Search Results (283)

Precursor-derived silicon carbide (SiC) ceramics have been widely used as absorbing materials, but the residual carbon sink produced by ceramicization limits their application under high-temperature and oxygen-containing conditions, such as the nozzle or jet vane of high-speed aircraft. In this paper, a novel molybdenum carbide/silicon carbide (Mo₂C/SiC) microwave-absorbing ceramic with a two-dimensional sheet structure was obtained through the pyrolysis of polycarbosilane-coated molybdenum sulfide (PCS@MoS₂). The results indicate that addition of an appropriate amount of MoS₂ can react with the free carbon generated during the pyrolysis of PCS, thereby reducing the material’s carbon content and forming Mo₂C. Concurrently, the layered structural characteristics of MoS₂ are utilized to create a two-dimensional composite structure within the material, which enhances the material’s absorption vastly. The as-prepared Mo₂C/SiC ceramics sintered at 1300 °C exhibit a minimum reflection loss (RL_min) of −46.49 dB at 8.96 GHz with a thickness of 2.6 mm. Additionally, the effective absorption bandwidth (EAB) of Mo₂C/SiC spans the entire X-band (8–12 GHz) due to the combined effect of multiple loss mechanisms. Full article

Radar sensors are critical for obstacle detection and navigation, especially for automated driving. Using the use-case “printing of heating coils on the inside of the front housing (primary radome)” needed for de-icing in winter, it is demonstrated that additive manufacturing (AM) can provide economic and functional benefits for manufacturing of the sensors. AM will allow significant cost reduction by eliminating parts and simplifying the manufacturing process. Different AM technologies for the coils were investigated, first, by applying the conductive traces by fused deposition modeling (FDM), and, second, by printing copper particle-free inks and pastes. The metal layers were electrically and mechanically characterized using a profilometer to measure the trace dimension and a four-point probe to measure the resistance. It was revealed that low-cost conductive filaments with low resistivity and current carrying capacity are commercially still not available. The best option sourced was a copper–polyester-based filament with 6000 µΩcm after printing. Therefore, low-cost particle-free copper inks and commercial copper flake paste were selected to print the heating coil. The Cu particle-free inks were amine-based Cu (II) formate complexes, where the Cu exists in an ionic form. Using contactless printing processes such as ink-jet printing or pneumatic dispensing, the traces could be deposited onto the low-melting temperature (225 °C) polymeric radome structure. After printing, the material needed to be sintered to form the conductive copper traces. To avoid damaging the polymer radome during sintering, two different processes were investigated: low-temperature (<150 °C) sintering in an oven for 30 min or fast laser sintering. The sintered Cu layers achieved the following specific electric resistivities when slowly sintered in the oven: paste 4 µΩcm and ink 8.8 µΩcm. Using laser sintering, the ink achieved 3.2 µΩcm because the locally high temperature provides better sintering. Also, the adhesion was significantly increased to (5 B). Therefore, laser sintering is the preferred technology. In addition, it allows fast processing directly after printing. Commercial equipment is available where printing and laser sintering is integrated. The potential of low-cost copper material and the integration in additive manufacturing of electronic systems using radar sensors as an example are demonstrated in this paper. Full article

Thermal conductivity is the key property of SiC-TRISO composite fuel. This study investigated the relationship between SiC phase transition, thermal conductivity, and microstructure across different temperatures. The physical phase, morphology, and microstructure of SiC and SiC-TRISO composite fuels were characterized by XRD and SEM. Meanwhile, EDS was employed to determine the chemical composition within SiC grains. The results showed the transformation of the β-SiC phase to α-SiC in the matrix with increasing sintering temperature, while Al, Y, and Ca concentrations within the SiC grains decreased. The highest λ value of SiC was achieved at a sintering temperature of 1750 °C, measuring 75.51 ${\displaystyle \frac{W}{m·K}}$ at room temperature and 43.36 ${\displaystyle \frac{W}{m·K}}$ at 500 °C. The incorporation of TRISO fuel lowered the $\lambda$ value of SiC-TRISO composite fuel, yielding 57.96 and 34.51 ${\displaystyle \frac{W}{m·K}}$ at room temperature and 500 °C, respectively. The outermost carbon layer of TRISO fuel interacts with the silicon carbide matrix and liquid phase, facilitating the phase transition from 3C-SiC to 6H-SiC and, subsequently, to 4H-SiC. This process accelerates the depletion of Al, Y, and Ca within the silicon carbide grains, encourages grain growth, and raises the free-carbon content, thereby decreasing the λ of the composite fuel. Full article

Continuous fiber-reinforced polymer (CoFRP) parts offer significant potential for reducing future product consumption and CO₂ emissions due to their high tensile properties and low density. Additive manufacturing enables the tool-free production of complex geometries with optimal material utilization, making it a promising approach for creating load-path-optimized CoFRP parts. Recent advancements have integrated continuous fibers into laser sintering processes, allowing for the support-free production of complex parts with improved material properties. However, additive manufacturing faces challenges such as long production times, small component dimensions, and defects like high void content. New processes, including Arburg Polymer Freeforming (APF), robotic direct extrusion (DES) and the integration of thermoplastic tapes, and laser sintering, have enabled the production of CoFRPs to address these issues. A comparison of these new processes with existing material extrusion methods is necessary to determine the most suitable approach for specific tasks. The fulfillment factor is used to compare composites with different matrix and fiber materials, representing the percentage of experimentally achieved material properties relative to the theoretical maximum according to the Voigt model. The fulfillment factor varies significantly across different processes and materials. For FFF processes, the fulfillment factor ranges from 20% to 77% for stiffness and 14% to 84% for strength, with an average of 52% and 37%, respectively. APF shows a high fulfillment factor for stiffness (94%) but is lower for strength (23%), attributed to poor fiber–matrix bonding and process-induced pores. The new DES process improves the fulfillment factor due to additional consolidation steps, achieving above-average values for strength (67%). The CoFRP produced by the novel LS process also shows a high fulfillment factor for stiffness (85%) and an average fulfillment factor for strength (39%), influenced by suboptimal process parameters and defects. Full article

The knowledge of Li diffusivities in electrode materials of Li-ion batteries (LIBs) is essential for a fundamental understanding of charging/discharging times, maximum capacities, stress formation and possible side reactions. The literature indicates that Li diffusion in the cathode material Li(Ni,Mn,Co)O₂ strongly increases during electrochemical delithiation. Such an increased Li diffusivity will be advantageous for performance if it is present already in the initial state after synthesis. In order to understand the influence of a varying initial Li content on Li diffusion, we performed Li tracer diffusion experiments on Li_xCoO₂ (LCO) and Li_xNi_1/3Mn_1/3Co_1/3O₂ (NMC, x = 1, 0.9, 0.65) cathode materials. The measurements were performed on polycrystalline sintered bulk materials, free of additives and binders, in order to study the intrinsic properties. The variation of Li content was achieved using reactive solid-state synthesis using pressed Li₂CO₃, NiO, Co₃O₄ and/or MnO₂ powders and high temperature sintering at 800 °C. XRD analyses showed that the resultant bulk samples exhibit the layered LCO or NMC phases with a low amount of cation intermixing. Moreover, the presence of additional NiO and Co₃O₄ phases was detected in NMC with a pronounced nominal Li deficiency of x = 0.65. As a tracer source, a ⁶Li tracer layer with the same chemical composition was deposited using ion beam sputtering. Secondary ion mass spectrometry in depth profile mode was used for isotopic analysis. The diffusivities followed the Arrhenius law with an activation enthalpy of about 0.8 eV and were nearly identical within error for all samples investigated in the temperature range up to 500 °C. For a diffusion mechanism based on structural Li vacancies, the results indicated that varying the Li content does not result in a change in the vacancy concentration. Consequently, the design and use of a cathode initially made of a Li-deficient material will not improve the kinetics of battery performance. The possible reasons for this unexpected result are discussed. Full article

Corundum–mullite refractory material is an important material in rotary kiln incinerators due to its excellent properties, e.g., high temperature stability and chemical resistance, etc. However, in the process of use, the complexity of the sintering process will inevitably produce a large amount of spent corundum–mullite refractory material. Therefore, it is important to study the failure mechanism of corundum–mullite refractory material to prolong its service life. In this manuscript, the scrapping mechanism for the corundum–mullite refractory material was studied by XRD, XPS, SEM-EDS, FTIR, etc. The results indicate that chemical corrosion caused by impurity elements, such as Fe, Ca, Mg, Ti, etc., is one of the important scrapping mechanisms. The corundum structure remains stable throughout the service life, while mullite exhibits the opposite phenomenon. The Al-O-Si bonds in the mullite structure are depolymerized by impurity elements to release free tetrahedral structures, including the [AlO₄] tetrahedron and [SiO₄] tetrahedron. In the intervention of iron, the free tetrahedra, including [AlO₄], [FeO₄], and [SiO₄] can bond with each other by sharing bridging oxygen (BO), probably forming Fe-O(BO)-Si, Fe-O(BO)-Al, and Al-O(BO)-Si in an Al₂O₃-SiO₂-Fe₂O₃-Me_xO_y (Me = Ca, Mg, Ti, etc.)-based amorphous phase. These findings provide theoretical support for prolonging the service life of refractory materials in rotary kiln incinerators. Full article

Ceramic detachments in cladding systems are indicative of adhesion loss between the ceramic tiles and the substrate or its adhesive mortar due to inadequate quality workmanship, the quality of the adhesive mortar or that of the ceramic material, whether acting simultaneously or not. The shear stresses resulting from the ceramic tiles’ expansion due to humidity accelerate this process. There is a shortage of studies on the quality of ceramic tiles and adhesive mortars. This study conducted elemental, physical and microstructural characterization tests on ceramic tiles and adhesive mortars that showed detachment up to two years after being laid. At first glance, the adhesive mortar samples had adequate traits and degree of hydration. The ceramic tiles, on the other hand, showed high porosity and high levels of amorphous and poorly sintered materials, with no crystalline phase. In a second analysis, scanning electron microscopy (SEM) tests associated with boiling plus autoclave moisture expansion tests executed on unused ceramic pieces of the same conformation proved to be more suitable for predicting expansion potential than standard tests. Due to the costs and difficulties in accessing and analyzing the SEM tests, chemical analysis of the ceramic tiles was executed using X-ray fluorescence (XRF) to assess the presence of the amorphous silica (free quartz) and alkaline oxides. Together with pressure and temperature determination tests (autoclave), they may represent another alternative that is easier to access and more cost-effective for predicting future expansion. Full article

This paper reviews recent advances in the synthesis of cobalt-free high-strength tungsten carbide (WC) composites as sustainable alternatives to conventional WC-Co composites. Due to the high cost of cobalt, limited supply, and environmental concerns, researchers are exploring nickel, iron, ceramic binders, and nanocomposites to obtain similar or superior mechanical properties. Various synthesis methods such as powder metallurgy, encapsulation, 3D printing, and spark plasma sintering (SPS) are discussed, with SPS standing out for its effectiveness in densifying and preventing WC grain growth. The results show that cobalt-free composites exhibit high strength, wear and corrosion resistance, and harsh environment stability, making them viable competitors for WC-Co materials. The use of nickel and iron with SPS is shown to enable the development of environmentally friendly, cost-effective materials. It is emphasized that microstructural control and phase management during sintering are critical to improve a material’s properties. The application potential of these composites covers mechanical engineering, metallurgy, oil and gas, and aerospace, emphasizing their broad industrial relevance. Full article

Al₂O₃ with SiC, silver, and graphene nanoplatelets (GNPs) powder mixture was produced by ball milling using ethanol as dispersion media. The GNP-reinforced Al₂O₃-SiC-Ag ceramic–metal composites were densified by spark plasma sintering technology (SPS). A homogeneous dispersion of GNPs in Al₂O₃-SiC-Ag was observed from the sintered samples, and the GNPs were embedded between the grains, which resulted in increasing the contact area. The trans-granular mechanism of crack propagation becomes increasingly dominant by adding GNPs. The hardness reaches 27 GPa, as tested by the Vickers microhardness method, which reflects an increase of 11% compared to Ag-GNPs-free Al₂O₃-SiC. On the other hand, by adding Ag-GNP content, the improvement in density is limited. Wear mechanisms, as determined through ball-on-flat testing, including adhesion, abrasion, and microcracks, are observed and discussed. The composite demonstrated remarkable self-lubricating properties, exhibiting a lower coefficient of friction (COF) and wear rate in an air environment compared to monolithic Al₂O₃-SiC. This improvement is attributed to the formation of a self-lubricating film, enabled by the uniform distribution of Ag and GNPs within the Al₂O₃-SiC matrix. The findings of this study propose a novel material design approach for developing self-lubricating ceramic composites with hybrid solid lubricants. Full article

This study investigates the thermoelectric properties of Se-rich β-Ag₂Se synthesized via a mechanochemical method followed by spark plasma sintering (SPS) in less than 30 min of the total reaction time. Importantly, only a short 10 min milling process followed by appropriate SPS was enough to produce single-phase Ag₂Se_1+x samples with varying selenium content (where x = 0, 0.01, 0.02, 0.04). The introduction of excess selenium significantly influenced the thermoelectric performance, optimizing the carrier concentration during synthesis and resulting in substantial thermoelectric improvements. The sample with nominal composition Ag₂Se_1.01 exhibited a high dimensionless figure-of-merit (ZT) >0.9 at 385 K, which is nearly six times higher than the reference sample (β-Ag₂Se). Our findings bring valuable insight into the technology of optimization of thermoelectric characteristics of Se-rich β-Ag₂Se, highlighting its potential for applications in thermoelectric devices. The study demonstrates the energetically efficient and environmental advantage of our mechanochemical route to produce Se-rich β-Ag₂Se, providing a solvent-free and commercially viable alternative synthesis for energy (thermoelectric and solar energy). Full article

LiCoO₂ (LCO) is a crucial active material for positive electrodes of commercial lithium-ion batteries. It is typically present in the form of micrometer-sized LCO particles, which are surrounded by binders and conductive agents with a thickness of tens of microns. In order to determine the intrinsic Li transport parameters of pure crystalline LCO, it is necessary to measure the Li diffusivity at room temperature in sintered LCO pellets free of additives. The LCO sintered bulk material consists of interconnected, about 3 µm clusters, composed of grains of about 70 nanometers in size. The Li chemical and tracer diffusivities are determined using electrochemical impedance spectroscopy (EIS) and potentiostatic intermittent titration technique (PITT), while the latter ones are in the range between 10⁻⁹ and 10⁻²⁸ m²s⁻¹, depending on the application of different relevant formulas and characteristic parameters. Consequently, it is essential to apply a classical non-electrochemical and Li selective method of tracer diffusion determination like ⁶Li depth profiling and secondary ion mass spectrometry (SIMS) for comparison. Li tracer diffusivities of about 10⁻²² m²s⁻¹ at room temperature are obtained by the extrapolation of the SIMS results from higher temperatures. This significantly narrows the range of reliable electrochemically determined Li tracer diffusivities to a more limited range, between 10⁻²¹ and 10⁻²² m²s⁻¹. Full article

Spark plasma sintering (SPS) is an effective technique for studying the diffusion bonding of diamond/Cu composites, and has the potential to advance the application of copper matrix composites. This study investigates the SPS diffusion bonding of diamond/Cu composites using a chromium (Cr) interlayer. The effects of process parameters on the microstructure and mechanical properties of the bonding interface were evaluated through shear strength testing and SEM analysis. The results show that shear strength increases with interlayer thickness up to a certain point, after which it decreases. As the bonding temperature, holding time, and bonding pressure increase, defects such as cracks and voids at the diffusion-bonded interface are reduced, resulting in improved shear strength. Under suitable conditions (10 μm interlayer, 810 °C, 60 min, and 10 MPa), the bonding interface is defect-free, achieving a maximum shear strength of 139.89 MPa and a thermal conductivity (TC) of 700.97 W/(m·K), indicating high-quality diffusion bonding. Full article

High resistance to tempering and extended service life are pivotal research directions for cutting tools utilized in the machining of industrial machine tool. The design of alloys and their manufacturing processes have become methods for the development of cutting tool materials. Carbon-free Fe-Co-Mo steel (FCM) has garnered attention due to its excellent magnetic properties and high-temperature performance, as well as its superior thermal conductivity, making it an ideal choice for applications in high-temperature and high-pressure environments. The µ-phase within this alloy exhibits exceptional high-temperature stability and resistance to aggregation. Its characteristics suggest that it has the potential to replace carbide reinforcement phases, which are prone to coarsening, in high-temperature applications of powder high-speed steel. This application of the µ-phase could lead to an enhancement in the resistance to tempering and the service life of powder metallurgy high-speed steel cutting tools. However, there is a relative scarcity of published research regarding the preparation of carbon-free high-speed steel via hot isostatic pressing (HIP) technology and the subsequent heat treatment processes. In this study, Fe-Co-Mo alloys reinforced with the intermetallic compound µ-phase were prepared at hot isostatic pressing sintering temperatures of 1200 °C, 1250 °C, and 1350 °C. Furthermore, to investigate the influence of the solid-solution treatment temperature on the microstructure and macroscopic properties of the alloy, the as-prepared materials were subjected to solution annealing treatment at different temperatures (1120 °C, 1150 °C, 1180 °C, and 1210 °C). The results demonstrate that by moderately reducing the sintering temperature, the segregation phenomenon of the reinforcing µ-phase was significantly reduced, leading to an optimization of the microstructural uniformity of the prepared sample, with the micro-scale µ-phase being uniformly dispersed within the α-Fe matrix. As the temperature of the solid-solution annealing increased, the microstructural uniformity was further enhanced, accompanied by a reduction in the quantity of the reinforcing phase and refinement of the grain size. Notably, after solid-solution annealing at 1180 °C, the hardness of the samples reached a peak value of 500.4 HV, attributed to the decrease in the reinforcing phase and grain refinement during the annealing process. Aging treatment at 600 °C for 3 h facilitated the uniform precipitation of the nano-scale µ-phase, resulting in a significant increase in sample hardness to approximately 900 HV. The prepared material exhibited excellent resistance to tempering, indicating its potential for application in high-temperature service environments. Full article

In conventional laser-based powder bed fusion of polymers (PBF-LB/P), aging of the powder due to preheating of the powder bed is a significant issue. This paper proposes a method for low-temperature PBF-LB/P using a semi-sintering process that minimizes powder aging caused by preheating. By partially semi-sintering the low-temperature powder bed, it was possible to execute the PBF-LB/P while avoiding the aging of most of the powder. Furthermore, the suppression of curling by the semi-sintered body eliminated the need to connect the base plate to the parts, which was necessary in previously reported low-temperature PBF-LB/P. Using the semi-sintering process, we successfully built cuboid and tensile test specimens in a polyamide 11 powder bed maintained below the crystallization temperature, where the powder hardly aged. The apparent densities of the built specimens were comparable to those produced using high-temperature PBF-LB/P. However, the elongation in the building direction of the built parts by the semi-sintering process should be improved. This study represents the first step toward the practical application of low-temperature PBF-LB/P using semi-sintering. Full article

In additive manufacturing, controlling hot cracking in non-weldable nickel-based superalloys poses a significant challenge for forming complex components. This study introduces a multiple preheating process for the forming surface in electron beam powder bed fusion (EB-PBF), employing a dual-band infrared surface temperature measurement technique instead of the conventional base plate thermocouple method. This new approach reduces the temperature drop during forming, decreasing surface cooling by 28.6% compared to traditional methods. Additionally, the precipitation of carbides and borides is reduced by 38.5% and 80.1%, respectively, lowering the sensitivity to liquefaction cracking. This technique enables crack-free forming at a lower powder bed preheating temperature (1000 °C), thereby improving the powder recycling rate by minimizing powder sintering. Microstructural analysis confirms that this method reduces low-melting eutectic formation and alleviates liquefaction cracking at high-angle grain boundaries caused by thermal cycling. Consequently, crack-free IN738 specimens with high-temperature durability were successfully achieved, providing a promising approach for the EB-PBF fabrication of crack-resistant IN738 components. Full article