Gas Pressure Sintering Research Articles

Effect of TiO2 addition on the microstructures, mechanical and dielectric properties of porous Si3N4-based ceramics

ABSTRACTPorous Si3N4-based ceramics with different TiO2 contents were prepared by gas pressure sintering method. The effects of TiO2 addition ranging from 0 to 25 wt-% on the phase compositions, microstructures, mechanical performance and dielectric properties were investigated. The addition of TiO2 significantly promoted the density which increased from 1.64 to about 2.3 g cm−3. The mechanical properties of porous Si3N4-based ceramics with TiO2 addition decreased first and then increased with the increase of TiO2 content, and the flexural strength and elastic modulus are more than 167.4 MPa and 72.8 GPa, respectively, which were higher than that of the Si3N4 ceramic without TiO2 addition. With the increase of TiO2 content, both the dielectric constant and dielectric loss increased, and the dielectric constant enhanced obviously. These results suggested that the TiO2 was beneficial for the improvement of mechanical properties and dielectric constant of porous Si3N4-based ceramics.

Effect of magnesium titanate content on microstructures, mechanical performances and dielectric properties of Si3N4-based composite ceramics

Abstract Silicon nitride-based composite ceramics with different contents of magnesium titanate have been fabricated via gas pressure sintering method. The phase compositions, microstructure, mechanical performances and dielectric properties of the composite ceramics were investigated. The density of the Si 3 N 4 -based composite ceramics firstly increased with additive of magnesium titanate powder up to 5 wt% and then gently decreased, and the mechanical properties firstly increased and then declined. Besides, the dielectric constant and dielectric loss increased with the increase of magnesium titanate contents. For the Si 3 N 4 -based composite ceramics with 5 wt% magnesium titanate powders, the flexural strength, elastic modulus, dielectric constant and dielectric loss reached 451 MPa, 274 GPa, 7.65, 0.0056, respectively. These results suggested that the magnesium titanate was beneficial for the improvement of mechanical performances and dielectric constant of Si 3 N 4 -based composite ceramics.

Effect of addition of micron-sized TiC particles on mechanical properties of Si3N4 matrix composites

Abstract The effect of micron-sized TiC particles on mechanical properties of Si 3 N 4 matrix composites was investigated. Si 3 N 4 TiC composites were fabricated by using powder metallurgy and gas pressure sintering (GPS) methods. In order to achieve high mechanical properties, the experimental data were carried out with five different Si 3 N 4 TiC contents. Hardness, fracture toughness and relative density of Si 3 N 4 TiC composites were characterized. The result indicated that the highest relative density (98.0%) and fracture toughness (K IC = 8.4 MPa m 1/2 ) were measured for monolithic silicon nitride and 5 wt% TiC, respectively. Qualitative and quantitative phase compositions were analyzed by X-ray diffraction (XRD). The experimental analyses indicated that the TiC phase reacted with Si 3 N 4 during the liquid phase sintering, resulting in formation of TiC 0.3 N 0.7 . The phase transition, crystal growth, microstructure and mechanical properties of composites were investigated by scanning electron microscopy (SEM). Due to the inhibition of the grain growth, the aspect ratio exhibited a higher value from SNT0 (aspect ratio = 4.54) to SNT5 (aspect ratio = 5.60). The toughening mechanism of Si 3 N 4 TiC composites was attributed to crack deflection and crack impedance by periodic compressive stress in the Si 3 N 4 matrix. However, the increase of the TiC content easily led to excessive connection microcrack, which would not be beneficial for the strength.

Mechanical, Electrical and Thermal Properties of α/β SiAlON-SiC Composites Fabricated by Gas Pressure Sintering Method

α/β SiAlON-SiC composites were produced by coating SiAlON based spray dried granules with varying amounts of SiC (0-10 vol % SiC) following gas pressure sintering technique. A two step sintering schedule between 1940-1990°C under 10 MPa N 2 gas pressure was designed in order to achieve high densification rate. All the composites and the reference SiAlON material were densified up to 99 %. α/β SiAlON - 10 vol. % SiC composite had the highest hardness value (Hv 10 : 16.33 GPa) due to the formation of α-SiAlON phases promoted with the increasing SiC content. No significant changes in the fracture toughness values of composites were observed. 3D segregated network of SiC particles along granules to establish electrical and thermal conductivity in the composites was successfully achieved with incorporation of 10 vol. % SiC. α/β SiAlON - 10 vol. % SiC composite exhibited the semiconductor characteristic and high thermal conductivity with the electrical resistivity of 10 2 Ω.m and thermal diffusivity of ~11 mm 2 /s measured at room temperature.

ChemInform Abstract: Green Light‐Excitable Ce‐Doped Nitridomagnesoaluminate Sr[Mg2Al2N4] Phosphor for White Light‐Emitting Diodes.

AbstractPolycrystalline samples of Sr1‐x[Mg2Al2N4]:xCe3+ (x = 0.005—0.08) are prepared by gas pressure sintering of Sr3N2, AlN, Mg3N2, and CeN (Mo crucible, 0.9 MPa N2, 1450 °C, 4 h).

The effect of z value on intergranular phase crystallization of αı/βı-SiAlON-TiN composites

Relationship between z value and intergranular phase (IGP) chemistry has not been completely developed to date for SiAlON based materials. In this study, the effect of z value of βı-SiAlON on crystallization and coalescence of IGP was investigated in αı/βı-SiAlON doped with Yb2O3-Sm2O3-CaO sintering additives and reinforced with TiN particles. αı/βı-SiAlON-TiN composites with compositions yielding different z values (0.3, 0.5, 0.7, 0.9 and 1.1) were prepared and sintered with gas pressure sintering route. Post sintering heat treatment was applied to sintered samples in order to enhance crystallization and coalescence of IGP. z value was proved an effective parameter on the intergranular phase chemistry and crystallization which are further influenced by post sintering heat treatment. No noticeable influence of heat treatment was observed on mechanical properties.

Highly improved reliability of amber light emitting diode with Ca -α-SiAlON phosphor in glass formed by gas pressure sintering for automotive applications.

Phosphor in glass (PiG) with 40 wt% of Ca-α-SiAlON phosphor and 60 wt% of Pb-free silicate glass was synthesized and mounted on a high-power blue LED to make an amber LED for automotive applications. Gas pressure sintering was applied after the conventional sintering process was used to achieve fully dense PiG plates. Changes in photoluminescence spectra and color coordination were inspected by varying the thickness of the plates that were mounted after optical polishing and machining. A trade-off between luminous flux and color purity was observed. The commercial feasibility of amber PiG packaged LED, which can satisfy international regulations for automotive components, was successfully demonstrated by examining the practical reliability under 85% humidity at an 85°C condition.

Effect of BN content on the mechanical and dielectric properties of porous BNp/Si3N4 ceramics

Porous BNp/Si3N4 ceramics were prepared by gas pressure sintering method as a candidate wave-transparent material. The effects of BN particle additions ranging from 0wt% to 15wt% on the phase compositions, microstructures, mechanical performance and dielectric properties were investigated. With the increase of BN content, the apparent porosity of the BN/Si3N4 ceramics increased greatly from 2.3% to 21.2%. Although the mechanical properties were partially weaken, the dielectric performance of the porous BN/Si3N4 was largely enhanced at the same time. For the 10% BN/Si3N4 porous composite ceramics, the permittivity and dielectric loss was 33.5% and 62.2% lower than that of the porous Si3N4 without BN content, respectively. Specifically, when 10% BN particles were added, the porous BNp/Si3N4 has a porosity of 20.5% and the permittivity and dielectric loss were 5.14 and 0.0085 respectively with bending strength of 190.1MPa and fracture toughness of 4.16MPam1/2, which can meet the required value of wave transparent materials.

Defects and luminescence control of AlN ceramic by Si-doping

Aluminum nitride (AlN) ceramic is a wide-gap semiconductor, which has a variety of native defects, in particular oxygen-related complexes. We have doped Si into AlN by gas pressure sintering (GPS) and investigated the influence of Si on the defects and luminescence of AlN. It was found that O impurities were reduced until 1.6% Si-doping, and then Si is incorporated into AlN by further doping. These mechanisms and related electrical and optical changes have been discussed.

Influence of YAG-Based Composite Sintering Aids on Gas Pressure Sintered BN<sub>w</sub>/Si<sub>3</sub>N<sub>4</sub> Composites

BNw/Si3N4 composites were fabricated by gas pressure sintering process using α-Si3N4 powder and self-made BN whisker as principal raw materials. The effects of different sintering additives such as YAG, MgO+YAG and RE2O3+YAG(RE=La, Sm, Dy) on the apparent porosity, microstructure, phase composition, mechanical performance and dielectric properties of the composites were analysed. The results revealed that composite sintering aids at certain ratio (MgO/RE2O3:YAG=1:5) was more conducive to ceramic densification than single YAG additive. The BNw/Si3N4 sintered with La2O3+YAG acquired the highest density and the maximum bending strength (272.46 MPa) and fracture toughness(4.9 MPa·m1/2). It was speculated that ceramic densification process was related to formation of different eutectic liquid phases with different viscosity. Additionally, when the apparent porosity of BNw/Si3N4 composites was 20% or less, dielectric properties of the material were mainly influenced by the porosity and the value of the permittivity and dielectric loss decreased with the increase of ceramic porosity.

Improvement of Luminescence Properties and Particle Growth of AlN Phosphors By Si-Doping

Aluminum Nitride (AlN) has been regarded as a promising material for ceramic phosphors. Recently, we have developed a high efficiency blue AlN phosphor by co-doping with Si and Eu [1-2]. It has been proven that particle have grown rapidly with the highest luminescent intensity on the proper Si doping. Now, it is important to understand the role of Si on AlN particle growth and luminescence enhancement. For this purpose, AlN powders doped with Si from 0 to 4 at. % were prepared using gas-pressure sintering (GPS) furnace at 1950°C in N2 atmosphere. Firstly, particle size, phases and luminescence spectra of Si-doped AlN powder were investigated using SEM, XRD and CL. Secondly, the defect were evaluated by EPR to identify the defects transitions. Finally, the particle growth mechanism was clarified by cross-sectional CL-EDS. From 0.4 to 2.0 % Si, particle growth of AlN powders as well as ultraviolet emissions at 290 and 350 nm are enhanced. The 350 nm emission may be attributed to native AlN defects such as vacancy-oxygen complexes. The shoulder at 290 nm and 450 nm can be ascribed to the different states of Si related defects in AlN by EPR analysis. Cross sectional CL images show that the 1.6 % Si-doped AlN yields 280 nm emission at the surfaces of AlN particles (Fig. 1). Many particles are connected to each other with these bright surfaces. It suggests that the Si-rich surface may act as the binder of AlN particles and enhance the Ostwald growth.

A new way of fabricating Si3N4 ceramics by aqueous tape casting and gas pressure sintering

Silicon nitride (Si3N4) ceramic was fabricated by aqueous tape casting and gas pressure sintering at 1850 °C for 60 min under a nitrogen atmosphere of 1.5 MPa. Effects of slurry properties on powder compaction and densification of the green tapes were investigated. The slurry of the Si3N4 powder with polyacrylic acid as dispersant showed shear-thinning rheological behavior. Casting of the slurry on glass plate produced flexible green tape with uniform powder distribution after dried in open air. The green tapes had relative high density, small pore size and homogeneous pore size distribution. Due to high compaction of the ceramic particles and homogeneous distribution of the pores in the green tapes, the sintered silicon nitride ceramics reached a relative density up to ∼99%. Elongated growth of β-Si3N4 grains with a small bimodal distribution, gave good flexural strength, elastic modulus and fracture toughness up to 764 MPa, 285 GPa and 7.05 MPa m1/2, respectively, to the silicon nitride (Si3N4) ceramic.

Injection molding of ultra-fine Si3N4 powder for gas-pressure sintering

The ceramic injection molding technique was used in the gas-pressure sintering of ultra-fine Si3N4 powder. The feedstock’s flowability, debinding rate, defect evolution, and microstructural evolution during production were explored. The results show that the solid volume loading of less than 50vol% and the surfactant mass fraction of 6wt% result in a perfect flowability of feedstock; this feedstock is suitable for injection molding. When the debinding time is 8 h at 40°C, approximately 50% of the wax can be solvent debinded. Defects detected during the preparation are traced to improper injection parameters, mold design, debinding parameters, residual stress, or inhomogeneous composition distribution in the green body. The bulk density, Vickers hardness, and fracture toughness of the gas-pressure-sintered Si3N4 ceramic reach 3.2 g/cm3, 16.5 GPa, and 7.2 MPa·m1/2, respectively.

Strong Energy-Transfer-Induced Enhancement of Luminescence Efficiency of Eu(2+)- and Mn(2+)-Codoped Gamma-AlON for Near-UV-LED-Pumped Solid State Lighting.

A series of Eu(2+)- and Mn(2+)-codoped γ-AlON (Al1.7O2.1N0.3) phosphors was synthesized at 1800 °C under 0.5 MPa N2 by using the gas-pressure sintering method (GPS). Eu(2+) and Mn(2+) ions were proved to enter into γ-AlON host lattice by means of XRD, CL, and EDS measurements. Under 365 nm excitation, two emission peaks located at 472 and 517 nm, resulting from 4f(6)5d(1) → 4f(7) and (4)T1(4G) → (6)A1 electron transitions of Eu(2+) and Mn(2+), respectively, can be observed. Energy transfer from Eu(2+) to Mn(2+) was evidenced by directly observing appreciable overlap between the excitation spectrum of Mn(2+) and the emission spectrum of Eu(2+) as well as by the decreased decay time of Eu(2+) with increasing Mn(2+) concentration. The critical energy-transfer distance between Eu(2+) and Mn(2+) and the energy-transfer efficiency were also calculated. The mechanism of energy transfer was identified as a resonant type via a dipole-dipole mechanism. The external quantum efficiency was increased 7 times (from 7% for γ-AlON:Mn(2+) to 49% for γ-AlON:Mn(2+),Eu(2+) under 365 nm excitation), and color-tunable emissions from blue-green to green-yellow were also realized with the Eu(2+) → Mn(2+) energy transfer in γ-AlON.

A route to increase fracture toughness of layered Si3N4/SiC composite using interlocked interfaces

Multilayer Si3N4/SiC composite with interwoven interfaces was designed and prepared via the aqueous tape casting and gas pressure sintering process. This composite had relatively uniform layer thickness, high strength (~564MPa), toughness (~16.8MPam1/2) and work-of-fracture (~4411J/m2). The first-level toughening mechanism started from the fracture surface of the multilayer composite. Obvious crack deflection was found in the Si3N4 and SiC layers. At the same time, the pullout and fractured rodlike β-Si3N4 grains were detected along the interfaces of the composite. The growth of the rodlike β-Si3N4 grains (the grain size>50μm) along the interfaces was determined by the vapor phase transport mechanism. Both rich SiO2 on the surfaces of the SiC/Si3N4 particles and high porosity of the SiC layers were contributed to the growth of β-Si3N4 grains.

Luminescence properties of a green-emitting Eu2+-doped Sr-containing SiAlON phosphors by gas pressure sintering

A green-emitting platelet Eu2+ doped SiAlON phosphor with a composition of Sr6Si25.6Al6.4N41.6O4.4 is developed by gas pressure sintering method at 1800 °C and 0.5 MPa of N2 for 10 h, which is highly luminescent under the excitation of ultraviolet to blue light and shows green emission, peaking around 510 nm with the FWHM of 65 nm, the emission is ascribed to the 4f65d1–4f7 transition of Eu2+ ions. The phase purity and crystal structure are investigated in this work. Quenching concentration of the phosphors is found to be 0.03 mol fraction. The thermal quenching properties of the green phosphors with different Eu2+ concentrations are also investigated. Better thermal stabilities can be obtained with lower Eu2+ concentration, the emission intensity at 200 °C remains 68 % of that measured at room temperature when Eu2+ concentration is 0.03, which is much better than standard orthosilicate phosphors. The internal and external quantum efficiencies of the green phosphor are 85 and 64 % upon the 460 nm excitation. The interesting properties of the phosphor make it a competitive candidate for UV or blue chip based white LEDs.

Mechanical, dielectric properties and thermal shock resistance of porous silicon oxynitride ceramics by gas pressure sintering

Porous silicon oxynitride (Si2N2O) ceramics were prepared by gas pressure sintering with 2.5–10.0mol% Li2O as additive. The influences of Li2O content on phase, microstructure, mechanical and dielectric properties of the Si2N2O ceramics were investigated. XRD analysis showed that the increase of Li2O content facilitate both the densification and the decomposition of Si2N2O into Si3N4. The elongated Si2N2O crystals contribute to the high flexural strength (161.3–228.4MPa at room temperature) of the products. The thermal shock resistance was greatly influenced by the sample porosity, and the highest critical temperature difference (ΔTc) could be up to 1300°C. The dielectric properties were mainly affected by the sample porosity. The as-sintered Si2N2O ceramics showed both low dielectric constant (ε<4.59) and loss tangent (tanδ<0.0049) with good mechanical properties and excellent thermal shock resistance, indicating the gas pressure sintered Si2N2O ceramics could be used as promising high temperature wave transparent material.

Effects of milling media on the mechanical properties of gas pressure sintered α/β-SiAlON

Three types of milling media, alumina, zirconia and cemented tungsten carbide (CTC), were used for the solid-state preparation of SiAlON from Si3N4 using Y2O3-Al2O3-AlN as sintering aids. Fully densified α and β dual-phase SiAlON composites were obtained by gas pressure sintering (GPS) at 1750 °C for 1.5 hrs. The relative contents of α- and β-SiAlON and grain boundary phases in the sintered samples were affected by the type of milling media, where the main grain boundary phases were α-, β-SiAlON and melilite (Y2Si3O3N4). Zirconium nitride was observed at the grain boundaries of the samples prepared using zirconia media, and WC and CO3W3C were observed with the sample prepared using CTC media due to contamination from the milling media. As a result, the overall mechanical properties, color, and microstructure of SiAlON were affected by the grain boundary phases and the α- to β-SiAlON ratio. The SNZ sample prepared using ZrO2 media showed better mechanical properties than the SNA and SNW samples. These results were compared with those obtained by spark plasma sintering, which was reported previously.

Effect of B&lt;sub&gt;4&lt;/sub&gt;C Content on Densification Behavior of SiC-B&lt;sub&gt;4&lt;/sub&gt;C Ceramics

The SiC-B4C multi-phase ceramics was fabricated by gas-pressure sintering technology. The rare-earth oxide Al2O3combined with Er2O3/SiO2was served as sintering aids. The results were shown that the combination of Al2O3/Er2O3/SiO2sintering additives were effective for densification of SiC-B4C multi-phase ceramics. The influence of B4C content on the phase constitution, microstructure and densification behavior of the SiC-B4C multi-phase ceramics were detailed. The lose weight and volume shrinkage rate of SiC-B4C multi-phase ceramics had similar evolvement trend when B4C content increased. Keywords: Gas-Pressure Sintering, SiC-B4C multi-phase ceramics, densification behavior.

Gas pressure sintering of BN/Si3N4 wave-transparent material with Y2O3–MgO nanopowders addition

BN/Si3N4 ceramics performed as wave-transparent material in spacecraft were fabricated with boron nitride powders, silicon nitride powders and Y2O3–MgO nanopowders by gas pressure sintering at 1700°C under 6MPa in N2 atmosphere. The effects of Y2O3–MgO nanopowders on densification, phase evolution, microstructure and mechanical properties of BN/Si3N4 material were investigated. The addition of Y2O3–MgO nanopowders was found beneficial to the mechanical properties of BN/Si3N4 composites. The BN/Si3N4 ceramics with 8wt% Y2O3–MgO nanopowders showed a relative density of 80.2%, combining a fracture toughness of 4.6MPam1/2 with an acceptable flexural strength of 396.5MPa.