Low loss alumina dielectrics by aqueous tape casting: The influence of composition on the loss tangent

Polycrystalline aluminas with various concentrations of oxide dopants CaO, MgO, and TiO₂, ranging from 0.05 wt.% to 5 wt.% (3 wt.% in case of MgO), as well as pure alumina references were prepared by tape casting of aqueous suspensions and sintered in air at 1600 °C for 4 h for applications as low dielectric loss electroceramics. Loss tangents were measured at room temperature in the frequency range between 1 and 100 kHz as the key parameter for the intended application. The values of loss tangents of doped materials were influenced by the concentration of dopants. The addition of 0.5 and 5 wt.% of TiO₂ and 3 wt.% of MgO decreased the value of loss tangent in the whole frequency range. The addition of these dopants eliminated abnormal grain growth and decreased the amount of residual porosity. By doing so, the dopants compensated the negative influence of process impurities and decreased the loss tangent values. The cations (Ti⁴⁺) with high solubility in the Al₂O₃ crystal lattice were preferably built into the grain boundary glass, thus efficiently reducing the concentration of polarizable defects in corundum matrix; the formation of vitreous phase had a positive effect on the value of loss tangent in TiO₂ doped samples. The increased values of loss tangent were related to lower density of prepared materials, and the presence of residual porosity. Other contributing factors were especially the formation of calcium-containing secondary crystalline phases, and the increased concentration of lattice defects due to incorporation of atoms with different valencies to alumina crystal lattice.     

Dielectric properties of alumina doped with TiO2 from 13 to 73 GHz

Recent studies show that alumina doped with TiO₂ exhibits promising dielectric properties, corresponding to low loss tangents and low temperature coefficients (or close to 0 ppmC⁻¹). This paper aims to confirm these trends and study the dielectric properties of alumina doped with TiO₂ from 0.5% to 12% wt. at high frequency, 13–73 GHz. This work demonstrates that alumina doped with TiO₂ corresponds to potential materials for frequency converter devices working at a high frequency (up to 50 GHz).     

Toward vacuum sintering of YAG transparent ceramic using divalent dopant as sintering aids: Investigation of microstructural evolution and optical property

Transparent YAG ceramics were fabricated by solid state reaction sintering using divalent dopants (CaO and MgO) as sintering additives without TEOS doping, and the effects of divalent dopants on their microstructure evolution and optical properties were investigated. It was found that CaO was more effective with respect to inhibiting grain growth than MgO, but not as effective as MgO in promoting densification. Fully dense, transparent YAG ceramics with excellent optical qualities could be achieved by optimizing the doping concentrations of CaO and MgO; the transmittance at 1064 nm was as high as 84.5% for 3 mm thick sample at the molar ratio of Ca: Mg=1:4, after sintered at 1840 °C for 8 h in vacuum.     

Microwave dielectric losses caused by lattice defects

Dielectric loss tangent at microwave frequency is mainly determined by the anharmonic terms in the crystal's potential energy. In addition, there is a kind of lattice defect that increases the dielectric loss tangent seriously. This paper presents the experimental results for two materials; the system Ba(Zn,Ta)O₃–BaZrO₃ and (Zr,Sn)TiO₄. The dielectric loss tangents of the system Ba(Zn,Ta)O₃–BaZrO₃ increases seriously when the B-site ions distribute disorderedly in the crystal. The doping of oxygen vacancies and acceptor ions in (Zr,Sn)TiO₄ increase tan δ by the way they increase the gradient and intercept of linear frequency dependency of tan δ. These experimental results are reasonably explained by Schlömann's theory. He predicted that the dielectric loss tangent increases when the ions are distributed disorderedly in a way that they break the periodic arrangement of charges in the crystal, and that the increase of tan δ is negligible if the disordered charge distribution maintains the charge neutrality within a short range of the lattice constant in the crystal.     

Effect of LiAl5O8 additions on the sintering and optical transparency of LiAlON

LiAl₅O₈ (zeta alumina) was reaction sintered with α-Al₂O₃ and AlN to produce γ-LiAlON. Zeta alumina transforms from a primitive to a face-centered cubic structure above 1290 °C with a lattice parameter similar to γ-AlON. Weight loss measurements combined with XRD suggest solubility of Li in the spinel structure at elevated temperatures. The Vickers hardness, at a 1-kg load, of the pressureless sintered LiAlON was 16.5 ± 0.5 GPa, independent of the grain size or amount of zeta alumina added, for LiAl₅O₈ additions ranging between 0 and 16 wt.%. In-line transmission in the visible and near-IR regions increased with increasing grain size, most likely due to pore coalescence.     

Energy storage density and tunable dielectric properties of BaTi0.85Sn0.15O3/MgO composite ceramics prepared by SPS

(100-x) wt.% BaTi_0.85Sn_0.15O₃–x wt.% MgO (BTS/MgO) composite ceramics were prepared by spark plasma sintering (SPS) technology. Phase constitution, microstructure, dielectric and electrical energy storage properties of BTS/MgO composite ceramics were investigated. The samples prepared by SPS had smaller grain size and presented layer-plate substructure. Dielectric permittivity and dielectric loss of BTS/MgO composite ceramics decreased significantly with the content of MgO increasing, and dielectric tunability maintained a relatively high value (>45%). Meanwhile, the dielectric breakdown strength was improved when addition of MgO in BTS matrix, which resulted in a significant improvement of energy storage density. The high dielectric breakdown strength of 190 kV/cm, energy storage density of 0.5107 J/cm³ and energy storage efficiency of 92.11% were obtained in 90 wt.% BaTi_0.85Sn_0.15O₃–10 wt.% MgO composite ceramics. Therefore, BTS/MgO composites with good tunable dielectric properties and electrical energy storage properties could be exploited for energy storage and phase shifter device applications.     

Densification and properties of ZrO2 nanoparticles added magnesia–doloma refractories

In this work the effect of nano- and microZrO₂ addition on the densification and hydration resistance of MgO–CaO refractories was investigated. 0, 2, 4, 6 and 8 wt% ZrO₂ was added to MgO–CaO refractories that contain 35 wt% CaO. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. Results show that with addition of ZrO₂ the bulk density and hydration resistance of the samples increased while apparent porosity decreased. Also the hydration resistance of the samples was appreciably improved by the addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of the microstructure. Also it revealed that the nanoZrO₂ addition was more effective than microZrO₂ due to its higher activity.     

The influence of MgO,Y2O3 and ZrO2 additions on densification and grain growth of submicrometre alumina sintered by SPS and HIP

Spark plasma sintering followed by hot isostatic pressing was applied for preparation of polycrystalline alumina with submicron grain size. The effect of additives known to influence both densification and grain growth of alumina, such as MgO, ZrO₂ and Y₂O₃ on microstructure development was studied. In the reference undoped alumina the SPS resulted in some microstructure refinement in comparison to conventionally sintered materials. Relative density >99% was achieved at temperatures >1200 °C, but high temperatures led to rapid grain growth. Addition of 500 ppm of MgO, ZrO₂ and Y₂O₃ led, under the same sintering conditions, to microstructure refinement, but inhibited densification. Doped materials with mean grain size <400 nm were prepared, but the relative density did not exceed 97.9%. Subsequent hot isostatic pressing (HIP) at 1200 and 1250 °C led to quick attainment of full density followed by rapid grain growth. The temperature of 1250 °C was required for complete densification of Y₂O₃ and ZrO₂-doped polycrystalline alumina by HIP (relative density >99.8%), and resulted in fully dense opaque materials with mean grain size<500 nm.     

Investigating the effect of porosity level and pore former type on the mechanical and corrosion resistance properties of agro-waste shaped porous alumina ceramics

The strength integrity and chemical stability of porous alumina ceramics operating under extreme service conditions are of major importance in understanding their service behavior if they are to stand the test of time. In the present study, the effect of porosity and different pore former type on the mechanical strength and corrosion resistance properties of porous alumina ceramics have been studied. Given the potential of agricultural wastes as pore-forming agents (PFAs), a series of porous alumina ceramics (Al₂O₃-xPFA; x=5, 10, 15 and 20 wt%) were successfully prepared from rice husk (RH) and sugarcane bagasse (SCB) through the powder metallurgy technique. Experimental results showed that the porosity (44–67%) and the pore size (70–178 µm) of porous alumina samples maintained a linear relationship with the PFA loading. Comprehensive mechanical strength characterization of the porous alumina samples was conducted not just as a function of porosity but also as a function of the different PFA type used. Overall, the mechanical properties showed an inverse relationship with the porosity as the developed porous alumina samples exhibited tensile and compressive strengths of 20.4–1.5 MPa and 179.5–10.9 MPa respectively. Moreover, higher strengths were observed in the SCB shaped samples up to the 15 wt% PFA mark, while beyond this point, the silica peak observed in the XRD pattern of the RH shaped samples favored their relatively high strength. The corrosion resistance characterization of the porous alumina samples in hot 10 wt% NaOH and 20 wt% H₂SO₄ solutions was also investigated by considering sample formulations with 5–15 wt% PFA addition. With increasing porosity, the mass loss range in RH and SCB shaped samples after corrosion in NaOH solution for 8 h were 1.25–3.6% and 0.44–2.9% respectively; on the other hand, after corrosion in H₂SO₄ solution for 8 h, the mass loss range in RH and SCB shaped samples were 0.62–1.5% and 0.68–3.3% respectively.     

Densification and Properties of Fe2O3 Nanoparticles added CaO Refractories

Up to 8 wt. % of Nano-iron oxide was added to CaO refractory matrix. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. The mechanical behavior was studied by a cold crushing strength (CCS) and flexural strength at 1200 °C test. As a result, it was found that the presence of Nano-iron oxide in the CaO refractory matrix induced 2CaO.Fe₂O₃ (C₂F), CaO.Fe₂O₃ (CF) and 3CaO.Al₂O₃ (C₃A) phase’s formation, which improved the sintering process. Nano-iron oxide also influenced the bonding structure through a direct bonding enhancement. On the Other hand, the presence of Nano-iron oxide resulting in improvement properties of CaO refractory matrix refractories such as bulk density, hydration resistance and cold crushing strength. The maximum flexural strength at 1200 °C is achieved by the samples containing 4 wt. % nano-Fe₂O₃.     

Reaction sintering of AlN–AlON composites

Sintering behavior of three different compositions in the AlN–Al₂O₃ system using Y₂O₃ as a sintering aid was investigated. Samples with various ratios of AlN/Al₂O₃ were sintered in nitrogen atmosphere using a gas pressure furnace in the temperature range 1750–1950 °C. The densification of the samples was studied by shrinkage and relative density measurements. Results showed that samples containing 1 and 70 wt.% alumina were sintered to near theoretical density at 1800 °C; whereas the sample with 20 wt.% alumina never reached densities higher than 93% in the temperature range considered. It was found that the AlN/Al₂O₃ ratio and the sintering temperature had a great influence on the microstructure and crystalline phases present in the samples, namely, AlN, γ-AlON, 27R, and YAG. In the sample with 20 wt.% alumina, porosity formation prevented further densification. These porosities were probably due to the release of oxygen during sintering.     

Effect of TiO2 additions on densification and mechanical properties of new mulltifunction resistant porcelains using economic raw materials

The aim of this work is to determine the effect of TiO₂ on sintering and mechanical proprieties of new multifunction resistant (MFR) porcelain prepared from local abundant raw materials. Based on a preliminary work, the new selected composition was 30 wt% kaolins (20 wt% kaolin halloysite type + 10 wt% kaolin Tamazart), 45 wt% k-feldspar and 25 wt% quartz and containing different contents of TiO₂ (3, 5 and 8 wt%). The sintering temperatures of mixtures were between 1140 and 1260 °C. Subsequently, the obtained phases in the elaborated samples were investigated by X-ray diffraction and Fourier transform infrared spectroscopy analyses, Raman spectroscopy and SEM analysis. The optimum sintering conditions gave a higher bulk density (2.47 g.cm⁻³) and excellent mechanical properties: The three point flexural strength (3PFS), Vickers micro-hardness (VMH) and apparent porosity (P_A) of porcelains sintered at 1160 °C were 238 MPa, 12.3 GPa and 2%, respectively. This obtained 3PFS value is drastically higher than that achieved for conventional porcelains (ranged between 60 and 80 MPa). Moreover, these two best 3PFS (238 MPa) and VMH (12.3 GPa) values achieved for this new MFR porcelains were considerably higher when compared to those values (3PFS=218 MPa and VMH=6.5 GPa) obtained by others for porcelain −30% ZrO₂ composite, even though their mixtures were hot pressed in vacuum at 970 °C for 2 min. Besides, the maximum value achieved for the new MFR porcelains is nearby that of the flexural strength of porcelain containing 5 wt% TiO₂ and 30 wt % alumina (about 240 MPa). In other words, the presence of 30 wt % alumina in their product well confirm the benefic effect of the used raw materials (saving 30 wt % alumina) on porcelain strengthening.     

Colossal permittivity in (In1/2Nb1/2)xTi1−xO2 ceramics prepared by a glycine nitrate process

(In + Nb) co-doped TiO₂ nanoparticles with very low dopant concentrations were prepared using a glycine nitrate process. A pure rutile—TiO₂ phase with a dense microstructure and homogeneous dispersion of dopants was achieved. By doping TiO₂ with 1.5% (In + Nb) ions, a very high dielectric permittivity of ε′ = 42,376 and low loss tangents of tanδ = 0.06 (at room temperature) were achieved. The large conduction activation energy at the grain boundary decreased with decreasing dopant concentration. The colossal permittivity was primarily attributed to the internal barrier layer capacitor (IBLC) effect. The dominant effect of interfacial polarization at the non–Ohmic sample–electrode contact was observed when the dopant concentration was ≤1.0 mol%. Interestingly, the sample–electrode contact and resistive–outer surface layer effects, i.e., surface barrier layer capacitor (SBLC) effect, has also an effect on the colossal dielectric response in (In + Nb) co-doped TiO₂ ceramics.     

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.     

Annealing induced discoloration of transparent YAG ceramics using divalent additives in solid-state reaction sintering

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y₃Al₅O₁₂ (YAG) ceramics. However, Si⁴⁺ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr³⁺ → Cr⁴⁺). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T = 84.6%, at 1064 nm) were obtained after vacuum sintering at 1840 °C for 8 h and without subsequent annealing in air. An absorption peak centered at ∼320 nm was observed before annealing, and it extended to ∼550 nm after annealing at 1450 °C for 10 h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca²⁺F⁺] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.     

Evolution of elastic properties and microstructural changes versus temperature in bonding phases of alumina and alumina–magnesia refractory castables

The microstructural evolutions of high alumina refractory concretes, based on the systems CaO–Al₂O₃ and CaO–Al₂O₃–MgO, have been studied by the way of ultrasonic high temperature measurements. Since such a refractory concrete can be considered as a composite material with two constituents, a continuous matrix (so called bonding phase) and aggregates, investigations of matrices made of mixtures containing cement, reactive alumina and/or magnesia, constitute a preliminary study which is presented in this paper. The elastic behaviour of these matrices has been followed from room temperature to 1550 °C via a specific ultrasonic method. During the first thermal treatment, different changes of slope are observed in the curve E = f(T). Between 200 °C and 400 °C, dehydration mechanisms involve a microstructural reorganisation correlated with a strong decrease of the elastic properties. At high temperature, the Young's modulus evolutions are associated with the expansive formations of CA₂^b and/or in-situ spinel at 1100 °C and then CA₆ (see endnote b) at 1450 °C, which directly depend on the CaO/Al₂O₃ and MgO/Al₂O₃ ratios in the mix. The forming of bond linkage between CA₆ and in-situ spinel grains in the matrix is believed to enhance the elastic properties at high temperature.     

Alumina porous nanomaterials obtained by colloidal processing using d-fructose as dispersant and porosity promoter

Recently, great effort has been devoted to obtain porous materials with customized pore size distribution, high surface area and submicrometer sized microstructures or nanostructures. In this work, the viability of colloidal processing routes to obtain porous bulk ceramics using alumina nanopowders and d-fructose as a dispersant and a porosity former has been explored.The rheological behaviour of nanosuspensions was studied in order to assure their stability and to analyse the influence of different parameters (solids loading, fructose content, pH, sonication time). Mesoporous green bodies were obtained by slip casting with d-fructose in concentrations ranging from 5 to 50 wt%. The drying and burning-out conditions were determined by DTA-TG measurements and the sintering cycles were selected from the dynamic sintering curve. Sintered alumina materials with high porosity (>60%), open microstructures, submicrometer sized porosity (d_p = 140–210 nm) and grain size lower than 500 nm, were obtained for pieces sintered at temperatures of 1300 and 1400 °C. The influence of different processing parameters on the porosity and the microstructure of the sintered materials is discussed.     

Thermal conductivity of porous alumina ceramics prepared using starch as a pore-forming agent

The thermal conductivity of porous alumina ceramics prepared using different types of starch (potato, wheat, corn, and rice starch) as pore-forming agents is investigated from room temperature up to 500 °C. The temperature dependence measured for alumina ceramics of different porosity (in the range 6–47%) is fitted with second-order polynomials and 1/T-type relations, and compared to available literature data for dense alumina. It is found that the porosity dependence of the relative thermal conductivity k_r = k/k₀ is well described by a modified exponential relation of the form k_r = exp(?1.5?/(1 ? ?)), where ? is the porosity. This finding is in agreement with other literature data and seems to indicate a common feature of all porous materials with microstructures resulting from fugitive pore-forming agents.     

Study of processing routes for WC-MgO composites with varying MgO contents consolidated by FAST/SPS

Two different preparation routes were applied to process WC-MgO composites with varying MgO contents (4.1 wt.% and 5.9 wt.% MgO). WC-MgO powder mixtures were synthesized by a milling process at 600 rpm for 6 h of partially oxidized WC (WC + WO₃), Mg₃N₂ and C. Alternatively, WC and MgO as initial powders were used. For consolidation of the powder mixtures the field-assisted sintering technology (FAST) was used. X-ray diffraction shows that samples out of different powder mixtures and sintered between 1600 °C and 1750 °C exhibited WC, MgO and the W₂C phase independent of the preparation route of the powder mixtures. A higher density and better mechanical properties (hardness and indentation fracture toughness) of WC-MgO were achieved of pure WC and MgO as initial powders were consolidated by FAST. It was found that a lower MgO content results in higher hardness values and in a slightly decreased indentation fracture toughness.     

Fused calcium zirconate for refractory applications

This article investigates dense CaZrO₃ produced by electric arc melting and compares it to solid state synthesized CaZrO₃. The fused CaZrO₃ had a considerable lower porosity than the solid state synthesized material, however due to evaporation of some CaO during the electric arc melting the stoichiometric ratio of CaO/ZrO₂ was slightly smaller than one, resulting in the formation of cubic ZrO₂ as a second phase. To determine the local phase distribution the materials were investigated with EBSD in addition to integral XRD. The vibration polishing of the ceramics resulted in excellent EBSD patterns. Die pressed cylinders of the fused material led to a porosity as low as 14.3%. The linear thermal expansion coefficient from 25 °C to 1000 °C was 10.45 × 10^?6 K^?1. The cold isostatic pressing of larger crucibles resulted in higher porosity of 15.9–16.5%. The EBSD analysis and the cold isostatic pressed crucibles can be applied in corrosion tests for titanium melts.