Single-step Sintering Research Articles

Optimization of sintering process to fabricate high-density ultrafine-grained (HfNbTaTiZrW)C ceramic: A comparative study

The fabrication of senary carbide (HfNbTaTiZrW)C was achieved by carbothermal reduction process combined with three different kinds of sintering paths, including conventional single-step sintering (CS) and two kinds of two-step sintering (TSS-1 and TSS-2). It is revealed that the synthesized nanosized powders (∼102 nm) through carbothermal reduction exhibited excellent sintering capability to obtain a high density single-phase solid solution with homogeneous metal cation distributions at macro- and sub-micrometer scales, while atomic-level heterogeneity existed with Zr segregation. Compared to CS samples with bimodal microstructure, the application of two-step sintering significantly reduced the amounts of intergranular pores and (ZrHf)O2, accompanied with grain refinement. In particular, a well-designed TSS-2 route comprising short-term high-temperature followed by low-temperature sintering achieved ultrafine-grained bulk with grain size of 1.44±0.22 μm and excellent mechanical properties of elastic modulus of 480 MPa, microhardness of 23.4 GPa, and fracture toughness of 3.8 MPa·m1/2.

Design of TiC intergranular-dispersed Al2O3 composites via in situ and two-step sintering with enhanced mechanical and electrical properties

To further enhance the mechanical properties of the Al2O3/TiC composites and reduce the percolation threshold induced by the controlling microstructures, composites containing 0–20 % TiC were fabricated in this work using the two-step sintering (TSS) method, in which TiC was in situ synthesized from carbon nanotubes and TiH2. From the acquired results, it was indicated that the TiC particles were completely dispersed at the grain boundaries. As the first-step temperature (T1) increased, the Al2O3 grains grew up, while the second-step temperature (T2) mainly affected the densification of the whole structure. The composites could be listed as follows, 1550/1400 > 1600/1400 > 1600/1300 > 1550/1300, which corresponds to a descending order of the balanced Young’s modulus, Vickers hardness, flexural strength, fracture toughness, and electrical conductivity. Moreover, the largest fracture toughness exhibited by 1600/1400-20 was 5.83 MPa m1/2, which increased by 69 % compared with monolithic Al2O3. The similarity of the Al2O3 grain size and the TiC particles size, as well as the complete grain-boundary dispersion of the relatively large TiC particles significantly contributed to the lower percolation value (10.5 %) of the Al2O3/TiC composites produced via the two-step sintering process compared with the composites fabricated by utilizing the conventional single-step sintering (percolation value: 11.2 %) method. The lower percolation value facilitates the fabrication of conductive Al2O3/TiC composites with excellent mechanical properties.

Comparison of properties of HAp and CaTiO3-HAp ceramic composite fabricated by different sintering conditions

A hydroxyapatite (HAp) ceramic and a 20% wt CaTiO3-HAp ceramic composite for potential application as biomaterials were produced using single-step sintering (SS) and two-step sintering (TSS) in air. The sample pellets were SS sintered at 1250 ?C with a holding time of 1 h. For TSS, the pellets were heated at 10?C/min to 1250?C (T1) and held at this temperature for 20 min (t1). The temperature was then rapidly lowered at 20?C/min to 1150?C (T2), and held at this temperature for 10 min (t2). The 20% wt CaTiO3-HAp ceramic composite obtained from TSS had a fine grain of 0.56 ? 0.04 mm due to the restraint of grain growth caused by the fast cooling from T1 to T2 and the hard CaTiO3 particles dispersed in the HAp matrix. The microstructure refinement of the 20% wt CaTiO3-HAp ceramic composite fabricated by TSS produced suitable sintered density due to increased locking at triple junctions. This ceramic composite exhibited fracture toughness of 2.10 MPa.m1/2 with a brittleness of 1.97 mm-1/2, indicating that crack propagation was suppressed. MG-63 osteoblast cells were able to adhere to the surface of both HAp and the 20% wt CaTiO3-HAp ceramic composite produced by TSS and were no different between 3 and 7 days of culture. The alkaline phosphatase activity of MG-63 cells was higher on both ceramics cultured in osteogenic differentiation medium than in complete growth medium, which indicated more efficient osteoinductivity.

Preparation of high-strength alumina-zirconia fibers by the sol-gel method combined with two-step sintering processes

The preparation of high-performance Al2O3-ZrO2 fibers is a significant and challenging task. In this study, continuous Al2O3-ZrO2 fibers were synthesized via the sol-gel method using aluminum powder and zirconium acetate as raw materials. It was observed that the amorphous state of Al2O3 transformed into γ-Al2O3 and then to α-Al2O3, while the amorphous ZrO2 directly transformed into t-ZrO2. The formation temperatures of t-ZrO2 and α-Al2O3 were both approximately 1100 °C. The optimal sintering temperature during the single-step sintering process for Al2O3-ZrO2 fibers was 1400 °C. Based on the experiment results, fully dense Al2O3-ZrO2 fibers with fine grains were successfully synthesized via a two-step sintering process. The average sizes of Al2O3 and ZrO2 grains were 153 nm and 62 nm, respectively. The Weibull strength of the Al2O3-ZrO2 fibers reached 2.2 GPa with a peak value of 2.6 GPa, which exhibited an increase of approximately 30% compared to that formed by the single-step sintering process. Furthermore, the nucleation, growth, and densification behaviors of Al2O3-ZrO2 fibers during the two-step sintering process were discussed.

Influence of bismuth oxide as a sintering aid on the densification of cold sintering of zirconia

In the past decades, Zirconia (ZrO2) has emerged as a promising technical ceramic, both as high temperature structural material and electrolyte for fuel cells, etc. The traditional synthesis of ZrO2 with spark plasma sintering (SPS) usually requires a sintering temperature as high as 1200 °C. General interest in lowering the sintering temperature to reduce energy consumption and thermal stresses has led to research on two promising routes – cold sintering via temperature-dependent chemical reactivity and sintering aids, which facilitates mass transport and improves densification. Here we combine both by developing a single-step sintering process benefitting from both water vapor through the in-situ conversion of Zr(OH)4 to ZrO2 and liquid phase Bi2O3 as a sintering aid. The resultant ZrO2 has a relative density above 80% with a sintering temperature as low as 900 °C, significantly higher than that of ZrO2 without sintering aids, which had a relative density of 54%, both sintered at 50 MPa. The dependence of porosity of sintered samples as a function of sintering pressure (range: 50 MPa–300 MPa) and temperature (range 400 °C–1200 °C) is mapped out as guidance for further material property design. A linear relationship between hardness and relative density was found, with a maximal hardness of 6.6 GPa achieved in samples with 30% porosity. In addition to sintered density, phase stabilization of tetragonal ZrO2 is enhanced at sintering temperature of 900 °C with water vapor and Bi2O3, respectively.

Thermoelectric converter with stepwise legs for high energy conversion efficiency

Due to industrial activity, an enormous amount of heat is wasted on the environment enabling energy loss and global warming. Thermoelectric (TE) technologies make possible the direct conversion of waste heat into electrical energy. The main bottlenecks of energy conversion by TE devices are low efficiency as well as the expensive and complex fabrication procedure of industrial converters. In this work, we fabricated a Bi2Te3-based TE converter with stepwise legs for highly efficient energy conversion using a single-step sintering method. Before the fabrication of the converter, we employed the numerical simulation and found that Co is the best choice among the other well-established metallic layers, that can be used for the highly conductive covering of the n- and p-type TE legs. The employment of this metal produces the lowest thermal stress that occurs in the stepwise converter at the cooling stage of a sintering process. The energy conversion efficiency of the designed converter estimated using the finite element method reaches 10.1% at a temperature difference of 375 K (Tc = 300 K). Moreover, the measured value of efficiency achieves 8.5%, which is 1.4 times higher than the commercial Bi2Te3-based converters and 9–14% higher than the other reported stepwise converters. The enhanced efficiency mainly originates from the effective stepwise design of the n- and p-type TE legs. This work offers a new path for the simple single-step fabrication of the Bi2Te3-based TE converters that should have great potential for energy harvesting.

Development of glass-ceramics from Soda lime silica glass waste with addition of kaolin and coloring oxide for Turquoise’s imitation

Turquoise, a hydrated phosphate of copper and aluminum, is an opaque, blue-to-green mineral with the chemical formula CuAl6(PO4)4(OH)80.4 H2O. Owing to its unique hue, turquoise is rare and valuable, and it has been appraised as a gemstone for thousands of years. However, with high porosity, low specific gravity, fracturable, and low hardness of just under six or slightly more than window glass, treatments are required before use as jewelry. Nowadays, imitations, and synthetics from glass, plastic, or pressed turquoise powder bonded with resin, are generally found in the market. In our present study, we aim to investigate an alternative method to form glass-ceramic for turquoise imitation by using a direct sintering method based on the soda-lime-silica glass waste with kaolin mixer and coloring oxide additives. In the preparation, the pre-mixed powders were poured into a stainless-steel die, then, the die was brought to the pressing machine for sample forming. A conventional furnace was applied for heating the samples to ∼1000 °C with a soaking time of 4 h. The obtained ingots were characterized by several techniques. The chemical compositions were determined by the Energy Dispersive X-ray Fluorescence (ED-XRF), and their chemical structures were evaluated by the X-ray diffractometer. Meanwhile, their chemical fingerprints were obtained by Raman spectroscopy. As for comparison, the natural turquoise, as well as the commercial imitation samples, selected from the local market were undergone the same characterization. We have found that with the proper content of the metal oxide additives, the products in unique blue and brown-turquoise appearances can be formed and cut to decorate the jewelry set for the niche market. Thus, an alternative method for recycling soda lime-silica waste glass based on a single-step sintering process at relatively low temperature enabled jewelry applications.

Influence of two-step sintering on power loss and permeability dispersion of MnZnNi ferrite

Manganese-zinc-nickel (MnZnNi) ferrite nanoparticles with the chemical composition Mn0.2Zn0.5Ni0.3Fe2O4 were prepared via co-precipitation method. Using the prepared nanoparticles, ring-shaped cores were formed and subjected to two-step sintering. Then, magnetic permeability, magnetic losses, density and microstructure of the samples were examined and compared with those of ferrites produced by the single-step sintering process. It was found that upon reducing the isothermal temperature from 1350 °C to 1250 °C, the density of the samples increased from 4.39 to 4.71 gr/cm3 and the average grain size diminished from 10.1 to 2.2 µm. The samples prepared through the two-step sintering method with a high temperature of 1350 °C and a lower temperature of 1250 °C exhibited a good performance in terms of loss and permeability. The two-step sintering process is an efficient method to produce high density and permeability plus low-loss ferrites.

Grain size engineered (Ba,Sr)(Zr,Ti)O3 ceramics with excellent energy storage properties for high-voltage pulsed capacitors

The macroscopic physical properties of functional ceramics are strongly affected by grain morphology, almost resulting from the sintering process. For a given ceramic material, it has been experimentally confirmed that the enhancement of dielectric breakdown strength Eb can largely increase energy storage density (W). An effective method to increase Eb of ceramic materials is the control of grain morphology. Herein, the (Ba0.95Sr0.05)(Zr0.2Ti0.8)O3 ceramics have been intensively investigated by different sintering techniques to improve grain morphology. Compared with single-step sintering, two-step sintering can sharply inhibit grain growth and make size distribution uniform, which is conducive to the increase in the Eb. A competitive energy storage capacity can be achieved with a high Wch of 3.78 J/cm3, a high Wdi of 3.50 J/cm3, and a high η of 92.6% by two-step sintering due to large Eb of 350 kV/cm, accompanied with good thermal stability till to 160 °C and good fatigue characteristics up to 105 cycles, indicating that two-step sintering is an effective strategy of engineering grain morphology.

Improved cathode performance and relaxation properties of LiMn<sub>2</sub>O<sub>4</sub> prepared by optimized ball-milling with single-step sintering

Improved cathode performance and relaxation properties of LiMn<sub>2</sub>O<sub>4</sub> prepared by optimized ball-milling with single-step sintering

Tunable ferroelectric properties in (K0.5Na0.5)NbO3 ceramics fabricated by a simple modification of the conventional sintering technique

ABSTRACT In this work, a modified sintering method, i.e. the three-step sintering technology, has been applied to fabricate the K0.5Na0.5NbO3 ceramics. The impact of the detailed sintering conditions on the sinterability, dielectric and ferroelectric properties of K0.5Na0.5NbO3 is systematically investigated. Attributed to the three-step sintering, the highest density of 4.07 g cm−3 can be observed, which is strongly improved compared with that of the single-step sintering technique and is increased up to 90% of the theoretical density. Furthermore, through the analysis on the tunability of the dielectric and ferroelectric performances, it is found that the maxima of permittivity and polarisation can be obtained in K0.5Na0.5NbO3 sintering at 1160–1220–1120°C. These results illustrate that both the dielectric and ferroelectric behaviours can be effectively modulated through the simple modification of sintering technique, which would provide a novel route to achieve tunable electrical properties in ferroelectrics.

Effects of sintering curves on microstructure, physical and mechanical properties and on low temperature degradation of zirconia-toughened alumina

The objective of this work was to study two-step sintering as a means of controlling the microstructure of coarse Al2O3 matrix composites containing submicrometric and nanometric inclusions of ZrO2 ranging from 0–30 wt. % by weight based on commercially available powders and evaluate its hydrothermal degradation as function of a water vapour pressure and its mechanical properties. The results showed that two-step sintering allowed a more efficient microstructural control than single-step sintering, resulting in good mechanical properties. The highest flexural strength was achieved for ZTA samples sintered in two-stage sintering conditions TSS2 with T1 = 1560 °C for 3 h, T2 = 1460 °C for 8 h. The studied composites showed good resistance to hydrothermal degradation compared to composites sintered in single step sintering conditions.

Two-step sintering of 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 lead-free piezoelectric material

Two-step sintering (TSS) as an efficient sintering method for obtaining dense microstructure while preventing excess grain growth was used for sintering 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 composition which is located near the morphotropic phase boundary of this binary system. In order to compare the obtained microstructure and piezoelectric properties, conventional single step sintering (SSS) was also examined. Microstructure evolution during sintering at different temperatures was investigated to find the optimum sintering temperature. Ferroelectric hysteresis loop as well as unipolar strain behavior of optimally sintered ceramics was studied. According to density measurement and microstructure studies of the prepared ceramics, TSS resulted in finer and more dense and uniform microstructure compared to SSS method. As a result remnant polarization of TSSed ceramic was increased by 35% and its coercive field was decreased by 16%. The inverse piezoelectric coefficient of the SSSed and TSSed was obtained 220 and 300 p.m./V, respectively. These values are high enough for practical applications such as actuators. The obtained results clearly showed that TSS is capable of sintering 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 at temperatures lower than which is required for SSS method. Therefore the composition stoichiometry is maintained after sintering and denser microstructure without abnormal grain growth is obtained which is responsible for improved electrical properties of the piezoceramics.

One-Step Enameling and Sintering of Low-Carbon Steels

Powder technology allows manufacturing complex components with small tolerances, saving material without subsequent machining. There is a growing trend in using sintered steel components in the automotive industry. Within 2020, about 2544 million US dollars was invested for manufacturing sintered components. Not only does this industry uses steel components, but the gas cooker industry also uses steel in its burners since they are robust and usually demanded by Americans, with forecasts of 1097 million gas cookers in 2020. Steel gas burners have a ceramic coating on their surface, which means that the burner is manufactured in two stages (casting and enameling). This work aims to manufacture the gas burners by powder metallurgy, enameling and sintering processes in a single step. To achieve this aim, the ASC100.29 iron powder has been characterized (flow rate, relative density and morphology); subsequently, the most suitable parameters for its compaction and an adequate sintering temperature were studied. Single-step sintering and enameling was achieved by compacting iron powder at 500 MPa and sintering at 850 °C for 5 min. The necessary porosity for mechanical anchoring of the coating to the substrate is achieved at this sintering temperature. Bending resistance tests, scratching, degradation under high temperature and basic solution and scanning electron microscopy were used to characterize and validate the obtained samples.

Nickel ferrite ceramics: combustion synthesis, sintering, characterization, and magnetic and electrical properties

ABSTRACT Nickel ferrite powders were prepared by combustion synthesis of Fe, Fe2O3 and NiO using NaClO4 as fuel. Nickel ferrite ceramics were fabricated by single-step and two-step sintering under various conditions. The effects of each sintering process on the densification, microstructure and properties of the ceramics were investigated. X-ray diffraction analysis of as-prepared powders and as-fabricated ceramics confirmed a single-phase NiFe2O4 with a cubic spinel crystal structure. Single-step sintering produced ceramics with a wider density range (3.63–3.97 g/cm3) than two-step sintering (3.74–3.84 g/cm3) and a wider grain size range (1.10–2.31 µm compared to 1.30–1.37 µm). Two-step sintering produced ceramics with greater microhardness (135.00-172.60 HV) than single-step sintering (65.33-153.00 HV). In both methods, higher sintering temperatures produced ceramics with greater saturation magnetization (Ms). The highest Ms values were 62.84 emu/g from single-step sintering at 1200°C and 70.00 emu/g from two-step sintering at T2 = 1150°C: Hc was, respectively, 31.18 and 22.00 Oe. The ceramic sintered by the two-step method at T2 of 1150°C presented superparamagnetic-like behavior of multi-domain magnetic materials, with high µi and very low Hc. The higher electrical resistivity (R), dielectric constant (εr) and quality factor of this ceramic support high-frequency data storage applications.

The Effects of Sintering Additives on the Sintering of 3Y-TZP Ceramic

The influence of 0.5 wt.% of manganese oxide (MnO2) addition as dopant into 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) were studied in comparison to undoped 3Y-TZP using different sintering profiles. Samples were sintered at 1200°C, 1300°C and 1400°C with holding time of 1 min, 12 min, 2 hours and 20 hours all for single step sintering. The optimal sintering profiles are A4 and C2 where 3Y-TZP doped with 0.5% wt. MnO2 sintered at 1400°C for 12 minutes and 1200 °C for 20 hours respectively. At these sintering parameters, the MnO2 doped 3Y-TZP sample exhibited relative density of ~97%, Vicker’s Hardness value of ~13 GPa, Fracture toughness value of ~4 MPam1/2 and Young’s modulus of ~210 GPa.

Multistep sintering: its role on density, phase homogeneity, microstructure and electrical properties of (1-x-y)BaTiO3-xPbZrO3-yNaVO3 System

Barium Titanate synthesized by the oxalate coprecipitation method was doped by other perovskite-based material PbZrO3 and NaVO3 to obtain a system of (1-x-y)BaTiO3-xPbZrO3-yNaVO3 with x = 0.15 and y = 0.025. TGA characterization shows an important weight loss that starts at 400°C and finished at 650°C. XRD analysis of calcined powder at 650°C shows that the samples mostly consist of single-phase BaTiO3 perovskite. Sintering process was carried out in two ways; first, a single step sintering up to 1250°C for 2 hours shows increasing secondary phases, until the BaTiO3 phase becomes the minority. Secondly, multistep sintering starting at 1050°C, holding for 2 hours, down to room temperature, and then repeated with similar processes for 1150°C and 1250°C. Characterizations including XRD, SEM for microstructure, and permittivity analysis of the sample after this thermal treatment shows that the majority phase is BaTiO3 with minor secondary BaVO2.9 phase and its electrical properties are enhanced.

Microstructure and impedance spectroscopy of high density holmium hafnate (Ho2Hf2O7) from nanoparticulate compacts

Nanoparticles of holmium hafnate (Ho2Hf2O7) synthesised employing an environmentally sustainable Leeds Alginate Process (LAP) were compacted into pellets and sintered using conventional single step sintering (SSS) technique for 2 h over the temperature range 1100 °C to 1500 °C in the interval of 100 °C and with unconventional two-step sintering (TSS) technique at (I) −1500 °C for 5 min followed by (II) – 1300 °C for 96 h. The relative density of the pellets after sintering was calculated using the Archimedes’ principle and crystal structure data was used to determine the theoretical density. Grain size was measured using ImageJ from SEM micrographs. The pellets processed by the TSS process have been found to be denser (98%) with lesser grain growth (0.77 μm) in comparison with those processed using the SSS process. Ionic conductivities of Ho2Hf2O7 pellets sintered by the two techniques were measured using ac-impedance spectroscopy over the temperature range of 340 °C to 750 °C in the frequency range of 100 mHz to 100 MHz for both heating and cooling cycles. The bulk (1.0 × 10−4 S.cm−1) and grain boundary (4.66 × 10−5 S.cm−1) conductivities of Ho2Hf2O7 at 700 °C prepared by SSS process is slightly higher than those processed by TSS process. The value of grain boundary activation energy is lower in the case of TSS process than that of SSS process. The results of this study indicates that the ionic conductivity of holmium hafnate pellets is not much affected by the sintering conditions but the TSS process is the better route for processing the holmium hafnate since the pellet could be sintered to remarkably high densities at lower temperature and exhibited lesser grain growth.

Transparent alumina ceramics fabricated by 3D printing and vacuum sintering

Transparent alumina ceramics were fabricated using an extrusion-based 3D printer and post-processing steps including debinding, vacuum sintering, and polishing. Printable slurry recipes and 3D printing parameters were optimized to fabricate quality green bodies of varying shapes and sizes. Two-step vacuum sintering profiles were found to increase density while reducing grain size and thus improving the transparency of the sintered alumina ceramics over single-step sintering profiles. The 3D printed and two-step vacuum sintered alumina ceramics achieved greater than 99 % relative density and total transmittance values of about 70 % at 800 nm and above, which was comparable to that of conventional CIP processed alumina ceramics. This demonstrates the capability of 3D printing to compete with conventional transparent ceramic forming methods along with the additional benefit of freedom of design and production of complex shapes.

Development of micro/nanostructured-Cu-TiO2-nanocomposite surfaces to improve pool boiling heat transfer performance

In the current study, a new single-step forced electrodeposition method is proposed for the fabrication of intended micro/nanostructured surfaces. In this method, high thermal conductive Cu-TiO2 nanoparticles (10 nm to 20 nm) are deposited on copper sample with higher current density followed by sintering at reducing atmosphere. Initially, the nanoparticles are deposited uniformly on the copper sample by employing single-step forced convection electrodeposition. Finally, a single-step sintering is employed to enhance the adhesion of the coated layer with the copper sample. During sintering (annealing), the growth in nano-grains is occurred which enhances the inter-connectivity between the grains and also between the grains and base copper surface. The coating thickness (19 to 42 μm), porosity (43 to 75%), wettability (65° to 38°), and roughness (0.38 to 1.32 μm) of the developed surfaces are increased with increase in current density during the electrodeposition. The electrodeposition is a simple and cost effective method and able to provide more dense and stable surfaces. After the surface characterization, their heat transfer performances are analyzed through pool boiling experiments. The maximum percentage reduction in boiling incipience temperature, maximum percent augmentation in critical heat flux and boiling heat transfer coefficient on coated surfaces are 66.7%, 95.71%, and 317%, respectively. Five repeated experimentations are performed at a period of 21 h. The highest variations of ±8.6% in CHF and ± 2.1 °C in wall temperature are observed. Heat transfer enhancement mechanism and effects of various parameters on pool boiling are discussed. The present outcomes are also compared with the published outcomes.