Two-step Sintering Process Research Articles

Preparation of high density ZnO ceramics by the Cold Sintering Process

Abstract High-density ZnO ceramics were prepared by a Two-Step Cold Sintering Process (TS-CSP) at an ultralow-temperature. The densification process of ZnO ceramics was demonstrated by TS-CSP. And the density, microstructure and electrical properties of the ZnO ceramics were investigated. The results indicate that most of the crystallization of ZnO ceramic can be completed in the first step of sintered at 150 °C/200 MPa for 30 min, with a relative density approaching 97.36%. This relative density was further improved to 99.43% after the second sintering step was deployed at 200 °C/200 MPa for 30 min, which displayed an average grains size of 1.5 μm and resistivity of 0.125 Ω cm. This work demonstrated an effective method to reduce CSP temperature and pressure in the production of high-performance ZnO ceramics.

Structure Inheritance in Nanoparticle Ink Direct-Writing Processes and Crack-Free Nano-Copper Interconnects Printed by a Single-Run Approach.

When nanoparticle conductive ink is used for printing interconnects, cracks and pores are common defects that deteriorate the electrical conductivity of the printed circuits. Influences of the ink solvent, the solid fraction of the ink, the pre-printing treatment and the sintering parameters on the interconnect morphology and conductivity were investigated. It was found that the impacts of all these factors coupled with each other throughout the whole procedure, from the pre-printing to the post-printing processes, and led to a structure inheritance effect. An optimum process route was developed for producing crack-free interconnects by a single-run direct-writing approach using home-made nano-copper ink. A weak gel was promoted in the ink before printing in the presence of long-chain polymers and bridging molecules by mechanical agitation. The fully developed gel network prevented the phase separation during ink extrusion and crack formations during drying. With the reducing agents in the ink and slow evaporation of the ink solvent, compact packing and neck joining of copper nanoparticles were obtained after a two-step sintering process. The crack-free interconnects successfully produced have a surface roughness smaller than 1.5 μm and the square resistances as low as 0.01 Ω/□.

Impact of sodium excess on electrical conductivity of Na3Zr2Si2PO12 + x Na2O ceramics

In order to industrialize NaSICON materials, modern fabrication techniques have to be used and one of those techniques for producing large-scale electrolyte sheets with 10–300 μm thickness is tape casting. Such technique however requires a sintering step at high temperatures leading to sodium depletion due to evaporation. The sodium loss becomes more significant for large-area and thin components. In order to investigate and compensate the sodium loss, NaSICON compositions with sodium excess were prepared, i.e. Na3Zr2Si2PO12 + x Na2O (0 ≤ x ≤ 0.2). The sodium loss can be reduced by applying a two-step sintering process (1250 °C for only 0.5 h and then at 1230 °C for 5 h). Several characterization techniques were used to analyze the resulting ceramics, the sodium depletion and its consequence on electrical conductivity. Chemical analyses indicated that all compositions were sodium deficient. Furthermore, the weight loss was investigated by thermogravimetric analysis confirming the reduction of weight loss by a factor 2 by applying a two-step sintering procedure with lower second sintering temperature. Initial thermodynamic calculations of the phase equilibria at high temperatures confirm the predominant evaporation of sodium. The highest electrical conductivity (1.6 ⋅ 10−3 S cm−1 at 25 °C) was measured for the composition showing the least sodium deficiency (x = 0.2). Furthermore, the activation energy of bulk and grain boundary conductivity decreased with increasing x in system.

A low-cost approach to fabricate SiC nanosheets by reactive sintering from Si powders and graphite

Abstracts Silicon carbide (SiC) nanosheet with 2D micro-morphology was fabricated by reactive sintering method and a two-step sintering process. The effect of molar ratio of C and Si (from 1/1 to 4/1) on the phase composition and microstructure was investigated. The obtained results indicated that the SiC whiskers with an average diameter about 73.6 nm generated on the graphite substrate when the molar ratio of Si/C was 1 in the initial batch mixture. As a result of the molar ratio of Si/C at 3/1–4/1, the morphology of SiC generated in graphite surface changed from original whiskers to nanosheets structure. SiC nanosheets were obtained with the average thickness of 33.7 nm (standard deviation: 6.1) in the case of the sample containing Si/C molar ratios of 4/1. The TEM results proved that SiC nanosheets grown along [111] direction. The formation mechanism of SiC nanosheets has been explained in detail based on a liquid Si/graphite substrate model.

Enhanced dielectric properties of BaTiO3 based on ultrafine powders by two-step calcination

As high-performance dielectric material, small sizes of barium titanate (BaTiO3) are desirable for the application of multilayer ceramic capacitors. Herein, we synthesized ultrafine BaTiO3 powders with mean particle size of (dav.) 156 nm, tetragonality (c/a) of 1.0092 and specific surface area (SBET) of 6.09 m2/g by two-step sintering process. Kinetic analysis of solid-state reaction reveals that the phase-boundary-controlled mechanism dominated the reaction process in two-step sintering process compared with one-step sintering process dominated by the first-order mechanism. It is also shown that enhanced nucleation in two-step calcination is beneficial for improving the tetragonality. In particular, the dielectric constant and dielectric loss of BaTiO3 ceramics at the Curie temperature are found to be about 12800 and 0.003. This work demonstrates a strategy to trigger a method which can effectively improve the dielectric properties of BaTiO3 ceramic.

Preparation of fine-grained Li4SiO4 pebbles by a combined sol–gel and hydrothermal method and their thermal cycling behavior

Li4SiO4 has been regarded as the vigorous competitors for tritium breeding materials due to its distinct advantages. In this work, Fine-grained Li4SiO4 pebbles were prepared by a combined sol–gel and hydrothermal method (SG–H method). According to our researches, hydrothermal treatment was helpful to gain the ultrafine powders, and two-step sintering process showed obvious advantages in the preparation of fine-grained Li4SiO4 pebbles. The related physical parameters of the samples were measured. Afterwards, thermal cycling tests were carried out to study the microstructure evolution and the variation of crushing load. SEM analyses and compression tests results indicated the Li4SiO4 pebbles obtained by SG–H method exhibited favorable stability under thermal cycling. It suggested that the obtained Li4SiO4 pebbles might meet the requirement of high service temperature in fusion reactor blanket.

Thermal conductivity of (1-x)BaTiO3-xPb(Zn1/3Nb2/3)O3 ceramics (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125 and 0.15)

Barium titanate BaTiO3 (BT) is one of the widely applied lead-free ferroelectric materials due to relatively high permittivity and low cost. Lead zinc-niobate Pb(Zn1/3Nb2/3)O3 (PZN) belongs to a class of disordered and inhomogeneous materials, i.e. is a relaxor. Low-lead (1-x)BT-xPZN ceramics (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125 and 0.15) were prepared by two-step conventional oxide mixed sintering process. Thermal conductivity (κ) of these ceramics was measured for the first time (except for x = 0) from room temperature to 200 °C. It was shown that the thermal conductivity decreases with both decreasing temperature and with increasing PZN content. κ(T) curves show anomaly at Curie temperature, which is connected with change of specific heat. Obtained results indicated that thermal conductivity of investigated samples is mainly related to phonons scattering by lattice defects and by atomic mass fluctuation resulted from PZN incorporation to BT.

Effect of initial microstructure on the sintering behavior of cubic zirconia nano-particles

Abstract The green bodies with two different packing structures (lower and higher density) were prepared from cubic zirconia nano-particles. The sintering trajectories during densification of normal and two-step sintering (TSS) were studied to obtain nanocrystalline cubic zirconia. In normal sintering process, both compacts exhibited the typical sintering trajectory reported in the literatures. However, in TSS, the two compacts followed different trajectories, and only the higher green density sample densified with the suppression of grain growth in TSS process. The density of the sample heated at the first firing temperature of 1100°C and subsequently heated at 1050°C for 10 h reached 98% with the grain size of 117 nm. The two-step sintering of the initial compact with higher density is supposed to be a better way to obtain dense material retained nanometric microstructure.

Effect of Sintering Atmosphere on the Synthesis Process, Electrical and Mechanical Properties of NiFe2O4/Nano-TiN Ceramics

NiFe2O4/nano-TiN ceramics were fabricated by a two-step cold-pressing sintering process. Effect of sintering atmosphere (air, nitrogen, argon) on the synthesis process of NiFe2O4/nano-TiN ceramics was investigated. The DSC-TG and XRD analysis results indicated that besides principal phases of NiO and NiFe2O4, new-phase Ni3TiO5 formed in the sintered ceramics under the three sintering atmospheres, and metallic phase including iron and nickel appeared in the sintered samples with inert gas (nitrogen, argon) sintering condition. Microstructure analysis results showed that considerable quantity of micropores appeared in the ceramic samples sintered under inert gases (argon, nitrogen), but lower porosity (3.0, 3.6%) and higher densities (4.78 g/cm3, 4.51 g/cm3) can be obtained, comparing to the both values (14.4%, 4.17 g/cm3) for the ceramic samples sintered under air atmosphere. Besides, the average bending strength and elastic modulus of the samples sintered under argon is 113.75 MPa and 7.13GPa, which is higher than that of 75.12 MPa, 5.42GPa and 91.96 MPa, 6.26GPa for the samples synthesized under air and nitrogen, separately. When changing sintering atmosphere from air to inert gases (argon, nitrogen), the fracture model of the 4 wt.%nano-TiN/NiFe2O4 ceramics synthesized at 1400 °C for 4 h transformed from intergranular fracture to intergranular-transgranular fracture.

Micro-stereolithography of KNN-based lead-free piezoceramics

The additive manufacturing technique of micro-stereolithography (µSL) is applied to fabricate alkaline niobate (K0.48Na0.52NbO3, KNN) based lead-free piezoceramics. Complex KNN-based piezoceramic green parts with fine dimensional characteristics, such as a piezoceramic transducer array and a hollow hemisphere, have been printed. The two-step debinding and sintering processes are employed to obtain crack-free dense KNN ceramics with complex shapes. These printed KNN ceramics exhibit superior piezoelectric and ferroelectric performance (εr = 2150, d33 = 170 pC/N, Pr = 12.1 μC/cm2, EC = 6.1 kV/cm, and TC = 230 °C), which are comparable to those of KNN piezoceramics made by traditional methods. These results suggest that the µSL technique may serve as an alternative method to manufacture KNN lead-free piezoceramic components, especially for the application with complex architectures.

Diffusion mechanism of oxygen ions in La2Zr2O7/YSZ composite ceramics

In this paper, the diffusion mechanism of oxygen ions in La2Zr2O7/YSZ (LZ/YSZ) composites was investigated experimentally and numerically. LZ/YSZ composite samples with multiple amounts of LZ were successfully synthesized via a two-step sintering process. The so-obtained samples preserved most of the tetragonal phase (82.2 mol%) in YSZ. Moreover, an adhesive grain boundary and good chemical compatibility between YSZ and LZ grains were verified in the composite samples. The measured impedance spectra revealed that pure YSZ is an excellent oxygen ionic conductor, whereas the LZ can serve as an effective oxygen ionic isolator at high temperatures as further confirmed via the molecular dynamics simulation. For LZ/YSZ composites, the measured ionic conductivity decreases nonlinearly with the increase of the volume fraction of LZ, indicating that the addition of LZ can significantly reduce the oxygen ionic conductivity of these composites. More specifically, the simulation results showed that pores and LZ crystals would reduce the diffusion coefficient of oxygen ions, especially for the oxygen ions near the LZ/YSZ interfaces or the pore surfaces. Furthermore, we demonstrated that the bulk diffusion coefficient in YSZ increases as a direct consequence of the application of tensile stress induced by the lattice distortion between LZ and YSZ. However, the ionic conductivity in the LZ/YSZ composite was not enhanced due to the dominant ‘blocking effect’ from pores and interfaces. Indeed, we experimentally proved that both LZ grains and closed pores would reduce the ionic conductivity as a result of the ‘blocking effect’.

Improvement in microstructure and mechanical properties of Ti(C, N) cermet prepared by two-step spark plasma sintering

A fine grained Ti(C, N) cermet tool material was prepared by two-step spark plasma sintering. Microstructure evolution and densification mechanisms of Ti(C, N) during spark plasma sintering were studied. Effect of two-step sintering process and Ni content on microstructure and mechanical properties were also investigated. The critical activated densification temperature of Ti(C, N) is about 1300 ℃, and the rapidest densification rate takes place at 1300 ℃~1400 ℃. Grains are in the size of 1 µm when the Ti(C, N) cermet was prepared by two-step spark plasma sintering. The optimal flexural strength, fracture toughness and Vickers hardness are 1094 ± 42 MPa, 7.2 ± 0.5 MPa m1/2 and 18.3 ± 0.4 GPa, respectively. The Ti(C, N) cermets containing more content of Ni have higher toughness, which is due to the remarkable toughening effect of crack bridging by large grains.

Enhancing the densification of ceria ceramic at low temperature via the cold sintering assisted two-step sintering process

The cold sintering process (CSP) in the transient aqueous environment of malonic acid is applied in this study to enhance the densification behavior of ceria ceramics at a significantly reduced sintering temperature. Dense ceria ceramics (> 98% relative density) could be achieved after CSP at 180 °C, followed by the sintering at 1100 °C/4 h. As expected, the CSP can improve the compaction of the particles before sintering, resulting in ceria ceramics with a fine microstructure of submicrometer scale of grain sizes. All samples show the single phase of cubic fluorite structure with no significant change of lattice parameter. Finally, electrical conductivity of CSP and conventionally sintered ceramics is investigated. This communication emphasizes the advantage of the CSP to reduce the sintering temperature without deterioration of the structure and properties of the final ceramics.

Light Burning Condition of Preparing Dolomite Clinker Using Natural Dolomite

The influence of the light burning temperature on the sintering property of nature dolomite has been investigated by two-step sintering process in the temperature range 1500 °C to 1600 °C. The resulting bulk densities and apparent porosities of the sintered dolomite samples were examined, and analyzing the sintered dolomite by scanning electron microscopy and X-ray diffraction were performed. The results showed light burned at 850 °C for 3 h, the main phases of the dolomite with 3-5 grain size were MgO, CaO and little CaCO3, and then fired at 1600 °C,the density of sintering dolomite reached to 3.38 g/cm3, the apparent property was 1.2 %, the size of MgO grain up to 3.75 μm . However when dolomite light burned at 1050 °C for 3 h, the main phases were MgO and CaO, and then fired at 1600 °C,the density of sintering dolomite only was 3.30 g/cm3, the apparent property was 2.3 %, the size of MgO only was 3.05 μm .

Precise prediction of dielectric property for CaZrO3 ceramic

As for CaZrO3 (CZ) ceramic, the reported dielectric property values, especially dielectric constants, were much different from 23 to 32, which is reliable and credible? Without precise property data, CZ can’t be further developed and utilized accurately. Herein, CZ ceramic was fabricated by a traditional two-step sintering process, then simulated and calculated the dielectric properties precisely at a microscopic polarization angle using the lattice vibrational spectra and the Clausius–Mossotti (C–M) as well as damping equations. The Raman and Fourier transform far-infrared modes were analyzed and used to predict the intrinsic properties, which were consistent well with the values calculated from C–M and damping equations. The intrinsic permittivity, after precise prediction, is about 20, which is reliable and credible. As for the dielectric loss, the value of about [Formula: see text] was obtained after precise calculation, which is similar to other results.

Sintering process effect on the BaTiO3 ceramic properties with the hydrothermally prepared powders

Ferroelectric barium titanate is an important traditional ferroelectric and dielectric material. Multilayer ceramic capacitors require nano-sized ceramics in technology. We synthesized nanocrystalline BaTiO3 powders by a hydrothermal method, pressed them into pellets and then, sintered nano sized BaTiO3 ceramics by conventional sintering method (CSM) and two-step sintering (TSS) process for comparison. The internal pore increases with the sintering temperature in the samples sintered with CSM while the samples sintered with TSS route show uniform grains without clear pores. BaTiO3 ceramics sintered with TSS process exhibit tetragonal structure and strong ferroelectric behaviors while those sintered with CSM mainly reveal cubic perovskite properties.

Preparation and electrochemical performances of xLi2MnO3·(1-x) LiNi0.45Co0.2Mn0.35O2 (0 ≤ x ≤ 1) for high-power lithium-ion batteries

In this paper, a series of cathode materials xLi2MnO3·(1-x) LiNi0.45Co0.2Mn0.35O2 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) were prepared by a carbonate co-precipitation method followed by a two-step sintering process. The effects of different x values on the phase structure, particles size distribution, tap density and electrochemical properties of as-prepared samples were systematically studied. The results display that the two phases between Li2MnO3 and LiNi0.45Co0.2Mn0.35O2 have a better synergistic effect: firstly, suitable amount of Li2MnO3 can stabilize the hexagonal α-NaFeO2 layered structure of OLO and inhibit the mixing positions of Li+ and Ni2+. Secondly, when the x value is 0.5 (MCO-0.5), spherical precursors have the most monodispersed particles size distribution (d = 0.72) and the highest tap density (1.82 g cm−3) among all prepared powders. Finally, 0.5Li2MnO3·0.5LiNi0.45Co0.2Mn0.35O2 (OLO-0.5) delivers a high initial discharge capacity (220 mAh g−1 at 0.1C) and a superior cyclic stability (~ 96% after 100 cycles). Moreover, OLO-0.5 provides the best rate capability owing to the highest discharge capacity when the C-rate increases from 0.1 to 10C.

A preliminary study on the preparation of nanostructured Ti-doped Li4SiO4 pebbles by two-step sintering process

Li4SiO4 has been widely studied as attractive tritium breeding materials due to its innate merits. Considering the potential advantages of nanostructure in tritium breeding materials, a distinctive process was developed to obtain nanostructured Li4SiO4 pebbles. In brief, ultrafine precursor powders were synthesized by solvothermal method without using surfactants, and then indirect wet method was adopted to generate the green spheres with homogeneous microstructure. After that, the suitable sintering conditions were defined by studying the effects of sintering parameters on the grain size evolution, and nanostructured Ti-doped Li4SiO4 pebbles were first obtained by two-step sintering method. This study will be expected to provide references for fabricating other Li-based tritium breeding materials.

Particle Characteristics and Densification of W6Mo5Cr4V2Co5Nb Overspray Powder

W6Mo5Cr4V2Co5Nb (825 K) alloy was prepared by a two-step sintering process from overspray 825 K alloy powder. The overspray powder characteristics and the microstructure and mechanical properties of the as-sintered 825 K alloy were investigated. Results showed that two types of carbides formed a network structure in the overspray powder, which had spherical or quasispherical shape: one was MC carbide that was rich in vanadium (V), and the other was M2C carbide enriched with vanadium (V) and tungsten (W). The sintered 825 K alloy contained M6C and MC carbides, of which M6C was rich in tungsten (W) and molybdenum (Mo), and both of these two carbides were uniformly distributed in the alloy matrix. The alloy had relative density of 98.43%, hardness of HRC 51.8, and superior bending strength of 2042 MPa. These mechanical properties can meet the requirements of most engineering applications.

A novel route to enhance the sinterability and its effect on microstructure, conductivity and chemical stability of BaCe0.4Zr0.4Y0.2O3-δ proton conductors

BaCeO3 based perovskites exhibit highest protonic conductivity in their class but suffer from low chemical stability. Addition of more stable BaZrO3 to BaCeO3 has been explored to obtain a good combination of conductivity and chemical stability. BaZrO3, however, is unfavourable towards densification due to its poor sinterability. In this study, a two-step sintering process and Zn addition have been explored vis-à-vis conventional sintering to obtain dense Y-doped BaCeO3-BaZrO3 solid solution (BaCe0.4Zr0.4Y0.2O3-δ) at comparatively lower temperatures. The conventional single-step sintering at 1450 °C for different sintering times (5, 10, 15 h) could not provide a dense sample. The two-step sintering process (1550 °C-5 min, 1450 °C-15 h), on the other hand, yielded a relative density of 95%. Addition of 4 mol.% Zn resulted in high sintered density (97%) at 1300 °C though it was not effective at 1200 °C due to insufficient shrinkage (densification). Zn addition also improved the chemical stability against CO2. The grain size of the sintered samples decreased with decreasing sintering temperature and or time. As a result the specific grain boundary conductivity decreased with decreasing sintering time. The overall conductivity was higher in the two-step processed sample compared to the Zn-doped sample.