Hot-pressed Materials Research Articles

Phase-change behavior of hot-pressed methylammonium lead bromide hybrid perovskites

The investigation of hot-pressed methylammonium lead bromide (MAPbBr3) is reported. The as-prepared materials consisted of microcrystallites (30–500 μm size) with a non-centrosymmetric pseudo-cubic tetragonal crystalline structure (P4 mm space group). The thermal analysis under air showed a thermal stability of MAPbBr3 perovskite till T = 250 °C and a broad endothermic event at T = 371 °C (melting point of solid PbBr2). Under increasing pressure at room temperature, the MA–Pb–Br system retained the pseudo-cubic tetragonal structure, the optical adsorption spectra displayed a strong absorption edge between 570 and 572 nm, and the Tauc plots revealed a direct semiconducting behavior with band energy gaps around 2.20(± 0.01) eV. The steady-state emission (PL) photoluminescence spectra (excitation wavelengths λexc = 380 nm) showed an emission band at ~ 574 nm. With increase in the temperature (under constant pressure and treatment time), the MA–Pb–Br system turned into the higher cubic symmetry (space group $$Pm\, \bar{3} m$$ ), the absorption edges shifted to 571–575 nm, and at T = 120 °C, the absorbance profile begins to increase at higher wavelengths (> 600 nm). By increasing the pressure at constant temperature, the perovskite adopted the cubic structure and a slight decrease in the energy band gap (2.17 eV for T = 150 °C and P = 15 MPa) was recorded. With increase in the hot-pressing time at T = 150 °C and P = 20 MPa, the optical absorption edges remained at ~ 575 nm; however, the decrease in the absorbance can indicate a degradation of the materials. The changes in the emission mechanism(s) of the hot-pressed materials can be related to the observed change in the crystalline structure and morphology.

Contact Multiphysics: A New SPS Paradigm to Enhance Material Fabrication by Design

Spark Plasma Sintering (SPS) is used extensively to fabricate a wide range of monolithic and advanced materials over a short time and at low temperatures which show inherent advantages over conventional hot-pressed materials. However, the presence of uncontrolled microstructure inhomogeneities, especially in relatively large SPS samples, greatly limits the successful transfer of this technology to an industrial process. The intricate complexity of the involved SPS phenomena, the nebulous role of contacts and the tight physical coupling between the powder and the device are the main concerns. This work has three aims: a) to introduce the concept of contact multiphysics in SPS: b) to illustrate that the current issues can be framed within the unifying concept of contact multiphysics: c) to point out that an in-depth understanding of contact multiphysics will contribute significantly to the shedding of the light on SPS phenomena, in order to solve the current limitations of SPS technology and to enable the desired SPS fabrication of materials by design.

Influence of reinforcement and processing on steel-based composites: Microstructure and mechanical response

ABSTRACTSteel matrix composite reinforced with 2–4 vol.% titanium diboride particles was fabricated successfully by powder metallurgy route through hot pressing method. Influence of sintering parameters on densification was investigated by measurement of density of resultant composites. Microstructural analysis of hot-pressed materials was performed. Hardness and deformation behavior under constant load were evaluated by conducting microhardness and nanoindentation tests. The addition of titanium diboride proved to be effective for enhancement of hardness and strength. Composite with 4 vol.% titanium diboride sintered at 1100°C resulted in improved hardness and elastic modulus which could be related to Orowan strengthening resulting from homogeneous distribution of fine titanium diboride particles in steel matrix. The results indicate that proposed method is economically feasible to process steel matrix composites with improved properties. A comparatively lower temperature and pressure offers better control of interface kinetics and microstructure.

Synthesis of boron suboxide (B6O) with alkaline earth metal oxide materials with improved properties

ABSTRACTThe pure boron suboxide (B6O) powder admixed with 1.08 vol% of MgO, CaO, and CaCO3 (corresponding to 1.5, 1.4, and 0.7 wt%, respectively) were synthesized by hot-pressing and the mechanical properties were evaluated. Pure B6O powders were sintered at a temperature of 1900°C, while the B6O-alkaline earth metal oxide admixed powders were sintered at a temperature of 1850°C. All sintering was done under an applied load of 80 MPa in an argon environment. The microstructures and phase analyses were studied by scanning electron microscopy and X-ray diffraction techniques, respectively. The addition of the alkaline earth metal oxide additives resulted in the formation of boride and borate phases, depending on the oxide used. The mechanical properties of the hot-pressed materials were characterized based on their density, hardness, and fracture toughness. Between 95 and 98% of the theoretical densities were achieved and the result indicate a good combination of hardness (between 31.6 and 32.1 GPa) and fracture toughness (between 6.1 and 6.8 MPa.m0.5) in the B6O-alkaline earth metal oxide materials. The introduction of the additive enhances the fracture toughness of the pure B6O-materials, while the fracture mode observed in the sintered materials is mainly transgranular.

Structure and properties of hot-pressed materials based on AlB12C2

The results are presented on the investigation of the structure and mechanical properties of hot-pressed materials based on AlB12C2 powders of sizes 50–150 nm. It is shown that the consolidated materials having mechanical properties comparable with boron carbide may be produced at essentially lower (by 270–300°C) sintering temperatures. It has been established that the heating rate at compressing affects the stoichiometry of the impurity phase containing Al, B, O, which define the difference in mechanical properties of the materials (a decrease of the heating rate results in the insignifical decrease of the material hardness and increase of the fracture toughness).

Production and properties of hot-pressed materials based on silicon carbide with additions of boron and titanium carbides

Special features of pressure sintering materials based on silicon carbide with additives of boron and titanium carbides have been investigated. The kinetic parameters of the densification have been established. Special features of the structure and physico-mechanical properties of hot-pressed materials of the SiC(8–20 wt %)–(B4C–TiC) system have been studied. Dense materials have been produced based on si-licon carbide with increased (3.8 MPa•m1/2) fracture toughness and low (0.07 Ω•m) electrical resistance.

Influence of binder on the porosity of carbon materials

The formation of porosity in carbon materials is considered. In hot-pressed materials with medium-sized grains, porosity is formed by the destruction of binder when blanks are fired to 600°C. In cold-pressed small-grain materials, shrinkage and binder destruction are responsible for the porosity. Up to 600°C, the yield of volatiles and shrinkage compensate the development of porosity, which mainly appears above 600°C. Using various binders, we may obtain carbon materials with small or large porosity. In addition, the binder affects the pore size and distribution. Methods of producing carbon materials with elevated porosity are considered, including the use of filler from a single size fraction and the use of pore-forming agents, Highly porous carbon materials are also produced from binder, with no filler. The raw materials employed are synthetic tars or mesophase pitch. The properties of carbon materials with different pore content are compared.

Direct powder forging of PM nickel-based superalloy: densification and recrystallisation

Powder metallurgy nickel-based superalloys have been widely used in high temperature applications. For these materials, a fully dense and fine-grained microstructure is important. Full densification can be achieved by a suitable processing technique, while the latter can be achieved through recrystallisation for which valuable guidance is provided by the information about recrystallisation nuclei. In this study, a fully dense powder metallurgy nickel-based superalloy component has been produced by a new manufacturing method—direct powder forging—using a single acting hydraulic press under normal atmosphere. Boundary misorientation of the produced material has been analysed to determine the degree of recrystallisation nucleation. A finite element model for direct powder forging has been developed in DEFORM-2D/3D and validated by comparing experimental and simulated load curves. The relationship of stress and strain state with densification and recrystallisation nucleation degree has been analysed. It was found that the direct powder forged FGH96 alloy has a much higher recystallisation nucleation degree and more recrystallised sub-grains, compared with those of the hot isostatic pressed material. Within the forged component, a higher recystallisation nucleation degree resides in the material near the container wall where greater values of shear strain rate have operated.

Sound-Absorbing Composites Based on Flax and Polymer Fibers

The paper considers sound absorption of the composite materials, which are based on the natural flax and polymer fibers. The thickness of a single layer of obtained nonwoven composite is about 10 mm. The analysis of absorption dependences is carried out for different thicknesses of the material. It is shown that in the low frequency range the absorption almost linearly increases depending on the thickness. The coefficient of the normal absorption increases with the frequency in the range from 50 Hz up to 1600 Hz. The hot-pressed material can be used for panel production. The sound absorption increases with the thickness of the pressed material. The measurements show the possibilities of application of the composite for the production of the parts of cabs’ interior.

Ceramic Material Based on Domestic Silicon Nitride Powder Prepared by an SHS Method

Research results are presented for development of hot-pressed high-temperature heat-resistant ceramic material based on silicon nitride in the system Si3N4–MgO, and physicomechanical properties are determined over a wide temperature range.

Study of aeroabrasive wear of hot-pressed materials of the B4C-TiB2 system

The results of the studies of aeroabrasive wear of hot-pressed materials of the B4C-TiB2 system at different angles of abrasive particles attacks have been considered. It has been shown that the materials wear resistance is essentially affected by the relation of phases in the composite. The formation of 5–10 wt % titanium diboride in a B4C composite have been defined to provide a high wear resistance because of the increase of the fracture toughness KIc from 3.8 to 4.4 MPa·m1/2 on retention of the high hardness (HV = 20–23 GPa).

High field ZnO varistors prepared by spark plasma sintering

ZnO varistors with submicrometre and nanoscaled microstructures and enhanced electrical properties were prepared by spark plasma sintering (SPS). The densification, grain size and switch field of the varistors were compared with those of hot pressed material. The switching field increased with decreasing grain size, and very rapidly below 500 nm. Switching fields up to 180 kV cm−1 were obtained for ceramics with submicrometre grain sizes (380 nm). This is nearly two orders of magnitude higher than those currently reported for commercial ZnO varistors. A nano powder, prepared by high energy milling, was sintered to a high density at much lower temperatures compared with the submicron powders and had a nanoscale grain size (45 nm). The nanoceramic broke down dielectrically under very high fields (>260 kV cm−1) before a varistive response was apparent.

Production of Porous β-Type Ti-40Nb Alloy for Biomedical Applications: Comparison of Selective Laser Melting and Hot Pressing.

We used selective laser melting (SLM) and hot pressing of mechanically-alloyed β-type Ti–40Nb powder to fabricate macroporous bulk specimens (solid cylinders). The total porosity, compressive strength, and compressive elastic modulus of the SLM-fabricated material were determined as 17% ± 1%, 968 ± 8 MPa, and 33 ± 2 GPa, respectively. The alloy’s elastic modulus is comparable to that of healthy cancellous bone. The comparable results for the hot-pressed material were 3% ± 2%, 1400 ± 19 MPa, and 77 ± 3 GPa. This difference in mechanical properties results from different porosity and phase composition of the two alloys. Both SLM-fabricated and hot-pressed cylinders demonstrated good in vitro biocompatibility. The presented results suggest that the SLM-fabricated alloy may be preferable to the hot-pressed alloy for biomedical applications, such as the manufacture of load-bearing metallic components for total joint replacements.

STUDY OF ELECTRON, PHONON AND CRYSTAL STABILITY VERSUS THERMOELECTRIC PROPERTIES IN Mg2X(X = Si, Sn) COMPOUNDS AND THEIR ALLOYS

We present results of extensive theoretical and experimental investigations of Mg 2 Si and Mg 2 Sn and their Mg 2 Si 1-x Sn x alloys. Electronic and phonon properties of binary compounds were studied by ab initio calculations. Then, both compounds were synthesized by the solid-state reaction and electrical resistivity and thermopower was measured at high temperature (300–900 K). In both the compounds, the theoretical bandgaps (0.56 eV in Mg 2 Si and 0.16 eV in Mg 2 Sn ) agree very well with the experimental values (0.6 eV in Mg 2 Si and 0.17 eV from activation law in Mg 2 Sn ) upon applying the modified Becke–Johnson semilocal exchange potential and including spin–orbit coupling in the calculations. Calculated phonon spectra support crystal stability of both compounds. For Mg 2 Si , the contributions from Si and Mg are spread over all the spectrum (0–10 THz), whereas in the case of Mg 2 Sn , a gap opens around 4 THz with Sn and Mg contributions dominating in lower and higher energy range, respectively. The calculated heat capacity at low temperature (0–300 K) fairly agrees with available experimental data. The crystal structure of Mg 2 Si 1-x Sn x with x = 0, 0.25, 0.4, 0.75, 1 was studied by X-ray diffraction measurements. The alloy compositions exist in the ranges 0 < x < 0.4 and 0.6 < x < 1 and the obtained samples are almost single phased. Detailed crystal stability study with temperature revealed that all powder samples started to decompose into MgO , Si and Sn at ~ 630 K. For hot pressed bulk materials, the decomposition is much slower than in powder compounds but it still appears. Interestingly, thermoelectric properties measurements performed in Mg 2 Si 1-x Sn x show that both electrical resistivity and thermopower curves are repeatable during temperature cycles up to 770 K. On the other hand, temperature-dependent X-ray powder diffraction suggests that these compounds are not stable. Furthermore, electronic structure calculations of almost 40 impurities (s- and p-block, 3d and 4d transition metal elements) diluted in Mg 2 Si and Mg 2 Si 0.75 Sn 0.25 were performed by the KKR-CPA method. Based on calculated impurity density of states the character of doping (n or p) is predicted, which, however, strongly depends on the substituted crystallographic site.

Enhancement of high temperature ductility of hot-pressed Cu–Cr–Nb alloy by hot rolling

Effect of hot rolling of the order of 90% reduction in thickness on the enhancement of ambient and high temperature mechanical properties of the vacuum hot-pressed dispersion hardened Cu–Cr–Nb alloy samples is studied. The vacuum hot-pressed samples are made by consolidation of alloy powders. The ambient and high temperature mechanical properties of the hot-pressed and hot-rolled samples are evaluated. Though the hot-rolled and hot-pressed materials exhibit comparable strength, high temperature ductility of the hot-rolled sample has greatly improved as compared to hot-pressed alloy. This attributes to the fragmentation of prior particle boundary network of the sintered materials and reduction in pore size and porosity level during hot rolling.

Large enhancement in the thermoelectric properties of Pb0.98Na0.02Te by optimizing the synthesis conditions

PbTe is known as a good thermoelectric material for waste heat recovery in the temperature range of 500 to 900 K. While various approaches such as nanostructuring for thermal conductivity reduction, resonant impurities, and band convergence by alloying for power factor enhancement have been proposed recently for enhancing the thermoelectric properties of PbTe, a systematic study on optimizing the synthesis conditions is also crucial to find a better base material, upon which those new approaches can be applied to further improve the material. In this paper, we systematically investigate the effect of various hot-press conditions on the thermoelectric properties of p-type 2% Na-doped PbTe, by varying the hot-press pressure from 70 to 130 MPa and the sintering time from 0.5 to 2 h. It is shown that the micro- and nano-scale structures in the hot-pressed material can be controlled by changing the sintering time and pressure. We demonstrate that by optimizing the hot-press conditions, the thermoelectric figure of merit of p-type 2% Na-doped PbTe can be enhanced up to zT = 1.74 at 774 K, which is about a 24% enhancement compared to the value of 1.4 presented by Pei et al. for the same material composition. Our electron transport modeling on bulk PbTe shows that this enhancement is due to the thermal conductivity reduction in both the electronic and lattice contributions. We believe that our findings can be accompanied with other recently-proposed techniques to further enhance the zT of this important thermoelectric material.

Thermoelectric Properties of Hot-Pressed Materials Based on Mg2Si n Sn1−n

Mg2Si n Sn1−n solid solutions consist of nontoxic widespread elements. In this work a number of samples of Mg2Si n Sn1−n solid solutions, where 1 ≥ n ≥ 0.7 with various carrier concentrations, were obtained using microcrystalline powder by hot pressing in vacuum. The Seebeck coefficient and the thermal and electrical conductivity were measured in the temperature range from 300 K to 800 K. It is shown that the specific thermoelectric figure of merit (the ratio of the thermoelectric figure of merit to the material density) of these samples weakly depends on the composition of the solid solution. Hence, whether a solid solution or pure Mg2Si is used depends on the application temperature of the material.

Preparation of Ni-Ti-Mo and Ni-Ti-Mo-Zr Alloys by High-Energy Ball Milling and Subsequent Hot Pressing

This work discusses on the preparation of Ni-45Ti-5Mo, Ni-40Ti-10Mo and Ni-46Ti-2Mo-2Zr (at-%) alloys by high-energy ball milling and hot pressing, which are potentially attractive for dental and medical applications. The milling process was performed in stainless steel balls (19mm diameter) and vials (225 mL) using a rotary speed of 300rpm and a ball-to-powder weight ratio of 10:1. Hot pressing under vacuum was performed in a BN-coated graphite crucible at 900°C for 1 h using a load of 20 MPa. The milled and hot-pressed materials were characterized by X-ray diffraction, electron scanning microscopy, and electron dispersive spectrometry. Peaks of B2-NiTi and Ni4Ti3were identified in XRD patterns of Ni-45Ti-5Mo, Ni-40Ti-10Mo and Ni-46Ti-2Mo-2Zr powders milled for 1h. The NiTi compound dissolved small Mo amounts lower than 4 at%, which were measured by EDS analysis. Moreover, it was identified the existence of an unknown Mo-rich phase in microstructures of the hot-pressed Ni-Ti-Mo alloys.

HIP法で固化成形したNi-Cr-Mo-B系耐食耐摩耗合金の諸特性に及ぼすCr，Mo量の影響

The effects of Cr and Mo contents on the properties of hot isostatic pressed (HIPed) materials with the compositions of Ni-X%Cr-Y%Mo-1.5%B (mass%) ((X,Y)=(10,36),(22,26),(34,16)) were investigated. Raw powders for these P/M materials were produced by gas atomization, followed by sieving less than 500 μm. The consolidation by HIP was carried out at 1423 K for 5 h under 147 MPa. Crystal structures and microstructures of these materials were investigated by X-ray diffraction analysis and scanning electron microscopy, respectively. Mechanical properties of Rockwell hardness and bending strength, corrosion resistance by immersion tests into various acid solutions and wear resistance by Ohgoshi-type abrasion test were also evaluated.Tetragonal M3B2 and M5B3 type complex borides were observed in the consolidated P/M materials. These complex borides with the size of approx. less than 5 μm were finely dispersed in Ni-based matrix. They showed Rockwell hardness from 40 to 50 HRC, which value increased with increasing Mo/Cr ratio, and bending strength comparable to that of P/M material with the composition of Co-1.6%C-30%Cr-8%W (ST-12). In addition, the present Ni-22%Cr-26%Mo-1.5%B P/M material with the most excellent mechanical properties revealed higher corrosion resistance than Ni-16%Cr-16%Mo-3.5%W-6.5%Fe (C-276) alloy and wear resistance comparable to ST-12.

The role of Al and Li concentration on the formation of cubic garnet solid electrolyte of nominal composition Li 7La 3Zr 2O 12

The effect of Al and Li concentration on the formation of cubic garnet of nominal composition Li 7La 3Zr 2O 12 was investigated. It was determined that at least 0.204 moles of Al is required to stabilize the cubic phase. It was observed for the cubic phase (stabilized by the addition of Al) that as Li content was increased from 6 to 7 moles it transformed to a tetragonal phase. Additionally, powders of cubic Li 6.24La 3Zr 2Al 0.24O 11.98 were hot-pressed at 1000 °C and 40 MPa. The hot-pressed material had a relative density of 98%. The room temperature total ionic conductivity of the hot-pressed material was 4.0 × 10 − 4 S/cm and the electronic conductivity was 2 × 10 − 8 S/cm.