Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Constrained sintering kinetics of 8 mol% Y₂O₃/92 mol% ZrO₂ (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films.Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol^?1) despite all probably being mainly controlled by grain boundary cation diffusion.     

Enhanced densification of thin tape cast Ceria-Gadolinium Oxide (CGO) layers by rheological optimization of slurries

Optimized CGO-based slurries are formulated and shaped into thin dense layers via a tape-casting process. The formulation is adjusted with respect to the rheological behaviour. The internal structure and flow properties of slurries are explored with the aim of identifying the required conditions to obtain thin dense CGO layers at reduced sintering temperatures (1200 °C). We demonstrate a correlation between the rheological properties of the slurries, the sintering behaviour and the microstructure of the resulting tapes. Remarkably, a dense CGO layer less than 20 µm thick is obtained with a non-congested slurry, having optimized ceramic loading and liquid-like behaviour.     

A novel approach to sintering (Ba,Ca)(Ti,Zr)O3 multilayer ceramic capacitors with Ni electrodes

In this study, a novel sintering technique combining rapid heating and constrained sintering was adopted to fire multilayer ceramic capacitors (MLCCs). It was demonstrated that chamber development can be significantly minimized, leading to a small internal residual stress in MLCCs when they were fired by the novel sintering technique instead of free sintering. The magnitude of tensile stress was closely related to the heating rate and the thickness of the constraining layer. The presence of in-plane tensile stress resulted from the constrained sintering in the x–y plane of the MLCCs, which then modified both the densification rate of the dielectric materials and the inner electrode. The thin inner electrode (<1 μm) with high continuity (>98%) and the fine grain size (1.5 μm) with narrow distribution (±0.10 μm) of BCTZ-based MLCC with a concave-free morphology can be attained by using such a rapid constrained sintering technique when BT is used as a constraining layer laminated on both sides of the multilayer BCTZ-based MLCC.     

Sintering process optimization for multi-layer CGO membranes by in situ techniques

The sintering of asymmetric CGO bi-layers (thin dense membrane on a porous support; Ce_0.9Gd_0.1O_1.95?δ = CGO) with Co₃O₄ as sintering additive has been optimized by combination of two in situ techniques. Optical dilatometry revealed that bi-layer shape and microstructure are dramatically changing in a narrow temperature range of less than 100 °C. Below 1030 °C, a higher densification rate in the dense membrane layer than in the porous support leads to concave shape, whereas the densification rate of the support is dominant above 1030 °C, leading to convex shape. A flat bi-layer could be prepared at 1030 °C, when shrinkage rates were similar. In situ van der Pauw measurements on tape cast layers during sintering allowed following the conductivity during sintering. A strong increase in conductivity and in activation energy E_a for conduction was observed between 900 and 1030 °C indicating an activation of the reactive sintering process and phase transformation of cobalt oxide.     

Effect of carbon contamination on the sintering of alumina ceramics

The present work aimed with the carbon contamination in alumina ceramics and its influence on sinterability of alumina in low vacuum and atmospheres of argon and nitrogen. The commercially available alumina was coated with carbon and sintered at different atmospheres to investigate the effect of carbon presence on alumina sintering behaviour. The sintering conditions were: heating/cooling rates 5 °C/min and 1.7 °C/min until the maximum temperature of 1400 °C and a dwell time of 2 h. The microstructure of the samples was investigated from fracture and surface, prior to polishing, chemical or thermal etching. The non-densified (porous) surface layer was found in the samples sintered in nitrogen and vacuum, however, sintering in argon atmosphere showed a negligible effect on the surface. The core of investigated specimens exposes a transgranular/intergranular fracture mode and is dense in all cases. In the case of vacuum sintering, the strong carbon diffusivity was also noticeable by the dark grey color of the samples. Interestingly, the formation of aluminium nitride took place during sintering of carbon coated alumina samples in a nitrogen atmosphere at 1400 °C. The thickness of the reactive porous layer was approximately 15 μm beneath the surface. Such a porous layer is inappropriate to the desired features of final ceramic products. Presented results lead to better understanding of the sintering behaviour of ceramic and to suitable selecting of the set-up by densification conditions.     

SPS densification and microstructure of ZrB2 composites derived from sol–gel ZrC coating

A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB₂ powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 °C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB₂ powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB₂ and a compositionally similar ZrB₂–ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB₂ reference sample was slow to densify until 1800 °C and was not fully dense even at 2000 °C, while the sol–gel modified ZrB₂ powder completed densification by 1800 °C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB₂ powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth.     

A novel route for manufacturing asymmetric BSCF-based perovskite structures by a combined tape and freeze casting method

In this work, we present an innovative route for the preparation of asymmetric structures by combining the tape and freeze casting methods A perovskite oxide ceramic based on BSCF (Ba_0.5S_0.5Co_0.8Fe_0.2O_3-δ) was chosen as the mother material. BSCF freeze cast substrates showed highly ordered and interconnected pores networks with open porosity from 66% to 79%. The top layer for the tape cast was altered by doping BSCF with Zr, thus forming a slightly different compound BSCFZ. The BSCFZ slurries containing 1.0 wt% dispersant and pH >6 were characterised by pseudoplastic behaviour, ideal for the preparation of tape cast layers. Coupling the BSCF freeze cast substrate with the BCSFZ tape cast green top layer to make asymmetric structures was achieved by the pre-sintering of the substrate between 900 and 1050 °C. This allowed for the densification and coupling of the BSCFZ top layer, resulting in suitable adhesion with no cracks or delamination. Higher pre-sintering temperatures than 1050 °C limited the densification of the BSCFZ top layer as the coefficient of expansion between the substrate and top layer did not match, leading to the formation of surfaces with high porosity.     

Densification behavior and properties of alumina–chrome ceramics: Effect of TiO2

Highly dense alumina–chrome bodies with low porosity are usually used as corrosion and thermal resistant refractories. Alumina–chrome refractory with molar ratio 1:1 was developed using chemical grade hydrated alumina and chromium (III) oxide by conventional sintering route. Batch materials were attrition milled, isostatically pressed and sintered in the temperature range from 1000 °C to 1700 °C with 2 h soaking at peak temperature. Phase development of the sintered materials with temperature was studied by X-ray diffraction. Sintering temperature, sintering condition and addition of sintering aid (TiO₂) have immense effect on the densification of the alumina–chrome refractory. Highly dense alumina–chrome refractory with almost nil apparent porosity was developed at 1500 °C in reducing atmosphere. Flexural strength of the sintered materials at room temperature and at 1200 °C was also measured. 1 wt% TiO₂ gives the optimum result with respect to densification and flexural strength.     

Phase transformation during the sintering of γ-alumina and the simulated Ni-laden waste sludge

This study investigated the fundamental phase transformation process when incorporating simulated nickel-laden waste sludge by alumina-based ceramic system. To match the required sintering conditions for commercial ceramic products as a development of waste-to-resource strategy, the investigation focused on the nickel incorporation behavior of γ-alumina in moderate to low temperature sintering environments (1250–750 °C) under a 3 h sintering process. X-ray diffraction (XRD) and Rietveld refinement were used to show the quantitative distribution of phases. The results demonstrated that γ-alumina interacted intensively with nickel oxide at sintering temperatures above 1000 °C. Furthermore, the densification effect of the sintering process is also illustrated, as this is crucial for developing the needed mechanical strength for practical applications. The findings in this study reveal that there is good potential for achieving the stabilization of nickel by thermal treatment with a γ-alumina containing precursor via moderate ceramic sintering temperatures.     

Densification behaviour and two stage master sintering curve in lithium sodium niobate ceramics

The Master Sintering Curve (MSC) has received much attention in recent years due to its ability to predict sintering behaviour of a given powder and green body process regardless of its thermal history. In this paper MSC, based on the combined stage sintering model is constructed for one of the most important lead-free piezoelectric viz. lithium sodium niobate, Na_1-xLi_xNbO₃ (x=0.12, LNN-12), ceramic using shrinkage data. The present study has been carried out to understand and control the densification behaviour during pressureless sintering. Two distinct stages of densification have been observed en route to the upper limit to sintering temperature. The activation energies of densification for the two temperature ranges viz. 800–1150 °C and 1150–1300°C were found to be 365 kJ/mol and 2530 kJ/mol, respectively, through the construction of MSC. The MSC should be useful in predicting the densification behaviour and the final density and for designing a reproducible fabrication schedule for the LNN-12 ceramics.     

Inkjet printing and nanoindentation of porous alumina multilayers

The objectives of this study were to analyse the effect of inkjet 3-D printing parameters, particularly the splat overlap distance, for the fabrication of defect-free porous Al₂O₃ ceramic multilayers, and to correlate the resulting porosities with the mechanical properties measured using nanoindentation. An aqua-based alumina ink was used in this study to fabricate the multilayers on dense alumina substrates by inkjet printing. The as-printed specimens were dried and sintered at 1200–1500 °C. The resulting microstructural features of each specimen and their corresponding porosities were studied using FIB-SEM. Elastic modulus and hardness were determined using the spherical nanoindentation technique. Results showed that defect-free porous alumina multilayers with excellent layer to layer and layer to substrate integrity were successfully fabricated. The porosity-dependence of the elastic modulus and hardness was shown to be consistent with values predicted using empirical expressions, despite the presence of abnormal grain growth at higher temperatures.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Ultra-high heating rate densification of nanocrystalline magnesia at high pressure and investigation on densification mechanisms

The pressure-assisted densification method based on combustion reaction heating was applied to prepare dense nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 50 nm was investigated, under the pressure range of 0–170 MPa, and the temperature range of 1620–1880 K with ultra-high heating rate (above 1600 K/min). The pressure was found to have an effect on enhancing densification while suppressing grain growth, and the higher sintering temperature lead to the larger grain size and lower density of the compact. Pure magnesia nanocrystalline ceramics with a relative density of 99.1% was obtained at 1620 K and 170 MPa, and the concurrent grain growth was almost completely restrained. Furthermore, the investigation on the pressure-dependent densification mechanisms including plastic flow, diffusion and power-law creep was also carried out. The result indicated the rate-controlling mechanism was the plastic flow accommodated by grain-boundary diffusion creep.     

Microstructural characterization of spark plasma sintered boron carbide ceramics

Fully dense boron carbide specimens were fabricated by the spark plasma sintering (SPS) technology in the absence of any sintering additives. Densification starts at 1500 °C and the highest densification rate is reached at about 1900 °C. The microstructure of the ceramic sintered at 2200 °C, with heating rates in the 50–400 °C/min range, displays abnormal grain growth, while for a 600 °C/min heating rate a homogeneous distribution of finely equiaxed grains with 4.05 ± 1.62 μm average size was obtained. TEM analysis revealed the presence of W-based amorphous and of crystalline boron-rich B₅₀N₂ secondary phases at triple-junctions. No grain-boundary films were detected by HRTEM. The formation of a transient liquid alumino-silicate phase stands apparently behind the early stage of densification.     

Effect of calcined colemanite additions on properties of hard porcelain body

Utilization of calcined colemanite as a sintering aid in hard porcelain body, prepared by slip casting route using aqueous suspension containing mixtures of kaolin, quartz and potassium feldspar, was investigated. After calcination, colemanite powders were added to the admixture of porcelain by partial substitution with potassium feldspars in the range of 0–5 wt% to discover its influences on the densification and technological properties of the final product. Through optimization of process parameters of ceramic suspensions by using appropriate dispersing agents and facilitating densification by calcined colemanite addition, hard porcelain possess high strength was obtained. When relationship between the additive concentration with temperature was established, it is found that porcelain formulations resulted in substantial reduction of firing temperature about 50 °C without compromising its quality by 1 wt% calcined colemanite addition.     

Enhanced sintering behavior of LSGM electrolyte and its performance for solid oxide fuel cells deposited by vacuum cold spray

How to obtain dense La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte at low sintering temperature (<1300 °C) is a challenge to improve solid oxide fuel cell (SOFC) performance at intermediate operation temperature. In this study, a double-layer design method for vacuum cold spray (VCS) prepared-LSGM electrolyte assisted with two-step sintering at a low temperature was proposed. The sintering behavior of VCS deposited LSGM layers at 1200 °C was investigated. The LSGM layers became denser in most regions except the appearance of some cracks. Subsequently, the effect of a second LSGM layer on the sintered top layer was studied to block cracks. Results showed that the co-sintered layer with a thickness of approximately 5 μm presented a maximum open circuit voltage of ∼0.956 V at 650 °C and a maximum power density of 592 mW/cm² at 750 °C. Result indicates that the sintering assisted VCS is a promising method to prepare the LSGM electrolyte applied in intermediate temperature SOFCs.     

Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment

Fe₂O₃ powders were introduced as sintering aid to fabricate yttria-stabilized zirconia (YSZ) hollow fiber membranes using a combined wet-spinning and post-sintering method. The obtained Fe₂O₃-YSZ hollow fiber membranes show enhanced performance for water treatment with fine crystal structure in terms of bending strength and pure water permeability. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) along with mechanical tests were employed to investigate the structural evolution in the sintering process and the effect of Fe₂O₃. It is suggested that the Fe₂O₃ dopants dissolve into YSZ at elevated temperatures, providing defect sites and vacancies for fast ion migration, favoring for densification and grain growth of the YSZ, which yields dense microstructures of fine crystallites at relatively low sintering temperature. The Fe₂O₃-YSZ hollow fiber membranes sintered at 1150 °C show a 3-fold increase of the permeate flux of pure water (F) (743 L m⁻² h⁻¹) along with comparable bending strength (152 MPa) compared to pure YSZ membranes. This modified method can reduce sintering costs and therefore fabrication costs which should pave the way for scale-up production for ceramic hollow fiber membranes.     

Self-propagating high-temperature synthesis of barium titanate and subsequent densification by spark plasma sintering (SPS)

This paper describes the self-propagating high-temperature synthesis (SHS) of perovskitic oxides, specifically BaTiO₃, and their subsequent densification by spark plasma sintering. With the final goal of obtaining dense nanostructured materials, SHS products were mechanically treated at different milling time conditions, before densification. It was found that the grain size of ball milled powders decreases with increasing milling time, this effect being more evident at early stages of milling. Depending upon the ball milling (BM) conditions adopted, crystallite size in the range 15–70 nm was obtained. After milling for 5 h, the resulting powders (20–30 nm) were sintered by SPS, at 700 A, for different periods of time. By properly varying sintering time in the interval 70–140 s, it is possible to obtain products with relative density in the range 66–99%, respectively. In particular, grain growth during sintering was found to be limited (below 50 nm) if the electric current is applied for time intervals equal to or less than 100 s. The observed dielectric properties are typical of a nanocrystalline BaTiO₃ ceramic.     

Low-temperature-sintered Bi0.5Na0.5TiO3-based lead-free ferroelectric ceramics with good piezoelectric properties

Li₂CO₃ has been used as a sintering aid for fabricating lead-free ferroelectric ceramic 0.93(Bi_0.5Na_0.5TiO₃)-0.07BaTiO₃. A small amount (0.5 wt%) of it can effectively lower the sintering temperature of the ceramic from 1200 °C to 980 °C. Unlike other low temperature-sintered ferroelectric ceramics, the ceramic retains its good dielectric and piezoelectric properties, giving a high dielectric constant (1570), low dielectric loss (4.8%) and large piezoelectric coefficient (180 pC/N). The “depolarization” temperature is also increased to 100 °C and the thermal stability of piezoelectricity is improved. Our results reveal that oxygen vacancies generated from the diffusion of the sintering aid into the lattices are crucial for realizing the low temperature sintering. Owing to the low sintering temperature and good dielectric and piezoelectric properties, the ceramics, especially of multilayered structure, should have great potential for practical applications.     

High-pressure spark plasma sintering (SPS) of transparent polycrystalline magnesium aluminate spinel (PMAS)

A high pressure SPS (spark plasma sintering) process was applied for consolidation of un-doped polycrystalline magnesium aluminate spinel. This approach allows fabricating a fully dense transparent ceramic with submicron grain size and high hardness values at a relatively low temperature (1200 °C). The light transmittance of the specimens increases with increasing applied pressure, while the hardness gradually decreases. The optimal combination of properties was achieved after sintering at 1200 °C at a heating rate of 5°/min, a holding time of 15 min and an applied pressure of 350–400 MPa. The specimens display the level of transmittance in the visible wavelengths and hardness values comparable with the best results reported in the literature for the two-stage fabrication process (pressureless sintering and hot isostatic pressing).