Hot-pressed Specimens Research Articles

Transport and thermoelectric properties of Bi2Te2.7Se0.3 prepared by mechanical alloying and hot pressing

Bi2Te2.7Se0.3 was prepared by mechanical alloying (MA) and hot pressing (HP), after which the effects of HP temperature on the transport and the thermoelectric properties were evaluated. Bismuth chalcogenide phases were synthesized afterMA for 4 h, and no secondary phases were observed. The relative densities of all hot-pressed specimens were higher than 95%. The carrier concentration and the mobility increased with increasing HP temperature, possibly due to the grain growth and the reduced number of lattice defects. All specimens showed n-type conductions in the temperature range from 323 K to 523 K. The electrical conductivity slightly decreased with increasing temperature, exhibiting a degenerate semiconductor behavior. With increasing HP temperature, an increase in the electrical conductivity and a decrease in the Seebeck coefficient were observed. The maximum dimensionless figure of merit obtained was 0.72 at 473 K for the Bi2Te2.7Se0.3 hot-pressed at 673 K due to its low thermal conductivity and high electrical conductivity.

Microstructure and mechanical properties of zirconia ceramics consolidated by a novel oscillatory pressure sintering

Zirconia ceramics with high density and fine, uniform grains were consolidated by a novel oscillatory pressure sintering (OPS) approach. Compared with the zirconia ceramics by hot pressing (HP) at the same sintering temperature, grain sizes of the OPS specimen were distributed in a narrower range with average size of 278nm; besides, the anisotropic grain growth was inhibited by oscillatory pressure, resulting in a smaller difference between the long radius and the short radius than the HP specimen. Due to microstructure evolution, the flexural strength of the OPS specimen reached approximately 1549MPa, and the load–displacement behaviors were also improved. Such evolutions in microstructure and mechanical performances were ascribed to the new densification mechanisms induced by oscillatory pressure.

Sintering of High‐Performance Silicon Nitride Ceramics Under Vibratory Pressure

To improve mechanical behaviors of silicon nitride ceramics, here we introduced a novel external field—vibratory pressure into the sintering of Si3N4 ceramics with advantages of higher density, more uniform distribution of interfacial phase, higher sintering motivation in the width direction, and therefore more favorable mechanical properties than traditional sintering methods. Grain size and aspect ratio of the two ceramics were investigated with linear intercept method. Flexural strength of the vibratory‐assisted hot‐pressing (VAHP) specimen increased from 936 ± 27.2 MPa to 1247 ± 28.9 MPa, and an increase of 10% was achieved in fracture toughness. It is believed that such VAHP method can provide a universal approach and new opportunities for the fabrication of covalent‐bonded ceramics or composites with enhanced performances.

Process controls for Bi2Te3-Sb2Te3 prepared by mechanical alloying and hot pressing

p-Type Bi2Te3-Sb2Te3 solid solutions were prepared by mechanical alloying (MA) and hot pressing (HP) under different process conditions, after which the transport and the thermoelectric properties were evaluated. The relative densities of all hot-pressed specimens were over 98%, and the microstructure and crystal orientation were independent of the HP direction. All specimens exhibited p-type conduction, and the electrical resistivity was observed to increase slightly with increasing temperature, indicating a degenerate semiconductor behavior. The carrier concentration decreased with increasing HP temperature while the mobility increased. The maximum figure of merit obtained was 0.86 at 323 K for Bi0.5Sb1.5Te3 hot-pressed at 648 K.

Densification behavior of high purity SiC by hot pressing

Densification behavior of 99.998% purity SiC by hot pressing without metallic additives was investigated. A theoretical densification of 92% was achieved at 2350°C with 50MPa in 3h. The densification was more affected by the sintering pressure rather than by the sintering temperature and was independent of initial particle size. Data were interpreted according to the power-law creep model. The β-SiC phase was preserved in the high purity, hot-pressed SiC specimen without β→α phase transformation even at 2350°C.

Fractographical assessment of densification mechanisms in hot pressed ZrB2-SiC composites

The controlling densification mechanisms of hot pressed monolithic ZrB2 ceramics and ZrB2-based composites, containing 15 and 30vol% SiC, at different consolidating temperatures were investigated, based on scanning electron microscopy micrographs of fracture surfaces, relative densities, and average grain size of ZrB2. For the hot pressed samples at 1700°C, particles fragmentation in the composite samples, mechanical interweaving, and rearrangement without sizeable chemical bonding were appointed as dominant densification mechanisms. Neck formation between ZrB2/ZrB2 was observed at 1850°C and plastic deformation of ZrB2 grains was nominated as controlling densification mechanism. Reduction of porosity in the hot pressed specimens at 2000°C was related to grain boundary diffusion mechanism. Colossal grain growth in monolithic ZrB2 ceramic proposed the occurrence of detrimental mechanisms such as grain coarsening and evaporation/condensation. Presence of intergranular SiC particles between ZrB2 grains impeded extremist grain growth.

Cone calorimeter study of polyethylene flame retarded with expandable graphite and intumescent fire-retardant additives

Polyethylene was flame retarded with two intumescent-type additives, 3,5-diaminobenzoic acid phosphate and ethylenediamine phosphate, which differ with respect to their decomposition onset temperatures, along with varying ratios of two grades of expandable graphite, also differing with respect to their onset temperatures for exfoliation. Hot-pressed sheet specimens were subjected to evaluation in a cone calorimeter. Although the best char yields were obtained with formulations containing the higher decomposition temperature intumescent, 3,5-diaminobenzoic acid phosphate, the overall best performance was realized using the lower decomposition temperature intumescent, ethylenediamine phosphate, when compounded together with the low exfoliation temperature expandable graphite. These results are attributed to the formation, at the burning surface, of a more cohesive char with better thermal and mass transfer barrier properties.

In-situ synthesis and sintering of mullite glass composites by SPS

The main subject of this work is an investigation of the effects of heating rate and current on the crystallisation of amorphous precursors in spark plasma sintering (SPS). For this, dry gel of Al2O3-SiO2 with a molar ratio of 1:1, was synthesized and sintered in-situ by SPS, and also by hot pressing (HP) for comparison. Phase analysis showed that the only crystalline product in both cases was mullite, whose Al2O3 content was lower in the SPS specimens. The microstructures showed a low volume fraction of large mullite fibers in the SPS specimens, whereas a high volume fraction of fine equiaxed grains was present in the HP specimen. The main difference in microstructure between HP and SPS specimens could be explained in terms of the higher heating rate of the SPS specimens. The size of the SPS die also affected the size and aspect ratio of the mullite fibers produced, which might have been due to either the different electrical current required or a difference in specimen temperature profile.

Simultaneous enhancement of mechanical and thermoelectric properties of polycrystalline magnesium silicide with conductive glass inclusion

Thermal fatigue and mechanical failure are two challenging aspects of thermoelectric (TE) module design and fabrication. Among the well-known TE materials with high conversion efficiencies are various brittle semiconductors or ceramics. However, formation of microcracks during fabrication and mechanical failure due to the high stress response in the course of thermal cycles are commonly observed in such material systems. In this work, we report the results of a novel technique to improve the mechanical and TE properties of magnesium silicide (Mg2Si) via addition of a small quantity (0.25–1vol.%) of conductive glass-frit. Mechanically alloyed and hot-pressed Mg2Si specimens separately doped with 2at.% Bi and 2at.% Al were sintered at 1173 and 1123K, respectively. The TE properties of both compounds were characterized by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (κ) in the temperature range 300–970K. The beneficial effects of addition of a minuscule quantity of Mg–Si–B–R-based (R=rare earth) conductive glass-frit to Al-doped Mg2Si samples were investigated. Both Al-doped and Bi-doped Mg2Si specimen were tested for mechanical reliability using diametric compression tests. Power factors times temperature (S2σT) of >2Wm–1K–1 were obtained from Al-doped samples containing conductive glass-frit. It was also found that addition of 1% of the conductive glass-frit results in significant improvement of the mechanical properties of Mg2Si by eliminating microcracks in the brittle Mg2Si system. Nearly 150% improvement was observed in the mechanical strength of the Al-doped samples reinforced with conductive glass-frit as compared to the samples without glass-frit.

LiCoPO4 mechanical properties evaluated by nanoindentation

LiCoPO4 is a promising cathode material for use in high voltage Li-ion batteries. One of the problems with LiCoPO4 is limited cycle life. One possible cause for the limited cycle life of LiCoPO4 is mechanical degradation. As a consequence, the mechanical properties, elastic modulus, E, and fracture toughness, KIC, of hot-pressed dense (~98%) polycrystalline (15–20μm) single phase LiCoPO4 were investigated. E for the hot-pressed LiCoPO4 specimen is ~137GPa while the E value for the LiCoPO4 specimen after annealing at 600°C is ~106GPa. The fracture toughness of the hot-pressed LiCoPO4 sample is ~0.41MPam1/2, which increased to ~0.53MPam1/2 after annealing. These low KIC values reveal that LiCoPO4 is a brittle material. It is believed that the decrease in E and increase in KIC with annealing is associated with a reduction in residual stress.

Fracture mode, microstructure and temperature-dependent elastic moduli for thermoelectric composites of PbTe–PbS with SiC nanoparticle additions

Twenty-six (Pb0.95Sn0.05Te)0.92(PbS)0.08–0.055% PbI2–SiC nanoparticle (SiCnp) composite thermoelectric specimens were either hot pressed or pulsed electric current sintered (PECS). Bloating (a thermally induced increase in porosity, P, for as-densified specimens) was observed during annealing at temperatures >603 K for hot-pressed specimens and PECS-processed specimens from wet milled powders, but in contrast seven out of seven specimens densified by PECS from dry milled powders showed no observable bloating following annealing at temperatures up to 936 K. In this study, bloating in the specimens was accessed via thermal annealing induced changes in (i) porosity measured by scanning electron microscopy on fractured specimen surfaces, (ii) specimen volume and (iii) elastic moduli. The moduli were measured by resonant ultrasound spectroscopy. SiCnp additions (1–3.5 vol.%) changed the fracture mode from intergranular to transgranular, inhibited grain growth, and limited bloating in the wet milled PECS specimens. Inhibition of bloating likely occurs due to cleaning of contamination from powder particle surfaces via PECS processing which has been reported previously in the literature.

Manufacturing Technology for Titanium Tubes from Billets Prepared by Electron-Beam Remelting

Technology is developed for manufacturing hot-pressed and cold-worked seamless tubes from titanium alloy billets prepared by electron-beam remelting, which makes it possible to reduce metal consumption and energy expended by lowering the number of production operations (excluding forging, machining, piercing, or expanding with preliminary drilling and heating before the above-mentioned operations). Billet mechanical properties and structure studied for hot-pressed and cold-worked pipe specimens show that the production parameters developed make it possible to obtain hot-pressed and cold-worked seamless tubes corresponding to specifications of domestic and overseas standard documents.

Evaluation of the Mechanical Properties of PMMA Reinforced with Carbon Nanotubes - Experiments and Modeling

The effective mechanical properties of polymethyl-methacrylate (PMMA) reinforced with carbon nanotubes (CNTs) were evaluated by means of two approaches: experiments and a micromechanical model. With various concentrations of CNTs, two specimen fabrication processes were examined: hot pressing (HP) and injection molding (IM). Experiments included a series of uniaxial tensile tests guided by an ASTM standard. Using displacement control, tests were carried out while images were taken of the gage area. The in-plane displacement fields were evaluated by means of Digital Image Correlation (DIC). A MATLAB program was then used to calculate strains, create stress-strain and strain-force curves and determine Young’s modulus E, Poisson’s ratio \(\nu \), the ultimate tensile stress \(\sigma _{uts}\) and the strain to failure \(\varepsilon _{f}\). In addition, simulations were carried out using a micromechanical model (High-Fidelity Generalized Method of Cells or HFGMC). A Repeating Unit Cell (RUC) consisting of one CNT and PMMA surrounding it was modeled and analyzed in order to determine the effective mechanical properties of the composite. This method allows for imperfect bonding between the phases which is controlled by two parameters. These damage parameters decrease the stress-strain response of the material. However, the increase of the volume fraction increases the composite response. These two conflicting effects appear to provide the observed decrease in Young’s modulus for low volume fractions as discussed. The effects of CNT concentration, geometry and orientation, as well as the interface between the phases, were examined. It was seen from the experimental results, for HP specimens, that for low concentrations of CNTs, E initially decreases and then increases significantly as the weight fraction increases. This behavior of E was quantitatively predicted by the HFGMC model. For IM specimens, Young’s modulus is nearly constant for low weight fractions of CNTs and then increases with weight fraction.

기계적 합금법에 의한 Mg3Bi2와 Mg3Sb2화합물 및 복합체의 제조

기계적 합금법을 이용하여 Mg, Sb, Bi을 planetary 볼밀에서 24~48시간 볼 밀링하여 <TEX>$Mg_3Sb_2$</TEX>와 <TEX>$Mg_3Bi_2$</TEX> 단일 결정상의 합금분말을 얻었다. 출발조성은 <TEX>$Mg_3Sb_2$</TEX>상의 경우 3Mg : 1.8Sb, <TEX>$Mg_3Bi_2$</TEX>상은 3Mg : 1.6Bi이었다. 얻어진 두 합금 분말을 여러 비율로 혼합하여 70 MPa의 압력 하, 723 K에서 1시간 가압소결하여 <TEX>$Mg_3Sb_2-Mg_3Bi_2$</TEX> 복합체를 제조하였다. 복합체의 주 결정상은 <TEX>$Mg_3Bi_2$</TEX>상과 유사한 안정한 상이었고 미량의 Bi상이 존재하였다. EDS 맵핑결과 가압소결체들의 조성분포는 불연속적이며 독립적으로 나타나서 <TEX>$Mg_3Sb_2-Mg_3Bi_2$</TEX>계 복합체가 제조되었다는 것을 확인할 수 있었다. Single phase crystalline powders of <TEX>$Mg_3Sb_2$</TEX> and <TEX>$Mg_3Bi_2$</TEX> were prepared by mechanical alloying Mg, Sb and Bi metals with planetary ball milling for 24~48 h. The compositions of starting raw materials for single phase <TEX>$Mg_3Sb_2$</TEX> and <TEX>$Mg_3Bi_2$</TEX> were 3Mg : 1.8Sb and 3Mg : 1.6Bi, respectively. Two types of mechanically alloyed powders obtained were mixed at some ratios for the fabrication of <TEX>$Mg_3Sb_2-Mg_3Bi_2$</TEX> composites and then hot pressed under uniaxial pressure of 70 MPa at 723 K for 1 h. The main phase of composites was a stable phase similar to <TEX>$Mg_3Bi_2$</TEX> phase with a small amount of Bi phase. The distributions of Sb and Bi elements on EDS mapping images were discontinuous and their compositional contours were clear, which means that the hot pressed specimens were composites composed of two compounds of <TEX>$Mg_3Sb_2$</TEX> and <TEX>$Mg_3Bi_2$</TEX>.

Effects of Heat Treatment on the Creep Crack Growth Behavior in Al/Al-4wt%Cu Functionally Graded Material

A conventional hot-pressing method was used to produce Al/Al-4wt%Cu functionally graded material (FGM). Heat treatment, which included solid solution treatment (T4) and aging treatment (T6), was carried out on hot-pressed (F) specimens. The creep crack growth tests were performed under constant loading of the creep-testing machines. The distribution of copper composition was investigated by line analysis via electron probe microanalysis. Fracture morphology and creep crack paths were studied by scanning electron microscopy. During heat treatment, the thickness of the graded transition layer increased due to copper composition redistribution. Creep crack growth retardation was found when crack propagated from the graded transition region to the Al-4wt%Cu layer. Greater improvement in creep crack growth resistance was achieved by the T4 and T6 states of Al/Al-4wt%Cu FGM. For T4 and T6 state specimens, the micro-cracks and crack kinking in the transition region were observed, which prevented creep crack growth.

기계적 합금법으로 제조한 Mg3-xZnxSb2의 열전물성

<TEX>$Mg_{3-x}Zn_xSb_2$</TEX> powders with x = 0-1.2 were fabricated by mechanical alloying in a planetary ball mill with a speed of 350 rpm for 24 hrs and then hot pressed under a pressure of 70 MPa at 773 K for 2 hrs. It was found that there were systematic shifts in the X-ray diffraction peaks of <TEX>$Mg_3Sb_2$</TEX> (x = 0) toward a higher angle with increasing Zn for both the powder and the bulk sample and finally the phase of <TEX>$Mg_{1.86}Zn_{1.14}Sb_2$</TEX> was formed at the Zn content of x = 1.2. The <TEX>$Mg_{3-x}Zn_xSb_2$</TEX> compounds had nano-sized grains of 21-30 nm for the powder and 28-66 nm for the hot pressed specimens. The electrical conductivity of hot pressed <TEX>$Mg_{3-x}Zn_xSb_2$</TEX> increased with increasing Zn content and temperature from 33 <TEX>$Sm^{-1}$</TEX> for x = 0 to 13,026 <TEX>$Sm^{-1}$</TEX> for x = 1.2 at 323 K. The samples for all the compositions from x = 0 to x = 1.2 had positive Seebeck coefficients, which decreased with increasing Zn content and temperature, which resulted from the increased charge carrier concentration. Most of the samples had relatively low thermal conductivities comparable to the high performance thermoelectric materials. The dimensionless figure of merit of <TEX>$Mg_{3-x}Zn_xSb_2$</TEX> was directly proportional to the Zn content except for the compound with Zn = 1.2 at high temperature. The <TEX>$Mg_{3-x}Zn_xSb_2$</TEX> compound with Zn = 0.8 had the largest value of ZT, 0.33 at 723 K.

Thermoelectric Properties of Half-Heusler TiCoSb&lt;sub&gt;1-x&lt;/sub&gt;Sn&lt;sub&gt;x&lt;/sub&gt; Synthesized by Mechanical Alloying Process

Half-heusler TiCoSb1-xSnx (0.00≤x≤0.05) alloys were prepared by mechanical alloying of elemental powders, and consolidated by vacuum hot pressing. Sn doped half-Heusler phases of TiCoSb was successfully produced in all doping ranges by vacuum hot pressing using as-milled powders without subsequent annealing. However, a little fraction of CoSb phase appeared after hot pressing. Thermoelectric properties as functions of temperature were evaluated for hot pressed specimens. Seebeck coefficients showed to change in conductivity by doping and the absolute value revealed relatively low. It is shown that electrical conductivity is relatively high and thermal conductivities are compatibly low. Sb doping up to x=0.02 in TiCoSb1-xSnx was shown to be effective on thermoelectric properties in this study.

Room temperature mechanical properties of polycrystalline YbAl3, a promising low temperature thermoelectric material

Intermetallic YbAl3 in the L12 (AuCu3) phase is a promising material for low temperature thermoelectric applications. However, there is no experimental data in the literature on the mechanical properties of YbAl3, although the design and development of thermoelectric modules incorporating YbAl3 will require mechanical property data. Using resonant ultrasound spectroscopy (RUS), the room temperature Young's modulus, shear modulus, bulk modulus and Poisson's ratio were determined as a function of volume fraction porosity, P, for specimens densified by both hot pressing (HP) and pulsed electric current sintering (PECS) polycrystalline specimens, where P ranged from 0.030 to 0.233 and mean grain sizes ranged from 0.5 to 1.4 μm. In addition, the longitudinal and acoustic wave speeds and the Debye temperature were measured. Using Vickers indentation, the hardness and fracture toughness of the specimens were also measured. Despite microstructural differences between the HP and PECS-processed specimens, the porosity dependence of the mechanical properties was a function of the total volume fraction porosity, P, independent of the details of the size and spatial distribution of pores within individual specimens.

Hot pressing effect on the shear bond strength of dental porcelain to CoCrMoSi alloy substrates with different surface treatments

The purpose of this study was to evaluate the effect of hot pressing on the shear bond strength of a CoCrMoSi alloy to a low-fusing feldspathic porcelain, for two types of surface treatments: polished and grit-blasted. Moreover, the shear strength of hot pressed porcelain was also compared with that of conventional vacuum sintered porcelain. Bond strength of metal–porcelain composites were assessed by the means of a shear test performed in a universal test machine until fracture. Fracture surfaces and interfaces were investigated by optical microscope, stereomicroscope and SEM/EDS. Data was analyzed with Shapiro–Wilk test to test the assumption of normality. The 2-way ANOVA followed by Tukey HSD multiple comparison test was used to compare shear bond strength results and the t-test was used to compare the porcelain shear strength (p<0.05). Hot pressed specimens exhibited significantly (p<0.001) higher bond strength values than those obtained by conventional PFM technique. Significant differences (p<0.001) were found in the shear bond strength between grit-blasted and polished specimens. Significant differences (p<0.05) were also found between the shear strength of vacuum sintered and hot pressed porcelain. This study revealed that metal–ceramic bond strength is maximized for hot pressed porcelain onto rough metal substrates, with lower variability in results. Hot pressing technique was also shown to enhance the cohesion of porcelain.

Studies of the hot-pressed TiN material by electron spectroscopies

We analyzed chemical composition and electron transport phenomena in surface and sub-surface region of hot-pressed TiN specimens using a combination of X-ray photoelectron spectroscopy (XPS) and elastic-peak electron spectroscopy (EPES) techniques. Both the surface chemical composition and the elements distribution in the bulk of TiN specimens were determined using XPS and XPS depth profiling analysis. In addition to TiN, a mixture of Ti oxynitride (TiOxNy) and TiO2 compounds as well as carbon contaminants have been detected in the surface region of the TiN specimens. The elemental depth-profiles disclosed uniform chemical bulk composition formed mainly by titanium and nitrogen as well as carbon and oxygen contaminants. Surface enrichment of Ti was evidenced as result of Ar ion-induced preferential sputtering of nitrogen. The inelastic mean free path (IMFP) data evaluated from the relative EPES for electron energies 0.5–2keV were uncorrected for surface excitations and compared with those calculated from the predictive TPP-2M formula for the measured surface composition. Except to the electron energy of 0.5keV, a good agreement was found between the measured and predicted IMFPs in the TiN specimen. The higher discrepancies in the measured IMFPs at the lowest energy can be explained by the surface excitation effect. The smallest root-mean-square-deviation and the mean percentage deviation of 2.8Å and 14.8%, respectively, were found between EPES IMFP data and those predicted for TiN with respect to the Ni standard.