Densities Of Ceramics Research Articles

Synthesis and Thermoelectric Properties of Y Doped SrTiO3by Modified Pechini's Method

AbstractThermoelectric properties of Sr1-xYxTiO3(x = 0-0.08) were calculated using a virtual crystal method. Using a Pechini's method, Sr1-xYxTiO3(x = 0-0.06) precursor powder was prepared. Using this method, we prepared pure SrTiO3at a lower temperature than would be using the conventional solid-state reaction method. The precursor solution was heated at 823 K for 5 h after drying at 353 K for 8 h to produce the precursor powder. The powder was sintered using a hot pressing technique. The relative densities of ceramics were more than 98%. The Seebeck coefficient and electrical conductivity of the samples were measured using the standard four-probe method in a flowing He gas atmosphere in a temperature range of 323 to 923 K. The conductivities of SrTiO3, Sr0.97Y0.03TiO3, and Sr0.94Y0.06TiO3at room temperature were 6.61 × 102, 5.61 × 103, and 1.58 × 104S/m, respectively. The Seebeck coefficients of SrTiO3, Sr0.97Y0.03TiO3, and Sr0.94Y0.06TiO3at room temperature were -548, -264, and -196 μV/K, respectively.

Alumina ceramics fabricated from bimodal alumina with additives

Superfine α-Al 2O 3 synthesized by a precipitation method and commercial α-Al 2O 3 were used to prepare bimodal alumina. Densification of bimodal alumina with MgO, SiO 2 as additives was investigated from 1400 to 1550 °C. The addition of superfine α-Al 2O 3 is not effective to enhance the densification at the sintering temperature of 1400 °C. With further increase of the sintering temperature, the densities of alumina ceramics from bimodal alumina with different contents of superfine α-Al 2O 3 are similar in values. The microstructure evolution, as well as mechanical properties of the alumina ceramics was also studied. For the 1450 °C sintered Al 2O 3 ceramics, fine grains located in the skeletal structure of the coarse grains and the increasing fine α-Al 2O 3 led to fine-grain structure. Spinel and mullite were observed in the fine-grain regions. For the 1500 °C sintered Al 2O 3 ceramic, there was significant grain growth. Mullite, spinel and MAS glass formed in the triple-grain junctions. Both 1450 and 1500 °C sintered Al 2O 3 ceramics have no liquid phase in grain boundaries. The addition of superfine α-Al 2O 3 can distinctly enhance the bending strength of alumina ceramics. The best mechanical properties were gained when the temperature was 1450 °C and then deteriorated with further increase of sintering temperature.

Thermal and optical properties of transparent magnesium oxide ceramics fabricated by post hot-isostatic pressing

The transparent magnesium-oxide (MgO) ceramic could be fabricated by a hot-isostatic press (HIP) technique of pressureless-sintered MgO compact, using non-agglomerated MgO powder with the average primary particle size of 57 nm. The relative density and average grain size of MgO compact pressureless-sintered at 1600 °C for 5 h were 96.7% and 10.7 μm, respectively. Owing to the HIPing operation (0.5 h) of pressureless-sintered compact at a temperature between 1500 and 1600 °C, relative densities of the MgO ceramics attained 99.9% or more with light transmission. The average grain size increased from 132 to 199 μm with HIPing temperature from 1500 to 1600 °C. The in-line transmission of the MgO specimen HIPed at 1600 °C for 0.5 h increased from 52 to 55% with increasing wavelength from 500 to 900 nm and corresponded to approximately 65%, compared to that of the MgO single crystal. The thermal conductivity of this MgO specimen attained 53.5 W m −1 K −1, whereas the thermal diffusivity decreased gradually to reach 0.0416 cm 2 s −1 with increasing test temperature up to 873 K.

Acetate Sol-Gel Synthesis of PMN Ceramics from Nb-Ethyleneglycol-Tartarate Complex

PbMgNb(OAc) x (OR) y precursor was prepared from Pb and Mg acetates by dissolution in acetic acid (AcOH) and by mixing with Nb-ethyleneglycol-tartarate (Pechini) complex. The sol was formed after hydrolysis by various addition of water at 80°C and it was transformed by polycondensation to PMN two-phase gel. By the calcination of gel at the temperature of 500°C for 2 hours, the two-phase system was created with perovskite and undesirable pyrochlore phases. The content of the Pb(Mg 1/3 Nb 2/3 )O 3 perovskite phase in PMN calcinates was increased from 0 to 60 vol.% by decreasing of water addition as it results from DTA, TG analysis, IR spectroscopy and XRD analysis. The Pb 1.83 (Mg 0.29 Nb 1.71 )O 6.39 pure pyrochlore phase was found in the calcinate prepared from gel with higher addition of water and hydrolyzing factor (h) equals 43. The content of perovskite phase in calcinate was approximately 60 vol.% at h = 21 and without water addition to sol. The partial decomposition of the pyrochlore phase with transformation into the perovskite phase (the maximal volume fraction of perovskite phase ∼ 70 vol.%) was observed above the sintering temperature of 1100°C. Densities of PMN ceramics sintered at 1200°C decreased with the content of py phase in PMN calcinates. The maximal value of ϵ r = 4200 (at 1 kHz) was found in the ceramic sample with the highest content of perovskite phase and density.

Transparent .BETA.-Tricalcium Phosphate Ceramics Prepared by Spark Plasma Sintering

β-tricalcium phosphate (Ca3(PO4)2, β-TCP) is known to be biocompatible and osteoconductive. In the present study, β-TCP ceramics were prepared by the spark plasma sintering (SPS) technique at temperatures ranging from 800 to 1000°C at a pressure of 60 MPa for 10 min at a heating rate of 25°C/min. No phases other than β-TCP were detected in the samples sintered at 800, 900 and 1000°C by X-ray diffractometry. The relative densities of the β-TCP ceramics sintered at 800, 900 and 1000°C were 70, 95 and more than 99%, respectively. Transparent β-TCP ceramics were obtained at 1000°C for 10 min. From the results obtained by scanning probe microscopy, the ceramics prepared at 800 and 900 °C were found to have a few pores. The transparent ceramics had an average grain size of 1.5 μm and seemed to be pore-free.

Effect of glass doping for the sinterability of Ba(Mg 1/3Ta 2/3)O 3 ceramics

Sintering of undoped Ba(Mg 1/3Ta 2/3)O 3 (BMT) ceramics at 1500 °C gives BMT ceramics with 96.0% theoretical density and an ordering structure, but the satellite phases of Ba 5Ta 4O 15, Ba 4Ta 2O 9, Ba 7Ta 6O 22, and Mg 4Ta 2O 9 are difficult to eliminate completely. The addition of MgO–CaO–Al 2O 3–SiO 2 (MCAS) glass as a sintering aid not only decreases the satellite phases of BMT ceramics but also improves the sinterability of BMT ceramics. The densification of the BMT ceramics can be performed well at lower temperatures of 1300–1350 °C. The satellite phases of Ba(Mg 1/3Ta 2/3)O 3 powder can almost be eliminated at 1350 °C when 3 wt.% MCAS is used as a sintering aid or at 1300 °C when 6 wt.% MCAS is used as a sintering aid. When the sintering temperature is higher than 1350 °C and the amount of MCAS addition is equal to or more than 3 wt.%, the densities of BMT ceramics decrease with the increase of sintering temperature because of the increase of satellite phases and the melting of MCAS glass.

Calcining influence on the powder properties of hydroxyapatite.

The effect of different calcination temperatures on the powder characteristics and the sintered density of synthetic hydroxyapatite (HA) powders, produced using two different processing routes, was examined. Powders were produced by either drying, milling and sieving an as-precipitated HA or by spray-drying a slurry of precipitated HA. Calcining the two powders at temperatures between 400 and 1000 degrees C did not significantly affect the powder particle size. The specific surface areas of the two powders, however, were reduced from 70-80 m2/g for a calcination temperature of 400 degrees C to approximately 5-7 m2/g for 1000 degrees C. Analysis of the surfaces of the HA powders using scanning electron microscopy (SEM) illustrated the coarsening and subsequent sintering of the sub-micron crystallites that constitute a powder particle as the calcination temperature increased, corresponding to the decrease in surface area of the powders. The sintered densities of the final ceramics were not significantly affected by calcining the powders. Microhardness measurements of ceramics prepared from powders calcined at different temperatures showed no significant variations with calcination temperature or powder processing method. The results of this study have illustrated that for applications where HA may be used in powder form, for example in plasma-spraying and for the production of HA-polymer composites, calcining the HA will significantly affect the powder properties, namely the surface area and morphology of the powders. For applications requiring HA in a dense ceramic form, for example as granules or blocks, calcining the powders does not significantly affect the properties of the final ceramic.

Effects of Firing Atmosphere and V<sub>2</sub>O<sub>5</sub> Addition on Sintered and Mechanical Properties of Mullite

High-purity mullite was sintered in oxidizing (air) and reducing (N2=H2=50%) atmospheres, and effects of the firing atmosphere and V2O5 addition on the sintering behavior and bending strength of mullite ceramics were investigated. The firing atmosphere did not affect the bulk density of the undoped mullite ceramics sintered at below 1650°C. SiO was formed by the reaction between mullite and H2 in the reducing firing. Higher bulk densities of the undoped mullite ceramics sintered at 1700°C in the reducing atmosphere than those sintered in the oxidizing atmosphere were assumed to be caused by the formation of glass phase in pores from the reaction between SiO and H2O. Addition of 0.1mass% V2O5 yielded mullite ceramics with the highest bulk density irrespective of the sintering atmosphere. High-temperature bending strength of the undoped mullite ceramics sintered in the reducing atmosphere showed a maximum at a lower temperature than that in the case of the ceramics sintered in the oxidizing atmosphere probably because of the presence of the glass phase.

The Influence of Mn Addition on the Trap Densities of Barium Titanate Ceramics.

The influence of Mn addition on the Positive Temperature Coefficient (PTC) effect of BaTiO3 ceramic semiconductors is investigated in this report. Mn free and Mn doped specimens were sintered at 1350°C for 1h in air. Mn addition significantly promoted the magnitude of the PTC effect. The activation energy was evaluated from Arrhenius plots of electrical conductivity. The trap densities of the grain boundaries above and below the Curie temperature, Tc, were estimated from the activation energy, permittivity, and mean radius of the grains. Addition of Mn increased the difference between the trap densities above and below Tc. SEM observation of the fractured surface of the specimens revealed the points to be analyzed which correspond to the grain boundaries, and AES spectra were obtained on these analysis points. As a result, no Mn was found at the grain boundaries. It appears that the Mn elements which dissolves in the grains increases the change of the trap density of the grain boundaries at Tc, and this could be one of the causes which promotes the magnitude of the PTC.