AlN Ceramics Research Articles

Comparison of ballistic performances of Al2O3 and AlN ceramics

Finite element analysis of the impact of 7.62AP bullets on ceramic/aluminum bi-layer hybrids was carried out using AUTODYN hydrocode. Johnson–Cook material model was used for the metal and Johnson–Holmquist model was used for the ceramic. The models were validated with experiments conducted on Al2O3/Al 5083 and AlN/Al 5083 bi-layer composites. It has been observed that AlN ceramics has superior performance in defeating the bullet in comparison to Al2O3 ceramics. Deformation of the backing plate and the corresponding plastic strain energies are considered in this study for the parameters of performance. The results will enable the design of materials for developing an efficient structure for the protection against the impact of 7.62AP bullets at ordnance velocities.

Enhanced mechanical properties of the nanostructured AlN-graphene composites rapidly sintered by high-frequency induction heating

Despite of many attractive properties of AlN, the current concern about the AlN focuses on its low fracture toughness below the ductile-brittle transition temperature. To improve its mechanical properties, the approach commonly utilized has been the addition of a second phase to form composites and to make nanostructured materials. In this respect, graphene was evaluated as the reinforcing agent of AlN ceramics using novel sintering method (high-frequency induction heated sintering method). Highly dense nanostructured AlN and AlN-graphene composites were achieved within two min at 1600°C. The effect of graphene on the microstructures and the mechanical properties (hardness and fracture toughness) of AlN-graphene composites was investigated using FE-SEM and Vickers hardness tester.

Imaging and spectroscopy of secondary electrons from AlN and β-SiAlON ceramics using fountain detector

To clarify the bright contrast of insulating ceramics in secondary electron (SE) image taken using scanning electron microscopy (SEM), the low-pass secondary electron signals and images of conductive AlN and insulating β-SiAlON powders taken by fountain detector (FD) were inspected. It was found that the background component of β-SiAlON is originally strong. This component may come from the SE acceleration according to the charging. The low energy SEs of 5–20 eV were strong in AlN particles. According to the wide acceptance angle of FSED, we could not detect clear energy shift in SE spectra. These observations suggest that the bright contrast of insulating materials is not an intrinsic character of insulators but the SE3 contribution.

Terahertz characterization of Y2O3-added AlN ceramics

Terahertz optical and dielectric properties of AlN ceramics fabricated by hot pressed sintering are investigated by THz time-domain spectroscopy in the frequency range of 0.2–3.5THz. The measured properties of the pure AlN ceramic are compared with those of Y2O3-added AlN ceramic. Two prominent resonance modes, which are essentially responsible for the dielectric properties of the Y2O3-added AlN in terahertz regime, are characterized at ωTO1/(2π)=2.76THz (92cm−1) and ωTO2/(2π)=18.2THz (605cm−1) and are well described by the pseudo-harmonic oscillator model through theoretical fitting. The resonance ωTO1 at 2.76THz is proposed to be due to the formation of a YAG (Y3Al5O12) secondary phase in Y2O3-added AlN ceramic. From the experimental results, good correlation is observed between the prominent peak of YAG secondary phase at 2.76THz and thermal conductivity. Additionally, there is a high correlation between densification and refractive index of AlN ceramics fabricated by hot pressed sintering.

Microstructure, mechanical properties and creep of magnesium alloy Elektron21 reinforced with AlN nanoparticles by ultrasound-assisted stirring

Ceramic AlN nanoparticles were added to a melt of magnesium alloy Elektron21 assisted by mechanical stirring and sonication for deagglomeration. The produced nanocomposite was investigated to determine its metallography, hardness, physical and mechanical properties, and compression creep resistance. Creep tests at a constant temperature of 240°C and constant stresses of between 70 and 200MPa revealed an improvement in creep strength after the addition of 1wt% of AlN nanoparticles. Minimum creep rates were determined and stress exponents were calculated. In order to determine the rate-controlling deformation mechanisms, the concept of a threshold stress for particle strengthened alloys was applied. True stress exponents were calculated for the alloy and for the composite.

Self-Propagating High-Temperature Synthesis of Composite Nanopowder AlN-BN from Systems "Sodium Azide – Halides of Aluminum and Boron"

The composite powder of AlN-BN is of interest from the point of view of the use for sintering the AlN-BN composite ceramics. The ceramics compared with AlN ceramics would have better thermal shock resistance, less britlleness, better machinability, good heat conductivity and tribological properties. Possibility of obtaining a composite powder of AlN-BN with the application of process of self-propagating high-temperature synthesis (SHS) from powder mixtures of NaN3 and precursors of aluminum and boron was investigated. Halide salts AlF3, Na3AlF6, KBF4 and NH4BF4 were used as the precursors. It was shown that these mixtures are capable of burning and obtaining agglomerated composite powders from nanosized (50-170 nm) particles.

Electrical Properties of AlN and SiO2 Co-Modified Barium Titanate Lead-Free Piezoelectric Ceramics

Ba1-xAlxTi1-ySiyO3-xNx(BAxTS4N) piezoceramics were prepared by solid-state reaction sintering. The effects of AlN and SiO2 co-doping on the phase structure and electrical properties of BaTiO3 ceramics were investigated and analyzed. The ceramics adopt a tetragonal crystal symmetry accompanied by the appearance of secondary phases Ba2TiSi2O8 and Ba4Ti2O27. The results of temperature dependence of dielectric constant indicate that the Curie temperature of BAxTS4N ceramics is independent of doping content. The BAxTS4N ceramic for the composition of x = 3% obtains the optimum ferroelectric properties with the remnant polarization (2Pr) of 14.69 μC/cm2 and the maximum piezoelectric coefficient (d33) of 230 pC/N.

Analysis of Distribution of Temperature and Stresses During the Friction Metallisation of AlN Ceramics with Titanium

Analysis of Distribution of Temperature and Stresses During the Friction Metallisation of AlN Ceramics with Titanium

Densification of AlN ceramics by spark plasma sintering under 1550 °C

We fabricated AlN ceramics at sintering temperatures with a minimum of 1450°C using spark plasma sintering (SPS) with Y2O3–CaO–B or Y2O3–CaO–LaB6 sintering additives. The relative densities of the AlN ceramics sintered at 1500°C for 10min with a uniaxial pressure of 50MPa exceeded 98%, indicating sufficient densification. However, with decreasing the sintering temperature to 1450°C, the relative densities were decreased to ∼90%. An increase in the uniaxial pressure to 100MPa or the extension of sintering time to 30min were found to promote the densification of AlN ceramics. The relative densities of the AlN ceramics sintered reached 99% even at a sintering temperature of 1450°C. The samples had dense and fine textures with an average grain size of ∼1μm. The thermal conductivities of the samples varied with relative density between 30 and 80Wm−1K−1.

Morphology of thick film metallization on aluminum nitride ceramics and composition of interface layer

Surface metallization of aluminum nitride is performed by a reactively thick film method. A layer of copper film is deposited on the surface of AlN ceramics. The morphology and composition of the metalized copper layer on ceramic substrate were investigated via scanning electron microscope (SEM) and X-ray diffraction (XRD). The interface layer between copper film and AlN substrate was examined through SEM, XRD and EDS. Reaction phases, Cu2O and intermediate phases (CuAlO2 and CuAl2O4), were detected at the interface layer. Subsequently, the formation process of these phases at the interface layer was discussed. And consequently, the relation within reaction phases and bonding strength was observed. Reaction phases play a key role in increasing bonding strength. The effect of these phases such as Cu2O and intermediate phases (CuAlO2 and CuAl2O4) on the mechanical property was discussed. Results show that the presence of Cu2O particles are detrimental to bonding strength. However, spinel CuAl2O4 and isolated CuAlO2 laminates at the interface layer can increase the bonding strength. The experimental results suggest that this thick film method is a useful technology for various metal-ceramics bonding.

Joining of Y<sub>2</sub>O<sub>3</sub>-Doped Aluminum Nitride with High Density Graphite by Spark Plasma Sintering

High density graphite disks and aluminum nitride ceramics powders have been utilized to obtain joints by Spark Plasma Sintering technique. The joining was carried out in vacuum, at temperatures of 1700°C, 1800°C and 1900°C, under the pressure of 50 MPa with a constant dwelling time of 5 minutes The AlN ceramics to be joined were also synthesized by ceramic technology standard route by using AlN powders and 2.5 % wt.Y2O3 powders as sintering additive, which were added in order to increase densification rate and by thus, thermal conductivity. The joining of AlN/C was performed both without and with the aid of a ceramic powder composite AlN+Y2O3+C film, as interlayer. Besides the crystalline phases (AlN and C), the Al5Y3O12 compounds with a cubic crystallographic structure were identified by X-ray diffraction. The optical microscopy images revealed that all samples, both without and with film as interlayer, had strong joined areas, without any defects and discontinuities at interfaces. The Vickers microhardness and Young Modulus values measured by nanoindentation have shown that using of the film as intermediary layer was leading to the highest values of mechanical properties (HV = 8 – 23 GPa and E= 227-512 GPa) at the AlN/C joints interfaces.

Vibration Assisted Hot‐Press Sintering of AlN Ceramics

AlN ceramics is very difficult to be sintered due to its strong covalent bonds and low self‐diffusion. In this work, a new hot‐press sintering system assisted by vibration was used to densify AlN ceramics. During hot‐pressing process an additional vibration caused by pressure fluctuation is imposed on the sample. The well‐densified AlN ceramics had been obtained at 1750°C under 15 MPa, or at 1800°C under 10 MPa which were much milder than traditional hot‐press conditions and would be beneficial to prolong working life of the graphite mold. The samples made by this method have a bending strength about 320 MPa. After annealing, it shows a thermal conductivity of 217 W/mK.

Electrical and thermal properties of SiC–AlN ceramics without sintering additives

Ceramics of SiC–AlN, consisting of α-SiC (6H and/or 4H) and 2H SiC–AlN solid solution (2Hss), were fabricated by conventional hot-pressing in an Ar atmosphere without sintering additives. The electrical resistivities of the SiC–AlN ceramics were 1.9×106Ωcm for 2vol% AlN, 3.7×109Ωcm for 10vol% AlN, and 1.1×1010Ωcm for 35vol% AlN, at room temperature. There was an increasing trend in resistivity with increasing AlN content which was most likely due to increased Al impurities at Si site acting as deep acceptors for trapping nitrogen-derived carriers. The thermal conductivity of the SiC–AlN ceramics showed a decreasing trend: 104.1W/mK at 2vol% AlN, 49.8W/mK at 10vol% AlN, and 35.1W/mK at 35vol% AlN content, due to increasing 2Hss content in the ceramics. There was a trade-off in improving both the thermal conductivity and electrical resistivity in SiC–AlN ceramics.

Effects of Y2O3 and Al2O3 sintering additives on the hot pressing behavior of AlN ceramics

Abstract AlN plates were fabricated by hot pressing AlN-Y2O3 and AlN-Y2O3-Al2O3 powder mixtures at 1700–1900 °C using yttria and alumina (Al2O3 particle sizes of 3 and 10 μm) powders as sintering additives. The physical and mechanical properties of the hot-pressed AlN ceramics were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The density, thermal conductivity, transverse rupture strength, and grain size were measured. The thermal conductivity of the hot–pressed AlN ceramics ranged between 83–92.7 W × (m × K)−1 and decreased with the addition of Al2O3, whereas the mechanical properties improved with the addition of Al2O3, which is known to have a beneficial effect of grain size refinement. The measured density values of 3.31 to 3.34 g × cm−3 were slightly higher than the theoretical density of 3.26 g × cm−3. This result was attributed to the use of a carbon mold under N2 atmosphere, which effectively removed oxygen atoms from the yttrium-aluminate phase that is expected to form at 1100 °C.

Sintering behavior of aluminum nitride ceramics with MgO–CaO–Al2O3–SiO2 glass additive

In this study, MgO–CaO–Al2O3–SiO2 (MCAS) glass oxide fabricated by a conventional melting process was used as a sintering additive to prepare dense AlN ceramics at lower temperatures. AlN specimen with 5wt.% MCAS glass oxide was produced by sintering in a dilatometer and a conventional furnace. The sintering behavior of the glass-doped AlN was investigated by means of dilatometric analysis, X-ray diffraction analysis, and field-emission scanning electron microscopy microstructural observation. Results revealed that the melted glass phase had a significant effect on the densification of the AlN ceramics by a liquid-phase sintering. This led to a reduction in the sintering temperature by 200°C from the 1800°C required for conventional AlN–rare earth or –alkaline earth oxide system.

Effect of Rare Earth Oxides on Electrical Properties of Spark Plasma Sintered AlN Ceramics

AlN ceramics doped with rare earth oxide (Sm2O3, Y2O3) were prepared by Spark Plasma Sintering (SPS) in nitrogen atmosphere using AlN powder as raw materials. The effect of rare earth oxide on the phase composition, microstructure and electrical resistivity of AlN ceramics were investigated. The results showed that those sintering additives promoted densification through liquid-phase sintering. AlN ceramics with electrical resistivity values of 1×10–1×10 Ω•cm were obtained by precipitating rare earth aluminates as a conducting pathway at the grain boundaries. With the increasing of Sm2O3 doping, the phase of samarium aluminate gradually changed from Sm4Al2O9 to SmAlO3 which had a higher resistivity than Sm4Al2O9. 3wt% Sm2O3 doped AlN ceramics had the minimum resistivity of 1.32×10 Ω•cm with the relative density of 99.33%. The yttrium aluminate phase gradually changed from Y3Al5O12 to Y4Al2O9 with the increase of Y2O3-doping. The electrical resistivity of different yttrium aluminates were about 1× 10 Ω•cm. Y2O3doped AlN sample with over 3wt% Y2O3 had an electrical resistivity about 1×10 Ω•cm. 4wt% Y2O3 doped AlN ceramic had the electrical resistivity of 1.60×10 Ω•cm with the maximum relative density of 99.08%. 268 无 机 材 料 学 报 第 30卷

Effect of Ga2O3 addition on the properties of Y2O3-doped AlN ceramics

Effect Ga2O3 addition on the densification and properties of Y2O3-doped AlN ceramics was investigated under the constraint of total sintering additives (Y2O3 and Ga2O3) of 4.5 wt%. Ga was detected in the AlN grain as well as the grain boundary phases. YAlO3 and Y4Al2O9 were observed as the secondary crystalline phases in all of the investigated compositions. As the substitution of Ga2O3 for Y2O3 increased, the quantity of the Y4Al2O9 phase decreased while that of YAlO3 was more or less similar. Neither additional secondary phases was identified, nor was the sinterability inhibited by the Ga2O3 addition; the linear shrinkage and apparent density were above 20 percent and 3.34-3.37 g/cm3, respectively. However, the optical reflectance and the elastic modulus generally decreased whereas the Poisson ratio increased significantly. The dielectric constant and the loss tangent of 4.0Y2O3-0.5Ga2O3-95.5Y2O3 at the resonant frequency of 8.22 GHz were 8.63 and 0.003, respectively.

Sintering behavior, microstructure, and thermal conductivity of dense AlN ceramics processed by spark plasma sintering with Y2O3–CaO–B additives

We fabricated dense aluminum nitride ceramics by spark plasma sintering (SPS) with Y2O3–CaO–B sintering additives and evaluated their microstructures and thermal conductivities. Samples were perfectly densified after 5min of SPS at 1650 or 1600°C, or 10min at 1550°C and had fine microstructures. Thermal conductivities of the samples were 80–90Wm−1K−1 and tended to increase with increasing sintering temperature.

The Relative Density and Electrical Properties of AlN with Additives Depending on the Composition of the Mixture and the Temperature of hot Pressing

The paper examines the effect of additions (TiO 2 , TiH 2 , TiN, and TiC up to 4 vol.%) and isothermal holding temperature on the dielectric properties of hot-pressed AlN ceramics in a wide frequency range. It is established that the service characteristics of AlN–TiO 2 ceramics show a parabolic dependence on titanium oxide additions: optimal porosity (0.1%), permittivity (9.7), and dielectric loss tangent (1.3  10 –3 ) are reached with 0.5 to 2 vol.%. TiO 2 . Additions of TiH 2 promote the formation of metallic films, mainly oriented along the pressing direction. The deviation of sintering temperature from the optimal value increases conduction-induced losses since structures with more defects and conducting phases form. It is shown that broadband (10 3 –10 7 Hz) dielectric spectroscopy can be used to monitor the composite’s microstructure: the frequency of migration polarization dispersion provided information on the effective thickness of the conducting channel and the slope () allowed the lattice and jump responses to be differentiated.

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.