Short-term Sintering Research Articles

Effect of sintering time on the marginal and internal fit of monolithic zirconia crowns containing 3–4 mol% Y2O3

BackgroundShort-term sintering may offer advantages including saving time and energy but there is limited evidence on the effect that altering sintering time has on the accuracy of monolithic zirconia crowns. The purpose of this in vitro study was to investigate the effect of shortened sintering time on the marginal and internal fit of 3Y-TZP and 4Y-TZP monolithic crowns.MethodsSixty monolithic zirconia crowns were fabricated for the maxillary first molar tooth on the prefabricated implant abutment. Groups were created according to the material composition: 3Y-TZP Generation 1, 3Y-TZP Generation 2 and 4Y-TZP. Two different sintering protocols were performed: same final sintering temperature (1500 °C) and various rates of heating (10 °C/min and 40 °C/min), cooling down speed (− 10 °C/min and − 40 °C/min), holding time (45 and 120 minutes), and total sintering time (approximately 2 and 7 hours, respectively). The marginal and internal fit of the crowns were determined using the silicone replica technique. Comparisons between groups were analyzed using two-way ANOVA. Pairwise multiple comparisons were performed using t-test (p < 0.05).ResultsThe mean marginal gap values of 4Y-TZP zirconia revealed statistically significant increase for the short-term sintering protocol (p < 0.0001), while no difference was observed between the sintering protocols for the mean marginal gap values of 3Y-TZP groups. Although all groups showed clinically acceptable gap values, altering the sintering time had an effect on marginal fit of the crowns manufactured from 4Y-TZP zirconia.ConclusionsShortening the sintering time may lead to differences within clinically acceptable limits. The manufacturer’s recommendations according to material composition should be implemented with care.

Enhanced dielectric properties of Ca0.9Sr0.1Cu3-yZnyTi4-xBxO12 ceramics by modifying grain boundary response

Ca0.9Sr0.1Cu3-yZnyTi4-xBxO12 ceramics (0.01 ≤ x ≤ 0.07, y = 0 and 0.05) were prepared using the citrate precursor process and sintered at 1100 °C for 3 h. X-ray diffraction (XRD) has confirmed that CaCu3Ti4O12 ceramics with single phase can be prepared for a short-term sintering. The results of XRD and Raman reveals the existence of secondary phase as x = 0.07. The energy dispersive spectrometer confirms the presence of Cu-rich phase at grain boundary, which leads to a weakened dielectric properties. B3+ doping can speed the grain growth, whereas Zn2+ doping decreases grain size slightly and high dense microstructures were observed. The introduction of Zn2+ leads to an increase of grain boundary resistance, a decrease of dielectric loss and an enhancement of frequency stability of dielectric properties. Enhanced dielectric properties with dielectric constant of ~13410 and dielectric loss of ~0.038 were observed for the sample at x = 0.04 and y = 0.05. Moreover, the nonlinear electrical characteristics are enhanced with proper concentration of Sr, B and Zn additions. The present results indicate that the improved dielectric properties of doped CCTO ceramics are mainly attributed to the improved microstructure and the modified grain boundary response.

Impact of the Structure on Mechanical and Tribological Properties of Sintered (Al–12 Si)–40 Sn Alloy

Interaction of the structure of the composite alloy (Al–12 Si)–40 Sn with its mechanical and tribological properties was studied. The alloy was obtained by the liquid-phase sintering of powder briquettes made of the powder mix of tin PO-2 and an atomized aluminum alloy of the eutectic composition Al–12 Si. Sintering was carried out at a temperature below the melting point of the eutectic; otherwise, the sample melted and lost its shape. It was found that the given sintering temperature does not make it possible to obtain samples with high density; their residual porosity was 6–8% and it almost was not decreased with the increase in the sintering time. The obtained material had low mechanical properties, which slightly improved with the increase in the sintering time to two hours. At the same time, the amount of exuded and evaporated tin noticeably increased. Taking into account the undesirable phenomena that occurred during a long exposure of sintered samples at high temperature, the hot densification was carried out after the short-term sintering. Densification of the sintered samples aiming at removal of the residual porosity was carried out in a closed mold under pressure above the ultimate strength of the alloy. It was established that such operation contributed both to significant improvement of strength and ductility of the studied material. In addition, the obtained nonporous material had high wear resistance under dry friction against a steel counterbody; it was especially noticeable during friction under high pressure. The wear rate of samples with a matrix made of the aluminum-silicon alloy was 30% lower as compared with the alloy with the matrix made of pure aluminum at all other conditions being equal.

Structural and Thermoelectric Properties of Nanostructured Nominally Stoichiometric Pb1−x Bi x Te Prepared by Mechanical Alloying

We have investigated the influence of substituting bismuth for lead in lead telluride. The material was nanostructured by mechanical alloying and compacted via short-term sintering or hot pressing. Synthesis of samples with substitution up to 6 at.% bismuth was carried out twice to ensure reproducibility. All relevant thermoelectric transport parameters were measured in a wide temperature range from 123 K or 173 K to 773 K. Two different techniques for measuring the electrical conductivity and Seebeck coefficient were used, one at low and the other at high temperature. Higher bismuth content of 4 at.% to 6 at.% was found to result in the best thermoelectric properties, with maximum ZT value of about 0.7 at 723 K for 4.0 at.% bismuth. The structure was examined in detail via x-ray diffraction analysis, Raman spectroscopy, and transmission electron microscopy. A key feature of the microstructure was the inhomogeneous distribution of bismuth in the lead telluride matrix and the occurrence of bismuth-rich regions on the nanoscale, related to remarkably increased carrier concentration and mobility.

Effect of preparation procedure and nanostructuring on the thermoelectric properties of the lead telluride-based material system AgPbmBiTe2+m (BLST-m)

We report on the preparation and thermoelectric properties of the quaternary system AgPbmBiTe2+m (Bismuth-Lead-Silver-Tellurium, BLST-m) that were nanostructured by mechanical alloying. Nanopowders of various compositions were compacted by three different methods: cold pressing/annealing, hot pressing, and short term sintering. The products are compared with respect to microstructure and sample density. The thermoelectric properties were measured: thermal conductivity in the temperature range from 300 K to 800 K and electrical conductivity and Seebeck coefficient between 100 K and 800 K. The compacting method and the composition had a substantial impact on carrier concentration and mobility as well as on the thermoelectric parameters. Room temperature Hall measurements yielded carrier concentrations in the order of 1019 cm−3, slightly increasing with increasing content of the additive silver bismuth telluride to the lead telluride base. ZT values close to the ones of bulk samples were achieved. X-ray diffraction and transmission electron microscopy (TEM) showed macroscopically homogeneous distributions of the constituting elements inside the nanopowders ensembles, indicating a solid solution. However, high resolution transmission electron microscopy (HRTEM) revealed disorder on the nanoscale inside individual nanopowders grains.

Tin Telluride-Based Nanocomposites of the Type AgSnmBiTe2+m (BTST-m) as Effective Lead-Free Thermoelectric Materials

In the medium temperature range tin telluride has emerged as a compensational thermoelectric material for the well established compound lead telluride. We present tin telluride-based nanocomposites of the composition AgSnmBiTe2+m (Bismuth–Tin–Silver–Tellurium, BTST-m) showing superior thermoelectric performance. Nanopowders with varying concentration ratios between silver bismuth telluride and tin telluride were synthesized by mechanical alloying. Three different compacting routes were applied: cold pressing/annealing, hot pressing, and short-term sintering. A strong interdependence between the compacting method and thermoelectric properties was found with hot pressed and short-term sintered samples exhibiting the best results. Alike for the already known antimony analogue, ZT could be increased to around 1. Scanning electron microscopy indicates that the observed tremendous reduction of thermal conductivity originates from precipitation of nanoparticles and a high number of grain boundaries. Powder X-ray diffraction and transmission electron microscopy revealed the formation of the rock salt-type structure with linearly varying lattice parameters depending on m according to a solid solution, in agreement with a microscopically homogeneous spatial distribution of the constituting elements. However, imaging on the nanostructure shows the formation of coherently ingrown nanoinclusions and diffuse scattering as an indication of short-range ordering.

Influence of Annealing and Sintering on the Thermoelectric and Mechanical Properties of Gas Atomized p-type Skutterudites

Filled cobalt-antimony based skutterudites are very promising thermoelectric materials for generator applications in an intermediate temperature range between 150 and 550°C. For applications, besides the functional thermoelectric properties also the mechanical properties and production methodsplay an important role. Both thermoelectric and mechanical properties can be varied by adjusting the doping level, filling elements and filling fraction. Furthermore, the properties can be influenced by the choice of preparation routes leading to the formation of certain nano- or microstructures.In this work we investigate the influence of powder treatment and sintering temperature on the properties of p-type In0.2Fe0.28Co3.72Sb12 skutterudites synthesized by gas atomization. After atomization the powder contains a large fraction of secondary phases which transform into the skutterudites phase during short term sintering. This phase transformation supports compaction and sintering. It is shown that the sintering temperature influences the resulting thermoelectric properties beyond pure effects of porosity. Best thermoelectric material propertiesare achieved at sintering temperatures between 550 and 600°C. Annealing the atomized powder prior to sintering leads to phase pure material for which the sintering temperature only influences the thermoelectric properties via the resulting sample porosity.

Influence of mechanochemical syntheses and compacting methods on the thermoelectric properties of nanostructured AgSnmSbTe2+m (TAST-m)

Tin telluride doped with silver antimony telluride (Tin-Antimony-Silver-Tellurium, TAST-m) was prepared by two mechanochemical methods: mechanical alloying and—as an alternative synthetic route—co-ball-milling, a recently developed way of combining the procedures of mechanical alloying of single elements with the usual ball-milling of polycrystalline compounds. For the two series of products with varying degrees of doping, three different compacting techniques, cold-pressing/annealing (CPA), hot-pressing (HP) and short-term sintering (STS), were applied, and their influence on the thermoelectric properties was investigated. Although short-term sintering seemed to be the most advantageous method in most cases, the influence of the density of an individual pellet on the thermoelectric transport properties needs to be considered. A structural characterization of the different series of nanopowders obtained by the two milling methods was carried out by x-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. The latter also allowed further insight into the microstructure and revealed a homogeneous distribution of silver and antimony in the tin telluride matrix.

Electro-migration of Zinc during current-assisted pressure sintering of β-Zn4Sb3 – effect of process parameters and its influence on the thermoelectric properties

ABSTRACTCompaction of β-Zn4Sb3 was carried out by current-assisted short term sintering under pressure (STS) using different material and process parameters. Surface Seebeck mapping (PSM) of the compacted specimens (along the uniaxial pressing direction) shows a wide gradation ranging from ∼40–180 μV/K due to electro-migration of Zinc along the current direction. The wide distribution of S corresponds to a mixture of Zn-Sb phases [1] which arise depending on the extent of Zinc migration during the STS process. Variation in the material and process parameters (average particle size, heating rate, compaction time/temperature) results in different spatial distribution of S. Measurements of electrical (σ), thermal (κ) conductivities and Seebeck (S) coefficients between room temperature and 523 K were carried out on two specimens having different average S values and distributions as observed by the PSM. The results indicate an increase in the lattice thermal conductivity (κL) and subsequent lower ZT in the specimens compared to the reported values for β-Zn4Sb3.

Transport Properties and Microstructure of Indium and Cerium added Cobalt-Antimony based Skutterudites

ABSTRACTIndium and cerium added cobalt-antimony based skutterudites with different filling fractions were synthesized using different annealing synthesis parameters. Phase homogeneity and microstructure of the resulting as-cast material were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The skutterudite material was further compacted using a current-assisted short-term sintering device. Temperature dependent measurements of the Seebeck coefficient, electrical and thermal conductivity were carried out on the compacted specimens in the temperature range of 350 K-700 K. Results indicate significant differences in the transport properties between the slowly cooled and quenched as‑cast materials and also with different filling fractions. Based on the measured transport properties the dimensionless figure of merit ZT was calculated for different filling fractions of indium and cerium. Among these compositions a ZTmax of 1.1 at 700 K was obtained.

Variations of Thermoelectric and Mechanical Properties of Large Lead Telluride Samples Produced by a Short-Term Sintering Method

Although lead telluride is a widely used thermoelectric (TE) material for generator applications in the intermediate temperature range, its mechanical properties have not been fully understood yet. Especially sintered PbTe samples have hardly been investigated with regard to their mechanical properties and homogeneity in general. The aim of the present study is to comprehend the TE and mechanical properties of large PbTe samples produced by a short-term sintering process. The potential and Seebeck microprobe technique was used to reveal the spatial distribution of the Seebeck coefficient on the samples’ surfaces at room temperature. Microhardness measurements were performed with a Vickers indenter. The results demonstrate that, despite a homogeneous density, the functional and mechanical properties vary along the samples, which is attributed to varying local parameters during sintering.