Hot-pressing Step Research Articles

Innovating in-situ characterization: a comprehensive measurement system for measuring the ZT and the contact resistance of vertical thermolegs exploiting the vertical transfer length method

In order to optimize their system design and manufacturing processes, it is crucial to undertake a thorough electrical and thermal characterization of micro thermoelectric generators (µTEGs). To address this need, a highly advanced and fully integrated in-situ measurement system has been developed. The main objectives of this system are to (1) enable the measurement of ZT and thereby of all thermoelectric (TE) properties of thermolegs made from powder-based TE materials and (2) at the same time accurately measure the contact resistance between the TE material and the electrical contacts. The µTEG fabrication concept used in this study is based on copper-cladded printed circuit board (PCB) material as a substrate, using the Cu layers for easy contact formation. In a first step, an innovative measurement concept, based on a distinctive vertical rendition of the well-established transfer length method, has been realized, allowing for the in-situ measurement of contact resistance between the TE material and the copper conductors on the PCB substrate. This enables a comprehensive assessment of the impact exerted by the applied force and temperature during e.g. a hot-pressing step for compacting the powder-based thermolegs during the manufacturing process. In a second step, a comprehensive measurement platform, referred to as the ZT-Card, has been devised to facilitate the evaluation of all relevant TE material properties—Seebeck voltage, electrical conductivity and thermal conductivity (all measured in vertical cross-plane orientation)—inherent to a highly miniaturized thermoleg. Additionally, the ZT-Card also allows for the assessment of contact resistance between the copper contacts and the TE material. Successful testing of this measurement system inspires confidence in the capabilities of the platform and will aid in future µTEG development.

Microwave Drying of Catalyst Layers for Fuel Cells and Electrolysers

Due to an increasing demand for sustainable energy, the polymer electrolyte membrane fuel cell (PEMFC) is considered a key technology in energy conversion with hydrogen as an energy carrier. The performance characteristics of a PEMFC is significantly influenced by the morphology of the catalyst layer[1], which is formed predominantly during the drying step in the manufacturing process.In order to improve the performance of PEMFC and hence reducing production costs, research effort as well as industrial interest has focused on the drying process. Moreover, energy efficiency of the drying step in the production of thin-film catalyst layers is considered a key driver of cost perspective for the mass-market introduction of PEM fuel cells and electrolyzers due to increasing energy costs and the need for reduction of environmental impact.In the presented work, an innovative drying approach for thin-film catalyst layers, used in PEMFC and electrolyzer applications, is investigated. The thin-film is either coated on a substrate and transferred onto a polymer electrolyte membrane in a second process step or is directly coated onto the polymer electrolyte membrane by doctor blade coating. Drying is performed by coupling of microwave radiation into the wet film, leading to a homogeneous and energy-efficient heating of the coated film. Energy losses of the microwave drying process are significantly reduced compared to conventional convective drying. Additionally, the microwave induced drying step offers maximum control during the process.Dried thin-films, produced in the lab scale, are transferred to a polymer electrolyte membrane by a hot-pressing step and are completed with gas diffusion layers to membrane electrode assemblies (MEA). Electrochemical performance of the MEA is characterized on a fuel cell test bench in single cell configuration. The morphology of the structure obtained from microwave drying is analyzed and compared to CCM samples dried in a conventional convective oven.The effect of a microwave-based drying process is discussed with regard to fuel cell performance and structure-property relations of the electrodes.

Automated Production of Catalyst Coated Membranes for PEM Water Electrolysis Using a Direct Ultra-Sonic Spray Coating Approach

The production of green hydrogen through polymer electrolyte water electrolysis (PEMWE) has attracted attention in recent years for its potential to enable the widespread use of green hydrogen [1, 2]. This technology particularly impacts important sectors of today’s society such as the agricultural and mobility sectors. Among the different components of an electrolysis cell, the membrane electrode assembly (MEA) is regarded as the most important component for its operation, since it is where the water-splitting reaction takes place. For this reason, the choice of materials and production approach play a highly important role to optimize its lifetime, performance, and cost [3]. Within this context, the MEA production via ultra-sonic spray coating has been lately gaining relevance due to its potential to yield high-activity electrode layers with a low catalyst loading.Conventionally, electrodes for PEMWE have been fabricated using other catalyst deposition techniques such as the doctor blade, airbrush spray coating or rod coating on a separate substrate, which is then transferred onto the membrane via hot pressing. However, the shortcomings of these approaches which include an additional hot press step were recently reported, particularly due to the complexity associated with electrodes with low catalyst loadings due to their sensitivity towards mechanical stress [1, 4]. On the other hand, effective approaches involving a direct ink deposition onto the membrane using ultra-sonic spray coating have been long reported for fuel cell applications [5, 6].In this work, we take the approach of direct ink deposition via ultra-sonic spray coating to produce iridium based MEAs for PEMWE. Furthermore, the dependency of the MEA electrochemical activity on the process and material choice are studied in an iterative manner. Of particular interest for this study is the material transfer rate from the catalyst ink to the membrane, the reproducibility of the developed spray coating process and the variation of materials used for the MEA fabrication. The spray process was performed with an ultra-sonic spray nozzle attached to an automized 3-axis rig and coupled with a syringe pump. The fabricated MEAs were characterized using a single cell setup with an active area of 4 cm2. The results of the first generation of the fabricated MEAs show that the material ratios in ink composition play an important role in the ink flowability through the piping which carries the ink to the ultra-sonic nozzle. Furthermore, this behavior has a direct and strong influence on the final composition of the catalyst layer, which consequently directly affects the cell performance.

Postprocessing of tungsten carbide‐nickel preforms fabricated via binder jetting of sintered‐agglomerated powder

AbstractThis study binder jets a tungsten carbide‐nickel (WC‐Ni) sintered‐agglomerated composite powder, and postprocesses the preforms using an initial sintering step followed by a hot isostatic pressing (HIP) step. The effects of sintering temperatures, sintering durations, and HIP temperatures on notable properties (e.g., porosity, microstructure, hardness, and oxidation behavior) are quantified. The highest average relative density produced in this study was 96.8%, and volumetric shrinkage of these coupons was about 64%. Microstructural characterization shows that the WC grains are homogenously distributed throughout the nickel matrix and grow to an average diameter of 1.6 (a 60% increase) during processing. X‐ray diffraction patterns indicate that no unwanted products were formed. Processed coupons achieved a maximum hardness of 54 Rockwell C, limited by their internal porosity. Oxidation tests result in the production of WO3 and NiWO4 at temperatures above 600°C. Methodologies and results from this study can be leveraged to additively manufacture highly dense, geometrically complex WC‐Ni parts with small carbide grains, low nickel content, desirable microstructure, and suitable functional properties.

Hot-pressing polyelectrolyte complexes into tunable dense saloplastics

Salt-plasticized polyelectrolyte complexes of sodium polystrenesulfonate (Na-PSS) and polydiallyldimethylammonium chloride (PDADMAC) were found to be fully dense (non-porous) when hot-pressed under optimal conditions. The quality of these saloplastics is determined by the molecular weights of polyelectrolytes, the salt type and concentration, as well as processing conditions like pressure and temperature. Higher molecular weights give more compact precipitates that are easy to process while decreasing the molecular weight led to particulate ones. Both the type and concentration of salt on complex formation and doping are explored and are found to play an equally important role. The effects of individual hot-pressing parameters, namely, pressure and temperature are studied thoroughly, facilitating the formation of uniform free-standing films over a wide range of thicknesses from 8 μm to 1 mm. For the first time, saloplastics were studied as free-standing films that are dense even at the nanometer scale. In their dry states, they were brittle and strong, showing a uniform Young's modulus over a range of relevant thicknesses, while the wet states were rubbery and elastic, showing a decreasing trend with thickness. In the wet state, the inverse trend is due to the quicker drying of thin films. Within a single hot-pressing step, protruding shapes and structures from the surface were efficiently made at different length scales. Moreover, saloplastics were also successfully reinforced with thin woven and nonwoven fibers for enhanced tensile strengths, higher than conventional thermoplastics. Overall, this work demonstrates an easy approach to fabricate dense saloplastics and their unique mechanical properties.

Preparation and Characterization of Polymer Composite Materials Based on PLA/TiO₂ for Antibacterial Packaging.

Polymer composite materials based on polylactic acid (PLA) filled with titanium dioxide (TiO2) nanoparticles were prepared. The aim of this work was to investigate the antibacterial action of TiO2 against a strain of E. coli (DH5α) to obtain information on their potential uses in food and agro-alimentary industry. PLA/TiO2 systems were prepared by a two-step process: Solvent casting followed by a hot-pressing step. Characterization was done as a function of particle size (21 nm and <100 nm) and particle content (0%, 1%, 5%, 10%, and 20%, wt %). Structural characterization carried out by X-ray diffraction (XRD) and Fourier Transformed Infrared spectroscopy (FTIR) did not reveal significant changes in polymer structure due to the presence of TiO2 nanoparticles. Thermal characterization indicated that thermal transitions, measured by differential scanning calorimetry (DSC), did not vary, irrespective of size or content, whereas thermogravimetric analysis (TGA) revealed a slight increase in the temperature of degradation with particle content. Bacterial growth and biofilm formation on the surface of the composites against DH5α Escherichia coli was studied. Results suggested that the presence of TiO2 nanoparticles decreases the amount of extracellular polymeric substance (EPS) and limits bacterial growth. The inhibition distances estimated with the Kirby-Bauer were doubled when 1% TiO2 nanoparticles were introduced in PLA, though no significant differences were obtained for higher contents in TiO2 NPs.

Ag nanowire electrode with patterned dry film photoresist insulator for flexible organic light-emitting diode with various designs

In this study, the utilization of a dry film photoresist (DFR) was explored for fabricating a patterned insulating layer in Ag nanowire (AgNW) bottom electrodes with various designs for flexible organic light-emitting diodes (OLEDs). With a single hot-pressing step, a pre-patterned DFR insulator was attached onto the AgNW electrodes, thus forming insulated electrodes in desired patterns. In the same hot-pressing step, AgNWs were also embedded into poly(methyl methacrylate), which sufficiently reduced the root mean square (RMS) roughness (~5 nm) for use in OLED lighting. Using DFR, AgNW electrodes with various shapes were fabricated to realize shaped flexible OLED lighting. The performance of OLED lighting employing the AgNW/patterned-DFR insulator electrode was comparable to that of indium‑tin-oxide-based OLED lighting. Further, the DFR insulator was utilized as a planarization layer in a paper-based OLED lighting, which operated well with a turn-on voltage of 3.5 V and maximum luminance of 157 cd/m2 at 7.5 V.

Flexible PbTiO3-nanowires/ P(VDF-TrFE) composite films and their dielectric, ferroelectric and pyroelectric properties

Flexible composite films with PbTiO3 monocrystalline nanowires as fillers and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) as matrix were prepared by solution-casting and subsequent hot-pressing process. It was found that hot-pressing step is necessary to eliminate the small portion of γ-phase in the solution-cast P(VDF-TrFE) film which could strongly suppress the switchability of the P(VDF-TrFE) matrix. Dielectric, ferroelectric and pyroelectric properties of the nanocomposites have been systematically studied by varying mass fraction of the nanowire fillers from 0 wt% to 50 wt%. Enhanced dielectric permittivities were obtained by incorporating PbTiO3 nanowires without sacrificing the dielectric loss properties. The relative permittivity and dielectric loss of the composites were greatly decreased after poling. P-E loops characteristic of ferroelectrics were observed in all the two-step processed films, showing a highest Pr of about 6.21 μC/cm2 at 30 wt% loading content. Pyroelectric coefficient p of the composites increases with the mass fraction of PbTiO3 nanowires as well as with the temperature, with a value of 52.7 μC/(m2 K) for pure P(VDF-TrFE) film at 45 °C and 72.8 μC/(m2 K) for 50 wt% PT/P(VDF-TrFE) composite film at 45 °C. The voltage FOM (FV) decreases as the loading of PbTiO3 nanowires increases (more prominently after 10 wt%), while detectivity FOM (FD) keeps a relatively high value on the same level (between 18 × 10−6 Pa−1/2–20 × 10−6 Pa−1/2) for 0–30 wt% loading content.

Fabrication of transparent MgAl2O4 from commercial nanopowders by hot-pressing without sintering additive

Transparent magnesium aluminate spinel (MgAl2O4) ceramics were successfully fabricated for the first time using commercially available nanopowders without sintering additive by a single hot-press step process. This method does not involve all the complexity of powder, green body pre-treatment and post-treatment, which are commonly used in the fabricating process. Effects of holding time (3–12 h) at the sintering temperature 1500 °C on the transmittance, microstructure and Vickers microhardness are investigated. The highest transmittance is achieved for the sample hot pressed at 1500 °C for 12 h, which exhibits 80.2% at the wavelength of 1100 nm. These results may provide a guide for understanding the microstructural development of transparent spinel without sintering additive.

A Structure and Durability Comparison of Membrane Electrode Assembly Fabrication Methods: Self-Assembled Versus Hot-Pressed

The most common means of fabricating membrane electrode assemblies (MEAs) for polymer electrolyte fuel cells (PEFCs) involves a hot-press step. The conditions used to perform the hot-press impacts the performance and durability of the fuel cell. However, the hot-press process is not essential for achieving operational MEAs and some practitioners dispense with the hot-press stage altogether by using a self-assembled approach. By performing the integration of the components in-situ during fuel cell assembly, there is the potential to lower the cost and time of manufacture. This study investigates the electrochemical performance and mechanical microstructure of MEAs that were either hot-pressed or self-assembled (non-hot-pressed) and compared at beginning-of-test (BOT) and end-of-test (EOT), following accelerated stress testing. Hot-pressed and self-assembled MEAs were found to show negligible difference in their performance and almost identical performance degradation. X-ray computed tomography (X-ray CT) showed distinct differences in the microstructure of the electrodes. In addition to a crack network in the catalyst layer, the self-assembled samples exhibit indentations that were not present in the hot-pressed sample. It was concluded that in-situ assembly of MEAs could be a suitable means of fabricating PEFC MEAs.

Electrospun Nafion/PVDF single-fiber blended membranes for regenerative H2/Br2 fuel cells

Nafion® perfluorosulfonic acid (PFSA) and poly(vinylidene fluoride) (PVDF) were electrospun simultaneously as a polymer solution mixture using a single needle spinneret. The nanofiber morphology was highly unusual, with bundled 2–5nm fibril strands of Nafion and PVDF aligned along the fiber axis. Membranes were made from fiber mats by a simple hot-pressing step, followed by thermal annealing, where the fibril morphology of Nafion and PVDF in the original nanofibers was retained in the membrane. The PVDF component in the final membrane served three roles, as an electrospinning carrier polymer for PFSA, a mechanical reinforcement, and a hydrophobic (uncharged) component to limit PFSA ionomer swelling. A series of single-fiber membranes with Nafion/PVDF contents ranging from 40/60 to 90/10wt%/wt% were prepared, characterized, and evaluated for use in a regenerative hydrogen/bromine fuel cell. As expected, there was a decrease in proton conductivity, water/electrolyte swelling, and Br2/Br3- permeability with increasing PVDF content. Membrane conductivity was lower than expected based on the weight fraction of PVDF, due presumably to an unusually large fraction of highly structured water. Nevertheless, a single-fiber Nafion/PVDF membrane with a thickness of 18µm and an 80/20wt%/wt% Nafion/PVDF composition performed well in a H2/Br2 regenerative fuel cell due to a combination of low area-specific resistance and low Br2/Br3- crossover. Thus, with an electrolyte containing 0.9M Br2 in 1.0M HBr, the maximum power output was 46% higher than that with a Nafion 212 membrane (1.31 vs. 0.90W/cm2).

Fabrication and microstructure of hot pressed laminated Y2O3/Nd:Y2O3/Y2O3 transparent ceramics

A transparent three-layered composite Y2O3/1at% Nd:Y2O3/Y2O3 ceramic was successfully fabricated by a hot-pressing method. After the hot-pressing step, the laminar structure was maintained very well, and no apparent deformation was found. The sample shows a fine microstructure (the average grain size is ∼1μm) without an apparent transition layer between a Nd-doped layer and an undoped layer as a result of the relatively low hot-pressing temperature of only 1600°C. For a 4-mm-thick polished specimen, the in-line transmittance reaches 59.7% at 400nm and 73.0% at 1100nm.

The manufacturing and properties of a nano-laminate derived from graphene powder

The work presents a method of consolidation of graphene flakes (platelets) into a bulk material showing high anisotropy of thermal, electrical and mechanical properties. Such materials can be used as directional high-temperature thermal insulators similar to graphite foils, but due to much finer microstructure they may exhibit different, possibly enhanced properties. The graphene flakes were consolidated by a filter pressing of propanol suspension followed by a hot-pressing of produced green bodies at 2200 °C under 25 MPa in a protective atmosphere.The hot-pressing step was necessary to force orientation of the flakes and to densify the material. Microstructural observations, mechanical strength and elastic properties assessment, as well as thermal and electrical properties analysis were performed. Scanning electron microscopy revealed that microstructure of the material consisted of highly-oriented layers of the graphene flakes. It resulted in a distinct anisotropy of thermal conductivity (360 vs. 3 W/mK), coefficient of thermal expansion (25·10−6 vs. −1·10−6 1/K) and electrical resistivity (60·10−6 vs. 850·10−6 Ω m) of the material in the in-plane and through plane direction, respectively. The material showed brittle behavior, but it could be machined.

Solid-state processing of oxidation-resistant molybdenum borosilicide composites for ultra-high-temperature applications

The high-temperature capabilities of multi-phase composites based on Mo5Si3Bx are examined after solid-state processing and pulsed laser deposition (PLD) coating fabrication approaches. These composites are prepared by mechanical alloying of elemental powders and densified by vacuum hot pressing, which is a scalable processing approach. Chemical analyses of the hot-pressed compacts reveal a consistent 15–22 percent loss of boron, which is primarily due to the high-temperature hot-pressing step. Composites possessing sufficient amounts of boron are evaluated by thermogravimetric studies in temperatures up to 1650 °C in air. One composition demonstrates oxidative stability after long-term (100 h) isothermal conditions, as well as thermal cycling to simulate solar-thermal operation. Hot-pressed samples of composites consisting of Mo5Si3Bx + MoSi2 + MoB are also employed as deposition targets for PLD trials. X-ray diffraction analysis of the resulting films indicates the absence of long-range crystallographic order.

High-density metallic nano-emitter arrays and their field emission characteristics

We report the fabrication and field emission properties of high-density nano-emitter arrays with on-chip electron extraction gate electrodes and up to 106 metallic nanotips that have an apex curvature radius of a few nanometers and a the tip density exceeding 108 cm−2. The gate electrode was fabricated on top of the nano-emitter arrays using a self-aligned polymer mask method. By applying a hot-press step for the polymer planarization, gate–nanotip alignment precision below 10 nm was achieved. Fabricated devices exhibited stable field electron emission with a current density of 0.1 A cm−2, indicating that these are promising for applications that require a miniature high-brightness electron source.

Conductivity and Wettability Changes of Ultrathin Nafion Films Subjected to Thermal Annealing and Liquid Water Exposure

The ionomer in the catalyst layer of polymer electrolyte fuel cells exists as ultrathin films (4–10 nm) covering the aggregates of Pt/C catalyst and may experience elevated temperatures during hot-pressing step of the electrode fabrication process. In this study, we report the effect of thermal annealing on pertinent ionomer-film properties: proton transport and surface wettability. Self-assembled, 10 nm thin Nafion ionomer films on silica substrate were heat treated at different temperatures in the range 40–160 °C. It was found that both surface and bulk rearrangement/reorganization occur simultaneously upon annealing. Surface reorganization was evident by the dramatic change in free-surface wettability from super-hydrophilic to hydrophobic. On the other hand, bulk rearrangement was evident from significant depression in the proton conductivity with increasing treatment temperature. Extended exposure, up to 18 h, to water in vapor form at high humidity did not change the protonic conductivity. On the other hand, exposure to liquid water resulted in an interesting finding of simultaneous regeneration of the film surface and bulk properties. The latter has implications for in operando behavior of ionomer in PEFC cathode catalyst layers, where local water and heat generation may strongly influence its properties.

Membrane Electrode Assemblies Based on Hydrocarbon Electrolytes with Nitrile Groups for Direct Methanol Fuel Cells

Sulfonated poly(aryl ether ether nitrile)s - sPAEEN are promising materials for reducing the methanol permeation from anode to cathode due to the strong polar interchain interactions caused by the nitrile functional groups. This feature allows the preparation of membranes with high proton conductivities and low crossover of methanol and water. Despite similar specific conductivities and lower thickness of the hydrocarbon membranes compared to Nafion 115, HC-MEAs prepared by a classic hot-pressing step show 40-70 % higher values for the cell resistances compared to Nafion 115 based MEAs. Therefore the MEA preparation procedure was changed to reduce losses due to poor lamination between the Nafion-based GDE and the hydrocarbon membrane. The impact of the type of sPAEEN membrane and the MEA preparation together with the cell performance data will be compared with state-of-the art Nafion-based cells and discussed in terms of consequences for the operating conditions for a DMFC system.

Microstructure design of UHMWPE-based materials: blending with graphite nanoplatelets

The main scope of this work was to develop a processing method to homogeneously distribute graphite nanoplatelets (GNP) within an ultra-high molecular weight polyethylene (UHMWPE) matrix. After this step, we estimated the toughness of the new nanocomposite material. Combining a sonication step, a micro-extrusion step and a hot-pressing step, we did not reach an optimal distribution state of the nanofillers. Nevertheless, the toughness of the nanocomposites evaluated by Charpy tester was higher than that of the reference UHMWPE (only processed by hot-pressing). We also found that our new processing procedure applied to neat UHMWPE leads to the higher toughness than that of the nanocomposite. Micro-extrusion appears as a promising processing tool for neat UHMWPE.

Fuel cell performances at optimized Nafion and Ru 85Se 15 loadings in cathode catalyst layer

The citric acid treated carbon supported Ru 85Se 15 catalysts are synthesized by microwave assisted polyol method at the optimal solution pH = 7. The catalyst coated membrane method with ultrasonic-spray technique is employed to prepare electrodes without hot press step. The cell performances with different Nafion contents and Ru loads in the cathode catalyst layer are systematically examined in both H 2/air and H 2/O 2 fuel cells at 65 °C under ambient pressure. The surface morphologies of membrane electrode assemblies and the interfacial characteristics of single cells are studied using scanning electron microscopy and electrochemical impedance spectroscopy, respectively. The Nafion content is optimized to be 33% when the catalyst loadings are lower than 0.8 and 0.6 mg Ru cm −2 in the H 2/air and H 2/O 2 fuel cells, respectively, but reduced to 20% at the higher Ru loads. The maximum peak power densities of 190 mW cm −2 at 620 mA cm −2 and 400 mW cm −2 at 1430 mA cm −2 are achieved in the H 2/air and H 2/O 2 fuel cells, respectively, with 0.27 mg Ru cm −2 and 33% Nafion. The best catalyst utilizations are obtained at 0.14 mg Ru cm −2 with 33% Nafion, resulting in the maximum peak power densities per unit mass Ru of 917 and 2460 mW mg −1 in the H 2/air and H 2/O 2 fuel cells, respectively.

Homogeneous templated grain growth of 0.65Pb(Mg 1/3Nb 2/3)O 3-0.35PbTiO 3 from nanocrystalline powders obtained by mechanochemical activation

The feasibility of a novel homogeneous templated grain growth approach for the processing of 0.65Pb(Mg 1/3Nb 2/3)O 3-0.35PbTiO 3 ceramics is studied. Novelty rests on the use of a nanocrystalline powder synthesized by mechanochemical activation for the matrix, and also for obtaining the templates via conventional sintering. Templates are (1 0 0) faceted cube-shaped microcrystals with an average size of 12 μm. The role of processing parameters such as the presence of PbO excess, and a hot-pressing step previous to the final thermal treatment is studied. Extensive growth of the templates and nearly full densification are demonstrated. The approach used here involves only conventional ceramic technology from a single source powder, and can be used as a means of obtaining larger crystals.