Sintering process as relevant parameter for Bi2O3 vaporization from ZnO-Bi2O3-based varistor ceramics

The relationship between sintering process and vaporization of the Bi₂O₃ in ZnO-Bi₂O₃-based varistor ceramics was investigated. The phase and microstructure evolution during the sintering process were examined. The results show that the higher the sintering temperature or the longer the sintering time is, the more the Bi₂O₃ is volatilized. The heating rate has little effects on the Bi₂O₃ volatilized in ZnO-Bi₂O₃-based varistor ceramics. This is in accordance with the relative X-ray diffraction peak area ratio analysis. The results also show that the sintering temperature has the greatest impact on the vaporization of the Bi₂O₃, then followed by sintering time, and the effect of the heating rate is the minimum.     

Microstructure and electrical properties of Y(NO3)3·6H2O-doped ZnO-Bi2O3-based varistor ceramics

Y(NO₃)₃·6H₂O-doped ZnO-Bi₂O₃-based varistor ceramics were prepared using a solid reaction route. The microstructure, electrical properties, degradation coefficient (D_V), and dielectric characteristics of varistor ceramics were studied in this paper. With increasing amounts of Y(NO₃)₃·6H₂O in the starting composition, Y-containing Bi-rich, Y₂O₃, and Sb₂O₄ phases were formed, and the average grain size decreased. Results also showed that with the addition of 0.16 mol% Y(NO₃)₃·6H₂O, Y(NO₃)₃·6H₂O -doped ZnO-based varistor ceramics exhibit comparatively better comprehensive electrical properties, such as a threshold voltage of 425 V/mm, a nonlinear coefficient of 73.9, a leakage current of 1.78 μA, and a degradation coefficient of 1.7. The dielectric characteristics and lightning surge test also received the same additional content of Y(NO₃)₃·6H₂O. The results confirmed that doping with rare earth nitrates instead of rare earth oxides is very promising route in preparing high-performance ZnO-Bi₂O₃-based varistor ceramics.     

Influence of B2O3 additives on microstructure and electrical properties of ZnO-Bi2O3-Sb2O3-based varistors

B₂O₃-doped ZnO-Bi₂O₃-Sb₂O₃-based varistors were fabricated by conventional ceramic technique. The microstructure and electrical properties were investigated by SEM, XRD and electrical measurements. With the addition of B₂O₃, the liquid-assisted sintering based on Bi₂O₃ was improved, and the Bi₂O₃-B₂O₃ glass and Zn₃(BO₃)₂ phase were formed on the grain boundaries. The doping of B₂O₃ markedly improved the varistor performance of the ZnO-Bi₂O₃-Sb₂O₃-based varistors. The nonlinear coefficient of the sample with 3.5 mol% B₂O₃ sintered at 1100 °C reached 56 and the leakage current was only 0.3 μA.     

Influence of glass additives on the sintering behavior and dielectric properties of BaO·(Nd0.8Bi0.2)2O3·4TiO2 ceramics

Effects of various glasses (Bi₂O₃–B₂O₃–ZnO–SiO₂ (BBXSZ, where X = the mole fraction of Bi₂O₃ in the glass) glasses with different Bi₂O₃ content) on both the sintering behavior and microwave dielectric properties of BaO·(Nd_1?xBi_x)₂O₃·4TiO₂ (BNBT) ceramics were investigated in developing low-temperature-fired dielectric ceramics for microwave devices. Glass wetting characteristics on the BNBT surface, X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a dilatometer were used to examine the effect of various glasses on BNBT densification and the chemical reaction between the glass and BNBT. The results indicate that BB25SZ glass can be used as a sintering aid to reduce the densification temperature of BNBT from 1300 to 900 °C without secondary phase formation. BNBT ceramics with 20 wt.% BB25SZ glass sintered at 900 °C show a relative density of 92%, a high-dielectric-constant of 84, a quality factor (Qxf) of 2999, and a temperature coefficient of resonant frequency of 23.7 ppm/°C.     

Influence of Yb2O3 addition on microstructure and corrosion resistance of 10Cu/（10NiO-NiFe2O4） cermets

10Cu/(10NiO-NiFe₂O₄) cermets doped with Yb₂O₃ were prepared by conventional powder metallurgy technique. The effects of Yb₂O₃ content and sintering temperature on the relative density, phase composition, microstructure of the sintered cermets and the corrosion resistance to Na₃AlF₆-Al₂O₃ melts were investigated by sintered density test, XRD analysis and SEM. YbFeO₃ phase, which distributes in the ceramics grain boundary as particles or film, is produced by the reaction between Yb₂O₃ and ceramics. The addition of Yb₂O₃ accelerates the sintering process of ceramics matrix, eliminates pores in the boundary and results in coarsened crystalline grain. The relative density of the cermets with about 1% (mass fraction) Yb₂O₃ sintered at 1275 °C increases to above 95%. Addition of about 1.0% Yb₂O₃ can inhibit obviously the corrosion of NiFe₂O₄ grain boundary and Cu phase in Na₃AlF₆-Al₂O₃ melts.     

Oxidation behavior of LPS-SiC ceramics sintered with AlN/Y2O3 as additive

In this work, the oxidation behavior of SiC ceramics sintered with additives based on AlN–Y₂O₃ system was investigated. SiC ceramics doped with different AlN:Y₂O₃ contents of 8.4:11.6 wt.% or 2.2:17.8 wt.% were sintered at 2080 °C for 1 h under nitrogen atmosphere, obtaining ceramics with relative density near to 96% in both compositions. Samples were oxidized at 1200 °C, 1300 °C or 1400 °C in air for up to 120 h. Oxidation was monitored by the weight gain of the samples as function of exposition time and temperature. A parabolic growth of the oxidation layer has been observed and the coefficient of the growth rate has been determined by relating the weight gain and the surface area. In oxidation testing performed at 1200 °C, samples containing lower Y₂O₃ amounts showed greater oxidation resistance; however, by raising the temperature (to 1400 °C), the samples containing higher Y₂O₃ amounts showed greater oxidation resistance. The oxidized layer characterized by X-ray diffraction presented SiO₂ and Y₂Si₂O₇ as crystalline phases. Furthermore, the activation energy for oxidation of 780 kJ/mol and 405 kJ/mol for AlN:Y₂O₃ contents of 8.4:11.6 wt.% or 2.2:17.8 wt.%, respectively.     

Intergranular glassy phase free SiC ceramics retains strength at 1500 °C

We report SiC ceramics sintered with AlN and Sc₂O₃ (or Lu₂O₃) that retain room temperature strength to 1500 °C. The intergranular glassy phase has not been observed in the ceramics due to a designed-sintering additive and, thus, the unsatisfactory effect of glassy phases on the high temperature strength is eliminated.     

Microwave Dielectric Properties of Nd(Zn1/2Ti1/2)O3 Ceramics with V2O5 Additives

The microwave dielectric properties and the microstructures of V₂O₅-doped Nd(Zn_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. Doping with V₂O₅ (up to 6 wt.%) can effectively promote the densification of Nd(Zn_1/2Ti_1/2)O₃ ceramics with low sintering temperature. At 1300 °C, Nd(Zn_1/2Ti_1/2)O₃ ceramic when added with 6 wt.% V₂O₅ possesses a dielectric constant of 29.9, a quality factor (Q × f) value of 105000 GHz (at 8 GHz), and a temperature coefficient of resonant frequency of ?45 ppm/°C. These V₂O₅-doped Nd(Zn_1/2Ti_1/2)O₃ ceramics can be used in microwave devices.     

Corrosion Resistance of APS- and HVOF-Sprayed Coatings in the Al2O3-TiO2 System

In this article, the results of corrosion investigations performed on thermally sprayed ceramic coatings with different compositions in the Al₂O₃-TiO₂ system (Al₂O₃, Al₂O₃-3%TiO₂, Al₂O₃-40%TiO₂, and TiO _x ) are presented. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high-velocity oxy-fuel (HVOF) spraying processes and characterized by means of optical microscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD). The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄, at room temperature, 60 °C, and 85 °C, as well as in hydrothermal conditions with deionized water at 100 °C and 200 °C. The corrosion stability of the coatings depended on coating characteristics (spraying method, microstructure, and crystalline phase composition) and the corrosive environment (media, test temperature, and duration). In contrast to expectations, APS-sprayed coatings were found to be more corrosion-resistant than the HVOF-sprayed coatings. Addition of TiO₂ to Al₂O₃ increased the corrosion stability, especially for the HVOF-sprayed coatings. In this work, TiO _x coatings were found to be more corrosion-resistant than the Al₂O₃-based coatings.     

Influence of La precursors on structure and properties of CeO2-ZrO2-Al2O3 composite oxides

Three La-doped CeO₂-ZrO₂-Al₂O₃ (CZA) composite oxide samples, namely, CZA-I, CZA-II and CZA-III, were prepared following a co-precipitation method in the presence of La₂O₃, La(NO₃)₃·6H₂O and H[La(EDTA)]·16H₂O precursors, respectively. When the precursor samples are sintered at 1000 °C, the as-prepared composite oxides mainly exhibit the CeO₂-ZrO₂ cubic fluorite phase, while the γ-Al₂O₃ and δ-Al₂O₃ phases appear when the precursor samples are subjected to sintering at 1100 and 1200 °C. CZA-III exhibits improved redox properties after high-temperature treatment compared with CZA-I and CZA-II. CZA-III presents the largest surface area of 97.46 m²/g among the three CZAs when the CZA-III precursor sample is sintered at 1000 °C. Furthermore, the corresponding oxygen storage capacity (OSC) is the largest with value of 400.27 μmol/g when CZA-III precursor sample is sintered at 1000 °C. Additionally, CZA-III exhibits the best thermal stability and the highest reduction temperature. However, by increasing the sintering temperature to 1200 °C, there is a dramatic decline in the properties of surface area and OSC. And a decrease for CZA-III in surface area by 58.94% and a decrease of the OSC value by 74.56% are observed.     

Microstructure and electrical properties of sol–gel derived Ni-doped CaCu3Ti4O12 ceramics

Dielectric properties and varistor performance of sol–gel prepared Ni-doped calcium copper titanate ceramics (CaCu₃Ni_xTi₄O_12+x, x=0, 0.1, 0.2, 0.3) were investigated. SEM and XRD were used in the microstructural studies of the specimens and the electrical properties were investigated for varistors. XRD patterns show that the CCTO ceramics were single phase with no Cu-rich phase. SEM results indicated that the samples had smaller grain sizes than those synthesized by traditional solid-state reaction methods. The experimental results show that the highest dielectric constant and lower dielectric loss occur when x=0.2. When x=0.3, the lowest leakage current is obtained and the maximum value reaches 0.295; meanwhile, the lowest threshold voltage and nonlinear coefficient are found, the minimum values of them are 1326 V/mm and 3.1, respectively.     

The influence of powder preparation condition on densification and microstructural properties of WC-Co- Al2O3 cermets

This study investigated how powder preparation during WC-10Co production with the addition of 10 wt% Al₂O₃ influenced its microstructural and mechanical properties. Powders were mixed with a mechanical shaker for 10 min and high energy milling for 2, 6, 10, 20, 30, and 50 h. The powders were then compacted at 200 MPa and sintered in a resistive dilatometric furnace for one hour, under an argon atmosphere, at a heating rate of 10 °C / min, and two sintering temperatures (1400 °C and 1550 °C). XRD and SEM/EDS analyses were carried out for both powders, which were sintered in order to examine their composition and morphology. The sintered powders were also characterized in terms of mechanical properties, densification, and dilatometric shrinkage. The results show that samples milled for 50 h and sintered at 1550 °C exhibited microstructures with denser phases than those of samples mixed in the shaker. The properties measured were around 68%, 45%, −0.30, and 280 HV for relative density, densification, dilatometric shrinkage, and hardness, respectively.     

Preparation and magnetic property of multi-walled carbon nanotube/a-Fe2O3 composites

In order to attain new functional nanomaterials with good magnetic property, multi-walled carbon nanotubes/hematite (MWNTs/α-Fe₂O₃) composites were synthesized using the co-deposition method. MWNTs/α-Fe₂O₃ composites were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy(TEM), Raman spectroscopy and vibrating sample magnetometry (VSM). The experimental results show that the structure and magnetic properties of the MWNTs/α-Fe₂O₃ composites are related to the heat treatment temperature. MWNTs are modified by α-Fe₂O₃ nano-particles and α-Fe₂O₃ nanorods with a diameter of 10?50 nm after being treated at 450 °C. When the heat treatment temperature exceeds 600 °C, MWNTs are only modified by Fe₃O₄ particles. Furthermore, the MWNTs composites treated at 450 °C and 600 °C have good magnetic behaviour.     

New dielectric material system of Nd(Mg1/2Ti1/2)O3–CaTiO3 with V2O5 addition for microwave applications

The effects of sintering aids additives on the microstructures and microwave dielectric properties of (1 ? x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ ceramics were investigated. The effects of V₂O₅ additions on the microwave dielectric properties and the microstructures of (1 ? x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ ceramics have been investigated. Doping with 0.5 wt% V₂O₅ can effectively promote the densification and the microwave dielectric properties of (1 ? x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃. It is found that CaTiO₃–Nd(Mg_1/2Ti_1/2)O₃ ceramics can be sintered at 1325 °C due to the liquid phase effect of a V₂O₅ additions. The dielectric constant decreases from 140 to 28 as x varies from 0.1 to 1.0. The microwave dielectric properties indicate the possibility of a phase transformation for x between 0.3 and 0.5. A low-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Investigation of Co-Substituted Nanosized Mn2Y-Hexaferrites Synthesized by Sol-Gel Autocombustion Method

The structural and electrical properties of Co-substituted and nano-sized Y-type hexagonal ferrites have been investigated in the present work. The samples with chemical composition Ba₂Co _x Mn_2?x Fe₁₂O₂₂ (x = 0.0, 0.5, 1.0, 1.5, 2.0) were prepared by sol-gel autocombustion method. The powdered samples and pellets were sintered simultaneously at 1000 °C for 5 h and characterized by means of DTA/TGA, FTIR spectroscopy, x-ray diffraction (XRD), field emission gun scanning electron microscopy, and energy dispersive x-ray spectroscopy. The XRD analysis confirms that the investigated ferrites have single phased Y-type hexagonal structure without showing any impurity phase. Lattice constants (a and c), cell volume (V), crystallite size (D), and x-ray density (ρ _x ) have also been calculated from the XRD data. DC electrical resistivity is measured within the temperature range of 30-100 °C for each sample and is observed to increase with increasing Co-substitution. The dielectric constant (∈) has also been measured which is observed to decrease with Co-substitution. Thus, high electrical resistivity and low dielectric constant make these materials suitable for multi-layer chip inductors and also for RF components and circuits.     

A Comparative Study on the Reduction Mechanism of Fe2O3 Under Different Heating Methods

The mechanisms for magnetizing roasting of Fe₂O₃ into Fe₃O₄ under microwave heating and electrical heating have been studied through thermogravimetric analyses, x-ray diffraction (XRD) measurements, and reaction kinetic calculations. In the reduction process, activated carbon was used as a reducing agent and argon as the protective gas. The results of heating tests indicated that the temperature heating speed during microwave heating is 50 times faster than that of electrical heating. The maximum conversion ratio of Fe₂O₃ by microwave heating is 24.5% higher than that by electrical heating. XRD results showed that the required temperature for Fe₂O₃ to completely convert into Fe₃O₄ by microwave heating is 200°C lower than that by electrical heating. Reaction kinetics parameters calculation results showed that the controlling step of microwave magnetizing is a phase boundary reaction of the contracted ball at 250–450°C with an apparent activation energy of 45 kJ/mol, whereas the controlling step of electrical magnetizing is a chemical reaction of stochastic coring at 450–650°C with an apparent activation energy of 225 kJ/mol.     

Y2O3 and Yb2O3 Co-doped Strontium Hafnate as a New Thermal Barrier Coating Material

Y₂O₃ and Yb₂O₃ co-doped strontium hafnate powder with chemistry of Sr(Hf_0.9Y_0.05Yb_0.05)O_2.95 (SHYY) was synthesized by a solid-state reaction at 1450 °C. The SHYY showed good phase stability not only from 200 to 1400 °C but also at a high temperature of 1450 °C for a long period, analyzed by differential scanning calorimetry and x-ray diffraction, respectively. The coefficient of thermal expansion of the sintered bulk SHYY was recorded by a high-temperature dilatometer and revealed a positive influence on phase transitions of SrHfO₃ by co-doping with Y₂O₃ and Yb₂O₃. The thermal conductivity of the bulk SHYY was approximately 16% lower in contrast to that of SrHfO₃ at 1000 °C. Good chemical compatibility was observed for SHYY with 8YSZ or Al₂O₃ powders after a 24 h heat treatment at 1250 °C. The phase stability and the microstructure evolution of the as-sprayed SHYY coating during annealing at 1400 °C were also investigated.     

Fabrication of transparent YAG ceramics by traditional solid-state-reaction method

Transparent polycrystalline YAG ceramics were fabricated by solid-state reaction method using commercial ultrafine yttria andα-Al2O3 powders.The starting materials were milled and calcined at 1 400℃,and sintered into transparent YAG ceramics at 1 750℃in the vacuum for 4 h.Neither the starting materials as-milled or those calcined into YAG phase at 1 500℃can be sintered into transparent ceramics.Wide grain boundaries emerge in the YAG ceramics sintered at 1 850℃for 4 h,at the edge of which YAG phases decompose into perovskite YAlO3(YAP)andα-Al2O3.     

Non-calcining process for CuAlO2 and CuAl0.9Ca0.1O2 ceramics

Synthesis of CuAlO₂ and CuAl_0.9Ca_0.1O₂ ceramics for thermoelectric application using a reaction-sintering process was investigated in this study. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. Some peaks of remained Al₂O₃ were detected in CuAlO₂. CaAl₄O₇ and CuO were detected in CuAl_0.9Ca_0.1O₂. Density values 2.83–3.04 g/cm³ were found in CuAlO₂ pellets sintered at 1350 °C. In CuAl_0.9Ca_0.1O₂ ceramics, density values 3.10–3.31 g/cm³ were found in pellets sintered at 1350 °C. Thin polygonal grains were observed in CuAl_0.9Ca_0.1O₂. The reaction-sintering process has proven a simple and effective method in preparing CuAlO₂ and CuAl_0.9Ca_0.1O₂ ceramics for thermoelectric application.     

Fabrication and characterization of nano-Y2O3, Al2O3, La2O3 dispersed mechanically alloyed and liquid phase sintered WNi for structural application

The investigation involves the fabrication of various nano oxide (Y₂O₃, Al₂O₃, La₂O₃) dispersed WNi alloys by mechanical alloying for 10 h and sintering in Ar atmosphere at 1400 °C, 1500 °C for 2 h. The selected composition for the study is W₈₉Ni₁₀(Y₂O₃)₁ (alloy A), W₈₉Ni₁₀(Al₂O₃)₁ (alloy B) and W₈₉Ni₁₀(La₂O₃)₁ (alloy C) (in weight%). Alloy A exhibit least crystallite size and maximum lattice strain at 10 h of milling. The lattice parameter of W exhibits expansion and contraction behavior during the milling. Microstructure of sintered alloys reveals the presence of both faceted and nearly spherical shaped grains. Bimodal grain size distribution is higher in alloy A at 1500 °C as compared to other alloys and 1400 °C sintering temperature. Texture study and Young's Modulus value reveals that hardness of alloy A is higher against alloy B and alloy C. Maximum % relative sintered density, mean hardness, compressive strength of 89%, 5.53 GPa, 2.25 GPa respectively has been achieved in alloy A at 1500 °C.