigated from the standpoints of phase diagram, heating characteristics of rare earth oxides, and morphology of intergranular oxide phase. In the millimeter-wave sintering of AlN, densification temperature decreased with the decrease of the ionic radius of rare earth ion and was closely related with the eutectic temperature in the RE 2O 3-Al 2O 3 binary system. The lowest densification temperature in the millimeter-wave sintering of AlN with Yb 2O 3 additive was attributed to the largest heating rate of Yb 2O 3·Al 2O 3 binary oxide under millimeter-wave radiation. Furthermore, the lowest densification temperature could be attained while selecting the Yb 2O 3 content so as to form the intergranular phase with the eutectic composition in the Yb 2O 3-Al 2O 3 binary system. The result showed good agreement with the above mentioned during the sintering of Si 3N 4 with Yb 2O 3-Al 2O 3 additive. From TEM observation, it was verified that film-like intergranular oxide phase formed under millimeter-wave radiation was favorable for attaining high thermal conductivity in the Yb 2O 3 added AlNs.

Intelligent Materials Synthesis Based on Millimeter-Wave Heating and SPS Methods

Characteristics of heating processing based on millimeter-wave or pulsed high current are discussed from the standpoint of the interaction between electromagnetic energy and solid materials. Capabilities of the electromagnetic processing are indicated by exemplifying several successful results such as millimeter-wave sintering of AlN, millimeter-wave post-annealing of aerosol-deposited PZT films and synthesis of single-phase nano-structured anatase by SPS (or pulsed high current heating). It is shown in these examples that well-characterized properties such as high thermal conductivity and preferential orientation are created by the inherent effect due to the electromagnetic field, which is called microwave or SPS effect in millimeter-wave or SPS processing.

High-temperature plasticity of SiC sintered with Lu2O3-AlN additives

Abstract SiC ceramics containing Lu2O3–AlN sintering additives were gas-pressure sintered, and creep testing in four-point bending geometry was performed at temperatures between 1200°C and 1500°C in air. Compared with the more conventional additive system Y2O3–AlN, the creep resistance of the Lu2O3-containing ceramics is higher. In particular, strain rates down to 610−10s−1 were measured after 60h at both 1300°C/300MPa and 1400°C/100MPa; in no case was steady-state creep reached. Annealing the ceramics at 1950°C for extended periods of time led to significant changes in the amount and distribution of intergranular phases. However, in contrast to earlier work using Y2O3–AlN additives, the influence of annealing on the creep resistance was negligible. Compliance testing as a function of temperature was conducted in order to monitor the softening behavior of materials with different additive compositions. In favorable cases, the drop in stiffness was only 12% up to a test temperature of 1450°C.

Low temperature sintering of AlN powdersynthesized by direct-nitridation

Low temperature sintering of AlN powdersynthesized by direct-nitridation

Low-temperature sintering behavior of the nano-sized AlN powder achieved by super-fine grinding mill with Y 2O 3 and CaO additives

For low-temperature sintering, mixtures of AlN powder doped with 3.53 mass% Y 2O 3 and 0–2.0 mass% CaO as sintering additives were pulverized and dispersed in a vertical super-fine grinding mill with very small ZrO 2 beads. The particle sizes achieved ranged between 50 and 100 nm after grinding for 90 min. The mixtures were then fired at 1000–1500 °C for 0–6 h under nitrogen gas pressure of 0.1 MPa. All nano-sized powders showed pronounced densification from 1300 °C as revealed by shrinkage measurement. The larger amounts of sintering additives enhanced AlN sintering at lower temperatures. Densified AlN ceramics with very fine and uniform grains of 0.3–0.4 μm were obtained at a firing temperature of 1500 °C for 6 h.

Migration of liquid phase in low temperature sintering of AlN

In the process of low-temperature sintering of AlN ceramics, the reaction of the sintering aids YF3 and CaF2 with superficial Al2O3, inherently contained in AlN lattice, results in formation of liquid phase. Nevertheless, the uniformly dispersed liquid phase is prone to migrate from the bulk to the surface of the samples, opposing densification. The analysis of the experimental results indicates that fresh liquid phase can continuously arrive from the bulk to the surface due to chemical reactions and crystallization which occur at the surface as well as wettalibilty and capillarity phenomena. The surface is depleted of liquid phase since the latter is consumed due crystallization and carbothermal reduction reactions with the elements of the atmosphere of the furnace N2 and C, resulting in formation of a dense layer of crystals of Al2Y4O9, CaYAl3O7 and Y2O3, grown perpendicularly to the surface. The chemical and structural features of this newly formed crystalline surface layer generate a significant difference of the wetting regimes and the capillary forces between the surface and the bulk, favouring pumping of the liquid from the bulk to the surface.

Preparation and characterization of silica-doped aluminum nitride–aluminum nitride polytypoid composites

Fine-grained AlN–AlN polytypoid composites doped with SiO 2 were prepared in situ by pressureless sintering of AlN–Al 2O 3 mixtures (3.7 and 10.5 mol% Al 2O 3) at 1870 °C using Y 2O 3 as sintering aid. SiO 2-doping (1 wt.%) was used to stabilize the AlN polytypoids and reduce the average grain size of the polytypoids. The composites consisted of elongated SiO 2 containing AlN polytypoids grains (30 and 61 vol.%) embedded in equi-axed AlN matrix. Vickers hardness and bending strength increased with polytypoid composite formation. The Young's modulus showed a maximum at 3.7 mol% Al 2O 3 (30 vol.% polytypoids). The fracture toughness increased up to 4.0 ± 0.3 MPa m 1/2 for the composites. The toughening was caused by crack deflection due to residual strain both in the polytypoids and the AlN matrix.

Phase Relations and Microstructural Development of Aluminum Nitride–Aluminum Nitride Polytypoid Composites in the Aluminum Nitride–Alumina–Yttria System

AlN–AlN polytypoid composite materials were prepared in situ using pressureless sintering of AlN–Al2O3 mixtures (3.7–16.6 mol% Al2O3) using Y2O3 (1.4–1.5 wt%) as a sintering additive. Materials fired at 1950°C consisted of elongated grains of AlN polytypoids embedded in equiaxed AlN grains. The Al2O3 content in the polytypoids varied systematically with the overall Al2O3 content, but equilibrium phase composition was not established because of slow nucleation rate and rapid grain growth of the polytypoid grains. The polytypoids, 24H and 39R, previously not reported, were identified using HRTEM. Solid solution of Y2O3 in the polytypoids was demonstrated, and Y2O3 was shown to influence the stability of the AlN polytypoids. The present phase observations were summarized in a phase diagram for a binary section in the ternary system AlN–Al2O3–Y2O3 parallel to the AlN–Al2O3 join. Fracture toughness estimated from indentation measurements gave no evidence for a strengthening mechanism due to the elongated polytypoids.

Hot-pressed AlN-Cu metal matrix composites and their thermal properties

AlN-Cu metal matrix composites containing AlN volume fractions between 0.1 and 0.5 were fabricated firstly by liquid phase sintering of AlN using Y2O3 as a sintering aid and then by hot pressing the powder mixtures of sintered AlN and Cu at 1050°C with a pressure of 40 MPa under flowing nitrogen. With Y2O3 additions of 1.5 to 10 wt%, the densification of AlN could be achieved by liquid phase sintering at 1900°C for 3 h and subsequently slow cooling. The sintered AlN showed a maximum thermal conductivity of 166 W/m/K at a Y2O3 level of 6 wt%. Dense AlN-Cu composites with AlN contents up to 40 vol% were achieved by hot pressing. The thermal conductivity and the coefficient of the thermal expansion (CTE) of the composites decreased with increasing AlN volume fractions, giving typical values of 235 W/m/K and 12.6 × 10−6/K at an AlN content of 40 vol%.

Sintering of AlN Using CaO‐Al2O3 as a Sintering Additive: Chemistry and Microstructural Development

The densification of aluminum nitride using Ca12Al14O33 as a sintering aid has been studied with emphasis on the effect of using coarse or fine powder, the amount of sintering aid, the sintering temperature, and embedding. Both crystalline and amorphous grain boundary phases were observed. Significant weight losses were observed for coarse‐grained samples, and if suitable embedding was not used. Porous and coarse‐grained ceramics with high contiguity and minor amounts of secondary phases were obtained by enhanced evaporation while dense ceramics were obtained limiting the evaporation. High weight losses in the graphite environment resulted in formation of a dense AlN surface layer.