Conventional Sintering Process Research Articles

Study of Structural, Thermal and Electrical Properties of Sr-Doped CaMnO<sub>3</sub>

Sr-doped CaMnO3 materials have wide applications due to their thermal and electrical properties. The importance of the synthesis of Sr-doped CaMnO3 material for various applications encourages researchers to evaluate and refine the synthesis process. In this study, Ca1-xSrxMnO3 (x = 0; 0.05; 0.1; 0.15; 0.2) system has been prepared by sol-gel method followed by conventional sintering process at 850°C for 8 hr. A thorough discussion has been made on the outcomes derived from the investigation on the structural, electrical, and thermal properties of Sr-doped CaMnO3 system using powder x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, DC fourprobe method, thermal expansion studies, and thermoelectric power analyses. The XRD patterns of all prepared samples exhibited single phase with orthorhombic crystal structure (space group Pnma). Rietveld refinements were performed for all the patterns by using Fullprof software to extract the structural properties. The values of unit cell volume of samples tend to increase with the increment of dopant concentration, whereas the crystallite size values were decreased with dopant concentration. The microstructures of all the samples were studied using SEM, and elemental compositions were confirmed from the EDS results. Linear thermal expansion coefficients of all the samples were found to have moderate values in the temperature range from 30°C to 800°C. The electrical properties of all the system of samples were studied in the temperature range from 30°C to 400°C using DC fourprobe conductivity setup. It was found that all the samples exhibited semiconductor nature. Sr-content on the A-site suppress the electrical resistivity up to 10% of concentration and 5% dopant content exhibited the lowest electrical resistivity. The values of Seebeck coefficient found to vary from -160 µV/K to -124 µV/K with the increase of dopant content in the parent compound.

Solution infiltrated lanthanum nickelate–GDC composite cathode via flash-light sintering for intermediate temperature solid oxide fuel cells

Solid oxide fuel cells (SOFCs) require high operating temperatures to minimize the oxygen reduction reaction resistance at cathode and increase the ionic conductivity of oxygen. However, at high operating temperatures, their stability during long-term operation degrades because of material deterioration and the mutual chemical reactions occurring at the interfaces. Herein, an electrode–electrolyte composite is fabricated by solution infiltration of lanthanum nickelate (LNO) electrode material into a GDC electrolyte layer to ensure more reaction sites compared with the conventional powder mixing of the electrode and electrolyte material. Further, the conventional thermal sintering process is replaced with the flash-light sintering process. Thus, the performance of the LNO–GDC cathode cell manufactured by flash-light sintering improves owing to suppression of grain growth of the infiltrated LNO particles. The microstructures of the infiltrated solution and composite layer of the electrolyte material are analyzed using field-emission scanning electron microscopy, and the crystallinity of the LNO nanoparticles is analyzed using high-resolution transmission electron microscopy. A maximum power density of 1.1 W/cm2 is achieved, an improvement of 58 % over the LSCF cell without infiltration at 750 ℃. This study is expected to contribute to the commercialization of SOFCs by replacing conventional thermal sintering.

Cold-sintered laterite-based geopolymers: Densification, microstructure and micromechanics

The consolidation of natural iron-rich aluminosilicates is enhanced using geopolymerisation, external pressure, and temperature in a ultra-low energy sintering context. Optimized formulations of laterite and Rice Husk Ash (RHA) are used to investigate the effects of the external pressure and temperature on the porosity, phases composition development, micromechanics and microstructure. FT-IR spectroscopy, scanning electron microscopy (SEM 0, Mercury Intrusion Porosimetry (MIP), Transmission Electronic Microscopy (TEM), and the Archimedes Porosimetry method were used to characterize the cold sintered products of geopolymerization of the iron-rich aluminosilicate binders and composites. Results showed that ultra-low energy sintering process allows the reduction of ∼ 99 % of capillary porosity thank to the physico-chemical bonding and the formation of enhanced nanosized ferrisilicates. The indentation modulus > 50 GPa, the hardness modulus of 3,18 GPa and the fracture toughness > 1 MPa m0.5 are in agreement with the compact microstructure, high bulk density (> 2.00 g.cm−3), and low water Absorption. The interlocking of globular units of H-N-(A, Fe)-S with nanosized ferrisilicates leads to high performance ceramics that cannot be achieved with conventional sintering process. Therefore, the cold sintering process demonstrated here is promising for the manufacturing of sustainable matrices using natural iron-rich aluminosilicates.

(B4C+Al2O3)/Al composites with excellent high temperature strength and thermal stability prepared by sintering in air atmosphere

B4C/Al composites are commonly used as neutron absorption materials. The introduction of amorphous Al2O3 through surface oxidation of Al powder can significantly enhance the mechanical properties of the composites; however, the thermal stability of the composite is compromised due to the inherent instability of amorphous Al2O3. In this study, (B4C + Al2O3)/Al composites were sintered in an air atmosphere with high pressure for the first time, generating stable γ-Al2O3. It was found that in comparison with the conventional vacuum sintering process, (B4C + Al2O3)/Al sintered in an air atmosphere exhibited increases in ultimate tensile strength of 42 % at room temperature and 90 % at 350 °C. Furthermore, due to the reaction between Al and oxygen, the composite exhibited exceptional thermal stability when subjected to annealing at 620 °C, demonstrating an ultimate tensile strength of 144 MPa at 350 °C, surpassing values reported in other studies.

Exploration of iron ore tailings with high volume in sustainable cement and ecofriendly cementitious material

In this work, sustainable cement clinkers and ecofriendly cementitious materials (ECMs) were prepared with iron ore tailings (IOTs). Alternative raw meals with IOTs were sintered to cement clinkers by conventional sintering processes. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM) and mechanical tests suggested that the cement clinker within 10 wt% IOTs had better quality than those without IOTs. In addition, the hydration and hydration products of the IOT-based cement were analyzed via XRD and SEM. Before preparing ECMs, IOTs were pretreated with a 100 mesh Tyler screen to remove silt and clay. Then, the pretreated IOTs and whole IOTs partly instead of fine aggregates, together with IOT-based cement were used to produce ECMs. The ratio of water-binder and compressive strength properties of the ECMs were investigated. It is suggested that the pretreated IOTs sand can be used as much as 60 wt%, while the amount of whole IOTs sand must be limited to 20 wt% before the performance dramatically decrease. These findings suggest that disposal of a high volume of Yeshan IOTs in sustainable construction building materials has feasibility and operational significance.

Surface Chemistry Modulation of BaGd0.8La0.2Co2O6-δ As Active Air Electrode for Solid Oxide Cells.

Modulation of the surface chemistry of air electrodes makes it possible to significantly improve the electrocatalytic performance of solid oxide cells (SOCs). Here, the surface chemistry of BaGd0.8La0.2Co2O6-δ (BGLC) double perovskite is modulated by treatment in an acidic citric acid solution. The treatment leads to corrosion on the surface of BGLC particles, and the effect is dependent on the acidity of the solution. As the acidity of solution is low, Ba cations are selectively dissolved out of the BGLC surface, while as the acidity increases, the corrosion becomes more homogeneous. The Ba surface deficiency remarkably increases the concentration of surface oxygen vacancies and electrocatalytic activity of BGLC. To avoid the loss of Ba-deficient surface during the conventional high temperature sintering process, a sintering-free fabrication route is utilized to directly assemble the Ba-deficient BGLC powder into an air electrode. A single cell with the surface Ba-deficient BGLC electrode shows a peak power density of 1.04 W cm-2 at 750 °C and an electrolysis current density of 1.48 A cm-2 at 1.3 V, much greater than 0.64 W cm-2 and 1.02 A cm-2 of the cell with the pristine BGLC, respectively. This work provides a simple and effective surface chemistry modulation strategy for the development of an efficient air electrode for SOCs.

Effects of diamond as a main carbon source on the fabrication and mechanical properties of reaction-bonded SiC

RBSC with a low residual Si less than 5 vol% was fabricated using SiC–C preform with diamond as a main carbon source by a conventional reaction sintering process at the temperature range between 1500 °C and 1800 °C. The diamond content in the SiC–C preform varied from 10 wt% to 17 wt%. The diamond as the main carbon source used in SiC–C preform effectively minimized the residual Si in RBSC by increasing the reaction of diamond due to the enhanced Si melt infiltration even in SiC–C preform with high carbon content. The Si melt infiltration should be performed above 1600 °C for the complete reaction of diamond particles with size of 0.5 μm because of the relatively low dissolution rate of diamond in Si melt. The flexural strength of RBSC with residual Si content less than 7 vol% exceeded 400 MPa. The flexural strength of RBSC was increased with decreasing residual Si content, but sharply decreased by the residual carbon accompanying pores in RBSC. RBSC with low residual Si content displayed moderate flexural strengths around 200 MPa even above Si melt temperature because of the formation of continuous SiC structure in RBSC by the enhanced grain growth of SiC. Vickers hardness of RBSC was steadily increased from 17.7 GPa to 24.6 GPa with decreasing residual Si.

Effect of dispersed particles on microstructure evolution and mechanical properties of 93W–4.6Ni–2.1Fe–0.3Co alloys

93W–4.6Ni–2.1Fe–0.3Co alloys reinforced by ZrC particles were prepared by the two-step sintering method in this article. The conventional sintering process conducted at a temperature of 1485 °C resulted in the formation of microvoids between the larger dispersed particles and the matrix, thereby leading to a degradation in the tensile property (959 MPa/11.5%). Through controlling the sintering temperature at 1440 °C, a dispersion-strengthened tungsten heavy alloys (WHAs) with excellent comprehensive tensile properties (974 MPa/27.0%) was obtained, mainly attributed to the synergistic effect of homogeneous dispersion strengthening and grain boundary purification. Under high compressive strain rates, dispersion-strengthened WHA demonstrates a superior yield strength compared to WHA, suggesting that dispersion-strengthened WHAs have potential applications in the armour-piercing field.

Effect of sintering mechanism on dielectric properties of Ba0.5Sr0.5TiO3-ZnAl2O4 composite ceramics synthesized by two-step sintering with low energy consumption

The application of Ba1-xSrxTiO3-based composites in microwave field has become a popular research topic. Ba0.5Sr0.5TiO3-ZnAl2O4 (BST-ZA) composite ceramics were successfully fabricated by both conventional sintering and two-step sintering process. The microstructure and dielectric properties of Ba0.5Sr0.5TiO3-ZnAl2O4 composite ceramics prepared by two sintering methods were compared. The samples prepared by the two-step sintering method have lower ion diffusion degree, shorter holding time and lower energy consumption, and it is easier to obtain samples with higher density and uniform grain size distribution, compared with those fabricated by the conventional sintering method. The dielectric permittivity and Curie temperature of two-step sintered samples are lower than those obtained by conventional sintering, while the dielectric tunability is almost unchanged (two-step sintering: 17.7 %, conventional sintering: 18.6 %). These results indicate that two-step sintering method is an efficient and energy-saving method in preparing BST-ZA composites.

Rapid Sintering of Inkjet Printed Cu Complex Inks Using Laser in Air

Cu complex inks offer a facile and cost-effective alternative to commercial nanoparticle inks for printed electronics application. Cu complex inks are prepared by mixing Cu formate tetrahydrate (metal precursor) with amino-2-propanol (complexing agent) in a molar ratio of 1:2. A carrier solvent consisting of ethanol and ethylene glycol is then added to help adjust the rheological properties of the ink for printing purpose. The Cu complex ink having a viscosity of 12 mPa.s is then inkjet printed onto a polyimide substrate (thickness: 125 μm). After printing 10 layers, the ink is pre-dried under air at 100°C for 5 minutes followed by sintering using an IR laser (942 nm) to enable rapid thermal decomposition of the Cu formate. After sintering using a 100 W line beam laser scanned at 3 mm/sec under air; a contiguous and homogenous Cu metallic trace is formed with a minimum bulk resistivity of 4.8 μΩcm (2.84 times of bulk Cu). The laser sintered trace exhibited a highly compact and homogenous sintered structure after thermal decomposition compared to the conventional oven sintering process. Th result is remarkable and proves the importance of faster decomposition process. In contrast, sintering the same trace using slow ramp rate within the traditional oven sintering pr process, it is observed that a Cu metallic trace is formed with particles in the range of 2-5 μm that are sparsely distributed due to low metal content of Cu complex inks (< 10 metal wt.%). Due to sparse distribution of particles, a low bulk resistivity value of 102 μΩcm is obtained.

In-situ synchrotron X-ray diffraction of hydroxyapatite-zirconia composite during conventional and flash sintering

We conducted in-situ X-ray diffraction to study the crystalline phase evolution in hydroxyapatite-zirconia composites during both conventional and flash sintering processes. Additionally, we examined the thermal history and microstructure of the composite under these sintering conditions. Despite both sintering methods reaching similar average temperatures, they yielded distinct results in terms of crystalline phase composition and microstructure. In the flash sintered samples, we observed a complete transformation of hydroxyapatite into α- tricalcium phosphate and a complete tetragonal to cubic phase transition in zirconia. Conversely, the conventionally sintered samples remained practically stable. Notably, the flash sintered samples exhibited needle-like microstructures, which were absent in their conventionally sintered counterparts. This divergence suggests that the application of an electric field plays a role in generating athermal effects during the sintering of hydroxyapatite-zirconia composites.

High-performance, stable buffer-layer-free La0.9Sr0.1Ga0.8Mg0.2O3 electrolyte-supported solid oxide cell with a nanostructured nickel-based hydrogen electrode

La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) with an extraordinary oxygen-ion conductivity has been extensively studied as an electrolyte material for intermediate temperature solid oxide cells (SOCs). However, the conventional high-temperature sintering process of electrodes results in detrimental reaction between LSGM and Ni-based hydrogen electrode and microstructural coarsening of the electrode. Herein, a buffer-layer-free LSGM electrolyte-supported single cell with a nanostructured Ni-Gd0.1Ce0.9O1.95 (GDC) electrode is developed using a sintering-free fabrication approach. The cell exhibits a peak power density of 1.23 W cm−2 at 800 °C and an electrolysis current density of 1.85 A cm−2 at 1.5 V with excellent operating stability. The good performance and durability is owing to the synergistic effects of the elimination of elemental interdiffusion at the electrode/electrolyte interface, polarization induced in situ formation of hetero-interfaces between Ni-GDC and LSGM, and remarkable structural stability of Ni-GDC. This study provides an innovative means for the development of efficient and durable buffer-layer-free LSGM-supported SOCs.

Influence of sintering process on microstructure and mechanical properties of alumina ceramics: Spark plasma and microwave hybrid sintering

Improvement in hardness of alumina has advantages in ballistic performance and wear resistance of the material. Spark plasma sintering (SPS) and microwave hybrid sintering (MHS) were examined for improving the hardness of alumina and results are compared with conventional sintering (CS). Results show that the highest hardness is produced by SPS (20.4 GPa) followed by conventional sintering (18.6 GPa) and microwave hybrid sintering (18.5 GPa) process. Two separate Hall-Petch relations satisfy hardness variation in both SPS, and MHS/CS. The rate of densification is found to be higher for microwave hybrid sintering than the conventional sintering. Sintering of alumina at 1500 °C, the microwave hybrid sintering produced a higher density and grain size (90.6 % & 0.8 μm at 0 h; 98.8 % & 2.3 μm at 1 h) in comparison with conventional sintering (87.4 % & 0.8 μm at 0 h; 98.3 % & 2.1 μm at 1 h) for same sintering (dwell) duration.

Exploring the possibilities of lead-free 0.98NaNbO3-0.02BiSmKZrO3 for electrical and optical applications using solid-state method

A new lead-free ceramic materials 0.98NaNbO3 - 0.02 (Bi(1-x)Smx)0.5K0·5ZrO3, are developed with varying levels of x (mole) (x = 0.01, 0.02, 0.03, and 0.04). The conventional sintering process involves adjusting the ratio of the constituent elements to optimize the material's properties. This results enhanced electrical conductivity and improved optical properties. Our findings show that the material exhibits an orthorhombic phase at x (mole) levels between 0.01 and 0.04. The FESEM and TEM analysis provided detailed information about the surface morphology and internal structure of the NaNbO3 ceramic materials. Additionally, the UV–Vis absorption spectra indicated that the band-gap range of 3.3 eV (± 0.04) (at x = 0.01–0.04) makes these materials suitable for various optoelectronic applications Furthermore, the luminescence behaviors of Sm3+ doped NaNbO3 materials were found to be influenced by the co-doping of Bi3+. Additionally, the dielectric constant (εr) and dielectric loss (tan δ) measurements indicated that the material undergoes two phase transitions from antiferroelectric to paraelectric within a temperature range of 120–250 °C. The investigation also compared the material's electric features, such as piezoelectricity and ferroelectricity, with its leakage current. The results showed that the co-doping of Bi3+ not only influenced the luminescence behaviors but also had an impact on the material's electrical properties. The synthesized proposed system 0.98 NaNbO3 - 0.02BiSmKZrO3 material also demonstrates excellent optical properties, with a high color purity of 80–81 % at x-0.02 under a 407 nm excitation wavelength. Additionally, the estimated color coordinates of the material fall within the reddish-orange domain, further enhancing its visual appeal.

Effect of phase transformation in cold sintering of aluminum hydroxide

Cold sintering of gibbsite and boehmite, which are difficult to densify by conventional sintering processes that require heating above 1000 °C owing to thermal decomposition, was attempted at various CSP temperatures. When boehmite particles were cold sintered, densification did not progress substantially, with a maximum relative density of 62% achieved at 200 °C. On the other hand, when gibbsite particles were cold sintered, the relative density of the hardened bodies increased significantly with temperature up to 150 °C, with a maximum value of 82%. However, when gibbsite was cold sintered at temperatures above 200 °C, phase transformation from gibbsite to boehmite took place, and the relative density of the hardened body decreased significantly. The precipitated boehmite particles bonded to each other to form a three-dimensional network, and the hardened body fabricated at 250 °C showed a compressive strength of 115 MPa despite a low relative density of 62%.

Preparation of extraordinary high-density indium-tin oxide target on the basis of pressure-less sintering method

Density is the most important parameter determining the performance of indium-tin oxide (ITO) targets. However, the full densification of ITO ceramics is extremely difficult. Previously reported strategies are based on the addition of components such as sintering aids or significant process adjustments. In this study, we aimed to realise the full densification of an ITO target by the fine regulation of parameters involved in the conventional pressure-less sintering process. The phase transitions, BET surface area, and particle size changes of indium-tin co-precipitated nanoprecursors sintered at different temperatures were studied. Moreover, the effects of the ITO powder state and sintering temperature on the density, resistivity, and microstructural uniformity of the ITO target were investigated. The results showed that the existence of rhombohedral In2O3 in the ITO powder and ceramic sintering temperature are vital to the full densification of the ITO target. Density was found to dominate ITO-target resistivity when the sintering temperature was <1585 °C. In this study, an extraordinary high-density (99.81 %), low-resistivity (1.707 × 10−4 Ω cm), and uniform-microstructured ITO target is prepared under optimised operation conditions.

Microstructure Characteristics of Porous NiTi Shape Memory Alloy Synthesized by Powder Metallurgy during Compressive Deformation at Room Temperature

Porous NiTi shape memory alloys (SMAs) possess compatible mechanical properties with human bones and can effectively reduce the risk of stress shielding and stress concentration; therefore, they have been termed promising candidates for orthopedic implants. However, microstructure characteristics of porous NiTi SMAs during plastic deformation have rarely been investigated. The present study aims to specifically investigate microstructure characteristics and the corresponding underlying mechanisms of fabricated porous NiTi SMAs via a conventional sintering (CS) process with NaCl space holder during compressive deformation at room temperature. To realize the aforementioned target, X-ray diffraction (XRD), scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) are applied in the present study. The results show that the fabricated porous NiTi SMA is 51.8% for porosity, 181.65 μm for the average pore size, and 0.78 μm for the average grain size. Many Ni4Ti3 and NiTi2 phases are formed in the mixed matrix with dominant B2 (NiTi) and some B19′ (NiTi). Severe inhomogeneous deformation happens within compressed specimens, leading to the occurrence of tangled dislocation and shear bands. Microcracks occur within fabricated porous NiTi SMAs at a deformation degree of 9.2%; then, they extend quickly to form macrocracks, which finally results in the failure of regions between pores. The observed nanocrystallization and amorphization around microcrack tips within the 12.5%-deformed sample can be attributed to the relatively small grain size and the grain segmentation effect via statistically stored dislocation (SSD) and geometrically necessary dislocation (GND).

Enhancing densification rate and the unusual 4H-SiC polytype stabilization in ultrafast high-temperature sintering of α-SiC

Novel Powder medium-based Ultrafast High-temperature Sintering (P-UHS) technique brings unexplored opportunities for the ultrarapid consolidation of large-sized and difficult-to-sinter ceramics with a complex shape. Herein, the P-UHS was demonstrated to be very effective means for the densification of large-sized α-SiC bulks (∼16 cm3) with boron and carbon additions under heating rates exceeding 1500 °C/min. Bulk ceramics with ∼97% density and homogenous microstructure were obtained within only 6 min. The ultrafast heating led to two orders of magnitude increase in the densification rate contributing to an energy saving of 90% if compared to conventional sintering process. In addition, the preferential 4H-SiC polytype stabilization was observed only when ultrafast heating rate was employed. The non-equilibrium transient chemistry and the ultrarapid shrinkage were considered as the major drivers for this unusual phenomenon.

Optimization of Sintering Process Parameters by Taguchi Method for Developing Al-CNT-Reinforced Powder Composites

In powder metallurgy, the sintering process is a high-power consuming and critical process for better mechanical properties of composites due to proper diffusion of atoms. In this context, different sintering processes were investigated along with their sintering condition. The present work focused on optimizing conventional sintering process parameters for carbon nanotubes (CNTs) reinforced aluminum composites using Taguchi optimization methods. The Taguchi L9 orthogonal array (OA) experiment was considered for the investigation. CNT’s wt.%, sintering temperature, and time were chosen as process parameters in the sintering process, while macro-hardness and relative density were evaluated as performance evaluation characteristics. The signal-to-noise ratio (S/N) and ANOVA statistical procedures were utilized to evaluate the effect of sintering parameters/levels on the micro-hardness and relative density of the Al/CNTs composite sintered. ANOVA statistical analyses revealed that the CNTs wt.% significantly influences relative density (83.58%), followed by temperature (14.58%), whereas CNTs wt.% significantly influenced micro-hardness (77.75%), followed by temperature (13.64%). The sintering of Al/CNTs composites using these optimum conditions is recommended to reduce power consumption and enhance the quality of the sintered composite.

Fabrication and hardness of in-situ Al3Ti–Al2O3 composite

In this work, an in-situ Al3Ti–Al2O3 composite was optimally synthesized from raw powders via mechanical milling and conventional sintering processes. The strong influence of milling time on the promotion of the phase reaction between the initial TiO2 and Al materials was proven by using X-ray diffraction and surface morphology analysis. The obtained results showed that the milling process did not initiate any reaction between the raw TiO2 and Al materials. However, the milling process was important for creating a homogeneous powder mixture and refining the particle size of the powders. The Al3Ti–Al2O3 composites were completely formed after conventional sintering at 750°C for 30 min for a milling time of over 4 h. The highest obtained microhardness of the composite was approximately 130 HV, which was suggested to be related to the microstructure of the bulk composite specimen consisting of two main phases, the Al3Ti matrix and the Al2O3 particles dispersed in the matrix. A small portion of an unidentified phase, a Ti-rich compound, was found in the matrix together with a tiny fraction of AlTi3. We suggest that the optimal sintering process and mechanical milling are important key factors in fabricating bulk hardness Al3Ti–Al2O3 composite materials.