Two-step Sintering Technique Research Articles

Co-precipitation synthesis and two-step sintering of YAG powders for transparent ceramics

Abstract Yttrium aluminum garnet (Y 3 Al 5 O 12 , YAG) precursor was synthesized by the co-precipitation method with ammonium hydrogen carbonate as the precipitant. The influence of aging and calcination temperature on the precursor composition and transformation temperature of the YAG phase was investigated. On that basis, a two-step sintering (TSS) method (heating the sample up to 1800 °C followed by holding it at 1600 °C for 8 h) was used to fabricate bulk transparent YAG ceramics in vacuum (10 −3 Pa) in this communication. A variety of techniques, such as X-ray powder diffraction, infrared spectra, scanning electron microscopy and UV–vis–NIR spectrophotometry were adopted to characterize the resulting YAG powders and ceramics. The results showed that aging had a dramatic effect on the precursor composition, which in turn influenced the transformation temperature of the YAG phase. Loosely agglomerated YAG powders with a mean particle size of 50 nm were obtained by calcinating the precursor without aging at 1000 °C. Finally, a transparent YAG ceramic specimen, achieving the in-line transmittance of 41% in the visible wavelength region and a nearly pore-free microstructure with uniform grains of about 4 μm, was produced via the TSS technique.

HIGH-FREQUENCY DIELECTRIC SPECTROSCOPY OF BaTiO3 CORE — SILICA SHELL NANOCOMPOSITES: PROBLEM OF INTERDIFFUSION

Three types of BaTiO3 core — amorphous nanoshell composite ceramics were processed from the same core-shell powder by standard sintering, spark-plasma sintering and two-step sintering techniques and characterized by XRD, HRSEM and broadband dielectric spectroscopy in the frequency range 103 – 1013 Hz including the THz and IR range. The samples differed by porosity and by the amount of interdiffusion from the cores to shells, in correlation with their increasing porosity. The dielectric spectra were also calculated using suitable models based on effective medium approximation. The measurements revealed a strong dielectric dispersion below the THz range, which cannot be explained by the modeling, and whose strength was in correlation with the degree of interdiffusion. It is assigned to an effect of the interdiffusion layers, giving rise to a strong interfacial polarization. It appears that the high-frequency dielectric spectroscopy is an extremely sensitive tool for detection of any gradient layers and sample inhomogeneities even in dielectric materials with negligible conductivity.

Suppression of Abnormal Grain Growth in WC-Co via Two-Step Liquid Phase Sintering

Microstructure refinement via two-step sintering and its mechanism have been investigated in 92WC–8Co (wt%) alloys. Samples were prepared using conventional and two-step sintering techniques. For conventional sintering, the powder compacts were sintered at 1400°C for various times. For two-step sintering, the powder compacts were heated up to 1500°C, immediately cooled down to 1400°C, and held for various times at 1400°C. In the conventional sintering, >99% relative density (RD) was obtained after 2 h; however, abnormal grain growth (AGG) occurred. In the two-step sintering, >99% RD was obtained without AGG for up to 2 h sintering times at 1400°C, showing a uniform microstructure with fine grains of ∼0.6 μm in average size. Computer calculation suggested a reduction of the maximum driving force for grain growth right after the first-step sintering at 1500°C. Retardation of AGG in two-step sintering is thus attributed to the change in average grain size and size distribution after the first step of sintering.

Pyroelectric Properties of Highly Ordered Pb(Sc0.5Ta0.5)O3 Ceramics by a Two-Step Sintering Technique

Dense Pb(Sc0.5Ta0.5)O3 (PST) ceramics were fabricated by a two-step sintering technique. X-ray diffraction analysis showed that the order of degree is 0.91 and micrograph indicated that the grain size is 1–3 μm. Under dc bias of 700 V/mm, the detective figure of merit reaches over 70 × 10−5 Pa−1/2, where the pyroelectric coefficient, dielectric constant, and dielectric loss are 650 × 10−8 C·(cm2·K)−1, 6600, and 0.019, respectively. Such superior pyroelectric properties and fine grain size give it a particular advantage in application for uncooled focal plane array detectors.

A novel low temperature synthesis technique of sol–gel derived nanocrystalline alumina abrasive

The effect of ball milling process, co-doped seed and two step sintering technique on the properties of sol–gel derived alumina abrasive sintered at low temperature was investigated. The results showed that ball milling time with 10 h can be effective in enhancing the activity of the precursor and the microstructural uniformity of sintered alumina abrasive. A small amount of Al2O3–(NH4)3AlF6 co-doped seed addition had potential synergistic effects for reducing α-Al2O3 phase transformation temperature and improving the mechanical property of alumina abrasive. A remarkable suppression of grain growth was achieved by controlling sintering temperature with two-step sintering method. Therefore, by using ball milling process, co-doping α-Al2O3–(NH4)3AlF6 seed and two-step sintering technique, the sol–gel derived uniform nanocrystalline alumina abrasive is easily achieved at low temperature. Nanocrystalline alumina abrasive prepared at these conditions exhibited excellent mechanical properties and wear resistance compared to fused corundum abrasive and those sol–gel derived corundum abrasive with conventional sintering methods.

Enhanced electrical conductivity of ultrafine-grained 8Y 2O 3 stabilized ZrO 2 produced by two-step sintering technique

Two-step sintering of nanocrystalline zirconia powder stabilized with 8 mol% yttria resulted in remarkable enhancement of the electrical conductivity and significant suppression of the accelerated grain growth. The electrical conductivity of the nearly full-dense structures obtained with an average grain size of 2.15 μm at 900 °C was ∼107 mS/cm. Two-step sintering resulted in reduction of the grain size to less than 300 nm and increase of the electrical conductivity to 209 mS/cm (i.e. >95% increase). These changes accompanied with a fracture toughness increase of more that 95% (from 1.61 to 3.16 MPa m 1/2), too.

Synthesis and two-step sintering behavior of sol–gel derived nanocrystalline corundum abrasives

Nanocrystalline corundum abrasive with mean crystal size of less than 100 nm was synthesized by two-step sintering method using sol–gel process. A remarkable suppression of grain growth was achieved by controlling sintering temperature and taking advantage of sintering aids during the final stage of a two-step sintering process. The grain size of the high densification samples (>99% theoretical density) produced by two-step sintering method was about 10 times less than the samples made by the conventional sintering technique. The microstructure of the samples was homogeneous without abnormal grain growth and the sol–gel derived corundum abrasive with two-step sintering technique exhibited excellent mechanical properties and wear resistance compared to those sol–gel derived corundum abrasive with conventional sintering methods and fused corundum abrasive.

Temperature dependence of dielectric properties for Sr0.3Ba0.7Bi3.7La0.3Ti4O15 ferroelectric ceramics

Bismuth layer structured ferroelectric ceramics, Sr0.3Ba0.7Bi3.7La0.3Ti4O15 (abbreviated as SBBLT), were fabricated by conventional and two-step sintering techniques. The effects of sintering processes on microstructure and dielectric properties of Sr0.3Ba0.7Bi3.7La0.3Ti4O15 ceramics were investigated. Dense and homogeneous ceramic materials with grain size of 1-3 μm were obtained. The decrease of oxygen vacancies in the two-step sintered ceramics decreases the conductivity, and also dielectric constant and dielectric loss at high temperature. The Curie temperature is shifted from 408°C of conventionally sintered ceramics to 345°C of two-step sintered ceramics at a soaking time of 15 h, which might attributed to smaller rattling space resulting from the decrease of unit-cell volume. The conductivity at high temperature decreases with increasing soaking time. The decrease in the conductivity is about one order of magnitude in two-step sintered samples.

Two-step sintering behavior of sol–gel derived nanocrystalline corundum abrasive with MgO–CaO–SiO2 additions

Nanocrystalline corundum abrasive with mean crystal size of less than 100 nm was synthesised by sol–gel process via two-step sintering technique. A remarkable suppression of grain growth was achieved by controlling sintering temperature and taking advantage of sintering aids (MgO–CaO–SiO2) during the final stage of a two-step sintering process. The grain size of the high densification samples (>99% theoretical density) produced by two-step sintering method was about seven times less than that of the samples made by the conventional sintering technique. The microstructure of the samples was homogeneous without abnormal grain growth and the obtained corundum abrasive exhibited excellent abradability compared to conventional sintering methods.

Improvement of translucency in Al 2O 3 ceramics by two-step sintering technique

Pure, 140, 500, and 2500 ppm MgO-doped Al 2O 3 specimens were sintered at 1700 °C in a vacuum with and without pre-sintering heat treatment (HT) at 800 °C for 50 h in air. Pre-sintering HT improved the transmittance in the visible (400–700 nm) range significantly. This enhanced transmittance was explained in terms of the removal of residual pores and the homogenization of the microstructure due to the lowering of the boundary mobility as a result of the MgO addition and the suppression of local densification through a pre-coarsening step.

Influence of Two-Step Sintering on Transparency of High Purity Alumina

Density, purity, grain, and grain size distribution are the key factors that influence the transparency of an optical ceramics. To achieve high transmittance, efforts should be made to eliminate or minimize scattering or absorption of light. The aim of this paper is to verify the effect of two-step sintering technique for translucent alumina. The density and transmittance of the samples were enhanced by pre-heat treating at 800°C for 50 hours before the high temperature sintering. Abnormal grain growth was successfully suppressed by a two-step sintering technique and a small amount of MgO as additives. It is a very effective technique for designing homogeneous microstructure and high transparency in high purity alumina.

Microstructural Tailoring and Characterization of a Calcium α‐SiAlON Composition

Three calcium α‐SiAlON microstructures—namely, fine‐grained, bimodal, and large elongated—were developed using powders of the same composition and then characterized. The evolution of grain size and morphology was determined to be a process of nucleation and growth that could be controlled with a two‐step sintering technique. The extent of texture was identified in the as‐hot‐pressed materials as a function of sintering conditions. Samples with different microstructures exhibited different hardness and fracture toughness. The true hardness was derived from the intrinsic relation between applied loads and indent sizes. The effect of microstructure on hardness and fracture toughness was analyzed.