Hot-pressing Process Research Articles

Development of Water-Soluble Composite Salt Sand Cores Made by a Hot-Pressed Sintering Process

Development of Water-Soluble Composite Salt Sand Cores Made by a Hot-Pressed Sintering Process

Large scalable, ultrathin and self-cleaning cellulose aerogel film for daytime radiative cooling

Passive cooling strategy shows great potential in mitigating global warming and reducing energy consumption. Because of the high emissivity in the atmospheric transparency window (λ ≈ 8–13 µm), cellulose is considered as a good candidate for radiative cooling. However, traditional cellulose coolers generally show poor solar reflection and can be polluted by dust outside, thereby resulting in poor daytime cooling efficiency. To address these drawbacks, we developed sustainable cellulose nanowhiskers (CNWs)/ZnO composite aerogel films with favorable optical performance, mechanical robustness, and self-cleaning function for efficient daytime radiative cooling, which can be achieved via freeze casting and hot-pressing process. Due to formation of multi-level porous structure and chemical bonds (Si-O-C/Si-O-Si), such aerogel film exhibited high solar reflectance (97%) and high infrared emittance (92.5%). It achieved a sub-ambient temperature drop of 6.9 °C under direct sunlight in hot weather. Most importantly, the surface roughness and low surface energy enable cellulose aerogel film hydrophobicity (contact angle = 133°), thereby resulting in an anti-dust function. This work provides insight into the design of sustainable thermal regulating materials to realize carbon neutrality.

Significant Progress for Hot-Deformed Nd-Fe-B Magnets: A Review.

High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due to their fine grain sizes of 200-400 nm, which are smaller by more than an order of magnitude compared to the traditional sintered Nd-Fe-B magnets. However, the current available coercivity of HD magnets is not as high as expected according to an empirical correlation between coercivity and grain size, only occupying about 25% of its full potential of the anisotropy field of the Nd2Fe14B phase. For the sake of achieving high-coercivity HD magnets, two major routes have been developed, namely the grain boundary diffusion process (GBDP) and the dual alloy diffusion process (DADP). In this review, the fundamentals and development of the HD Nd-Fe-B magnets are comprehensively summarized and discussed based on worldwide scientific research. The advances in the GBDP and DADP are investigated and summarized based on the latest progress and results. Additionally, the mechanisms of coercivity enhancement are discussed based on the numerous results of micromagnetic simulations to understand the structure-property relationships of the HD Nd-Fe-B magnets. Lastly, the magnetization reversal behaviors, based on the observation of magneto-optic Kerr effect microscopy, are analyzed to pinpoint the weak regions in the microstructure of the HD Nd-Fe-B magnets.

A high-performance, full-degradable bioinspired newspaper-based composite enhanced by borate ester bonds

Sustainable construction materials with high performance serve as an important role in our dairy life. However, constructing strong, green and cost-effective structural materials from waste feedstocks is still challenging. Inspired by the borate chemistry in natural plant cell walls, a high-performance, full-degradable and multifunctional newspaper (NP)-based laminated composite was successfully fabricated by boric acid (BA) and poly(vinyl alcohol) (PVA) via borate ester bonds and hydrogen bonds under simple hot-pressing process. Owing to the strong covalent borate ester bonds and abundant hydrogen bonds crosslinking, the resultant b-NP/PVA/BA composite exhibited outstanding tensile strength (147.83 ± 1.60 MPa), elongation at break (15.43 ± 0.56%) and toughness (17.80 ± 0.78 MJ/m3), which exceeded most wood-based composites and all reported NP-based composites. The b-NP/PVA/BA composite also showed excellent self-healing property and could be biodegraded completely within 4 months in soil. Furthermore, the obtained composite was useful for panel overlayers due to its excellent mechanical enhancement effect, antimildew and healthy properties. Notably, the cost-effective and green raw materials, as well as the facile and eco-friendly preparation, offered a competitive strategy to develop high-performance and full-biodegradable composites from waste resources, which can be extensively utilized in the fields of building, furniture and packaging.

Mo2C interface layer: effect on the interface strength and cutting performance of diamond/Fe-Ni-WC composites

The weak interfacial strength between Fe-based matrix and diamonds causes diamond particles to fall off prematurely, significantly affecting the cutting efficiency of diamond composites. This work aimed to optimize the interfacial microstructure and mechanical properties of the diamond/Fe-Ni-WC composites by coating a Mo2C layer on the diamond particles. The Mo2C-coated diamonds were prepared via the molten salt method. Diamond/Fe-Ni-WC composites were sintered by the hot-pressing process under a vacuum. The bending strength of composites with Mo2C-coated diamonds was improved from 804 to 996 MPa with the increased coating time. The energy dispersive spectroscopy scanning indicated that the Mo2C coating layer changed the interfacial composition between the matrix and the diamonds from Fe–C alloy to Mo2C, improving the interface strength. Moreover, the diamond protrusion heights of composites were raised by adding a Mo2C interface, resulting in an improvement in the cutting efficiency of impregnated diamond tools.

Preparation and Properties of Medium-Density Fiberboards Bonded with Vanillin Crosslinked Chitosan.

An eco-friendly medium-density fiberboard (MDF) was prepared using vanillin (V) crosslinked chitosan (CS) adhesive through a hot-pressing process. The cross-linking mechanism and the effect of different proportions of added chitosan/vanillin on the mechanical properties and dimensional stability of MDF were investigated. The results showed that vanillin and chitosan are crosslinked to form a three-dimensional network structure due to the Schiff base reaction between the aldehyde group of vanillin and the amino group of chitosan. At the same time, when the mass ratio between vanillin/chitosan was 2:1, MDF obtained the best mechanical properties, the maximum modulus of rupture (MOR) of 20.64 MPa, the mean modulus of elasticity (MOE) of 3005 MPa, the mean internal bonding (IB) of 0.86 MPa, and the mean thickness swelling (TS) of 14.7%. Therefore, the MDF bonded with V-crosslinked CS can be a promising candidate for environmentally-friendly wood-based panels.

Ion conducting elastomer designed from thiourea-based dynamic covalent bonds with reprocessing capability

Developing ion conducting elastomer (ICE) that possesses reprocessing capability is highly desirable to avoid resource wastage and environmental crises. However, elastomers with covalent cross-linking are difficult to reprocess due to their thermosets properties. To address this challenge, a thiourea-based dynamic covalent network was employed to design the ICE. An analysis of the interaction between the lithium salt and the polymer chain resulted in the selection of LiTFSI. Furthermore, the finding from the analysis confirmed that not only the formation but also the behavior of the dynamic covalent network was affected by the interactions between the polymer chain and lithium salts. The resulting dynamic covalent elastomer can be repeatedly reprocessed through various methods (hot-press and solution processes) and exhibits proper dimensional stability and ionic conductivity (∼4.6 × 10−4 S/cm at 25 °C).

Consolidation of aerospace-grade high-temperature thermoplastic carbon fiber composites via nano-engineered electrothermal heating

A carbon nanotube (CNT) electrothermal heater is shown to enable consolidation of high-temperature thermoplastic composite laminates for the first time, leveraging the CNT film's stable operation in air above 500 °C. This out-of-oven (OoO) conductive heating method without applied pressure (i.e., vacuum bag only) is used to process aerospace-grade carbon fiber/polyether ether ketone (PEEK) composite prepreg, which requires heating to 380–400 °C and is normally processed with significant applied pressure (∼2 ± 1 MPa) via a hot-press or autoclave. Micro-computed tomography verifies that the processed laminates exhibit low porosity of ≤0.1 vol%, which more than meets the desired target of <1 vol% voids. A range of thermophysical (including degree of crystallinity) and mechanical (interfacial shear strength and bending) tests show equal or improved properties vs. both hot press and autoclave processing. The CNT heaters in the OoO approach are observed to most closely match the desired processing cycle, with the observed benefit of being the only process studied to hit the target range of % crystallinity. The energy consumption comparison indicates a 98% energy saving of the CNT heater compared to the hot press. This study demonstrates autoclave-quality processing of high-temperature thermoplastic composite structures via conformal CNT heaters that can be fabricated efficiently and effectively in an out-of-autoclave (OoA) process.

Study of the hot-pressing sintering process of diamond/copper composites and their thermal conductivity

Diamond/copper composites have been widely studied as high thermal conductivity (TC) materials for heat dissipation in electronic integrated devices. In this study, diamond/copper composites with fin structures were prepared by hot-pressing sintering. The relative density and TC of the composites were analyzed for different diamond particle sizes, diamond volume fractions, sintering temperatures and sintering pressures. The relative density of the composites decreases significantly with increasing diamond particle size and volume fraction. The TC of the composite was found to be a maximum of 564.2 W/m·K at 1400 N sintering pressure and 900 °C sintering temperature when the diamond particle size was 230 µm and the volume fraction was 60%. In addition, the coefficient of thermal expansion (CTE) of the diamond/copper composite was 7.01 × 10−6 K−1 at 1400 N sintering pressure and 900 °C sintering temperature, which meets the packaging requirements for electronic integrated devices. The heat dissipation experiment of the diamond/Cu microchannel heat sink (MCHS) showed that the surface temperature of the heat source can be controlled at 59.9 °C when the inlet velocity was 0.4 m/s, achieving a cooling of 40.1 °C.

Correlation of microstructure, hardness, and electrical conductivity of hypereutectic Al-Si/B4C composites manufactured by hot pressing technique and subjected to hot extrusion

ABSTRACT B4C (5, 10 and 15 wt %) particles were added to hypereutectic Alumix-231® metal powders to produce composite material. The mixed powders were blended in a triaxial mixer. The production of compact samples was carried out by a hot press process. A hot extrusion secondary treatment was applied to some of the produced samples. Density measurements, microstructure examinations, and measurements of hardness and electrical conductivity were conducted on the samples. Primary Si particles and Cu-, Mg- rich intermetallic phases were observed within the microstructure of the sample produced from pure Alumix-231®. B4C particles seemed to be concentrated at the grain boundaries in the hot press samples. Through the secondary process of extrusion, primary Si particles and B4C particles exhibited mechanical fiberization-like dispersion. The samples with a density of over 97% were produced. The hardness of the samples increased with increasing wt % B4C particle ratio. The measured hardness values of the extruded samples were higher than the hot press samples. The extruded samples also showed high electrical conductivity values.

Enhanced mechanical properties of porcelain ceramic tile/Kevlar fabric composite with bio‐inspired shell‐like structure

AbstractCeramic materials with high strength, toughness, and excellent impact resistance are urgently required for many structural applications, but these mechanical properties are difficult to achieve in traditional ceramic tiles due to their inherent brittleness. Inspired by the specific structure of shells, the multilayered ceramic tile/Kevlar fabric composite with a bio‐inspired shell structure was successfully fabricated via a surface hydroxylation followed by simple hot press process. It is found that the composites have representative step‐like fracture behaviors rather than brittle fracture, which has been proven to possess a better ability of mechanical performance and noncatastrophic failure behavior compared to same‐thickness ceramic tile. Specifically, the bending strength, fracture toughness, and fracture work of the composite with a 15‐tier structure come to 836.5 ± 12.5 MPa, 14.6 ± .2 MPa·m1/2, and 7228.8 ± 108.4 J·m1/2, which are even better than those of reported advanced materials. Such fracture‐resistant behaviors are correspondent to the strengthening effects of the crack deflection, interfacial debonding, and fiber pull out, accompanied by bio‐inspired structure and appropriate bonding state between brittle or ductile layers. This resin or fabric content can be used as well as the slip systems to transfer the internal stress in time to consume more fracture energy per unit length and prevent risky brittle fracture, while carrying loads. We expect these findings to provide vital guidance for promoting the applications of traditional ceramics in bio‐inspired high‐performance composites for actual ceramic manufacturers.

Optimization of precision molding process parameters of viscoelastic materials based on BP neural network improved by genetic algorithm

Micro-structured optical components (MOC) are popular due to their unique optical and mechanical properties. Glass molding processing (GMP) is currently the main method of MOC manufacturing. In GMP, the process parameters determine the efficiency and quality of the glass processing. However, the optimization of their multi-objective molding process parameters has not been studied comprehensively enough. In this paper, a thermocompression model of a viscoelastic material and a mold is developed using coupled thermal-structural analysis in Abaqus. In addition, the data was analyzed to investigate the influence of process parameters on GMP performance. Then, a BP neural network model was built, and the model was trained and tested. A prediction model that can reasonably reveal the relationship between the process parameters and the molding effect was obtained. A multi-objective optimization algorithm GA was used to optimize the design of the hot-pressing process parameters. At last, a set of process parameters with the best forming effect was obtained. This study provides valuable insights into optimizing glass molding process parameters for practical production.

High microwave dielectric constant and improved thermal stability of functionalized Zn0.15Nb0.3Ti0.55O2-filled polyolefin composites

This paper investigated the structure, microwave dielectric properties and thermal properties of xZn0.15Nb0.3Ti0.55O2 @VTMS/polyolefin composites in the range of x = 83–91 wt%. Zn0.15Nb0.3Ti0.55O2 (ZNT) powder underwent conventional sintering process before being laminated with 1,2-PB/EPDM/SBS through hot-press process after being modified by Vinyltrimethoxysilane (VTMS). In this study, we analyzed the surface character, microstructure, microwave dielectric properties and thermal performances of the composite material. The findings indicate that a core-shell structure was formed to enhance the compatibility between ceramic particles and polyolefin matrix. Furthermore, the modified ceramic formed an interfacial layer with resin matrix that limited the movement of resin chain segments and improved the thermal stability of the composites. However, further increase of ceramic content more ceramic agglomerates and pores introduced more ceramic agglomerates and pores resulting in reduced cross-linking density of the resin. Consequently, a high-Dk composite was obtained when x = 89 wt% at 10 GHz: Dk = 16.79, Df = 0.0023 and τε = −120 ppm/℃, which is promising for microwave applications.

Poly-Lactic Acid/Graphene Anode for Lithium-Ion Batteries Manufactured with a Facile Hot-Pressed Solvent-Free Process

For environmental and cost purposes, solvent-free electrode manufacturing techniques are needed for lithium-ion cell technology. In this work, we present a stand-alone lithium-ion anode, containing graphene and Poly-lactic acid (PLA) as active and binding material, respectively, manufactured in a free-solvent process. To this purpose, PLA and graphene were thoroughly mixed and a hot-press was used to form the resulting electrode. At a half-cell configuration, the electrodes exhibited a stable reversible specific capacity of more than 300 mAh g−1 at C/15 for over 450 cycles and a promising C-rate performance of around 90 mAh g−1 at 6 C of constant current mode. After cyclic voltammetry analysis of the electrochemical behavior and the kinetics of the prepared electrodes, the Li atom diffusion coefficient was calculated around 1.2 × 10−8 cm2/s during lithiation and 0.6 × 10−8 cm2/s during delithiation. Finally, we show that this electrode manufacturing technique can be upscaled for higher mass loading and corresponding areal capacity at least up to 1 mAh/cm2 and thus it can be considered for practical applications.

Poly(L-lactic acid)/graphene composite films with asymmetric sandwich structure for thermal management and electromagnetic interference shielding

The blowout growth of the power density and transmission efficiency of microelectronic devices makes the problems of “overheating” and electromagnetic pollution of electronic devices increasingly prominent, and it is raising an urgent demand to improve the thermal conductivity and electromagnetic interference (EMI) shielding performance of packaging materials. Additionally, the environmental damage caused by fossil-based electronic waste should not be ignored. Hence, a scalable production strategy was proposed to prepare sandwich structure poly(L-lactic acid) (PLLA) composite films with high thermal conductivity and excellent EMI shielding efficiency (SE), which was achieved via solution pouring method and stacking hot-pressing process, in which PLLA/graphene nanoplatelets (PLLA/GNPs) with different GNPs contents was the top and substrate layers, and PLLA/poly(D-lactic acid)/Fe3O4 (PLLA/PDLA/Fe3O4) with a small amount of PDLA and Fe3O4 was the middle layer. Owing to the asymmetric distribution of filler and the successful introduction of magnetic particles, when the GNPs and Fe3O4 contents of the whole film are respectively only 5.61 wt% and 4.39 wt%, and the asymmetric ratio is 1:9, the in-plane thermal conductivity (K‖) and EMI SE of the composite film reaches 7.49 W m−1 K−1 and 41.7 dB, respectively, higher than those of PLLA film with symmetric structure (3.31 W m−1 K−1 and 33.0 dB) at the same contents of fillers. Furthermore, a small amount of PDLA (4.39 wt%) promotes the formation of stereocomplex (SC) crystallites in the middle layer and further induces the interlayer SC microcrystals during the stacking hot-pressing process, enhancing the interlayer interaction to maintain the mechanical robustness of the PLLA film. This work provides a new direction for the manufacture of polymer based composite films with high thermal conductivity and directional EMI shielding, and has broad application potential in the field of large-scale production of green electronic products.

OPTIMIZATION OF THE MANUFACTURING OF METASEQUOIA-BASED THREE-LAYER STRUCTURE PARQUET FLOORING BY A RESPONSE SURFACE METHODOLOGY

On the basis of a single-factor experiment, a mathematical model was established by the response surface analysis method based on the Box-Behnken experimental design principle. The effects of three factors, including hot-pressing temperature, hot-pressing time, and hot-pressing pressure, and their interactions on the modulus of rupture (MOR) of Metasequoia-based three-layered structure parquet flooring were studied. The results show that the quadratic polynomial model in the regression equation is significant, and the correlation between the value predicted by the model and the experimental value is 91.17%. The optimized best hot-pressing process parameters are determined to be as follows: hot-pressing temperature of 96.03°C, hot-pressing time of 6.70 min, and hot-pressing pressure of 8 kg·cm-2. Under these conditions, the best MOR are obtained, reaching a value of 102.05 MPa. The theoretically predicted value is in good agreement with the experimental results.

Investigating the deformation behavior of hot-pressed Ti/Al/Ti laminated composite

In this paper, the deformation behavior of Ti/Al/Ti composite fabricated by the hot-pressing process was investigated by simulation and experimental analyses. Furthermore, the elastic properties of laminated composites were predicted by using different representative volume elements (RVEs) subjected to compression and tension. The SEM images displayed well-bonded straight interfaces. Also, XRD and EDS analyses revealed that TiAl3 was the only intermetallic compound that grew at the Ti/Al interfaces. The simulation results showed that stress concentration and strain localization existed at the interfaces during hot compression. In addition, the elastic behavior of simulated RVEs agreed relatively well with the experimentally-obtained values. Nevertheless, the predicted values of effective elastic constants of large RVE were consistent with the experimental values. This is due to the actual thickness, volume fraction, and stacking order of composite layers.

Improved toughness and impact resistance of bio-inspired porcelain ceramic composites with shell-like structure

Combining high strength, excellent toughness, and outstanding impact resistance still remain challenges in architecture ceramics limited by their inherent brittleness. Inspired by the natural structure of shells, the multilayered ceramic/resin/fabric composite was fabricated via surface hydroxylation followed by a simple hot press process. It was found that the optimized ceramic/epoxy resin/Kevlar fabric composite showed up to 44.89%, 52.49%, 753.77%, and 148.33% improvements to 84.53 ± 5.0 MPa, 2.76 ± 0.04 MPa·m1/2, 53.19 ± 0.80 J·m1/2 and 1.49 ± 0.10 J, respectively in the strength, toughness, fracture work, and impact energy compared to traditional ceramic. This unique combination of properties in this composite was ascribed to the layered design, elastic contents, and appropriate bonding state, corresponding to several strengthening and toughening mechanisms, such as crack deflection, interfacial debonding, and fiber pull-out. ​These properties, including typical step-like fracture and non-catastrophic failure, advancing the applications of traditional ceramics in high-performance composites manufacturers.

Densification of polycrystalline alumina with dense dislocation arrays via stainless steel sealed powder metallurgy hot isostatic press

Aluminum oxide (alumina, Al2O3) is one of the most widely used ceramic materials owing to its excellent dielectric, chemical resistance, and low cost. In contemporary semiconductor fabrication processing, in particular, Al2O3 plays a key role in high-value processing components, such as an electrostatic chuck. Here, we report for the first time a novel densifying technique for Al2O3 that can be referred to as the powder metallurgy hot isostatic press (PM-HIP) process. Unlike the conventional HIP process, our PM-HIP process can directly convert Al2O3 powders into fully densified ceramics via a one-step process. More importantly, during the PM-HIP process, thermomechanical energy can easily be transferred to each of the particles simultaneously, which facilitates the sinter process (i.e., necking and diffusion) and the clipping of lattice planes. As a result, highly dense dislocation arrays are embedded within the entire volume of the sintered body. This correlates with unique material properties-processing relationships in the Al2O3 ceramics sintered via PM-HIP. The Al2O3 ceramics sintered by PM-HIP exhibited unique mechanical and electrical properties, highlighting its potential for use in electrostatic chucks.

Fabrication and Characterization of EVA Resins as Adhesives in Plywood.

The practical problem of free formaldehyde pollution in the plywood industry is that polyethylene films have been shown to be able to replace some urea-formaldehyde resins for wood adhesives. To broaden the variety of thermoplastic plywood, reduce the hot-press temperature, and save energy consumption, an ethylene-vinyl acetate (EVA) film was selected as a wood adhesive to manufacture a novel wood-plastic composite plywood via hot-press and secondary press processes. The effects of the hot-press and secondary press processes at different levels on the physical-mechanical properties of EVA plywood (tensile shear strength, 24 h water absorption, and immersion peel performance) were evaluated. The results showed that the properties of the resulting plywood using the EVA film as an adhesive could meet the type III plywood standard. The optimum hot-press time was 1 min/mm, the hot-press temperature was 110-120 °C, the hot-press pressure was 1 MPa, the dosage film was 163 g/m2, the secondary press time was 5 min, the secondary press pressure was 0.5 MPa, and the secondary press temperature was 25 °C. EVA plywood can be used in indoor environments.