Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Porous ZnAl2O4 spinel nanorods: High sensitivity humidity sensors

Porous zinc aluminate (ZnAl₂O₄) spinel nanorods were synthesized via a homogeneous co-precipitation approach followed by a heat treatment at 900 °C. The porous rod-like nanostructures not only increase the efficiency of adsorption sites, but also promote the dissociation of water adsorbed on nanorod boundaries and pore-walls. Moreover, they also provide an effective and fast channel for the transport of water vapor and liquid. Therefore, the impedance signal of the sensors based on the porous nanorods presents high sensitivity, good linearity, small hysteresis, and fast response/recovery time to humidity. Additionally, the sensors are also relatively stable to humidity for a long time. This study demonstrates that porous ZnAl₂O₄ nanorods are a promising platform for the construction of humidity sensors.     

Sintering behavior and microwave dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics sintered under air and N2 atmosphere

The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) and Bi₂(Zn_1/3Nb_2/3−xV_x)₂O₇ (BZNV, x = 0.001) sintered under air and N₂ atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V₂O₅ and N₂ atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N₂ atmosphere with _r = 76.1, Q = 850 and Q_f = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with _r = 76.7, Q = 890 and Q_f = 3580 GHz. The temperature coefficient of resonant frequency τ_f was not obviously influenced by the different atmospheres. For BZN ceramics the τ_f was −79.8 ppm/°C while τ_f is −87.5 ppm/°C for BZNV ceramics.     

ZnLi2/3Ti4/3O4: A new low loss spinel microwave dielectric ceramic

A new low loss spinel microwave dielectric ceramic with composition of ZnLi_2/3Ti_4/3O₄ was synthesized by the conventional solid-state ceramic route. The ceramic can be well densified after sintering above 1075 °C for 2 h in air. X-ray diffraction data show that ZnLi_2/3Ti_4/3O₄ ceramic has a cubic structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.40172 Å, V = 593.07 Å³, Z = 8 and ρ = 4.43 g/cm³. The best microwave dielectric properties can be obtained in ceramic with relative permittivity of 20.6, Q × f value of 106,700 GHz and τ_f value of −48 ppm/°C. The addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1075 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added ZnLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Development of MgAl2O4 spinel coating on graphite surface to improve its water-wettability and oxidation resistance

Low water-wettability and oxidation resistance of graphite have limited its application in carbon containing refractory castables. The aims of this study are the improvement of water-wettability and the oxidation resistance of natural flaky graphite by applying an oxide coating on its surface. To develop the coating, magnesium aluminate spinel sol was formulated via a citrate–nitrate route and graphite powder was then added to the sol. The mixture was heat treated in appropriate temperature and atmosphere to get the polycrystalline MgAl₂O₄ coating on graphite particles surface. The microstructure of coating was studied by X-ray diffractometer, SEM and TEM. The water-wettability was evaluated by measuring the water drop contact angle and plotting the zeta potential vs. pH. The results showed the development of a stable nanocrystalline MgAl₂O₄ spinel coating which improved the water-wettability and oxidation resistance of graphite significantly. Also, characterization of the coating is explained with emphasis on its application importance.     

Synthesis and densification behaviour of magnesium aluminate spinel: Effect of Dy2O3

Magnesium aluminate spinels have been developed by reaction sintering of calcined alumina and calcined magnesia and its densification behaviour was studied in presence of Dy₂O₃. Green bar made from stoichiometric spinel composition with and without Dy₂O₃ were subjected to dilatometric study, densification study and microstructural evaluation by SEM. It was found that Dy₂O₃ additive does not have significant effect on the spinelisation but favours the densification of the spinel. Microstructure of sintered spinel without any additive is non-uniform with some exaggerated grain growth. Dy₂O₃ prevents the exaggerated grain growth and thereby helps in the densification process.     

Grain growth in microwave sintered Si3N4 ceramics sintered from different starting powders

Silicon nitride ceramics have been produced by microwave sintering at 28 GHz with Y₂O₃, Al₂O₃ and MgO as sintering additives. The effect of initial β content of the Si₃N₄ starting powder on the microstructural development has been assessed by scanning electron microscopy (SEM) and quantitative image analysis. Phase transformation behaviour was assessed by X-ray diffraction. Mechanical properties of the sintered bodies were determined through assessment of hardness and fracture toughness. It was found that the samples sintered from powders with lower initial β content developed larger grains than those from higher β content powders, due to fewer nucleation sites during the →β transformation. However, attempts to develop a more bimodal microstructure by using a mixture of the two grades of powder, in an effort to increase both fracture toughness and fracture strength, were unsuccessful. In this case the microstructure was similar to that developed in the materials produced from higher β content powders. The mechanical properties of the sintered bodies were very similar, despite differences in microstructure. This was attributed to the strong bonding between the grains and grain boundary phase resulting in crack paths in all the materials that were predominantly transgranular, with little debonding or crack deflection. Under these circumstances the effect of larger grains is eliminated.     

Spinelisation and properties of Al2O3–MgAl2O4–C refractory: Effect of MgO and Al2O3 reactants

The effect of particle size of MgO and Al₂O₃ on the spinel formation associated with permanent linear change on reheating (PLCR) and microstructure of Al₂O₃–MgAl₂O₄–C refractory is investigated as a function of heating cycle at 1600 °C with 2 h holding at each cycle. It was found that rate of spinel formation and associated volume expansion is very much dependent on the reactivity and particle size of the reactant. When the reactants are very fine and reactive there is considerable amount of spinel formation, whereas coarser reactants with lower reactivity show negligible formation of spinel phase and associated expansion. Magnesia and alumina with moderate reactivity develops optimum PLCR of the refractory. It continuously increases with the number of heating cycles. The SEM photomicrographs show that in Al₂O₃–MgAl₂O₄–C refractory the spinel phase is formed in between the calcined bauxite grain and the EDX analysis indicates that the spinel phase formed is stoichiometric in nature.     

Synthesis of CuGa2O4 nanoparticles by precursor and self-propagating combustion methods

Copper gallate spinels, CuGa₂O₄, have been synthesized by two wet chemical routes: precursor method and self-propagating combustion involving a glycine-nitrate system. All complex precursors have been characterized by chemical analysis, infrared spectroscopy (IR), ultraviolet visible spectroscopy (UV–vis), electron paramagnetic resonance spectroscopy (EPR), thermal analysis and scanning electron microscopy (SEM). The copper gallate spinel oxides have been further investigated by X-ray diffraction (XRD), SEM, IR, UV–vis, magnetic measurements and EPR. The crystallite size of the copper gallate was found about 280 Å.     

The effects of the addition of tetragonal-ZrO2 on the mechanical properties of MgAl2O4 – ZrO2 composites

The effects of the addition of tetragonal 3mol% Y₂O₃ – ZrO₂ into a MgAl₂O₄ spinel matrix were investigated. MgAl₂O₄ spinel's lacking mechanical properties prevent further utilization in many structural and refractory applications even though it has excellent chemical and thermal stability. The addition of tetragonal-ZrO₂ was observed to improve the hardness, fracture toughness and biaxial flexural strength of MgAl₂O₄ materials. Moreover, the additions of 3mol% Y₂O₃ – ZrO₂ resulted in a reduction of the mean grain size.     

Microwave assisted sintering of CaCu3Ti4O12

CaCu₃Ti₄O₁₂ electroceramic was prepared by a microwave assisted solid-state reaction technique from CaCO₃, CuO and TiO₂ powders. Processing involved the preparation of raw material, mixing and milling, calcination, pellet forming and sintering processes. Conventional furnace and microwave assisted sintering processes were employed in order to improve phase structures, morphology and dielectric properties of CaCu₃Ti₄O₁₂ ceramics. Surface and fracture FESEM analysis showed that the microwave assisted sintered CaCu₃Ti₄O₁₂ produced better densification and more uniform grain size compared to the conventional sintered sample.     

Low-temperature preparation and microwave dielectric properties of cold sintered Li2Mg3TiO6 nanocrystalline ceramics

This study aims to fabricate Li₂Mg₃TiO₆ ceramics with ultrafine grains using a novel cold sintering process combined with a post-annealing treatment at a temperature <?950?°C. In this study, phase composition, sintering behavior, microstructure evolution, and microwave dielectric properties of the resultant nanocrystalline ceramics were investigated for the first time. The as-compacted green pellets at 180?°C yielded a high relative density of ~ 90% and the ceramics that were post-sintered over a broad temperature range (800–950?°C) possessed highly dense microstructure with a relative density of ~ 96%. The average grain size varied from 100 to 1200?nm for the samples sintered at 800–950?°C. Furthermore, the quality (Q × f) values of the obtained specimens exhibited a strong positive dependency on the grain size, which increased from 17,790 to 47,960?GHz for grain sizes ranging between 100 and 1200?nm, while the dielectric permittivity (ε_r) and temperature coefficient of the resonant frequency (τ_f) values did not undergo any significant changes over this range of grain size.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Fine-grained transparent MgAl2O4 spinel obtained by spark plasma sintering of commercially available nanopowders

A high transmittance/small grain size combination for pure spinel ceramics from commercially available nanopowders without sintering aids can be obtained by SPS sintering. By using a low heating rate ≤10 °C/min and a sintering temperature ≤1300 °C, a transparent polycrystalline MgAl₂O₄ spinel was fabricated by SPS with an in-line transmission of 74% and 84% for 550 nm (visible) and 2000 nm (NIR) wavelengths respectively. A small average grain size of about 250 nm was obtained and the pores located at the multiple grain junctions have a mean size of about 20 nm. The high in-line transmission is linked not only to the low residual porosity but particularly to the very small size of pores.     

Highly-efficient electromagnetic interference shielding and microwave dielectric behavior of a (Bi2O3 + B2O3)-doped MWCNT/BaTiO3 ceramic nanocomposite

Microwave dielectric properties along with electromagnetic interference shielding effectiveness (EMI SE) of a multi-walled carbon nanotube (MWCNT)/barium titanate (BaTiO₃) nanocomposite are investigated in this paper. Appropriate amount of sintering additive (Bi₂O₃ +?B₂O₃) was doped into some nanocomposites to reduce the sintering temperatures. The dielectric properties of the nanocomposites with various MWCNT and sintering additive contents were evaluated at different microwave frequency ranges. It was found that the incorporation of optimized amount of (Bi₂O₃ +?B₂O₃) can give rise to significantly good dielectric properties. Results also indicated that incorporation of 6?wt% (Bi₂O₃ +?B₂O₃) into 1.5?mm-thick nanocomposite containing 8?wt% MWCNT led to an EMI SE greater than 28?dB, suggesting this novel nanocomposite as a promising candidate for microwave absorption and electromagnetic interference applications.     

The comparison of mechanical behavior of MgO-MgAl2O4 with MgO-ZrO2 and MgO-MgAl2O4-ZrSiO4 composite refractories

Mechanical properties of different compositions obtained from the additions of 5, 10, 20 and 30 wt.% zircon (ZrSiO₄) into the MgO-spinel composite refractories and ZrO₂ into MgO have been examined, the variations that occurred have been determined, and the parameters affecting those factors have been investigated with the reasons. The density, strength, Young's modulus, fracture toughness, fracture surface energy and work of fracture were measured and evaluated. Microstructural variations and fracture surfaces have been examined and the formation of new phases has been identified depending on the additive type and quantity. The relationships between mechanical properties and structural variations for different compositions have been examined. In MgO-spinel materials, strength, Young's modulus and fracture toughness values decrease up to 20% spinel addition and stay almost constant for further loads. ZrO₂ addition displays same trend but not as effective as spinel. Besides, since ZrO₂ is stable in cubic form, it does not show any toughening mechanism. Forsterite formation is the most important factor for 2-fold improvement in the mechanical properties of MgO-spinel-zircon refractories. The more the zircon addition, the more the mechanical properties improve. The generation of natural bonding between matrix particles with forsterite formation, on the other hand, causes the fracture path to turn to transgranular fracture with an increase in fracture surface energy and a decrease in work of fracture, among which the latter is considered as an indicator of thermal shock resistance of the materials being high.     

Preparation of Sr2SiO4:Eu phosphors by microwave-assisted sintering and their luminescent properties

An Eu³⁺ activated strontium silicate phosphor was synthesized using a microwave-assisted sintering with a flux NH₄Cl. X-ray powder diffraction analysis confirmed the formation of pure Sr₂SiO₄ phase without second phase or phases of starting materials as Sr_1.9SiO₄:Eu³⁺_0.1 powders sintered at various temperatures in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave-assisted sintering. In the PL studies, the excitation spectrum of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors exhibited a broad band in the UV region centered at about 270 nm which was consistent with the absorption spectra. Both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 617 nm under excitation of 395 nm, with similar luminescent intensity. The results showed that microwave processing has the potential to decrease the sintering time and required energy input for the production of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors without degrading photoluminescence.     

Influence of porosity on the electrical properties of La9.33(SiO4)6O2 oxyapatite

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for fuel cells as SOFC. The influence of porosity on the electrical properties of La_9.33(SiO₄)₆O₂ oxyapatite is reported here. Hot pressed pellets with various densification ratios have been characterized by the complex impedance method. The high frequency response associated with the bulk contribution is much more affected by the porosity than the grain boundaries contribution: as a consequence, the electrical behaviour of the samples has been considered in assimilating the porous ceramics to composite materials made of apatite with various amounts of air inclusions. Thus, the porosity dependence of conductivity, activation energy and permittivity are reported here. A percolation threshold has been highlighted for porosity values greater than 30%, involving great lowering of the electrical performances.     

Effect of Y2O3 additive on conventional and microwave sintering of mullite

Mullite has become a strong candidate material for advanced structural and functional ceramics. Much interest has recently focused on sintering aids for mullite. The aim of this study was to evaluate the effect of Y₂O₃ as a sintering aid in the conventional and microwave sintering of mullite. To accomplish this study, a highly pure industrial mullite was used. Mullite with and without Y₂O₃ was pressed under a cold isostatic pressure of 200 MPa. Samples were sintered conventionally at 1400, 1450, 1500, 1550 and 1600 °C for 2 h and microwave-sintered for up to 40 min using a large range of power. The microstructure and physical properties of the microwave-sintered samples were compared to those of the conventionally sintered samples. The results showed that Y₂O₃ improved the densification of mullite bodies in the conventional and microwave sintering processes, but high densifications were achieved in just a few minutes when Y₂O₃ was used with microwave processing.