Step Sintering Process Research Articles

Enhanced mechanical properties of R–SiC honeycomb ceramics with in situ AlN–SiC solid solution

Recrystallized SiC (R–SiC) honeycomb ceramics were prepared using Al4SiC4 as additives through a two–step sintering process, consisting of pre–nitridation treatment at 1800 °C for 180 min, followed by heat treatment for 60 min at temperatures ranging from 2000 to 2200 °C. The influences of firing temperature on phase compositions, microstructures, porosity, and mechanical properties of honeycomb ceramics were evaluated. The results revealed that AlN, SiC, and C were formed in situ after nitriding at 1800 °C. As the subsequent heat treatment temperature increased from 2000 to 2200 °C, the sublimation rate of submicron SiC increased, as well as AlN solid solubility. AlN diffused into SiC lattice and formed the SiC–AlN solid solution, which accelerated the conversion of 6H–SiC to 4H–SiC. According to first–principles calculations, the formation of solid solution contributed to the sintering process, which performed through the reduction of the surface energy of the grains. In addition, the pore size of specimens increased with elevating the heating temperature, and the distribution transitioned from unimodal to bimodal. Simultaneously, AlN and C activated the SiC lattice and facilitated mass transfer rate. The SiC sintering necks were well–developed at 2200 °C, resulting in the optimized overall performance of SiC honeycomb ceramics with a porosity of 52.7% and compressive strength of 17.98 MPa.

Fabrication of porous apatite-wollastonite glass ceramics using a two steps sintering process

The aim of this work was to produce porous apatite-wollastonite (AW) glass-ceramic scaffolds via a two step sintering process and to assess their suitability for bone replacement applications. Apatite-wollastonite, polyvinyl chloride (PVC) and poly (methyl methacrylate) (PMMA) were used to fabricate bonelike porous scaffolds using the burning out method. Compression tests were carried out on AW scaffolds after sintering. Scanning electron microscopy was used to observe the pores distribution and microstructure of the sintered scaffolds. The scaffolds containing 70% AW and 30% PMMA have the highest compressive strength and apparent density. All scaffolds present a microstructure with interconnected micro and macroporosity (less than 50 µm).

LiCoO2 Cathode Material Prepared through Two Step Sintering Process

LiCoO<sub>2</sub> cathode material has been continuously applied in commercial LIBs cells. It has high gravimetric and volumetric density. In this research, an economical approach to obtain LiCoO<sub>2</sub> is proposed. Pure cobalt oxide (Co<sub>3</sub>O<sub>4</sub>) precursor was obtained via atmospheric precipitation of cobalt sulfate and thermal decomposition of the as-obtained hydroxide precursor. The next heat treatment was performed to obtain LiCoO<sub>2</sub> powder. To investigate the characteristic of the precursor and the final product, XRD, FTIR, and SEM analysis were conducted. The final product has hexagonal structure and quasi spherical morphology. The size of the particle is in micron. The charge-discharge analysis of LiCoO<sub>2</sub> was conducted in LiCoO<sub>2</sub>/Graphite system where the initial capacity of LiCoO<sub>2</sub> is 120 mAh/g at the current density of 0.1 C (20 mA/g). Overall, this method can be used for large scale LiCoO­<sub>2</sub> cell production.

Binder-jetting additive manufacturing of Mg alloy densified by two-step sintering process

Difficult to effectively densify in a short time is an important issue that limits the application of magnesium (Mg) alloy fabricated by binder jetting additive manufacturing (AM) process. In this study, two-step sintering (TSS) process was utilized to improve the density of printed Mg alloy samples. Results show that the relative density, mechanical properties and corrosion resistance of the samples after different TSS process were dramatically improved in a short time. A suitable sintering temperature of first sintering step can rapidly increase the density of the samples in a short time. During the second step sintering process, little exceeding the liquidus temperature can further increase the density of the sample, but little lower than the liquidus temperature will lead to a sharp increase in grain size. The sample after sintering at 680 °C for 30 min then sintering at 610 °C for 6 h shows the highest relative density (0.995), the highest strength (394 MPa), the lowest corrosion rate (101 mm/year) and the smallest corrosion current density (60.4 μA/cm2). Compared with one-step sintering process, TTS process has higher sintering efficiency.

Rutile TiO2 microwave dielectric ceramics prepared via cold sintering assisted two step sintering

In this work, a sintering route named cold sintering assisted two step sintering process (CSP-TS) is presented to prepare rutile TiO2 ceramics with submicron grain sizes. Cold sintering process at 300 °C with tetrabutyl titanate and water as the liquid phase yields a ‘green body’ with a relatively high density of ∼80 %, and finally dense (98.5–99.8 %) rutile TiO2 ceramics with grain sizes of ∼600 nm can be obtained in the second sintering process at 950−1000 °C. The microstructural analysis with SEM and TEM indicates that the CSP-TS samples sintered at 950 °C have an obvious phenomenon of recrystallization, accompanying by a decrease of amorphous phases and a formation of clear grain boundaries. Besides, the rutile TiO2 ceramics prepared by CSP-TS possess excellent microwave dielectric properties with relative permittivity of 92.0–98.4 and Q × f values of 27,800−31,900 GHz. Therefore, it is feasible to utilize CSP-TS to prepare ceramics with small grain sizes at low sintering temperatures.

Microstructure and mechanical properties of TiC steel-bonded carbides prepared by two-step sintering technique

In this study, a two-step sintering technique was applied to prepare TiC steel-bonded carbides. The first step sintering process was heating to a liquid sintering temperature of 1390 °C without soaking time for achieving a critical density, and then unstable micropores remained in the samples, in which the rapidly densification of sample primarily depended on liquid pore filling. The second step sintering was performed at a lower temperature of 1330 °C for 10 h to complete densification process due to the grain-boundary diffusion. Compared with traditional vacuum sintering method, the growth of TiC particle was inhibited significantly and the evaporation of metal binder was suppressed through the two-step sintering technique. Overall, the prepared samples possessed excellent mechanical properties with the TRS and hardness of 2093.3 MPa and 87.3 HRA, respectively.

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho2Zr2O7) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho2Zr2O7 pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10−3 Scm−1) and grain boundary (2.50⨯10−3 Scm−1) conductivities of Ho2Zr2O7 prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.

Cr doped ZnO: Investigation of magnetic behaviour through SQUID and ESR studies

Polycrystalline Zn1-xCrxO (0.01 ≤ x ≤ 0.09) samples synthesised by solid state reaction technique were sintered at different temperatures following slow step sintering schedule, investigated for optical and magnetic properties using suitable characterisation techniques. Cr2O3 and CrO2 phases have been detected in the Raman spectra of Zn1-xCrxO samples with x ≥ 0.05. Photoluminescence study indicated improved optical property of the samples compared to undoped ZnO. While low percentage Cr doped samples show diamagnetic behaviour, different types of magnetic orderings are observed in the samples with higher percentage of dopants (x ≥ 0.05) for different sintering temperatures. The spin system and magnetic properties were analysed through Electron Spin Resonance study; g-value of 1.97 indicates Cr in 3+ valence state in ZnO. Presence of both Cr3+ and Cr4+ in ZnO understood to facilitate super exchange interactions to promote room temperature ferromagnetism. ESR study ensures improved magnetic homogeneity through slow step sintering process.

Properties of Porous Magnesium Using Polymethyl Methacrylate (PMMA) as a Space Holder

Porous magnesium has been recognized as a promising biodegradation metal for bone substitute application because of its excellent biocompatibility, low density, ability to biodegrade in vivo and excellent mechanical properties. In the present work, porous magnesium was fabricated by powder metallurgy using spherical polymethyl methacrylate (PMMA) as a space holder. To determine the optimum sintering temperature for porous magnesium fabrication, the porous magnesium was fabricated using double step sintering process at various sintering temperature (550°C, 585°C and 620°C) in first stage of research. The porous magnesium fabricated was then characterized for morphology, porosity, density and compressive strength. Density of porous magnesium increases and the porosity decreases with increasing sintering temperature. The mechanical characterization indicated that porous magnesium sintered at 620°C exhibited the highest compressive strength and density with optimum range of porosity of 39.37%. For second stage of research, porous magnesium with porosities of 39.38 – 40.82% was produced with different sizes of PMMA particles (38-63 µm, 63-90 µm and 90-125 µm). The compressive yield strength ranges between 19.95 MPa to 23.28 MPa and increases with decreasing PMMA particles size and porosity. These results proven that the PMMA has a potential to be used as space holder in porous magnesium for biomedical application.

Optimization of ITO glass/TiO2 based DSSC photo-anodes through electrophoretic deposition and sintering techniques

Abstract In the current study, TiO 2 layers were deposited onto Indium Tin Oxide (ITO) coated glass substrates at varied voltage levels ranging from 2.5 V DC to 7.5 V DC , using Electrophoretic deposition (EPD) process. Subsequently, the deposited TiO 2 layers were either sintered by using a single step (450 °C) or double step (350 °C, 450 °C) sintering process. The fabricated films were later dye stained and integrated with Dye Sensitized Solar Cells (DSSCs). Upon the morphological and electrical characterization of the samples, it was revealed that the DSSCs containing a semiconductor layer deposited at 5 V, having a thickness of ~20 µm and an average pore size of ~30 nm produced the highest power conversion efficiency (of 4.2% ) amongst all the EPD variants employed, owing to the optimized thickness and dye loading properties. The efficiency further increased to 4.5% , when the single step sintering process was replaced by two step sintering processes.

Preparation and characterization of CaO–Al2O3–SiO2 glass-ceramics from molybdenum tailings

Molybdenum tailings (MTs) are by-products of the molybdenum mining industry, and further treatment are necessary before their disposal for avoiding contamination of the environment, such as landfills, plant covers, or using material productions. Here, MTs were blended with other chemical raw materials to manufacture a decorative glass-ceramic of CaO–Al2O3–SiO2 by sintering processes. The effects of Fe3O4 and TiO2 oxides in MTs on the melting and crystallization behavior of the glasses were investigated by DSC, XRD, FESEM, FTIR, and TEM. The results showed that the crystallization temperature (Tc) and crystallization activation energy (E) of the glasses decreased when the amount of MTs increased from 41 to 49 wt% (M1-M5). Wollastonite (CaSiO3) and catena-silicate (Ca2.87Fe0.13(SiO3)3), which constituted as the major and minor crystalline phases, were precipitated in the prepared glass-ceramic. In the five specimens tested, the crystalline phases were evenly distributed throughout the surface and sufficiently interlocked together. This one step sintering process, in which the glass ceramic was nucleated and crystallized at 1110 °C for 2 h, generated a material with an appearance suitable for building decoration.

Oxidation and thermo physical studies of non-stoichiometric thorium uranium oxides prepared by gel combustion method

Thorium uranium oxide powder with uranium concentrations 10, 20, 30, 40, 50 and 70mol% was prepared by gel combustion method. The sintering behaviour of the powders studied by dilatometry indicated that samples containing uranium up to 30mol% had similar shrinkage behaviour with single step sintering. However, samples containing 40, 50 and 70% uranium followed two step sintering process. Though the air calcined powder shows two phase system beyond 30% uranium containing samples, sintering results into single phase system for samples containing up to 50% of uranium. The temperature programmed reduction-oxidation experiments showed that compositions containing up to 50-mol% of uranium show single reducible species throughout the temperature range. X-ray photoelectron spectroscopic studies of these sintered pellets showed presence of uranium in 6+ and 4+ oxidation states in these oxides. The experimentally determined heat capacities of solid solutions were found to have higher values than that of computed using Neumann–Kopp’s molar additivity rule.

Microstructure, stability and biocompatibility of hydroxyapatite – titania nanocomposites formed by two step sintering process

In the present work, we report the characterization of TiO2-hydroxyapatite (HA) nanocomposites obtained by a two-step sintering (TSS) process of a mixture of HA and titanium hydride (TiH2) powders. The reactions underwent by TiH2 in the presence of HA and hydrogen release, and subsequently, titanium oxidation was examined by thermal analysis. A longer holding time in the second sintering stage enabled obtaining a homogenous TiO2-HA (36% rutile) composite with a thermal expansion coefficient of 11.46·10−6C−1 in the 40–1000°C range. Unconventional TSS process hinders HA decomposition to detrimental tricalcium phosphate (TCP). Wear rate of ceramics was determined by tribological measurements and the material biocompatibility was evaluated using MTT assay. Overall, cell viability results correlated with morphological observations indicated a good biocompatibility of HA-based composites at all tested concentrations. Incorporation of the TiO2 phase in HA by TSS process was found to be an efficient way to prepare bioceramics with improved performances.

Improved dielectric and piezoelectric properties of K/Na excessed (Na,K)NbO3 lead-free ceramics by the two step sintering process

The Na0.5K0.5NbO3 (NKN) ceramics with K/Na excess dopants were prepared by employing the two-step sintering process. The K/Na excessed NKN ceramics were researched by varying the sintering temperature, in order to optimize their piezoelectric properties by employing a conventional sintering method. Also, the effects of dwell time for the two-step sintering process on the ceramic properties were studied. The crystalline structure of the NKN ceramics with K/Na excess dopants were measured by X-ray diffraction (XRD) analysis. The microstructure was examined by field emission scanning electron microscopy (FE-SEM). In addition, piezoelectric coefficient, d33 and relative dielectric permittivity ɛr, will be discussed. In this research, we expected to improve the piezoelectric and dielectric properties of NKN lead-free piezoelectric ceramics by employing excess dopants of K/Na and the two-step sintering process.

Understanding Hydrogen Peroxide Decomposition Activity of Transition Metal Oxide Catalysts in Oxygen Reduction Reaction

Energy storage is essential for large-scale integration of renewable energy such as solar and wind into the electricity grid. Fuel cells and metal-air batteries are promising technologies for grid-scale energy storage owing to their high energy densities and environmental friendliness. However, it is well known that the slow kinetics of the oxygen reduction reaction (ORR) at the cathode limits the efficiency of these fuel cells and metal-air batteries. Among transition metal oxides, calcium-doped lanthanum cobalt oxide is one of the promising catalysts for ORR because of the ability of these materials to support both oxygen evolution and oxygen reduction reactions [1]. In previous work, we demonstrated that in transition metal oxide–carbon composites catalysts, carbon is the primary electrocatalyst for the two electron electro-reduction of oxygen to hydrogen peroxide and the transition metal oxide decomposes the hydroperoxide to generate additional oxygen [2]. Therefore, we have focused on understanding the role of mixed metal oxides on decomposition of hydrogen peroxide for designing more active carbon-based catalyst systems. Specifically, we have studied the effect of manganese substitution of cobalt in calcium-doped lanthanum cobalt oxide on the hydrogen decomposition rate of the catalysts. Manganese-substituted compounds of the formula La0.6Ca0.4Co1-xMnxO3 compound with x value = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1 were synthesized by the sol-gel method using the Pechini process. As previously described [3], a two-step heat treatment was employed for the synthesis of these compounds. In the first step the material subjected to controlled combustion at 120-170 oC, then the material was collected, ground and heat-treated in a second step sintering process at 700oC for 2 hours. The decomposition characteristics of hydrogen peroxide on various transition metal oxides was measured by oxygen reduction studies on rotating-ring-disk (coated glassy carbon) electrode, and by steady state polarization studies of the oxidation and reduction of hydrogen peroxide. We have demonstrated that the mixed potential of the oxide electrode under open circuit conditions could be predicted from the kinetics of the conjugate reactions of peroxide oxidation and reduction. We also find that the higher rate of oxidation of hydroperoxide correlated directly with the improved oxygen reduction activity in the composite catalyst. The presentation will discuss these findings. Figure 1

Investigation of mechanical properties based on grain growth and microstructure evolution of alumina ceramics during two step sintering process

Alumina ceramics having small grain size and high density yield good mechanical properties, which are required in most mechanical applications. Two Step Sintering (TSS) is used to develop dense alumina ceramics. In this research work the effect of sintering temperatures on microstructure and density of the alumina specimens developed by using TSS has been investigated. It has been observed that TSS is more efficient in controlling grain growth and increasing the density as compared to One Step Sintering (OSS) of alumina. Scanning electron micrographs of sintered alumina specimens have been compared. It has been observed that TSS proves to be a better technique for increasing density and controlling grain growth of alumina ceramics than OSS. More relative density, hardness, fracture toughness and small grain size was achieved by using TSS over OSS technique.

Introducing the fluorine doped natural hydroxyapatite-titania nanobiocomposite ceramic

In the present research, natural hydroxyapatite (NHA) was synthesized from bovine bones and then fluorine was doped into the NHA matrix to produce fluorine doped NHA (FNHA; natural fluor-hydroxyapatite) in optimum conditions. At the end an FNHA-TiO2 nanobiocomposite ceramic with excellent biocompatibility and good chemical stability was synthesized through a mechanochemical route and a subsequent two step sintering (TSS) process. Thermal gravimetric analysis (TGA), Differential scanning calorimetry (DSC), X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), inductive coupled plasma (ICP), and energy-dispersive X-ray spectroscopy (EDX) were used as the means for gathering and analysis of the results. According to the obtained results, TiO2 can prevent early decomposition of FNHA by the formation of the CaTiO3 phases and hence strengthen the interactions between the apatite particles which results in the increase of the mechanical properties. Besides, TiO2 provides more SiOH nucleation sites for the formation of the apatite layers and hence more bioactivity.

Effects of High-Energy Ball Milling Time on the Sintering Process of FeSe Superconductors

FeSe superconducting bulks were prepared with high energy ball milling (HEBM) aided sintering process, within which process, tetragonal β-FeSe superconducting phase could be formed directly with one step sintering process, and the formation of hexagonal δ-FeSe non-superconducting phase was effectively avoided. The influences of HEBM time on the sintering process of FeSe bulks were systematically investigated. With different HEBM time, the phase composition and morphology of precursor powders changed correspondingly, which thus influenced the final phase composition and superconducting properties of FeSe superconducting bulks. Due to the formation of FeSe bulks with larger tetragonal phase content and higher superconducting transition temperature, HEBM time of 6.0 h was recognized as the optimal parameter. Shorter HEBM time could lead to the insufficient decrease of particle size and low density. While longer HEBM time caused the formation of amorphous hexagonal δ-FeSe, which crystallized during sintering process. Thus no more tetragonal FeSe could be obtained. The FeSe superconducting bulk with the critical temperature Tc(onset) of 8.0 K was obtained with the HEBM time of 6 h, and sintering temperature of 700 oC for 12 h.

High performance of Ni0.9Mn1.8Mg0.3O4 spinel nanoceramic microbeads via inkjet printing and two step sintering

The effect of two step sintering process on the microstructure and electrical properties of inkjet printing high performance Ni0.9Mn1.8Mg0.3O4 ceramic microbead materials with lower thermal time constant and higher material constant is investigated.

Microstructure and mechanical properties of Ti–22Al–25Nb alloy fabricated by elemental powder metallurgy

In order to better understand the relationship of processing–structure–mechanical properties of Ti–22Al–25Nb alloy fabricated by elemental powder metallurgy (EPM), the effects of sintering parameters on the microstructure and mechanical properties of the Ti–22Al–25Nb alloy were investigated. The blended powders with a nominal composition of Ti–22Al–25Nb (at%) obtained by 190rpm/4.0h milling under a protective argon atmosphere were used for the fabrication of Ti–22Al–25Nb alloy via hot-press sintering. The step sintering process was carried out at 630°C/20MPa/1.0h preliminary, and then 950–1300°C/35MPa/1.0–2.5h followed by furnace cooling. Microstructure were characterized using the scanning electron microscope (SEM) and transmission electron microscopy (TEM). Tensile tests were performed at room temperature and 650°C, respectively. Results showed that the typical lamellar microstructure consisting of α2, O and B2 phases were observed in the Ti–22Al–25Nb alloys sintered at 1250°C/35MPa/1.5h. Moreover, the forming mechanism of O (orthorhombic Ti2AlNb) phase and reactive mechanism were analyzed. The elongations of the sintered Ti–22Al–25Nb alloys were less than 5.09%, indicating the bad ductility at room temperature. However, The Ti–22Al–25Nb alloy sintered at 1250°C for 1.5h exhibited a good combination property with the tensile strength and elongation of 620.53MPa, 7.44% at 650°C, respectively.