Fabrication and performance of dielectric tape based on CaO-B2O3-SiO2 glass/Al2O3 for LTCC applications

A non-aqueous tape-casting process for fabricating CaO-B₂O₃-SiO₂ glass/Al₂O₃ dielectric tape for LTCC applications was investigated. An isopropanol/ethanol/xylene ternary solvent-based slurry was developed by using castor oil, poly(vinyl butyral), and dibutyl phthalate as dispersant, binder, and plasticizer, respectively. The effects of dispersant concentration, binder content, plasticizer/binder ratio, and solid loading, on the properties of the casting slurry and resultant tape were systematically investigated. The results showed that the optimal values for the dispersant and binder contents, plasticizer/binder ratio, and solid loading were 2.0 wt%, 7.5 wt%, 0.6, and 62 wt%, respectively. The resultant flexible and uniform, 120-μm-thick CaO-B₂O₃-SiO₂ glass/Al₂O₃ tape had a density of 1.90 g/cm^?3, tensile strength of 1.66 MPa, and average surface roughness of 310 nm. Laminated tapes sintered at 875 °C for 15 min exhibited excellent properties: relative density of 97.3%, ε_r of 7.98, tan δ of 1.3 × 10^?3 (10 MHz), flexural strength of 205 MPa, and thermal expansion coefficient of 5.47 ppm/°C. The material demonstrated good chemical compatibility with Ag electrodes, indicating a significant potential in LTCC applications.     

The effect of magnesium compounds (MgO and MgAl2O4) on the synthesis of Lanthanum magnesium hexaaluminate (LaMgAl11O19) by solid-state reaction method

In the present study, the lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉)(LaMA) powder was synthesized by the solid–state reaction method using two types of magnesium compounds, including magnesium oxide (MgO) and magnesium aluminate (MgAl₂O₄) spinel (MAS). The effect of substitution of magnesium oxide with MAS on the synthesis temperature, intermediate compounds and morphology of synthesized powders were investigated. The microstructural results showed that the intermediate compounds of lanthanum aluminate (LaAlO₃), aluminum oxide and MAS were formed in the presence of magnesium oxide, whereas in the latter case, the LaAlO₃ intermediate phase was not observed and La₄Al₂MgO₁₀ was formed at about 810 °C. Also in both cases, a single LaMA phase with the platelet-like morphology was formed. The thickness of the LaMA platelets decreased from 300 nm to 125 nm and the synthesis temperature increased from 1330 °C to 1355 °C, by replacing MgO with MAS.     

Fast synthesis of MgAl2O4‒W and MgAl2O4‒W‒W2B composite powders by self-propagating high-temperature synthesis reactions

MgAl₂O₄?W and MgAl₂O₄?W?W₂B composite powders were obtained rapidly in a single step by self-propagating high-temperature synthesis of WO₃?Mg?xAl₂O₃ and WO₃?B₂O₃?Mg?yAl₂O₃ systems. The addition of various Al₂O₃ contents (x and y-values) to the starting materials was considered as the main synthesis parameter. Thermodynamic calculations revealed that the adiabatic temperature of both systems was decreased with increasing Al₂O₃ content. The XRD results indicated that after acid leaching of the WO₃?Mg?xAl₂O₃ combustion products, W and MgAl₂O₄ were formed as the main phases and WO₂, MgWO₄ and Al₂O₃ as the minor constituents in the final composite. On the other hand, MgAl₂O₄?W composites were synthesized in the WO₃?B₂O₃?Mg?yAl₂O₃ system at y<1.4 mol. By increasing the y-value to 2.1 mol, W₂B was formed as a new product leading to production of MgAl₂O₄?W?W₂B composite. The formation of spinel was confirmed by the Fourier transformed infrared spectroscopy analysis. Microstructure observations represented the uniform distribution of MgAl₂O₄ blocks within the fine spherical W particles. The melting of Al₂O₃ was found as a vital step for rapid synthesis of MgAl₂O₃ by the SHS route. Finally, the possible formation mechanism of MgAl₂O₄ during the combustion synthesis was proposed.     

Kinetic analysis of magnesium aluminate spinel formation: Effect of MgO:Al2O3 ratio and titania dopant

The mechanistic pathway of MgO-Al₂O₃ reaction in solid state to form MgAl₂O₄ spinel was investigated to correlate the kinetic parameters with ratio of reactants (MgO:Al₂O₃) and with the presence of a doping agent, TiO₂. The time-temperature-expansion data of oxide compacts was analyzed using several model free analyses and model based (linear and non-linear) kinetic algorithms. These indicated that spinel formation process can be best described by single step with n-dimensional Avrami equation for every MgO:Al₂O₃ ratio, irrespective of titania dopant. The activation energy (E_a) of the process was proportional to % spinel formed in each system and validated with quantitative XRD analysis. The higher value of Avrami coefficient (n) in 90 wt% Al₂O₃ compositions has been explained with geometric considerations of powder packing. Incorporations of 1% TiO₂ in the MgO: Al₂O₃ oxide compact did not markedly affect the reaction model, frequency factor and Activation energy.     

Investigation on the anti-reduction mechanism of Ti4+ in high dielectric constant system Ca0.9La0.067TiO3 by doping with Al2O3

Ca_0.9La_0.067TiO₃ (abbreviated as CLT) ceramics doped with different amount of Al₂O₃ were prepared via the solid state reaction method. The anti-reduction mechanism of Ti⁴⁺ in CLT ceramics was carefully investigated. X-ray diffraction (XRD) was used to analyze the phase composition and lattice structure. Meanwhile, the Rietveld method was taken to calculate the lattice parameters. X-ray photoelectron spectroscopy (XPS) was employed to study the valence variation of Ti ions in CLT ceramics without and with Al₂O₃. The results showed that Al³⁺ substituted for Ti⁴⁺ to form solid solution and the solid solubility limit of Al³⁺ is near 1.11 mol%. Furthermore, the reduction of Ti⁴⁺ in CLT ceramics was restrained by acceptor doping process and the Q × f values of CLT ceramics were improved significantly. The CLT ceramic doped with 1.11 mol% Al₂O₃ exhibited good microwave dielectric properties: ε_r = 141, Q × f = 6848 GHz, τ_f = 576 ppm/°C.     

The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

MgAl₂O₄ bulk samples were fabricated by two different approaches to investigate the effect of slip casting and sintering temperature on their transparency. Three MgAl₂O₄ samples containing 1 wt% LiF, as the sintering aid, were prepared by the spark plasma sintering process (SPS) at 1400 °C and 1500 °C, under 100 MPa, for 15 min. Also, another MgAl₂O₄ sample was prepared by slip casting followed by SPS under similar conditions. It was observed that utilizing slip casting led to more transparency (10% in the visible region and 20% in the IR region) due to the more homogeneous structure. It was also observed that by reducing the SPS temperature from 1500 °C to 1400 °C, the transparency increased (20% in the IR region) because of the lower grain growth rate at the lower temperature.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Microstructure and elastic modulus of electrospun Al2O3-SiO2-B2O3 composite nanofibers with mullite-type structure prepared at elevated temperatures

In the present work, Al₂O₃-SiO₂-B₂O₃ composite nanofibers with mullite-type structure were prepared using electrospinning technique. The microstructure and elastic modulus of the composite nanofibers obtained at elevated temperatures were studied. The results showed that Al₄B₂O₉ phase formed at 900 °C and then transformed to Al₁₈B₄O₃₃ at 1100 °C. Mullite was also detected in the nanofibers prepared at 1100 °C. Amorphous SiO₂ existed in all samples even the calcination temperature reached up to 1400 °C. The continuous and uniform structure of the composite nanofibers was kept after calcining at different temperatures, while rougher surface was evident due to the growth of the grain caused by the elevated temperature. An increase of elastic modulus of the samples from 9.47 ± 1.91 GPa to 27.30 ± 2.61 GPa was observed when calcination temperatures increased from 800 °C to 1400 °C.     

Investigation on a postmortem resin-bonded Al-Si-Al2O3 sliding gate with functional gradient feature

A resin-bonded Al-Si-Al₂O₃ sliding gate was designed on the basis of sintered alumina containing both Al and Si fine powders, and the sliding gate achieved good application results in the practical process of steel pouring. Moreover, the postmortem sliding gate was characterized and analysed by X-ray diffraction, scanning electron microscope, and energy dispersive X-ray spectroscopy. The results show that the postmortem sliding gate presents a functional gradient feature with a reinforcement zone–transition zone–plastic zone phase distribution, in which the phases in the reinforcement zone from 0 mm to 5 mm are Al₂O₃, Al₄O₄C, SiC, and Al₄C₃; i.e., the Al, Si, and carbon in the composite totally converted into non-oxide phases. Further, phases in the transition zone from 5 mm to 10 mm are Al₂O₃, SiC, and Al₄C₃, whereas phases in the plastic zone from 10 mm to 15 mm are Al₂O₃, SiC, Al₄C₃, Al and Si. The formation mechanism of the grade distribution of phases in the postmortem sliding gate is described as follows. During operation, Al and Si reacted with C so that Al₄C₃ and SiC formed in situ; then, Al₄C₃ further reacted with Al₂O₃, whereby Al₄O₄C was formed as the reinforcement phase at the 0–5-mm zone with a high temperature. As the temperature decreased from the hot face to interior, a part of the free Al and Si remained in the form of plastic phases.     

Distribution of carbon contamination in MgAl2O4 spinel occurring during spark-plasma-sintering (SPS) processing: I – Effect of heating rate and post-annealing

Carbon contamination from the carbon paper/dies during spark-plasma-sintering (SPS) processing was examined in the MgAl₂O₄ spinel. The carbon contamination sensitively changes with the heating rate during the SPS processing. At the high heating rate of 100 °C/min, the carbon contamination having organized structures occurred over almost the entire area from the surface to deep inside the SPSed spinel disk. In contrast, at the slow heating rate of 10 °C/min, the carbon contamination having disordered structures occurred only around the surface area. The carbon phases transform into high pressure CO/CO₂ gases by post-annealing in air and lead to pore formation along the grain junctions. The pore formation significantly occurs at the high heating rate due to the large amount of the contaminant carbon phases. This suggests that if once the carbon contamination was formed in the materials, it is very difficult to remove the carbon phases from the materials.     

Fabrication of porous Al2O3-based ceramics using ball-shaped powders by preceramic polymer process in N2 atmosphere

Porous Al₂O₃-based ceramics were successfully fabricated using ball-shaped powders by preceramic polymer process in N₂ atmosphere. These results showed that the amorphous Si-O-C ceramics were formed on the surface of ball-shaped Al₂O₃ particles by the pyrolysis of the silicone resin during sintering in N₂ atmosphere, which played a role in connecting the Al₂O₃ particles by forming the sintering necks. When the sintering temperatures increased from 1100 °C to 1600 °C, the formed Si-O-C ceramics still existed in the amorphous state and had no crystallization. Interestingly, the amorphous β-SiC formed at 1300 °C and its amount gradually increased with further increasing temperatures. The linear shrinkage rate of the samples varied from 0.49% to 0.73% and the weight loss rate increased from 2.01% to 10.77%. The apparent porosity remarkably varied with the range of 24.9% and 34.5%, as the bulk-density varied from 2.66 to 2.47 g/cm³. The bending strength gradually increased from 9.36 to 22.51 MPa with increasing temperatures from 1100 °C to 1500 °C, however, the bending strength remarkably decreased at 1600 °C, which was attributed to the comprehensive function of the high porosity, broken Al₂O₃ particles and weak connection between Al₂O₃ particles in the samples.     

Theoretical analysis and synthesis of Al4O4C and Al2CO phase in the resin bonded Al-Al2O3 refractory in N2-flowing

In flowing nitrogen, Al₄O₄C and Al₂CO bonded Al₂O₃-based composite was successfully prepared by a gaseous phase mass transfer pathway at 1600 °C for 3 h after an Al-AlN core-shell structure was formed in the resin bonded Al-Al₂O₃ refractory at 580 °C for 8 h. The formation mechanism of Al₄O₄C and Al₂CO phase is as follows. An Al-AlN core–shell structure is built at 580 °C for 8 h and broken at higher temperatures, and then, Al(g) reacts with C from the resin and N₂ to form Al₄C₃ and AlN, respectively. Owing to the exothermic reaction of the Al₄C₃ and AlN formation, the reaction temperature in the resin bonded Al-Al₂O₃ refractory is above the practical environmental temperature; for instance, the reaction temperature is above 1715 °C at 1600 °C in this work. Therefore, Al₄C₃ reacts with Al₂O₃ to generate Al₄O₄C and then Al₄O₄C is transformed into Al₂OC by the reaction ${\mathrm{Al}}_4{\mathrm{O}}_{\mathrm{4}}{C(s)+\text{Al}}_4{\mathrm{C}}_{\mathrm{3}}(s)\to {\mathrm{4Al}}_{\mathrm{2}}\text{OC}(s)$ at elevated temperatures. Al₂OC solid solution is finally formed through the dissolution of AlN into Al₂OC.     

Synthesis of Al2O3 nanowires by heat-treating Al4O4C in a carbon-containing environment

A novel route was developed for the synthesis of Al₂O₃ nanowires by heat-treating Al₄O₄C via Si atom doping in a furnace with a carbon heater. The nanowires had dimensions of 20–60 nm in diameter and a few hundred microns in length, and were tunable by adjusting the heating temperature. Al₂O₃ nanowires exhibited a curved, or twisted, structure and the interface of the bending part had a defective microstructure. The study of their growth mechanism indicates that the Al₂O₃ nanowires grew by a vapour–liquid–solid (VLS) growth process.     

Enhancing copper infiltration into alumina using spark plasma sintering to achieve high performance Al2O3/Cu composites

Al₂O₃/Cu (with 30 wt% of Cu) composites were prepared using a combined liquid infiltration and spark plasma sintering (SPS) method using pre-processed composite powders. Crystalline structures, morphology and physical/mechanical properties of the sintered composites were studied and compared with those obtained from similar composites prepared using a standard liquid infiltration process without any external pressure. Results showed that densities of the Al₂O₃/Cu composites prepared without applying pressure were quite low. Whereas the composites sintered using the SPS (with a high pressure during sintering in 10 min) showed dense structures, and Cu phases were homogenously infiltrated and dispersed with a network from inside the Al₂O₃ skeleton structures. Fracture toughness of Al₂O₃/Cu composites prepared without using external pressure (with a sintering time of 1.5 h) was 4.2 MPa m^1/2, whereas that using the SPS process was 6.5 MPa m^1/2. These toughness readings were increased by 18% and 82%, respectively, compared with that of pure alumina. Hardness, density and electrical resistivity of the samples prepared without pressure were 693 HV, 82.5% and 0.01 Ω m, whereas those using the SPS process were 842 HV, 99.1%, 0.002 Ω m, respectively. The enhancement in these properties using the SPS process are mainly due to the efficient pressurized infiltration of Cu phases into the network of Al₂O₃ skeleton structures, and also due to high intensity discharge plasma which produces fully densified composites in a short time.     

Facile modified synthesis and intermediate temperature electrical properties of Sn0.95Al0.05P2O7/KSn2(PO4)3 composite electrolyte

In this study, Sn_0.95Al_0.05P₂O₇ and a novel dense Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite electrolytes were synthesized. The structural characterization of X–ray diffraction (XRD) and microstructual properties of scanning electron microscopy (SEM) were carried out. The XRD results indicated that an in-situ reaction between Sn_0.95Al_0.05P₂O₇ and inorganic melt salt take place to form the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite. The intermediate temperature electrical properties were determined by using impedance spectroscopy, oxygen concentration cell and hydrogen concentration discharge cell. Finally, the H₂/O₂ fuel cell using the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ as electrolyte membrane was constructed and the obtained maximum power output densities were 67.7 mW cm^?2 and 142.1 mW cm^?2 at 650 °C and 700 °C, respectively.     

Low-temperature sintering of Ti1−xCux/3Nb2x/3O2 (x = 0.23) microwave dielectric ceramics with CuO and B2O3 addition

The influence of CuO and B₂O₃ addition on the sintering behavior, microstructure and microwave dielectric properties of Ti_1?xCu_x/3Nb_2x/3O₂ (TCN, x = 0.23) ceramic have been investigated. It was found that the addition of CuO and B₂O₃ successfully reduced the sintering temperature of TCN ceramics from 950 to 875 °C. X-ray diffraction studies showed that addition of CuO-B₂O₃ has no effect on the phase composition. The TCN ceramics with 0.5 wt% CuO-B₂O₃ addition showed a high dielectric constant of 95.63, τ_f value of + 329 ppm/°C and a good Q × f value of 8700 GHz after sintered at 875 °C for 5 h, cofirable with silver electrode.     

Crystal structure and properties of Al2O3-Cr2O3 solid solutions with different Cr2O3 contents

To enable the comprehensive application of Al₂O₃-Cr₂O₃ solid solutions, the crystal structures and properties of Al₂O₃-Cr₂O₃ solid solutions with different Cr₂O₃ contents were studied. It was observed that Al₂O₃ and Cr₂O₃ form a complete substitutional solid solution over the entire composition range at 1650 °C, with no compounds being formed. Lattice parameters “a” and “c” both increase linearly with an increase in the Cr₂O₃ content. The doping of the Cr³⁺ ions causes a more severe lattice strain in the c-axis direction. The diffraction angles of the diffraction peaks decrease in a linear manner with the increase in the Cr₂O₃ content. The relationship between the theoretical density of the solid solution and the Cr₂O₃ content could be fitted using a second-order polynomial. It was also observed that the linear expansion coefficient of the solid solutions decreases with an increase in the Cr₂O₃ content.     

Experimental phase relations between MgO-saturated magnesium-aluminate spinel (MgAl2O4) and CuOx-rich liquid

Experimental phase equilibrium data for the Cu-O-Al₂O₃-MgO system is required to improve the performance of MgAl₂O₄-containing refractories and slagging in non-ferrous smelting. In this work, the phase relations of MgAl₂O₄ in the Cu-O-Al₂O₃-MgO system were studied experimentally in air within a temperature range of 1100–1400 °C using the equilibration and quenching method. The chemical compositions of the phases in the quenched samples were determined using electron probe microanalysis (EPMA). Less than 1 wt% of Al₂O₃ or MgO were found in the oxide liquid phase, whereas the solid MgAl₂O₄ and MgO phases contained up to 23 wt% and 30 wt% of ‘Cu₂O’, respectively. Discrepancies between these results and the corresponding calculated values generated by the MTDATA 6.0 software and Mtox database Version 8.2 ranged from 3 wt% to 19 wt%. The results of this work indicate that the MgAl₂O₄ spinel is chemically stable in the presence of a CuO_x-rich liquid under the conditions studied.     

Experiments and transient finite element simulation of γ-Y2Si2O7/B2O3-Al2O3-SiO2 glass coating on porous Si3N4 substrate under thermal shock

A dense γ-Y₂Si₂O₇/B₂O₃-Al₂O₃-SiO₂ glass coating was fabricated by slurry spraying method on porous Si₃N₄ ceramic for water resistance. Thermal shock failure was recognized as one of the key failure modes for porous Si₃N₄ radome materials. In this paper, thermal shock resistance of the coated porous Si₃N₄ ceramics were investigated through rapid quenching thermal shock experiments and transient finite element analysis. Thermal shock resistance of the coating was tested at 700 °C, 800 °C, 900 °C and 1000 °C. Results showed that the cracks initiated within the coating after thermal shock from 800 °C to room temperature, thus leading to the reduction of the water resistance. Based on the finite element simulation results, thermal shock failure tended to occur in the coating layer with increasing temperature gradient, and the critical thermal shock failure temperature was measured as 872.24 °C. The results obtained from finite element analysis agree well with that from the thermal shock tests, indicating accuracy and feasibility of this numerical simulation method. Effects of thermo-physical properties for the coating material on its thermal shock resistance were also discussed. Thermal expansion coefficient of the coating material played a more decisive role in decreasing the tangent tensile stress.     

Study on energy consumption of Al2O3 coating prepared by cathode plasma electrolytic deposition

Al₂O₃ coatings with large specific surface were prepared on cast nickel-based superalloy K418 by cathode plasma electrolytic deposition (CPED) in aqueous solutions at different concentrations. The significance of energy consumption and a simple calculation method during CPED were proposed, and the influence of electrolyte concentration on current density-voltage curve, energy consumption, and microstructure of coatings were studied. It was found that increasing the concentration of electrolyte can effectively reduce the current density at the initial stage while prolonging the deposition time and stepping up the energy consumption of whole coating preparation. The morphology observation results showed that the pore size of Al₂O₃ coatings enlarges with the increase of the concentration, and the optimum electrolyte concentration is 0.5–1 mol L^?1. Under this condition, the new method of oxidation pretreatment at 950 ℃ on samples for 30 min can efficiently decrease the current density during the early stage of preparation, which is beneficial to the deposition of complex-shaped samples with large size.