Effect of sintering temperature and dopant concentration on microstructure and electrical conductivity of ultra-fine-grained ZrO2–Sc2O3 ceramics

Phase transformations in ZrO₂ + xSc₂O₃ solid solutions (6.5 < x < 11 mol%) at sintering of ceramics obtained from nanopowders produced by laser evaporation of the ceramic targets have been studied. The Sc₂O₃ concentration increasing from 6.5 to 11 mol% is accompanied by the sintering temperature decreasing and the average grain size growth from 130 nm to 760 nm. At concentration of about 7 mol% Sc₂O₃ an abrupt increase of the average grain size and electric conductivity is observed. The sinterability of the ZrO₂ − хSc₂O₃ ceramics is affected by the prehistory of nanopowders preparation. The characteristics of ceramics obtained from nanopowders evaporated from the targets based on (ZrO₂ + xmol% Sc₂O₃) mixture and on the (ZrO₂ − 11mol% Sc₂O₃) solid solution significantly differ, namely, in the latter the sintering temperature is markedly lower and the shrinkage rate is higher. Besides, its average grain size is substantially lower and the conductivity is higher.     

Influence of stabilizing ions and sintering process on the thermal conductivity of α-SiAlON ceramics

α-SiAlON ceramics with different stabilizing ions of Yb, Dy, Nd, Y, Ca, and binary stabilizing ions of (Yb + Ca) and (Yb + Nd) were prepared by spark plasma sintering at 1600°C and gas pressure sintering at 1800 and 1900°C, and their thermal conductivity was investigated. It was found that α-SiAlON ceramics with larger and heavier stabilizing ions had lower thermal conductivity and the thermal conductivity could be further reduced by using binary stabilizing ions, which can be explained by phonon scattering from point defects. At the same time, the samples prepared at lower sintering temperatures showed smaller grain sizes and lower thermal conductivity. The relationship between the thermal diffusivity of samples and temperature was studied, where the dependence of inverse thermal diffusivity on temperature was better fitted by a quadratic fitting function than the usual linear one over a wide temperature range from 25 to 800°C.     

Effect of sintering time on the microstructure and stability of Al2O3–ZrO2 composite powders under microwave-assisted sintering

The function of ceramic coating is closely related to the construction technology and the quality of ceramic powders. Generally, Al₂O₃–ZrO₂ powders are rapidly sprayed on the material surface at high temperatures to obtain better performance. Improving the quality of Al₂O₃–ZrO₂ powders can make them more widely used in ceramic coating. In this paper, microwave sintering was used to enhance the sintering process of the powders, and the effect of sintering time on the microstructure, properties, and stability of Al₂O₃–ZrO₂ powders was investigated. The results proved that microwave heating could improve the crystallinity and stability of the samples. At 900 °C, the tetragonal phase content in samples with different sintering times were 63.05%, 63.25%, 62.39%, and 63.22%, respectively. The average particle sizes obtained by Gaussian fitting are 1.04 μm, 0.83 μm, 0.88 μm, 0.86 μm, respectively. The Gaussian fitting particle size data was consistent with the normal distribution. Compared with the particle size of raw material (1.10 μm), the particles were refined, and the dispersion effect was noticeable. Therefore, the best sintering time for microwave sintering Al₂O₃ stabilized zirconia was 2 h. This paper aims to provide reasonable data support for improving the preparation of high-quality Al₂O₃-PSZ ceramic powders and to guide the industrial production of Al₂O₃-PSZ powders.     

Effect of ZnO on the microstructure and electrical properties of WO3–Bi4Ti3O12 ceramics

The aim of the present work is to explore the possibility of incorporate a small amount of ZnO to improve the microstructure control of W-doped BIT-based materials. Two different processing routes have been used according to previous results reported for other materials: reaction and sintering in one single step and a previous calcination step. The sintering behaviour of the samples, the obtained crystalline phases and the microstructure analysis indicate that the reaction between ZnO and Bi₂O₃ plays a critical role during sintering. Both Bi₂Ti₂O₇ and Zn₂TiO₄ secondary phases are stabilized when adding ZnO. Actually, when WO₃ and ZnO are incorporated simultaneously to BIT materials, they interact stabilizing the Bi₂Ti₂O₇ phase and avoiding the incorporation of W⁶⁺ into the BIT lattice. As a consequence, the electrical conductivity of the samples with ZnO is two orders of magnitude higher than that of the samples doped only with WO₃, suggesting that WO₃ does not form a solid solution with BIT. The curve dielectric constant vs temperature also reveals the role played by the Bi₂Ti₂O₇ phase.     

Effect of Li2CO3–Bi2O3 doping on the low-temperature sintering,structure, and dielectric and mechanical properties of MgO–TiO2(w) ceramics

Dense MgO–12% TiO₂(w) ceramics containing 12 wt% TiO₂, which were doped with Li₂CO₃–Bi₂O₃ composite sintering aids, were prepared at a low sintering temperature of 950 °C in this study. The effects of sintering additives on the sintering characteristics, phase composition, microstructure, and dielectric and mechanical properties of the ceramic samples were systematically investigated, and the influences of their phase composition and microstructure on the dielectric and mechanical properties were examined. The introduction of sintering aids produced a new Bi₄Ti₃O₁₂ phase in the sample structure, while the residual Bi₂O₃ mixed with the newly formed Mg₂TiO₄ and Bi₄Ti₃O₁₂ phases distributed at MgO grain boundaries formed a structure surrounding MgO grains. This structure filled the pores in the ceramic sample, which increased its density and enhanced the mechanical properties. At a Li₂CO₃–Bi₂O₃ content of 15 wt%, the density, flexural strength, and Vickers hardness of the ceramic samples reached their maximum values of 3.4 g/cm³, 218.9 MPa, and 778.7 HV, respectively. However, the further increase in the Li₂CO₃–Bi₂O₃ content deteriorated their dielectric properties although the dielectric constant and dielectric loss remained below 13.4 and 2.1 × 10⁻³, respectively. The findings of this work indicate that Li₂CO₃–Bi₂O₃ sintering aids can significantly lower the sintering temperature of MgO–12% TiO₂(w) ceramics and control their dielectric and mechanical properties through microstructural changes.     

Effect of the sintering temperature on microstructure and properties of Al2O3–Cu–Ni hybrid composites obtained by PPS

In the present research, the influence of sintering temperature on the microstructure and properties of Al₂O₃–Cu–Ni hybrid composites prepared by the Pulse Plasma Sintering (PPS) technique were described. In this research, three temperatures have been selected: 1250°C, 1300°C, and 1350°C. SEM observations were carried out to determine the distribution of the metallic phase in the composite depending on the sintering temperature. The conducted experiments and microscopic observations enabled a better understanding of the phenomena occurring between the ceramic matrix and metallic phase in the obtained materials. The mechanical properties like a hardness and fracture toughness were measured. The technology applied allowed us to obtain ceramic-metal composites with a homogeneous microstructure. It was found that the sintering temperature influences the selected physical and mechanical properties of the composites produced. It was found that samples produced at 1300°C are characterized by the highest relative density and the mechanical properties.     

Effect of CaO on the optical quality and microstructure of transparent MgO·1.5Al2O3 spinel ceramics prepared by reactive sintering

Transparent MgO·1.5Al₂O₃ spinel ceramics were successfully prepared via reactive sintering of Al₂O₃ and MgO raw powders followed by hot isostatic pressing (HIP) using CaO as the sintering additive. The effects of CaO on the densification process, microstructure and optical quality of samples were investigated. It was found that the amount of CaO played an important role in the sintering process. By adding 0.05?wt% CaO, the sample with high transmittance (82.3% at 400?nm), small grain size (<5?μm) and high strength (228?±?15?MPa) was obtained after HIPing at 1550?°C. However, when the amount of CaO increased to 0.1?wt%, non-cubic and columnar-shaped grains generated at low HIP temperatures (1550–1650?°C), which severely reduced the optical quality of resulting samples. The grains were calcium aluminates, whose formation was closely related to the molar ratio of Al₂O₃/MgO, CaO amount and sintereing temperature.     

Effect of sintering conditions and heat treatment on the properties,microstructure and machining performance of α-β-SiAlON ceramics

The properties of SiAlON ceramics are strongly affected by the composition and structure of the intergranular phase, which are controlled by dopants, sintering conditions and starting silicon nitride (Si₃N₄) powder characteristics. In this study, 25α:75β SiAlON compositions were designed with different molar ratios of Y:Sm:Ca (9:0.5:0.5 and 3:6:1). The effects of cation ratios, different cooling profiles (50 °C/min and 5 °C/min) and further heat treatment under different conditions (at 1600 °C for 2, 4 and 6 h) on the final phase composition, the type of the intergranular phase (amorphous or crystalline) formation, the resulting microstructures and the machining performance were studied. It is found that slow cooling and heat treatment have a great influence on crystallisation behaviour and in turn the crystallisation enhance the machining performance of SiAlON materials in cutting tool applications.     

Effect of a Bi–Cr–O synthetic multi-phase on the microstructure and electrical properties of ZnO–Bi2O3 varistor ceramics

ZnO–Bi₂O₃–MnO₂ (ZBM)-based varistors were fabricated via doping a novel synthetic multi-phase (SMP) additive produced by calcining the mixture of 18Bi₂O₃·Cr₂O₃ at a given temperature. The effects of the SMP on the microstructural and electrical properties of ZBM varistors were investigated. It was found that the SMP dopant was a compound crystalline phases including Bi–Cr–O phases (Bi_7.38Cr_0.62O_12+x and CrBi₁₈O₃₀) and small amounts of Bi₂O₃ rather than a synthesized polycrystal. The Bi–Cr–O phases were not emerged for samples with x=1, indicating that the amount of it is tiny and the small Bi₂O₃ may accelerate ZnO grain growth. With more SMP doping (x>1) in the ZBM ceramics, it acted as a barrier inhibiting grain growth. For samples with x=5, excellent electrical properties were obtained: the nonlinear coefficient α increased up to 50.19 corresponding to the highly barrier height of 2.62 eV; the leakage current I_L reduced to 0.3 μA. The dielectric constant ε_a is proportional to the ratio of the grain size d to the thickness of the depletion layer width t, which explained the ε_a increased at f=1 kHz for the samples with x=1 and 5. The improvement of the electrical properties can be explained by the oxygen absorption mechanism.     

Low sintering of X7R ceramics based on barium titanate with SiO2–B2O3–Li2O sintering additives in reducing atmosphere

Development of a low-temperature sintered dielectric material derived from barium titanate for X7R characterized dielectric ceramics application is discussed in this paper. By addition of SiO₂–B₂O₃–Li₂O sintering additives to commercial BaTiO₃ powder, more than 95% of the theoretical density was obtained at a sintering temperature of 950 °C in H₂/N₂ atmosphere. The influence of the composition and procedures on the microstructures, lattice parameters and properties of ceramics materials were systemically studied. After explaining the reason for lower isolated resistivity (IR) in the previous experiment, several methods are tried out to improve the IR properties, which have reached the application requirement level of 10¹² Ω cm. These ceramics sintered between 900 °C and 950 °C in H₂/N₂ atmosphere are promising candidates for fabrication of Cu electrode MLCCs.     

Effect of P2O5 on evolution of microstructure of MgO–Al2O3–SiO2 glass ceramics

Abstract

MgO–Al₂O₃–SiO₂ (MAS) cordierite based glass ceramics were prepared by volume crystallisation. X-ray diffraction, Scanning electron microscopy and Energy diffraction scanning were used to investigate crystallisation behaviour and the influence of P₂O₅ on microstructure MAS based glass ceramics. The results showed that P⁵⁺ could promote the phase separation of MAS glass and that the glass was divided into two areas, such as Mg₄Al₂Ti₉O₂₅ and the containing P⁵⁺ area at <900°C. Mg₄Al₂Ti₉O₂₅ and Mg₃(PO₄)₂ in the area were both advantageous to the precipitation of μ cordierite, which further transformed to α cordierite due to P⁵⁺ in the residual glassy phase. However, P⁵⁺ inhibited the presence of cordierite when the heat treatment temperature was >900°C.     

Hot-pressed ZrB2–SiC composite ceramics: Effect of various Ta-containing additives on the microstructure and mechanical properties

The ZrB₂–SiC composites have been commercially used at ultrahigh temperatures, but it often failed due to their poor toughness. In order to solve this problem, four types of Ta-containing additives (Ta, TaC, TaB₂ and TaSi₂) were used as the “third phase” to regulate the microstructure, so as to enhance the mechanical properties of hot-pressed ZrB₂–20SiC-based ceramics (in vol. %). The incorporation of the additives generated a core–shell structure, which comprised of a ZrB₂ core and a (Zr, Ta)B₂ solid solution shell. The additives helped refine the ZrB₂ grains in addition to the metallic Ta and release the internal stress field generated by the thermal misfit. The interfacial structure was modified by the formation of the coherent core/shell interface and the semi-coherent interface of adjacent ZrB₂ grains and the semi-coherent ZrB₂/(Zr, Ta)C interface in the TaC-additive composite. The addition of TaB₂ or TaC hardened the ZrB₂–20SiC ceramics, whereas the addition of Ta or TaSi₂ reduced the hardness. The fracture toughness was enhanced by the formation of the Ta-containing phases. These phases reduced the stress intensity factor of the crack tip, which was proportional to the intrinsic residual stress. However, the crack-propagation mechanism would be changed by the incorporation of various Ta-containing additives. The decrease in the crack deflection, which was induced by the stronger interfacial bonding force and the significant consumption of SiC, resulted in relatively low toughness in the Ta- and TaC-included samples. The weaker interfacial bonding force in the TaB₂- and TaSi₂-included samples caused an increase in deflection and generated branching, which enhanced the toughness of the TaSi₂-included composites to ～4.72 MPa?m^1/2.     

Influence of Ge and GeO2 on the microstructure and varistor properties of TiO2–Ta2O5–CaCO3 ceramics

This study investigated the effect of elemental crystal Ge or/and GeO₂ doping on the microstructure and varistor properties of TiO₂–Ta₂O₅–CaCO₃ varistor ceramics, which were prepared via the traditional ball milling–molding–sintering process. X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy demonstrated that co-doping with Ge and GeO₂ changed the microstructure of TiO₂–Ta₂O₅–CaCO₃ ceramics, thereby increasing the nonlinear coefficient and decreasing the breakdown voltage. The optimum doping concentrations of Ta₂O₅, CaCO₃, Ge, and GeO₂ exhibited the highest nonlinear coefficient (α=14.6), a lower breakdown voltage (E_B=18.7 V mm⁻¹), the least leakage current (J_L=10.5 μA cm⁻²), and the highest grain boundary barrier (Φ_B=1.05 eV). In addition, Ge and GeO₂ function as sintering aids, which reduce the sintering temperature because of their low melting points.     

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.     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

The microstructure and thermal conductivity of porous β-SiAlON ceramics fabricated by pressureless sintering with Y-α-SiAlON as the sintering additive

Porous β-SiAlON ceramics are fabricated by pressureless sintering with Y-α-SiAlON ceramic powders obtained by the combustion synthesis method as the main additives. The microstructure and composition of porous β-SiAlON ceramics are tailored by changing the addition of the synthesized Y-α-SiAlON ceramic powders. The formation mechanism of porous β-SiAlON ceramics illustrates that their bimodal grain size distributions are determined by the β-SiAlON seed crystals which are derived from the starting synthesized powders and the first precipitates in the liquid phase, respectively. A small addition of Y-α-SiAlON is conducive to the reduction of the thermal conductivity of porous β-SiAlON ceramics. Furthermore, the higher the addition of Y-α-SiAlON ceramics powders, the lower the open porosity, columnar grain size, and dielectric loss, but the larger the density, flexural strength, dielectric constant, and thermal conductivity.     

Effect of MnO2 addition on microstructure and electrical properties of ZnO–V2O5-based varistor ceramics

The microstructure and electrical properties of ternary system ZnO–0.5 mol% V₂O₅–MnO₂ ceramics sintered were investigated in accordance with MnO₂ content by sintering at 900 °C. For all samples, the microstructure of the ternary system ZnO–V₂O₅–MnO₂ ceramics consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂. The incorporation of MnO₂ to the binary system ZnO–V₂O₅ ceramics was found to restrict the abnormal grain growth of ZnO. The breakdown field in the E–J characteristics increased from 175 to 992 V/cm with the increase of MnO₂ content. The incorporation of MnO₂ improved non-ohmic properties by increasing non-ohmic coefficient. The highest non-ohmic coefficient (27.2) in the ternary system ZnO–0.5 mol% V₂O₅–MnO₂ was obtained for MnO₂ content of 2.0 mol%.     

Effect of SrTiO3 concentration and sintering temperature on microstructure and dielectric constant of Ba1−xSrxTiO3

The Ba_1−xSr_xTiO₃ materials have received increased attention as one of the most important materials for electroceramic components, such as high dielectric ceramic capacitors, tunable phase shifters and PTCR. In this paper, the effect of SrTiO₃ concentration and sintering temperature on the microstructure and dielectric constant of Ba_1−xSr_xTiO₃ materials at the Curie temperature have been investigated. When Ba_1−xSr_xTiO₃ materials were sintered at 1350 °C, the peak value of the dielectric constant, ϵ_max, monotonically decreased with increasing SrTiO₃ concentration. At the sintering temperature of 1400 °C the dielectric constant maximum at the T_C increased with an increase in the x value, reaching the highest value at around x=0.4 and then decreased. As sintering temperature increased to 1450 °C, ϵ_max increased with increasing SrTiO₃ concentration up to x=0.6. The dielectric properties of Ba_1−xSr_xTiO₃ materials were discussed in terms of SrTiO₃ concentration and microstructure.     

Gelcasting and pressureless sintering of silicon carbide ceramics using Al2O3–Y2O3 as the sintering additives

In this paper, silicon carbide ceramics were prepared by aqueous gelcasting and pressureless sintering using Al₂O₃ and Y₂O₃ as the sintering additives. In order to develop well dispersed SiC slurries in the presence of sintering additives, the Al₂O₃ and Y₂O₃ powder was treated in the citric acid solution in advance. Zeta potential measurement showed that the isoelectric point (IEP) of Al₂O₃ and Y₂O₃ powder moved toward low pH region after treatment. Rheological measurement confirmed that the addition of as-treated powder showed very limited influence on the slurry properties as compared to that of untreated powder. SiC slurries with solid content of 54 vol% and enough fluidity can be developed. After gelcasting and pressureless sintering, SiC ceramics with nearly full density, fine grained and homogeneous microstructure can be obtained. Results showed that the surface treatment of Al₂O₃ and Y₂O₃ with citric acid is effective for the gelcasting process of SiC.     

Effect of SnO–P2O5–MgO glass addition on the ionic conductivity of Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte

NASICON-type structured compounds Li_1+xM_xTi_2-x(PO₄)₃ (M = Al, Fe, Y, etc.) have captured much attention due to their air stability, wide electrochemical window and high lithium ion conductivity. Especially, Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) is a potential solid electrolyte due to its high ionic conductivity. However, its actual density usually has a certain gap with the theoretical density, leading the poor ionic conductivity of LATP. Herein, LATP solid electrolyte with series of SnO–P₂O₅–MgO (SPM, 0.4 wt%, 0.7 wt%, 1.0 wt%, 1.3 wt%) glass addition was successfully synthesized to improve the density and ionic conductivity. The SPM addition change Al/Ti–O bond and P–O bond distances, leading to gradual shrinkage of octahedral AlO₆ and tetrahedral PO₄. The bulk conductivity of the samples increases gradually with SPM glass addition from 0.4 wt% to 1.3 wt%. Both SPM and the second-phase LiTiPO₅, caused by glass addition, are conducive to the improvement of compactness. The relative density of LATP samples increases first from 0 wt% to 0.7 wt%, and then decreases from 0.7 wt% to 1.3 wt% with SPM glass addition. The grain boundary conductivity also changes accordingly. Especially, the highest ionic conductivity of 2.45 × 10^?4 S cm^?1, and a relative density of 96.72% with a low activation energy of 0.34 eV is obtained in LATP with 0.7 wt% SPM. Increasing the density of LATP solid electrolyte is crucial to improve the ionic conductivity of electrolytes and SPM glass addition can promote the development of dense oxide ceramic electrolytes.