Conventional Hot Pressing Research Articles

Densification mechanisms of UO2 consolidated by spark plasma sintering

Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, Qact, and the stress exponent, n, for SPS of nearly stoichiometric UO2. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate Qact and n, respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare Qact. The average values were Qact =96 kJ/mol (CHR), Qact = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250−450 kJ/mol), creep (350−550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol).This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS.

Effect of vacuum hot pressing on the bonding quality and heat transfer performance of plywood

Vacuum technology was applied to the hot-pressing technology of plywood production with phenolic resin (PF). For plywood with the same number of layers, veneers were hot-pressed with conventional and vacuum hot-pressing processes respectively at the same hot-pressing time. Varying layers (3, 9 and 15-layers) and spread quantities (220 g m−2, 260 g m−2, 300 g m−2) were applied. Heat transfer performance of the assembly, polymerization extent of PF, and bonding quality of plywood were examined by thermocouple and humidity sensors, Fourier transform infrared spectroscopy (FTIR), fluorescence microscope and bonding strength measurement. The results indicated that, compared to conventional hot pressing, the temperature of the central layer of the assembly vacuum hot-pressed was significantly increased by about 20–30 °C and the adhesive cured more quickly. The hot-pressing method had a significant influence on the bonding strength of plywood, with an enhancement of the bonding strength to wood failure ratio of 48.37% and 33.55%, respectively. The adhesive between adjacent laminates formed a continuously dense layer with more connection points, leading to excellent mechanical meshing and high bonding quality. In addition, vacuum hot pressing can eliminate the influence of adhesive position on bonding strength and make the mechanical properties of plywood tend to be uniform in thickness direction. Under the experimental conditions, the variable energy cost of 15-ply plywood with 1 m3 produced by vacuum hot pressing was 3.83% less than that of conventional hot pressing, and the plywood could still maintain good bonding strength at low spread quantity.

Oscillatory pressure sintering: A new method for preparing WC-Co cemented carbides

WC-10 wt% Co cemented carbides were prepared by a new oscillatory pressure sintering (OPS) process in the present work, and the influence of sintering temperature on their microstructure and mechanical properties was mainly investigated. The results were also compared to the sample prepared via conventional hot pressing (HP) under a similar consolidation condition. The optimum OPS temperature was ∼1310 °C; the denser sample with uniform microstructure could be obtained; the sample exhibited the highest flexural strength of 2431 MPa, Vickers hardness of 21.1 GPa and fracture toughness of 11.42 MPa m1/2. Moreover, the OPS-fabricated sample displayed much better mechanical properties than that of HP-fabricated sample, which might greatly be related to the improved grain boundary sliding, plastic deformation and mass diffusion during the OPS process. This work demonstrates a new approach to prepare high-performance cemented carbides.

Densification and Phase Transformation in Multi-Layered Graded Si3N4-TiN Components Produced by Field-Assisted Sintering.

The structural and/or functional design of multiphase ceramics, along with their processing, are timely research topics in the area of field-assisted sintering techniques, such as spark plasma sintering, especially for systems containing both electrically insulating and conductive phases. In the present study, spark plasma sintering of Si3N4–TiN composites was investigated by changing the TiN particle size and electrical current waveform. Their combined effects on both the densification behavior and α-to-β phase conversion of the Si3N4 matrix was studied and compared by means of a thermodynamic approach and dilatometric measurements. Through the control of TiN phase characteristics and heating mode, double-layered Si3N4-based components were also prepared using a one-step spark plasma sintering process, which was compared with conventional hot-pressing. It was shown that the size of the conductive TiN phase has a significant influence on the particle rearrangement, with the formation of a liquid phase, and the solution–diffusion–precipitation process, through the field-induced local heating and electrowetting mechanisms. Moreover, the contribution of current pulsing to the densification and α-to-β conversion of the layered Si3N4-based components was mostly dependent upon the particle size distribution and content of the TiN phase, indicating that the electric-field effect is dependent upon current path.

Oscillating pressure sintering of W–Ni–Fe refractory alloy

In this paper, we report the fabrication of 90W–7Ni–3Fe refractory alloy using a novel oscillating pressure sintering (OPS) technique. The technique is similar to conventional hot pressing (HP), excepting that an oscillating pressure replaces the static pressure in HP. The results show that compared to HP, OPS can significantly improve the sintering ability of the alloy by decreasing the sintering temperature required for the densification of the alloy and inhibiting grain growth. The sample obtained at 1250 °C exhibits an HV hardness of 450 HV0.5, which is not only much higher than that for the HP prepared sample, but also much higher than materials with similar compositions ever reported. The results clearly demonstrate that OPS is a very promising technique for the fabrication of high performance refractory alloys.

Influence of Distinct Manufacturing Processes on the Microstructure of Ni-Based Metal Matrix Composites Submitted to Long Thermal Exposure

Metal Matrix Composites (MMCs) are known for their remarkable properties, by combining materials from different classes. Ni-based MMCs are a promising group of heat-resistant materials, targeting aerospace applications. A discontinuously reinforced Inconel X-750/TiC 15 vol.% MMC was proposed for use in lighter, creep resistant turbine elements, with the aim to endure service temperatures up to 1073 K (800 °C). However, their microstructural stability at high temperatures for long periods of time remained to be further investigated. To address this need, specimens were produced by both conventional hot pressing and spark plasma sintering, using powders milled by low and high energy processes, followed by long isothermal aging. The treatments were conducted at 973 and 1073 K, for times between 50 and 1000 hours. The resulting samples were investigated with XRD and EDS techniques for phase analysis. In addition, measurements of hardness were made to monitor changes in mechanical behavior. It was found that, for each different manufacturing process, the amount, distribution and size of γ’ and other precipitates notably vary during the overaging process. Consequently, the amount of elements kept in solid solution also shifted with time. Furthermore, the study shows how distinct initial microstructures, resulting from diverse fabrication processes, differently impact the microstructural stability over long times of exposure to high temperatures.

Partial strengthening method for cold stamped B-pillar with minimal shape change

The partial strengthening method for pre-stamped panels both avoids excessive strength enhancement over the entire product and substantially reduces the processing cost compared to conventional hot press forming. In this paper, we propose two types of partial strengthening processes to selectively increase the local strength of a cold stamped B-pillar by utilizing an induction coil system and water spray quenching to induce the martensitic transformation in the target region. Thermo-mechanical FE analyses were conducted to evaluate the effect of the process parameters such as the quenching rate, quenching time, and heating width on the shape changes during the partial strengthening processes while simultaneously considering the thermal dilatation and transformation plasticity.

Carbon Fiber Reinforced Thermoplastics Molding by Using Direct Resistance Heating to Carbon Nanofilaments Grafted Carbon Fiber

In the automobile industry, carbon fiber reinforced thermoplastics (CFRTP) have attracted attention as potential materials to reduce the weight of the automobile body. In order to apply CFRTP to mass-produced automobile parts, it is necessary to develop the reduction of molding time and the impregnation method into the carbon fiber (CF) for the thermoplastic resin, which has relatively high viscosity. Although the conventional hot press molding uses only the heat transfer from the mold to the molding materials, it is expected to develop a new molding method for CFRTP using heat generation of the materials themselves to overcome these issues. As a method of heating the carbon fiber, there is a direct resistance heating method, in which carbon fiber is directly energized and heated by Joule heat. We have developed resistance welding methods in which carbon nanotube (CNT) grafted carbon fiber (CNT-CF) is used for the heating elements, and revealed that the higher welded strength is obtained by using CNT-CF instead of CF. Therefore, the carbon nanofilaments (CNF) grafted carbon fiber (CNF-CF) including CNF-CF is expected not only to be used as a resistance heating medium at the time of joining but also as a reinforcing fiber and as a self-heating member at the time of molding. In this study, we develop the CFRTP molding method by using direct resistance heating to CNF-CF in the hot press molding. CFRTP ([0°]20) with the volume fractions (Vf) of 40% are molded by conventional hot press and hot press with direct resistance heating to reinforcing fiber. CF or CNF-CF is used for reinforcement. CFRTP molded by hot press with direct resistance heating to CNF-CF indicated lower void content than CFRTP molded by hot press with direct resistance heating to CF. Compared to CFRTP molding by only hot press, hot press molding with direct resistance heating to CNF-CF can mold CFRTP with low void content.

Significantly enhanced dielectric permittivity and suppressed dielectric loss in Na1/2Bi1/2Cu3Ti4O12/poly(vinylidene fluoride) nanocomposites

We report significantly enhanced dielectric permittivity and suppressed dielectric loss in Na1/2Bi1/2Cu3Ti4O12/poly(vinylidene fluoride) (NBCTO/PVDF) nanocomposites. NBCTO nanoparticles (≈100 nm) synthesized via a chemical combustion method were used as a filler to enhance the dielectric permittivity of PVDF polymer. NBCTO/PVDF nanocomposites with various volume fractions of NBCTO were fabricated using conventional mixed powders and hot pressing. Microstructural analysis revealed good dispersion of NBCTO nanoparticles in the PVDF matrix compared to that of NBCTO micro-particles. The dielectric permittivity of NBCTO/PVDF nanocomposites increased with increasing NBCTO filler content. Interestingly, at 103 Hz, the nanocomposite with 50 vol% of NBCTO exhibited a largely enhanced dielectric permittivity of about 261 and a low dielectric loss of 0.4. Greatly increased dielectric permittivity in the NBCTO/PVDF nanocomposites can be ascribed to large interfacial spaces and short interparticle distances.

Micro electrical discharge drilling characteristics of conductive SiC–Ti2CN composite

An electrically conductive SiC-Ti2CN composite was fabricated from β-SiC and TiN powders with 2 vol% equimolar Y2O3-Sc2O3 additives by conventional hot-pressing. The composite (electrical resistivity of nearly 10-4 Ω·cm) was wire electro discharge machined into a specimen having a thickness of 500 μm. Entrance clearance, machining time, number of shorts, material removal rate (MRR), counts, and the effects of the capacitance and voltage of conductive SiC-Ti2CN composite during micro electrical discharge drilling were measured and compared to those of SUS304. As the unit discharge energy increased, entrance clearance, machining time, and number of shorts decreased. A decrement in the number of shorts was the main reason for decrements in entrance clearance and machining time. MRR was proportional to unit discharge energy. The study to determine the location of shorts during electro discharge drilling was also conducted. It was founded that the short circuit occurred mainly at the beginning (0–1000 cts) because of low electrical density, and right before the exit (3000–5000 cts) owing to the difficulty of removing debris. Both the capacitance and voltage tended to be inversely proportional to the number of shorts.

Fabrication of Fe–Cu matrix diamond composite by microwave hot pressing sintering

The FeCu pre-alloyed diamond tools drills were fabricated and characterized using microwave hot press sintering (MHPS) and conventional hot press sintering (CHPS). The characteristic of densification, microstructure and mechanical properties of samples were assessed. Compared to the CHPS, the MHPS showed evident advantages, including more uniform element distribution, shorter sintering time, lower sintering temperature and preferable mechanical properties. The influence of different temperature on the mechanical properties has been investigated, revealing that high sintering temperature is benefit for the mechanical properties of samples. Experimental results also indicate that the samples show excellent performance under MHPS. And the pre-alloyed metallic powder could be heated to the designed sintering temperature at a high heating rate (30 °C/min). The hardness and relative density were also detected to be 106.5 HRB and 96.2% by MHPS at 860 °C. And the rearrangement of solid particles is a determinant factor to impact the relative density and hardness. The possible mechanism was discussed.

Novel proton-conducting nanocomposites for hydrogen separation membranes

Design of oxide and nanocomposite materials with high mixed protonic-electronic conductivity such as lanthanide niobates and tungstates is encouraging approach in developing hydrogen separation membranes. This work aims at elucidating the relation of structure, oxygen and protonic mobility of such materials. La0.99Ca0.01NbO4, LaNb3O9 and Nd5.5WO11.25−δ were synthesized by Pechini and citrate route. Nanocomposites with LaNb3O9 and Ni + Cu were prepared by ultrasonic dispersion or mechanical treatment in a high energy mill and wet impregnation, then sintered using conventional thermal sintering and hot pressing. All obtained materials were characterized using XRD, SEM, TEM with EDX analysis, IR and Raman spectroscopy. The oxygen and proton mobility were studied by isotope exchange, unit cell volume and weight relaxation techniques. The proton conductivity was studied by Van der Pauw technique. The main phases were scheelite for La0.99Ca0.01NbO4, perovskite for LaNb3O9 and fluorite for Nd5.5WO11.25−δ, extended defects were observed agreeing with IR and Raman spectroscopy data. The oxygen mobility studies revealed two types of bulk oxygen related to two phases in samples. D2O exchange studies demonstrated very fast protonic transport in samples. H2O desorption experiments revealed the working temperature range being 300–450 °C. The protonic conductivity values (~10−4 Ω−1 cm−1 at 400 °C) agree with the literature data and are sufficiently high for the practical application. Proton tracer and chemical diffusion coefficients values are ~10−11 and ~10−3 cm2/s at working temperatures, respectively. Successful test of proton conducting membrane with Nd5.5WO11.25−δ based functional layer showing promising performance has been carried out.

Extreme Electromechanical Characteristics and Microstructural Features of Ferro-Piezoceramics Based on Lead Titanate

Microstructural features and electrophysical parameters of a number of solid solutions based on PbTiO3 are investigated. Solid solutions (Pb1–3/2x + z/2Nd x )(Ti1–y–zMn y Inz)O3 and Pb1–xCa x Ti1–y(W1/2Co1/2) y O3 with extreme values of the mechanical Q-factor and electromechanical anisotropy are chosen as the objects of investigation. The ferro-piezoceramic samples are obtained through conventional sintering and hot pressing. X-ray structural and microstructural studies are performed, and the complex elastic, dielectric, and piezoelectric parameters of experimental samples are measured. The frequency dependences of complex parameters of the experimental samples have been studied at frequencies of up to 40 MHz. It is established that the extreme values of the mechanical Q-factor and piezoelectric anisotropy of the investigated PbTiO3-based solid solutions are due to microstructural features associated with the technological regimes of ferropiezoceramic fabrication.

Optimum sintering temperature of high quality silicon nitride ceramics under oscillatory pressure

High quality silicon nitride ceramics were fabricated by an oscillatory pressure sintering (OPS) process and the effect of sintering temperature on the microstructure and mechanical properties of Si3N4 ceramics was investigated. Different from the specimen fabricated by the conventional hot pressing (HP) method, only β-Si3N4 grains were detected in all the OPS-fabricated specimens. Driving forces of densification and grain growth were accelerated aided by oscillatory pressure. The optimum sintering temperature for silicon nitride ceramics under oscillatory pressure is considered to be 1770°C; the highest average aspect ratio of β-Si3N4 grains (4.74) was achieved from the highly densified compact (99.86% of theoretical density). The specimen prepared at this temperature accompanied with the highest flexural strength, hardness and fracture toughness of 1348MPa, 16.04GPa and 12.8MPam1/2, respectively. The presence of oscillatory pressure is detected to promote the elimination of defects and accelerate the densification and grain growth kinetics of grains, thus contributing to the improvement of mechanical properties.

An oscillatory pressure sintering of zirconia powder: Densification trajectories and mechanical properties

AbstractThe densification trajectories and mechanical properties of zirconia ceramics obtained by oscillatory pressure sintering (OPS) process were investigated, during the sintering process an oscillatory pressure was applied at three stages. Current results indicated that at intermediate stage the oscillatory pressure revealed a favorable improvement of mechanical properties compared with conventional hot pressing (HP) and pressureless sintering (PS) procedures, while the enhancement was not obvious at initial stage. When the oscillatory pressure was applied at final stage, the OPS specimens exhibited the highest bending strength and hardness of 1455 ± 99MPa and 16.6 ± 0.31GPa compared with the PS and HP specimens. Considering the high elastic modulus and Moiré patterns observed in the OPS specimen, the oscillatory pressure applied at intermediate and final stages was detected to facilitate the sliding of grain boundary, plastic deformation of monolithic grains, the removal of pores and the strengthening of atomic bonds.

HIPed TiO2 dense pellets with improved photocatalytic performance

The effects of heating method and temperature on physical, structural and photocatalytic behaviors of TiO2 pellets prepared by conventional heating and hot isostatic pressing have been evaluated. The pellets of submicron TiO2 powders were heated to 600, 650, 700, 750 and 1000°C using both processing methods in order to compare anatase to rutile phase transformation and densification behaviors. Bulk densities and porosities were calculated using the Archimedes method. XRD analysis were performed to calculate anatase/rutile ratios. Microstructures were characterized using SEM. Photocatalytic experiments have been performed under full spectrum irradiation. Degraded methylene blue samples were periodically monitored through UV–vis spectrophotometer to determine degradation kinetics. Anatase to rutile transformation is slightly faster and densification is better for lower temperatures for conventional heating, however HIPing gives better densification above 750°C as it also retards rutile transformation. Mixed phase structures and HIPed samples showed the best photocatalytic performance which makes this method advantageous.

Effect of electric current on densification behavior of conductive ceramic powders consolidated by spark plasma sintering

The densification mechanism of conductive powders is revealed by comparing the electric current-assisted spark plasma sintering (SPS) of ZrN powder with conventional hot pressing (HP) carried out with the same powder and under the same pressure and temperature. To determine the actual temperature inside ZrN powder, a sacrificial thermocouple is directly inserted into the powder during the SPS process. The spatial distribution of the electric current passing through the powder is calculated using the finite element modeling. The porosity-interparticle neck area geometrical relationship is applied to estimate the electric current density inside the powder volume subjected to SPS. For the first time, by taking into account the explicit influence of the electric current effect on the SPS densification mechanism, the constitutive equations describing the electric current-assisted hot pressing of powders are developed. The densification mechanism of ZrN is determined by the inverse regression of the new SPS constitutive equations and by utilizing the experimental results on ZrN powder consolidation with and without the participation of the electric current effect.

Facile and economical fabrication of conductive polyamide 6 composites with segregated expanded graphite networks for efficient electromagnetic interference shielding

An efficient electromagnetic interference (EMI) shielding composite based on economical expanded graphite (EG) and polyamide 6 (PA6) was fabricated by a facial strategy in this work. A typical segregated structure was constructed by conventional mechanical mixing and hot pressing. EG sheets were selectively located at the boundary between PA6 regions, and thus constructed a well-connected segregate EG conductive network in the composite. Owing to the advantage of segregated network structure, EG effectively improved the EMI shielding effectiveness (SE) of PA6 at fairly low content. The effective EMI SE of 25 dB was achieved at 2.27 vol% EG loading. This result indicated the segregated PA6/EG composite can serve as an economical material for EMI shielding application.

Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering

The influence of sintering temperature on the microstructure and mechanical properties of Al2O3−20wt% ZrO2 composites fabricated by oscillatory pressure sintering (OPS) was investigated by means of X-ray diffraction, scanning electron microscopy, three-point bending test and Vickers indentation. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The optimum oscillatory pressure sintering temperature was found to be 1600°C; almost fully dense materials (99.94% of theoretical density) with homogeneous microstructure could be achieved. The highest flexural strength, fracture toughness and hardness of such composites reached 1145MPa, 5.74MPam1/2 and 19.08GPa when sintered at 1600°C, respectively. Furthermore, the oscillatory pressure sintering temperature could be decreased by more than 50°C as compared with the HP method, OPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.

Improved mechanical properties of Al2O3-25 vol% SiCw composites prepared by oscillatory pressure sintering

Abstract 25 vol% SiC whiskers reinforced Al2O3 composites (Al2O3-25 vol% SiCw) were prepared in the present work by using a novel oscillatory pressure sintering (OPS) technique. For comparison, such composites were also prepared by the conventional hot pressing (HP) process. It was found that the use of the external oscillatory pressure led to a higher densification, a more homogenous microstructure and a smaller grain size of Al2O3 matrix compared to the HP-fabricated specimen, thus improving room temperature flexural strength (741 ± 36 MPa), fracture toughness (8.18 ± 0.43 MPa m1/2) and hardness (21.32 ± 0.40 GPa) of the OPS-fabricated specimen. Hence, we believe that the novel OPS process provides a promising approach to fabricate high-performance ceramic matrix composites.