Fabrication of high-density NiFe2O4 ceramics by slip casting and pressureless sintering

High-density NiFe₂O₄ ceramics with homogeneous microstructure were produced by slip casting and pressureless sintering. The slurry stability, sintering behavior, and microstructure of NiFe₂O₄ ceramics were investigated. A stable slurry can be obtained by adding 12.5 wt% NiFe₂O₄ nanoparticle and 5 wt% nano-binder at a slurry pH around 11.0. The linear shrinkage and linear shrinkage rate for both NiFe₂O₄ ceramic green bodies shaped by cold press molding and slip casting showed nearly the same trends. The temperature associated with the maximum linear shrinkage rate of slip casted green body was 1263.5°C, which was lower than that of cold press molded sample (1272.0°C). The sintering activation energy of slip casted green body was also lower than that of cold press molded sample (279.18 vs 288.47 kJ mol⁻¹), owing to high density and homogeneity of slip-casted green compact. A high-density NiFe₂O₄ ceramics with uniform grain size distribution can be produced by slip casting and pressureless sintering at 1350°C for 6 hours, attributed to the ability of slip casting to minimize agglomerates and micropores. It demonstrated that slip casting was more suitable to prepare high-density NiFe₂O₄ ceramics with good homogeneity.     

Synthesis and characterization of dense and fine nickel ferrite ceramics through two-step sintering

Two-step sintering (TSS) has been employed in the current study to suppress the accelerated grain growth of NiFe₂O₄ nanopowder compacts in the final sintering stage. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range from 1200 °C to 1300 °C is effective for the first-step sintering (T₁) due to its highest densification rate. The second-step sintering temperature (T₂) should be within the kinetic window, where grain boundary diffusion is maintained but grain boundary migration is suppressed. The grain sizes of high density (≥98% theoretical density) NiFe₂O₄ compacts produced by TSS are smaller than 700 nm, while that of those formed by CS are over 2.5 μm. The evidence indicates that the saturation magnetization of nearly full NiFe₂O₄ ceramics is independent of grain size and likewise high, with the corresponding values of approximately 54 emu/g. The Vickers hardness and fracture toughness both increase with the decrease of grain size and porosity.     

Effect of TiN content on properties of NiFe2O4-based ceramic inert anode for aluminum electrolysis

NiFe₂O₄-based ceramic inert anodes for aluminum electrolysis doped with various TiN nanoparticles were prepared by a two-step cold-pressing sintering process to investigate how TiN affected the sintering behavior and properties of the composites. The differential scanning calorimetry-thermogravimetry (DSC-TG), X-ray diffraction (XRD), and microstructure analysis results indicated that the Ti and N were evenly distributed in the NiFe₂O₄ matrix to form a solid solution. The maximum linear shrinkage and linear shrinkage rate were enhanced with the increase of TiN nanoparticles contents, and the sintering activation energy of initial stage was lowered from 382.63 to 279.58 kJ mol⁻¹ with the TiN nanoparticles additive range from 0 to 9 wt%. When the content of TiN nanoparticles was 7 wt%, the relative density, bending strength, and elastic modulus reached their maximum values of 97.24%, 73.88 MPa, and 3.77 GPa, respectively, whereas the minimum static corrosion rate of NiFe₂O₄-based ceramic of 0.00114 g cm⁻² h⁻¹ was obtained, mainly attributed to the relatively dense and stable microstructure. The electrical conductivity of NiFe₂O₄-based ceramics presented a clear ascending trend with increasing TiN nanoparticles content and elevated temperature, attributed to the increased concentration and migration rate of carrier.     

Laser sintering of Al2O3-based green ceramics prepared by different pre-sintering temperatures

Al₂O₃-based green ceramics are prepared by isostatic cold pressing technology. The prepared green ceramics are pre-sintered at the temperature from room temperature to 1100°C, and then Al₂O₃ ceramics are prepared by laser sintering. The effects of pre-sintering temperatures and laser parameters on mechanical properties and the sintering quality are analyzed. The results show that good crystallinity of Al₂O₃ particles is obtained at a higher pre-sintering temperature. The flexural strength and density of green ceramics increase with the temperature of heat treatment. The flexural strength decreases slightly at ∼200°C due to the paraffin binder disintegration. The pre-sintering temperature and laser processing parameters have a significant influence on the sintering quality. With the increase of laser power and laser frequency, dynamic grain growth occurs, and then grains are refined. The majority of plate-like grains are transformed into long cylindrical-like grains in the severe densification process. However, porous flocculation microstructures are generated on the samples pre-sintered at 1100°C after laser sintering, which is due to the material gasification in atmospheric environment during sintering by infrared laser. More uniform microstructure and better sintering quality of samples pre-sintered at 500°C can be achieved after laser sintering with a relatively narrower grain size distribution.     

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

In this study, calcium lanthanum sulfide (CaLa₂S₄, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.     

Transient liquid-phase sintering of AlN ceramics with CaF2 additive

Transient liquid-phase (TLP) sintering of CaF₂ additive on the densification behaviors and microstructural development of AlN ceramics are investigated. It is found that 1 wt% CaF₂ can effectively promote densification process. Increasing content of CaF₂ results in finer grain size and slower densification during intermediate sintering stage. XRD results show that grain-boundary phase of CaAl₄O₇ is formed at 1150 °C from reactions of AlN–CaF₂–Al₂O₃. With further temperature increasing, the grain-boundary phases of CA2 and CaAl₁₂O₁₈, which were formed from the reaction between CaF₂ and oxide layers, experienced transformations firstly into CaAl₄O₇ above 1600 °C and into CaAl₂O₄ at higher temperature. SEM and TEM results show that formed grain-boundary phases can evaporate from sintering bodies during further soaking, leaving clean grain boundaries. The efficiency of TLP sintering mechanism is further manifested by the preparation of transparent AlN ceramics with good combination properties.     

Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

Preparation and Performance Study of Mullite/Al2O3 Composite Ceramics for Solar Thermal Transmission Pipeline

For lowering sintering temperature of mullite/Al₂O₃ composite ceramics for solar thermal transmission pipeline, kaolin, potassium feldspar, quartz, and γ‐Al₂O₃ were used as raw materials to in situ synthesize the composite ceramics with pressureless sintering method. Densification, mechanical properties, thermal expansion coefficient, thermal shock resistance, phase composition, and microstructure were investigated. The experiment results demonstrated that the introduction of potassium feldspar and quartz decreased the lowest sintering temperatures greatly to 1300°C. The optimum sample A3 sintered at 1340°C obtained the best performances. The water absorption, apparent porosity, bulk density, bending strength, and thermal expansion coefficient of A3 were 0.04%, 0.12%, 2.71 g/cm³, 94.82 MPa, and 5.83 × 10^?6/°C, respectively. After 30 thermal shock cycles (wind cooling from 1100°C to room temperature), no cracks were observed on the surfaces of the sample, and the bending strength increased by ?7.96%. XRD analysis indicated that the main phases of samples before and after 30 thermal shock cycles were consistently mullite, corundum, and α‐cristobalite, while the content of mullite increased after thermal shock. SEM micrographs illustrated that the mullite grains growth and micro‐cracks appeared after thermal shock endowed the composite ceramics with excellent thermal shock resistance.     

Application of the cold sintering process to the electrolyte material BaCe0.8Zr0.1Y0.1O3-δ

This paper describes and discusses the application of the original sintering process named cold sintering to the electrolyte material BaCe_0.8Zr_0.1Y_0.1O_3-δ to enhance its densification at a temperature below that needed in a conventional sintering. This new technique enables the acceleration of the densification resulting in a more compacted microstructure with an unexpected high relative density of 83 % at only 180 °C. A subsequent annealing at 1200 °C further enhances the densification which reaches 94 %. The electrochemical performance of CSP sintered ceramics was investigated and optimized by varying different process parameters. The comparison with the conventional sintered material reveals an increase of the total conductivity by mostly increasing the grain boundary one. This result emphasizes the benefits of CSP to not only reduce the sintering temperature but also to enhance the electrochemical properties.     

Investigation of grain boundary diffusion and grain growth of lithium zinc ferrites with low activation energy

Activation energy and diffusion kinetics are important factors for grain growth and densification. Here, Bi₂O₃ was introduced into Li_0.43Zn_0.27Ti_0.13Fe_2.17O₄ ferrite ceramics via a presintered process to lower the reaction activation energy and to achieve low temperature sintering. Interestingly, Bi³⁺ ions entered the lattice and substituted for Fe³⁺ in the B‐site (i.e., a pure LiZn spinel ferrite). Also, SEM image results show that Bi₂O₃‐substituted LiZn ferrite ceramics have low critical temperature for grain growth (920°C), which is very advantageous for LTCC technology. This indicates that Bi₂O₃ is an excellent dopant for ceramics. Furthermore, to promote normal grain growth of the ceramics at low temperatures, different volumes of V₂O₅ additive were added at the final sintering stage. Results indicate that an optimal volume of V₂O₅ additive promotes grain growth (with no abnormal grains) and enhances magnetic performances of the ceramics at low sintering temperature. Finally, adding the optimal volume of V₂O₅ additive resulted in a homogeneous and compact LiZnTiBi ferrite ceramic with larger grains (average size of ~8 μm), high 4πM_s (~4100 gauss), and low ΔH (~190 Oe) obtained (at 900°C). Moreover, the doping method reported in this study also provides a reference for other low temperature sintered ceramics.     

Fabrication of transparent Y2O3 ceramics by two-step spark plasma sintering

Transparent Y₂O₃ ceramics were successfully fabricated by spark plasma sintering applying a two-step pressure and heating profile. Through the shrinkage curve of the single-step SPS profile, it was confirmed that shrinkage occurred at 800°C–1250°C, and it was selected as the two-step pressure profile. After the first-step SPS stage at 1250°C, the second-step SPS stage, which had the highest real in-line transmittance, was completed at 1500°C. The two-step SPS profile improved the shrinkage behavior and was able to achieve sufficient densification without excessive coarsening. As a result, the normalized real in-line transmittance to 1 mm was 80.6% at 1100 nm, which is close to the theoretical transmittance of 81.6%. The two-step pressure and heating profile in the SPS process was a significant advantage in manufacturing ceramics that were transparent and had sufficient densification.     

A lithium aluminium borate composite microwave dielectric ceramic with low permittivity,near-zero shrinkage,and low sintering temperature

A low temperature co-fired dielectric material with low shrinkage during the sintering process can enhance the circuit design of electronic devices. Lithium aluminium borate composite ceramic with a composition of Li₂O:Al₂O₃:B₂O₃ = 1:1:2 (abbreviated: LAB) was prepared by a traditional solid-state reaction method. These ceramics have a low sintering temperature (675–750 °C), low permittivity, and near-zero shrinkage. When the sintering temperature was 725 °C, the LAB ceramics exhibited a small shrinkage of ?2.4% and the best microwave dielectric properties with ε_r = 3.9, Q × f = 35 500 GHz, and τ_?= ?64 ppm/°C. The LAB ceramics sintered at 700 °C have near-zero shrinkage of ? 0.4% and good microwave dielectric properties. The ceramics transformed from (Li₂B₄O₇ and Al₂O₃) to (Li₂Al₂B₄O₁₀ and Li₄Al₄B₆O₁₇) phases with increasing the sintering temperature, which may be the reason why they show marginal shrinkage. In addition, the ceramics could be co-fired with Ag, indicating that this material is a good candidate for low-temperature co-fired ceramic devices.     

Two-step pulsed electric current sintering of transparent Al2O3 ceramics

Abstract

Fully densified Al₂O₃ ceramics with fine grain size were obtained by pulsed electric current sintering through a two-step heating profile (referred to as TS-PECS). Highly transparent Al₂O₃ polycrystals with fine grain size (400 nm) were successfully fabricated by the TS-PECS process, namely, sintering at 1000°C for 1 h and followed at 1200°C for 20 min under uniaxial pressure of 100 MPa. Effects of the first step temperature and heating rate were discussed for bulk density, grain size and transparency. The temperature in the first step strongly affects densification and grain growth of Al₂O₃. On the other hand, heating rate, even of 100 K min^?1, in TS-PECS does not give significant influences on densification and grain growth of Al₂O₃. Inline transmittance at 640 nm in wavelength normalised to 1 mm in thickness is increased by decreasing heating rate even in TS-PECS.     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Effect of TiO2 Doping on the Sintering Process,Mechanical and Magnetic Properties of NiFe2O4 Ferrite Ceramics

TiO₂‐doped NiFe₂O₄ samples were prepared via ball‐milling and two‐step sintering processes. Besides NiFe₂O₄ phase, two new phases, NiTiO₃ and Fe₂TiO₅, formed in TiO₂‐doped samples. The temperature of sintering onset for 1.0 wt% TiO₂‐doped samples is 230°C lower than that of undoped samples. Early‐stage synthesis process of TiO₂‐doped NiFe₂O₄ ceramics is controlled by grain boundary diffusion mechanism. Increasing TiO₂ content from 0 to 1.0 wt%, the apparent activation energy decreased from 813.919 KJ/mol to 639.361 KJ/mol. The values of relative density and bending strength reached their maximum value with 1.0 wt% TiO₂. Saturation magnetization, residual magnetization ratio and coercivity decrease with increasing TiO₂ content.     

Sintering behavior of Ni–Zn ferrite powders prepared by molten-salt synthesis

Ni_0.5Zn_0.5Fe₂O₄ powders were prepared by a novel molten-salt synthesis method. The effects of calcination processes of the powders on their sintering behaviors were investigated. Compared with the synthesis by traditional solid-state reaction, the proposed molten-salt method can significantly reduce the synthesis temperature of Ni_0.5Zn_0.5Fe₂O₄ from 800 to 550°C below, and the prepared powders have relatively high sintering activity at low temperature, which can thus decrease the sintering temperature. However, the abnormal growth of grains is easy to occur during sintering, thus resulting in uneven grain size. In particular, during the molten-salt synthesis, the holding time for calcination is a dominant factor affecting the activity and crystallization degree of the resultant powders. From the point of view of increasing the density of sintered bodies, the optimal conditions for synthesizing Ni_0.5Zn_0.5Fe₂O₄ powder by the proposed molten-salt synthesis is 400°C for 6 h. In addition, the saturate magnetization of the finally obtained ferrite ceramics has nothing to do with the preparation processes, while their coercivity depends on their densification and grain size caused by their different processing routes.     

Cold sintering ZnO based varistor ceramics with controlled grain growth to realize superior breakdown electric field

Controlling the grain growth and grain boundary morphology is of great importance in the manipulation of electrical properties of electro-ceramics. However, it has been a challenge to achieve dense varistor ceramics with grain sizes in submicrons and nanometers using conventional thermal sintering at high temperatures. Here we present a strategy to fabricate dense ZnO based ceramics with controlled grain growth and thin grain boundaries using cold sintering process (CSP). With CSP, the sintering temperature of ZnO based ceramics dramatically drops from 1100 °C to 300 °C. The Bi₂O₃, Mn₂O₃, and CoO dopants suppress the grain growth of ZnO under CSP conditions, and Bi-rich intergranular films (2?5 nm) can be observed along grain boundaries. The cold sintered ZnO-Bi₂O₃-Mn₂O₃-CoO ceramic shows a non-linear coefficient of 33.5, and a superior breakdown electric field of 3550 V/mm. This work thus demonstrates that CSP is a promising technique for designing new submicron-/nano-ceramics with superior performances.     

Fabrication of Er:Y2O3 transparent ceramics for 2.7 μm mid-infrared solid-state lasers

Laser grade 7 at.% Er:Y₂O₃ transparent ceramics with submicron grain size were fabricated by using one-step vacuum sintering followed by hot isostatic pressing (HIPing) technique. Through studying the sintering trajectory of Er:Y₂O₃ ceramics, the sintering temperature zone where sufficient relative density (>96%), no pore-boundary separation, and sub-micron grain size (<1 μm) ceramic samples could be identified. The samples pre-sintered in this zone were readily densified by HIPing. To maximum the densification and achieve high transparency, it is critical to suppress the final-stage grain growth. After HIPing at 1520 °C, the Er:Y₂O₃ ceramics were fully densified without further grain growth, and exhibited in-line transmission of about 81.6% at 2000 nm. Continuous wave (CW) room temperature laser operation of the Er:Y₂O₃ transparent ceramic at 2.7 μm was demonstrated.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Effect of Sm2O3 on microstructure,thermal shock resistance and thermal conductivity of cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline

Cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline were fabricated via pressureless sintering at a low sintering temperature with added Sm₂O₃. The effects of Sm₂O₃ on sintering behaviors, mechanical property, phase transformation, microstructure, thermal shock resistance and thermal conductivity of the composite ceramics were investigated. TEM analysis results demonstrated that Sm³⁺ located in glass and grain boundaries to facilitate the densification via the liquid-phase sintering mechanism and improve bending strength by grain refinement, respectively. Proper addition (3 wt%) of Sm₂O₃ could promote the crystallization of cordierite, and improve thermal shock resistance of the composite ceramics with an increasing rate of 16.70% for bending strength after 30 thermal shock cycles (air cooling from 1100 °C to RT). The composite ceramics possessed a superior thermal shock resistance, where a large amount of particles were formed to suppress crack initiation and propagation during thermal shock. Cordierite-mullite-corundum composite ceramics with proper Sm₂O₃ addition (3 wt%) had a lower thermal conductivity than that of composite ceramics without Sm₂O₃ addition by strengthening the scattering of phonon, which could reduce the heat loss during solar heat transmission process.