Conventional Hot Pressing Research Articles

Fine-grained BCZT piezoelectric ceramics by combining high-energy mechanochemical synthesis and hot-press sintering

Different stoichiometries of lead-free BaZr0.2Ti0.8O3–Ba0.7Ca0.3TiO3 (BCZT) prepared by mechanosynthesis and sintered by either conventional sintering (CS) or hot pressing (HP) techniques were studied to establish the dependence of piezoelectric and dielectric properties on sintering parameters and microstructure. All synthesized stoichiometries showed a pseudocubic perovskite phase with homogeneously distributed A- and B-cations in the structure. The BCZT retained the pseudocubic symmetry after sintering and an average grain size <1.8 µm was obtained in all cases. HP sintering hindered the secondary phase segregation observed in the CS ceramics and increased the relative density. Piezoelectric coefficients (d33) ranging from 5.1 to 21 pC/N and from 10.0 to 88.0 pC/N were obtained for CS and HP ceramics, respectively, despite the pseudocubic symmetry and the fine grain size. The higher d33 values for the HP ceramics are a consequence of the higher density, better chemical homogeneity and lower sintering temperature and time required for the mechanosynthesized BCZT powders with high sintering activity.

Influence of pre-treatment with non-thermal atmospheric pressure plasma on bond strength of TP340 titanium-PEEK direct bonding

Direct bonding of a TP340 titanium to PEEK by hot pressing via pre-treatment of non-thermal atmospheric pressure plasma jet has been demonstrated. The plasma irradiation effect on the bonding surface on the bond strength after hot pressing was investigated. The tensile shear strength of TP340-PEEK joined by hot pressing after plasma pre-treatment was measured by comparing specimens bonded using conventional hot pressing and those bonded using adhesives. The plasma treatment to the TP340 side resulted in the formation of TiO2, which is chemically fed to oxide formation due to the irradiation of oxygen radicals generated by the plasma, resulting in a bond strength of less than 1 MPa, similar to the bond strength of the untreated specimens. The plasma irradiation effect on the PEEK side on the bond strength of TP340-PEEK bonded samples was also investigated. The bonding strength was increased by plasma irradiation to PEEK. As the plasma irradiation time was increased, the bonding strength gradually increased to 9.2 MPa, which is about 19 times higher than the bonding strength without plasma irradiation. These results suggest that oxygen radicals in the atmospheric pressure RF plasma jet produced oxygen-containing surface functional groups on the PEEK surface, which increased the strength of the TP340-PEEK direct joining.

Investigation on electrochemical corrosion behavior and mechanical properties of Fe-nano particles produced by high-energy ball milling technique

Abstract This study focuses on converting iron particles from grinding sludge, after removing impurities, into Fe-nanoparticles using high-energy ball billing. The goal is to examine the corrosion behaviors and mechanical properties of these Fe-nanoparticles. Nanostructured Fe-powder was synthesized through a process involving 10 h of high-energy ball milling, followed by conventional hot pressing and sintering. Structural and microstructural properties were thoroughly examined using techniques such as X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and elemental diffraction spectroscopy. Upon sintering, SEM and TEM analyses unveiled the formation of a nanostructured alloy within the samples. Notably, the milled sample exhibited high hardness value, measuring at 155 HV. However, it is noteworthy that the un-milled sample demonstrated superior compression strength compared to its milled counterpart. Furthermore, the corrosion behavior of the samples was evaluated through electrochemical corrosion studies. Interestingly, the sample subjected to 10 h of milling (coin number 5) displayed a significantly lower corrosion rate, measuring at 1.3921 mm/year, suggesting enhanced corrosion resistance attributed to the nano structuring process.

Enhancing thermoelectric properties of Ca3Co4O9 ceramics through oscillatory pressure sintering

Oscillatory Pressure Sintering (OPS) has been demonstrated to be a favorable manufacturing approach for the control of fine and uniform distribution of grains in various ceramic systems, which can lead to enhanced densification and thus improved physical and mechanical properties. In this study, OPS was utilized to prepare pure Ca3Co4O9 thermoelectric material. The results show that the electrical properties and thermoelectric properties of Ca3Co4O9 prepared by OPS are superior to those of samples sintered by conventional hot pressing (HP) without oscillation. This may be attributed to the better alignment of plate-like grains in the OPS samples, which increase electrical conductivity, and also the higher density of grain boundaries that enhance the phonon scattering and, thus, the reduction of thermal conductivity. This study suggests that a straightforward oscillatory pressure sintering approach can be utilized to obtain high-quality and high-performance thermoelectric materials without the need for doping, templating, or high-temperature sintering.

Sintering behavior and mechanical properties of β-SiC ceramics under oscillatory pressure

Sintering behavior and mechanical properties of β-SiC ceramics under oscillatory pressure are investigated in this study. Compared with conventional hot pressing (HP), samples sintered under oscillatory pressure show higher densification rate and density, and thus lowering sintering temperature. Grain size of samples sintered via hot oscillatory pressing (HOP) is smaller than that of samples sintered via conventional HP. Both samples sintered via HOP (HOP-1850) and HP (HP-1850) at 1850 °C contain large number of defects (nano-spaced stacking faults (nSFs), nanotwins, and dislocations). However, sample HOP-1850 possesses more nSFs and high-order nanotwins in grain interior. In addition, dislocation density in the interior of grains containing nSFs and nanotwins for sample HOP-1850 is nearly twice that of sample HP-1850. Compared with sample HP-1850, sample HOP-1850 shows higher hardness (25.03 ± 0.63 GPa) and toughness (5.08 ± 0.02 MPa∙m1/2). The highest flexural strength of sintered samples under oscillatory pressure is 813 ± 44 MPa, which is 30 % higher than that of hot-pressed sintered sample under the same temperature.

Reducing Bonding Temperature and Energy Consumption in Electronic Packaging Using Flash Electro-Thermal Carbon Fiber Heating Elements.

Semiconductor packaging based on an epoxy molding compound (EMC) currently has several disadvantages including warpage, limited processing area, and variability that all negatively affect cost and production yield. We propose a facile EMC molding process method using a flash electro-thermal carbon fiber heating (FE-CH) device based on carbon fiber-based papers to manufacture an EMC molded to a copper substrate (EMC/Cu bi-layer package) via Joule heating, and using this device, a modified cure cycle that combines the conventional cure cycle (CCC) with rapid cooling was performed using FE-CH to reduce the curvature of the EMC/Cu bi-layer package. Compared to the conventional hot press process, which uses 3.17 MW of power, the FE-CH process only uses 32.87 kW, resulting in a power consumption reduction of over 100 times when following the CCC. Furthermore, the FE-CH-cured EMC/Cu bi-layer package exhibits mechanical properties equivalent to those of a hot press-cured specimen, including the degree of cure, elastic modulus, curvature, bonding temperature, residual strain, and peel strength. The modified cure cycle using the FE-CH results in a 31% reduction in residual strain, a 32% reduction in curvature, and a 47% increase in peel strength compared to the CCC, indicating that this new process method is very promising for reducing a semiconductor package's price by reducing the process cost and warpage.

Aqueous Cold Sintering of Li-Based Compounds.

Aqueous cold sintering of two lithium-based compounds, the electrolyte Li6.25La3Zr2Al0.25O12 (LLZAO) and cathode material LiCoO2 (LCO), is reported. For LLZAO, a relative density of ∼87% was achieved, whereas LCO was sintered to ∼95% with 20 wt % LLZAO as a flux/binder. As-cold sintered LLZAO exhibited a low total conductivity (10-8 S/cm) attributed to an insulating grain boundary blocking layer of Li2CO3. The blocking layer was reduced with a post-annealing process or, more effectively, by replacing deionized water with 5 M LiCl during cold sintering to achieve a total conductivity of ∼3 × 10-5 S/cm (similar to the bulk conductivity). For LCO-LLZAO composites, scanning electron microscopy and X-ray computer tomography indicated a continuous LCO matrix with the LLZAO phase evenly distributed but isolated throughout the ceramics. [001] texturing during cold sintering resulted in an order of magnitude difference in electronic conductivity between directions perpendicular and parallel to the c-axis at room temperature. The electronic conductivity (∼10-2 S/cm) of cold sintered LCO-LLZAO ceramics at room temperature was comparable to that of single crystals and higher than those synthesized via either conventional sintering or hot pressing.

Oscillatory pressure-assisted sinter forging for preparation of high-performance SiC whisker reinforced Al2O3composites

In this work, a novel process, oscillatory pressure-assisted sinter forging (OPSF), for the preparation of high-performance ceramic composites was reported. Compared with the samples made by conventional sinter forging (SF) and hot oscillatory pressing (HOP), the SiC whisker reinforced Al₂O₃ composites (SiC_w–Al₂O₃) prepared by OPSF at the same temperature exhibited a higher density and significantly improved the mechanical properties. The improvements in densification and performance are attributed to simultaneous enhanced shear deformation at both macro- and micro-scales, resulting from the combination of die-free configuration and oscillatory pressure of OPSF. And the strength of grain boundary is greatly increased when the temperature reaches 1600 ℃ of OPSF, due to that the grain-boundary sliding became pronounced at higher temperatures. The current results shed light on a powerful technique for preparing ceramic composites, which is likely applicable to other systems.

Lignin-based epoxy composite vitrimers with light-controlled remoldability

Vitrimers open new possibilities in the reprocess of epoxy and other thermosets. However, direct heating is not practical on many occasions, and the waste vitrimers would cause great harm to the environment. In this work, we propose to use kraft lignin (KL) to fabricate vitrimer with reprocessability and environmental friendliness. The lignin-based epoxy vitrimer was fabricated by blending epoxy-modified KL and poly(ethylene glycol) bis(carboxymethyl) ether (PEG-DCM). The obtained lignin-based epoxy vitrimer (EML/PEG-DCM) showed good light-to-heat capability. Under the infrared radiation (808 nm, 1 W cm−2) for only 30 s, the surface temperature of EML/PEG-DCM was over ∼148 °C, and reached the maximum at ∼231 °C for 5 min. This good light-to-heat effect can activate the dynamic 3D cross-linking networks and repair the vitrimer. The energy consumption of the light-controlled remolding process is only one-thousandth of the conventional hot-press. This study not only helps to explore the natural characteristics of lignins, promoting their functional and intelligent utilization, but also provides a new raw material platform for the development of green vitrimer materials.

Microstructure and properties of AlCoCrNiFe high-entropy alloy sintered by hot oscillating pressing

In this paper, we report for the first time the successful preparation of bulk AlCoCrNiFe high-entropy alloys (HEAs) by hot oscillating pressing (HOP). The effects of sintering temperature on sintering microstructure, Vickers hardness and corrosion resistance properties of the resulting samples were systematically characterized and compared with samples prepared by conventional hot pressing (HP). The structure of HOPed samples was both FCC and BCC phases (disordered and ordered phases). As the sintering temperature increased, the original particle boundary of HOPed samples was gradually disappeared, and its microstructure uniformity was improved. The significant microstructural coarsening (thickness of phase lamellar increasing) was occurred and the FCC phase volume fraction was increased. Moreover, the density of samples was increased as well. When the sintering temperature was 1100 °C, the highest density could be reached up to about 99.23%. Then its maximum Vickers hardness was nearly 488.80 HV1. While the internal defects of the material were reduced because its original particle boundary disappeared and microstructure uniformity improved, which were conducive to the corrosion resistance improvement. Thus, its minimum corrosion current density and corrosion rate in 3.5 wt% NaCl solution were only 1.5 μA/cm2 and 0.01 mm/year, respectively. The results revealed that the samples prepared by HOP at the same sintering temperature exhibited higher density, fewer original particle boundaries, higher hardness, and better corrosion resistance than the sample prepared by HP. The current study suggests that HOP is promising for making high-quality AlCoCrNiFe HEAs.

SHAPE, ORIENTATION, INTERACTION, OR DISPERSION: VALORIZATION OF THE INFLUENCE FACTORS IN NATURAL RUBBER NANOCOMPOSITES

ABSTRACT The addition of nanoparticles as reinforcing fillers in elastomers yields nanocomposites with unique property profiles, which opens the door for various new application fields. Major factors influencing the performance of nanocomposites are studied by varying the type and shape of nanoparticles and their dispersion in the natural rubber matrix. The industrial applicability of these nanocomposites is put into focus using two types of graphene and a nanoscale carbon black, all commercially available, and scalable processing techniques in the form of a highly filled masterbatch production via latex premixing by simple stirring or ultrasonically assisted dispersing with surfactant followed by conventional two-roll milling and hot pressing. Different processing and measurement methods reveal the potential for possible improvements: rheology, curing behavior, static and dynamic mechanical properties, swelling, and fire behavior. The aspect ratio of the nanoparticles and their interaction with the surrounding matrix prove to be crucial for the development of superior nanocomposites. An enhanced dispersing method enables the utilization of the improvement potential at low filler loadings (3 parts per hundred of rubber [phr]) and yields multifunctional rubber nanocomposites: two-dimensional layered particles (graphene) result in anisotropic material behavior with strong reinforcement in the in-plane direction (157% increase in the Young's modulus). The peak heat release rate in the cone calorimeter is reduced by 55% by incorporating 3 phr of few-layer graphene via an optimized dispersing process.

Holding time optimization of spark plasma sintering for PZT-based soft ceramics

The microstructure, density, dielectric and piezoelectric properties of the soft PZT-based ceramics sintered by conventional ceramic technology (CCT), hot pressing (HP), and spark plasma sintering (SPS) were studied. For SPS method, the holding time of the sample at the fixed sintering temperature (970 °C) was optimized. It was found that despite their highest density the ceramics sintered by SPS method were characterized by lower dielectric and piezoelectric parameters compared to other sintering technologies used. Possible reasons for the influence of technological factors on macroscopic responses are discussed.

Sintering behavior of alumina whisker reinforced zirconia ceramics in hot oscillatory pressing

Alumina whisker reinforced zirconia ceramic composite was prepared by both hot oscillatory pressing (HOP) and conventional hot pressing (HP). The results show that compared with HP, HOP can significantly increase the final density and densification rate of the material. Analysis of densification kinetics reveals that the predominant densification mechanism transits from grain boundary sliding in the beginning to the diffusion in the later stage. The main effect of the oscillating pressure is to increase the densification rate in the process of grain boundary sliding. The current study suggests that HOP is a promising technique for densifying whisker reinforced ceramics.

Hot oscillatory pressing of B4C ceramics for improving densification and mechanical properties

AbstractB4C ceramics were prepared by conventional hot pressing (HP) and hot oscillatory pressing (HOP). The results show that at the temperature and pressure studied here, the main densification mechanism of HP is grain‐boundary sliding, whereas that of HOP is plastic deformation caused by dislocation gliding, indicating that the oscillatory pressure promoted plastic deformation of the material. The hardness of B4C prepared by HOP reaches 39.5 GPa, which is much higher than the hardness of sample prepared by HP. The current study suggests that HOP is promising for preparing high‐performance B4C ceramics.

Synthesis of low-cost Ti4O7 membrane electrode for electrooxidation of tetracycline under flow-through conditions: Performance, kinetics and mechanism

The high cost of anode material and the limitation of liquid-mass transfer on anode surface in electrochemical advanced oxidation processes always restrict the removal efficiency of organics. To overcome the limitations, this study synthesized a low-cost Ti4O7 membrane electrode by simple sol-gel method, and developed a flow-through electrooxidation system based on Ti4O7 membrane electrode. The membrane electrode presented high crystallinity, high specific surface area (10.18 m2/g), concentrated pore sizes distribution (0.1–1 µm), and high oxygen evolution potential (2.2 V vs. SHE). Furthermore, compared with stirring conditions, the flow-through conditions could enhance the liquid-mass transfer on anode surface, resulting in a high tetracycline degradation efficiency (97.24%) and a low energy consumption (0.18 kWh/gDOC). High degradation rates of tetracycline (more than 95%) were both observed with the initial tetracycline concentration ranged from 10 to 50 mg/L. The increase of pipeline pressure and current density had positive influence on tetracycline degradation. Hydroxyl radicals and sulfate radicals produced on the electrode surface are the main oxidants for tetracycline degradation. The degradation pathway of tetracycline included oxygenation, hydroxylation, demethylation, decarbonylation, ring-open, and C-N bond cleavage. Compared with materials prepared by conventional hot pressing and plasma spraying method, this simply synthesized Ti4O7 membrane electrode exhibited efficient oxidation capability and competitive electric cost for contaminants degradation, demonstrating its practicability, feasibility, and facilitation for the potential application.

Densification mechanism and microstructure development of Fe-Ni alloys consolidated by field assisted sintering

Fe-xNi alloys (x = 0–5.0 in wt%) were consolidated by conventional sintering (CS), hot pressing (HP), and field assisted (activated) sintering technique (FAS or FAST), and their densification, microstructure, mechanical properties were comparatively investigated, particularly focusing on the field or current effects of FAS technique. The Ni addition promoted the densification and suppressed the grain growth in Fe-Ni alloys consolidated by both CS and FAS, and the FAS produced the Fe-Ni alloys with higher apparent density and smaller grain size. Consequently, the Fe-Ni alloys fabricated by FAS exhibited a significant improvement in the bulk hardness compared to those consolidated by CS and HP. The element distribution mapping by energy dispersive spectroscopy (EDS) indicated that Ni was mainly present along the grain boundaries in the FAS specimen. Furthermore, the diffusion couple experiment showed that the FAS enhanced the Ni diffusion along the grain boundaries. The obtained results indicated that the electric current together with mechanical pressure in FAS promoted the Ni diffusion along the grain boundaries, which resulted in the enhanced densification with suppressed grain growth.

Sintering behavior and mechanical properties of alumina ceramics exposed to oscillatory pressure at different sintering stages

The effect of oscillatory pressure on the sintering behavior and mechanical properties of alumina ceramics was studied at different stages of sintering. Compared to ceramics prepared via conventional hot pressing, samples consolidated by hot oscillatory pressing exhibit the higher density and smaller grain size along with superior hardness and fracture strength. The oscillatory pressure was beneficial to improve the sintering behavior of alumina ceramics, which was independently of the step of sintering. Furthermore, the higher was the temperature at which the oscillatory pressure was applied, the better were the mechanical properties and, consequently, the performance of ceramics.

Chemical and technological aspects of increasing the functional characteristics of hard piezoceramics

Objectives. Ferroelectrically hard piezoelectric ceramics are in demand for high-power applications in piezotransformers, ultrasonic emitters, and piezo motors, which requires a combination of high piezoelectric characteristics and mechanical quality factors in it. The aim of this research was to reveal the main regularities in the microstructure and functional characteristic formation of ferroelectrically hard piezoceramics based on two widespread chemical systems, Pb(ZrxTi1-x)O3 and (Na1-xKx)NbO3, through various technological modes of production. In this study, two fundamentally different technological ways of forming a dense microstructure on the example of above systems have been employed to obtain the best set of dielectric, piezoelectric, and mechanical parameters for practical applications. In the case of lead-containing ceramics, various sintering technologies have been used, including conventional ceramic, hot pressing, and spark plasma sintering.Methods. The microstructure of the piezoelectric ceramics was investigated using electron microscopy, and the functional characteristics were assessed in terms of mechanical and piezoelectric properties. The density values were determined by hydrostatic weighing in octane, the relative dielectric permittivity was measured using an LCR meter, and the values of the piezoelectric coefficient and mechanical quality factor were gathered using the resonance–antiresonance method.Results. This research has identified that spark plasma sintering technology makes it possible to obtain high-density samples, which contain a homogeneous microstructure and double the figure-of-merit values, for use in high-power piezoelectric devices that operate at piezoresonance frequencies. It also found that the addition of a small amount of CuNb2O6 (x = 0.025) to lead-free solid solutions leads to the formation of a liquid phase during sintering, thereby creating a compacted microstructure with relative density values (96%) that have practical limitations in conventional ceramic technology. An increase in both the piezoelectric and mechanical properties, which leads to a twofold increase in the values of the quality indicator, was also observed.Conclusions. It is possible to increase, and even to double, the functional characteristics of both lead-containing and lead-free ferroelectrically hard piezoceramics by varying the technology used in the manufacturing process. By using spark plasma sintering technology with lead-containing ceramics, it is possible to reduce the optimum sintering temperature by 200 °C and the sintering time by more than 20 times, thus reducing production costs.

Preparation of Cu2Se thin films by vacuum evaporation and hot-pressing

Abstract A method using vacuum evaporation and hot-pressing to prepare Cu2Se thin films on Cu substrates is developed. The effects of the heating temperature, heating time, and pressure on the crystallinities of the Cu2Se thin films were studied. The structure, morphology, and composition properties of the Cu2Se thin films were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results showed that the crystallinities of the Cu2Se thin films improved as the heating temperature was increased from 200 °C to 280 °C. A low pressure of 2 MPa and a short time of 5 min were sufficient to obtain highly crystalline Cu2Se thin films at 280 °C. The fabricated Cu2Se/Cu structure could directly convert heat to electricity with a thermoelectric potential of 12.6 mV at 400 K and 17.8 mV at 500 K. Compared with conventional hot-pressing of bulk materials, this method requires lower temperatures and smaller pressures.

Improving the Properties of Fast-Growing Chinese Fir by Vacuum Hot Pressing Treatment

Chinese fir was compressed by vacuum hot pressing and conventional hot pressing at different temperatures (180°C, 200°C and 220°C), respectively. The color parameters of the heat-compressed sample were measured, the relative mechanical properties of the material were tested and changes in the chemistry of fir were investigated using Fourier transform infrared spectroscopy (FTIR) and Xray photoelectron spectroscopy (XPS). The results indicated that the color difference between compressed and untreated wood increased gradually with the increase of temperature. Compared with the conventional hot pressing treatment, the color difference (ΔE*) of the Chinese fir treated by vacuum hot pressing decreased by 43.73%, 69.91%, and 77.17%, respectively. The mechanical properties (bending elastic modulus and bending strength) of Chinese fir treated by vacuum hot pressing were significantly improved. The 24-hour water absorption thickness expansion rate of fir treated by vacuum hot pressing is smaller than that of conventional hot pressing. It is implied that vacuum hot pressing treatment is an effective method to produce compressed wood, which can improve the mechanical properties and dimensional stability of wood, and reduce the influence of carbonization on wood color.