Formation,microstructure and properties of aluminum borate ceramics obtained from alumina and boric acid

The formation of aluminum borates (Al₁₈B₄O₃₃ and Al₄B₂O₉) from alumina and boron oxide occurs between 600 and 800 °C. These materials have refractory properties and corrosion resistance. The objective of this work is to develop materials from the Al₂O₃-B₂O₃ system, employing alumina and boric acid as starting powders, to study the critical processing variables and describe the developed microstructure and properties.Three formulations (13, 19.5 and 26 wt% B₂O₃) were studied. In order to confirm the formation of borates, the differential thermal analysis and thermogravimetric analysis were carried out. Afterwards, uniaxially pressed disc-shaped specimens were fired at four temperatures above the formation temperature. The textural properties of the ceramics were evaluated by the immersion method, this permit to evaluate the sintering processes. Then the degree of borate formation was confirmed by X-ray diffraction.Finally, the developed microstructures were characterized by scanning electron microscopy, and the diametral compression behavior was evaluated.A series of porous (≈50%) refractory materials from the Al₂O₃-B₂O₃ system were developed. The processing strategy resulted in materials with Al₁₈B₄O₃₃ as the main crystalline phase. Needle grains with diameters between 0.2 and 1 µm and an aspect ratio over 20:1 were obtained. Thus, based on the information gathered from our research, aluminum borate ceramic materials can be designed for structural, insulating or filtering applications employing only alumina and boric acid as boron oxide source.     

Microstructural evolution and kinetics analysis of aluminum borate ceramics via solid-state reaction synthesis

Aluminum borate ceramics (ABCs) with a skeleton structure of rod-like crystals were established via a solid-state reaction synthesis. The influence of heat treatment systems on the phase composition and microstructural evolution of an anisotropic grain were firstly studied. Next, the impact of boric acid content on the activation energy of reaction, microstructure, mechanical properties, and neutron shielding capability were investigated. The results demonstrate that orthorhombic Al₁₈B₄O₃₃ formed between 900°C and 1000°C which was the stable phase during heating treatment. In addition, increasing the boric acid content was favorable for reducing the activation energy of forming aluminum borate, resulting in increased in situ formation and growth of aluminum borate whiskers in fired samples. Next, mircropores could be generated by the decomposition of boric acid at high temperatures, which had a considerable influence on the physical properties of the sample. Furthermore, the Monte Carlo particle transport program simulation and neutron shielding experiments showed that the prepared ABCs could shield neutron effectively manifested as possessing a high neutron absorption cross-section in the case of porous materials. Moreover, ABCs have potential application value as thermal insulation material in the nuclear energy field.     

Novel aluminum borate foams with controllable structures as exquisite high-temperature thermal insulators

In this contribution, a manageable foam-casting technique for the preparation of novel aluminum borate foams (ABFs) as thermal insulators with highly controllable performances is presented. ABFs were fabricated from α-Al₂O₃ and 2Al₂O₃·B₂O₃ with the addition of various amounts of foaming agents, thickening agents, and slurry solid contents. The dispersions and rheological properties of the slurries were then examined, followed by exploration of microstructural evolution and testing of mechanical/thermal properties. It should be noted that the generated micro-pores generated and interlocking rod-like 9Al₂O₃·2B₂O₃ crystals may lead to superior mechanical tolerances and lower thermal conductivities for the ABFs. In general, the as-prepared ABFs with porosities ranging from 73.8 to 96.3 vol%, compressive strengths of 8.20–0.15 MPa, and thermal conductivities of 0.228–0.046 W/(mK) (200–800 °C) could render them suitable for application as high-temperature thermal insulating materials.     

Mechanical behavior and microstructure of porous needle: Aluminum borate (Al18B4O33) and Al2O3-Al18B4O33 composites

In this article we assess and compare the complex mechanical behavior of two complex microstructure ceramics material formed within the reaction sintering frameworkTwo comparable pairs of materials with respectively similar microstructures were obtained by reaction sintering from boric acid and alumina. Two single phase porous ceramics were compared with two composite (1:1) porous ceramic. The first and second phases were aluminum borate needles (Al₁₈B₄O₃₃) and alumina (Al₂O₃).The four with comparable grain size and analogous apparent porosities: in diameter (≈ 0.7 µm) and in volume fraction (≈ 45%). The mechanical behavior was studied by means of the diametral compression test at low displacement rate and explained in terms of the texture, microstructure features evaluated by mercury intrusion porosimetry and scanning electron microscopy.Single Al₁₈B₄O₃₃ phase porous materials presented higher mechanical strengths than the composite materials. Within the respective microstructural configurations the whisker thickness did not affect significantly the mechanical behavior and parameters. A well-defined fragile behavior was observed and described in the composite material. On the other hand the single Al₁₈B₄O₃₃ needle porous material presented a distinctive behavior with local discontinuities without loss of integrity in the diametral stress behavior, and achieved strength up to 50% higher than the corresponding composite.     

Design and development of ceramic-based composites with tailored properties for cutting tool inserts

A successful approach to the development of tailored cutting tool materials requires the development of innovative concepts at each step of manufacturing, from the material design, synthesis of composite powders, to their processing and sintering. In this paper, a computational design approach is applied in the development of reinforced ceramic-based cutting tool inserts with tailored structural and thermal properties. Several potential filler materials are considered at the material design stage for the improvement of structural and thermal properties of a selected matrix material. Properties, such as an improved thermal conductivity and reduced coefficient of thermal expansion are essential for an effective cutting tool insert to absorb thermal shock at varying temperatures. In addition, structural properties such as elastic modulus have to be maintained within a moderate range. A mean-field homogenization theory and effective medium approximation using an in-house code are applied for predicting potential optimum structural and thermal properties for the required application. This is done by considering the effect of inclusions as a function of volume fraction and particle size in the ceramic base matrix. Single inclusion composites such as alumina-silicon (Al₂O₃-SiC) and alumina-cubic boron nitride (Al₂O₃-cBN) as well as hybrid composite such as alumina-silicon-cubic boron nitride (Al₂O₃-SiC-cBN) are developed using the Spark Plasma Sintering (SPS) process in line with the designed range of filler size and volume fraction to validate the computational results. It is found that the computational material design approach is precise enough in predicting the target properties of a designed hybrid composite material for cutting tool inserts.     

A lithium aluminium borate composite microwave dielectric ceramic with low permittivity,near-zero shrinkage,and low sintering temperature

A low temperature co-fired dielectric material with low shrinkage during the sintering process can enhance the circuit design of electronic devices. Lithium aluminium borate composite ceramic with a composition of Li₂O:Al₂O₃:B₂O₃ = 1:1:2 (abbreviated: LAB) was prepared by a traditional solid-state reaction method. These ceramics have a low sintering temperature (675–750 °C), low permittivity, and near-zero shrinkage. When the sintering temperature was 725 °C, the LAB ceramics exhibited a small shrinkage of ?2.4% and the best microwave dielectric properties with ε_r = 3.9, Q × f = 35 500 GHz, and τ_?= ?64 ppm/°C. The LAB ceramics sintered at 700 °C have near-zero shrinkage of ? 0.4% and good microwave dielectric properties. The ceramics transformed from (Li₂B₄O₇ and Al₂O₃) to (Li₂Al₂B₄O₁₀ and Li₄Al₄B₆O₁₇) phases with increasing the sintering temperature, which may be the reason why they show marginal shrinkage. In addition, the ceramics could be co-fired with Ag, indicating that this material is a good candidate for low-temperature co-fired ceramic devices.     

Exploring the potential of the mechanical/thermal properties and co-shielding ability of Bi2O3-doped aluminum borate ceramics against neutron/gamma radiation

The efficient optimisation of radiation shielding materials (RSMs), which protect people from potential radiant threats, is highly desirable; however, it remains challenging. This study addresses the low-cost fabrication of the ceramic-based RSMs, aluminium borate-based ceramics using Bi₂O₃ as a novel simultaneous shielding agent and sintering promoter. The phase compositions, microstructures, sintering kinetics, and performances of the as-prepared Bi₂O₃ doped aluminium borate ceramics (BDABCs) are systematically researched. Finally, co-shielding tests for neutron and gamma radiation are performed. The results demonstrate that Bi₂O₃ can positively influence the sintering densification process of BDABCs via the evident reduction in the sintering activation energy. The migration of the Bi₂O₃–B₂O₃ liquid phase affects the pore structure, crystal morphology, and thermal conductivity of the samples. The obtained BDABCs exhibited highly reliable mechanical properties with a maximum elastic modulus and modulus of rupture of 124.3 GPa and 54.9 MPa, respectively; controllable thermal conductivity from 1.32 to 6.16 W m^?1 K^?1; and 12 wt% Bi₂O₃-doped sample (1400 °C × 3 h, 1.5 cm) shows the best radiation shielding performance, including 58.6% neutron and 26.6% γ rays. The obtained results manifest the enormous potential of BDABCs as structural materials and functional RSMs.     

Physical,thermal, optical,structural and nuclear radiation shielding properties of Sm2O3 reinforced borotellurite glasses

To observe direct effect of samarium (III) oxide reinforcement on physical, thermal, optical, structural and nuclear radiation attenuation properties, a broad-range experimental and numerical investigations were performed with a group of novel borotellurite glasses. FTIR spectra of powdered samples were taken at 250-4000 cm^-1. The transmittance and absorption characteristics, optical band gaps, and Urbach energies were measured. The glass transition temperatures, crystallization temperatures and melting temperature values of the samples were determined. Nuclear radiation shielding properties have been determined for gamma-ray, neutrons and heavy charged particles. The lowest transmittance and highest absorbance were reported for the TBVS1.5 sample with highest Sm₂O₃ additive. In addition, obtained results from the nuclear radiation shielding calculations have showed that TBVS1.5 sample has superior nuclear radiation shielding properties against gamma-ray, neutron and heavy charged particles. The increasing Sm2O3 additive has visibly improved the nuclear radiation attenuation properties by keeping other material properties within usable limits.     

Aluminum borate whisker-based lattices with a hierarchical pore structure obtained via digital light processing

To satisfy the ever-increasing demand for aluminum borate porous ceramics with complex shapes and tunable pore structures in a diverse set of fields, aluminum borate whisker-based lattices with hierarchical pore structures were fabricated by a combination of in situ reaction and digital light processing three-dimensional (3D) printing. The optimal dispersant concentration and exposure parameters for 3D printing were determined based on analyses of the rheological properties and working curves of the Al₂O₃–B₂O₃ photosensitive slurries. The effects of the B₂O₃/Al₂O₃ molar ratio on the morphology and properties of aluminum borate lattices were investigated. The results showed that the addition of an excess of B₂O₃ was beneficial to the growth of aluminum borate whiskers. When the B₂O₃/Al₂O₃ molar ratio was set to 6:9, the resultant aluminum borate lattices exhibited a typical hierarchical pore structure, including inherent large pores in the lattices and small pores formed by interlocked aluminum borate whiskers generated in situ within the struts. This unique hierarchical pore structure endowed the ceramic lattices with a high compressive strength (1.18 MPa) and porosity (82.58%), as well as non-brittle fracture characteristics. Owing to these outstanding properties, aluminum borate whisker-based lattices are promising candidates for high-temperature thermal insulation, catalyst supports, acoustic absorption, and particle filtration.     

Design and preparation of BaO–Al2O3–SiO2–B2O3/Quartz LTCC composites with tailored coefficient of thermal expansion

In this work, by focusing on widespread problem of thermal mismatch caused by different coefficients of thermal expansion (CTE) in electronic packaging materials, low-temperature co-fired ceramic (LTCC) materials with tunable CTE values were designed. By substituting Ba²⁺ with Sr²⁺ and replacing quartz with alumina and zirconia, respectively, BaO–Al₂O₃–SiO₂–B₂O₃/quartz LTCC composites with CTE of 7.05–9.52 × 10^?6/°C were developed. Results show that major crystalline phases of LTCC composite materials are quartz and hexacelsian. By replacing quartz with alumina or zirconia, sintering behavior and subsequently thermal expansion and dielectric properties were modulated. On the other hand, substituting Ba²⁺ with Sr²⁺ can be beneficial to the densification of composite materials. The introduction of Sr²⁺ triggered mixed alkali effect and hindered the crystallization of hexacelsian phase, which can further improve mechanical properties. Finally, sandwich structure module of BaO–Al₂O₃–SiO₂–B₂O₃/quartz with gradient CTE values was obtained, which showed potential for electronic packaging with sustained thermal compatibility under cyclic temperatures.     

Microstructural differences between electrospun alumina borate nanofibers prepared by solutions with different PVP contents

Alumina borate nanofibers were fabricated by electrospinning combined with sol-gel method using aluminum acetate (Al(OH)₂(OOCCH₃)·1/3H₃BO₃, stabilized with boric acid) and polyvinyl pyrrolidone (PVP) as raw materials. As-spun composite nanofibers were electrospun from the spinning solutions prepared with different PVP contents. The as-spun nanofibers were calcined at 1000 ℃ and the microstructures of the calcined nanofibers were investigated. The results showed that with the content of PVP increased, the diameters of the alumina borate nanofibers increased, and the temperatures at which the Al₄B₂O₉ phase formed and Al₄B₂O₉ transformed to Al₁₈B₄O₃₃ also increased. However, the crystallinity of the calcined nanofibers decreased with the increase of the PVP content. The grains became smaller and more uniform due to the increasing amount of voids and cracks generated by the decomposition of PVP.     

In-situ elongated aluminium borate grains toughening WC- 1.87 wt % Al2O3- 4.13 wt % ZrO2 composite via spark plasma sintering

In this work, a novel way was developed to facilitate the sintering of a binderless cemented carbide with Al₂O₃ contain while improving its toughness. WC- 1.87 wt % Al₂O₃- 4.13 wt %ZrO₂ cemented carbides with 1 wt % B₂O₃ as additives were consolidated by high energy ball-milling and spark plasma sintering the as-milled composite powders. The effects of 1 wt % B₂O₃ content on the sintering behaviour, microstructure and mechanical properties of the obtained cemented carbides were investigated. The presence of B₂O₃ significantly lowers the sintering temperature by forming liquid phase and react with Al₂O₃ which contributed to obtaining fully dense specimens at 1350 °C and maintains fine grain sizes of WC until the temperature exceeding 1450 °C. The sintering temperature of the specimens with optimum mechanical properties has been also reduced comparing that of the original WC- 1.87 wt % Al₂O₃- 4.13 wt %ZrO₂ cemented carbides. Furthermore, the addition of B₂O₃ triggered the reaction between B₂O₃ and Al₂O₃ resulting in forming in-situ elongated aluminium borate grains (A₄B₂O₉ and A₁₈B₄O₃₃ whiskers), which promoted the toughness. The specimens sintered at 1450 °C exhibited optimal mechanical properties: the Vickers hardness and fracture toughness were 19.26 GPa and 11.49 MPa m^1/2, respectively.     

Fabrication of rigid media tube filter bound with aluminum borate using Al2O3–B2O3–MgO frit for filtration of molten aluminum

We performed a study of a tube filter binding agent for molten aluminum (rigid media tube filter, RMF). In this study, we examined the formation mechanism of aluminum borate (9Al₂O₃·2B₂O₃; 9A2B) from Al₂O₃–B₂O₃–MgO frit composition, which used MgO instead of CaO in the Al₂O₃–B₂O₃–CaO frit in our previous report. The results indicated that crystallization of 9A2B is easier in Al₂O₃–B₂O₃–MgO compared to Al₂O₃–B₂O₃–CaO, in that crystals are formed with only a heating process, and the amount of crystallization increases depending on the calcination temperature and the heat load over the retention time. A tube filter made of Al₂O₃–B₂O₃–MgO frit contained a larger amount of 9A2B crystals and therefore its strength did not deteriorate until a very high temperature of 1200 °C was reached, and its coefficient of thermal expansion was low. This is an advantageous property for RMF used under constraints and high temperatures. In addition, when the amount of 9A2B crystal formation increases, the wettability of molten aluminum decreases, and it becomes more difficult for molten aluminum to impregnate the filter material, but we found that corrosion resistance was high.     

Growth behavior of aluminum borate whiskers on zirconia toughened alumina (ZTA) particle surface

In this work, B₄C-covered zirconia-toughened alumina (ZTA) particles are prepared and oxidised at 1050 °C for different times (0, 2, 4, and 8 h) in air. The X-ray diffraction and electron probe micro-analysis results show that the covering layer is mainly composed of oxide B₂O₃ intermetallics, residual B₄C particles, and Al₁₈B₄O₃₃ whiskers. The scanning electron microscopy results show that the growth of Al₁₈B₄O₃₃ whiskers on the ZTA particles enhances with increasing heat preservation time; the optimum holding time is determined to be 8 h Al₂O₃ in the ZTA particles diffuse into the covering layer and combine with B₂O₃ to form Al₁₈B₄O₃₃ whiskers; the Al₁₈B₄O₃₃ whiskers grow via the liquid-solid mechanism.     

Thermal stress durability and optical characteristics of a promising solar air receiver based black alumina ceramics

In solar thermal technology, solar receiver materials with high solar absorptivity and photocatalytic efficiency have recently become a mandatory requirement for promoting and maximizing the released thermal and electrical energy. This paper presents a detailed study of the optical performance and thermal stress durability of the promising solar receiver material, black Al₂O₃/CuO ceramics. Different Al₂O₃/CuO ceramics with different CuO content (10–40 wt%) were obtained by the pressureless sintering method. Optical properties such as solar absorbance and reflectance, band gap energy and photoluminescence are inclusively investigated. The thermal stress resistance of the obtained ceramic receivers is simulated using the finite element modeling (FEM) method at different temperatures. Results indicated that adding and increasing the content of CuO to Al₂O₃ has a significant role in transforming alumina from a non-solar light-absorbed material to a solar absorber material with high absorptivity in the Ultraviolet–Visible-Near Infrared (UV-VIS-NIR) spectrum. Composite with 40 wt% CuO has recorded the maximum absorbance of 75% in the visible light region. Moreover, Al₂O₃/CuO ceramics gave multiple graded band-gaps in the range of (1.6–5 eV). Composites with high wt% of CuO have the most increased photocatalytic activity and light emissivity efficiency. Thermal stress analysis of the different ceramics showed outstanding stress durability with uniform heat distribution. Hence, black Al₂O₃/CuO ceramics can be considered ideal solar absorber materials with high sustainability and durability at high temperatures.     

High temperature properties of B6O-materials

B₆O is a potential superhard material with a hardness of 45 GPa measured on single crystals. Recently it was found that different oxides can be utilized as an effective sintering additive which allows densification under low pressures. In this work the effect of addition of Y₂O₃/Al₂O₃ on high temperature properties was investigated using impulse excitation technique (IET), hardness measurements and dilatometric measurements. The IET technique reveals the softening of the residual B₂O₃ in the materials without additives at 450 °C; in the materials with Y₂O₃/Al₂O₃ the softening is observed at only about 800 °C. This data agrees with the values found for different borate glasses.The materials showed no pronounced reduction of hardness at these temperatures. This is additional evidence, supporting previous observations that the material consists of pure grain boundaries between B₆O grains. Hardness values (HV5) of up to 17 GPa at 1000 °C were observed.     

Composition Dependence of Thermal Expansion and Glass Transition Temperature for Barium Aluminoborate Glasses

The glass transition temperatures and the thermal expansion coefficients below and above the glass transition range are determined for barium aluminoborate glasses over a wide range of compositions containing up to 60 mol % BaO and 35 mol % Al₂O₃. The behavior of these characteristics is studied depending on the Al₂O₃ concentration at constant BaO/B₂O₃ ratios (from 0.2 to 1) and upon replacement of B₂O₃ by BaO at constant Al₂O₃ contents (up to 30 mol %). The influence of composition on the properties is interpreted within the concepts of the barium aluminoborate glass structure.     

Al2O3–B4C–Al Composite Material Systems via Pressureless Infiltration Methods

Modification of the Al₂O₃–Al system's chemistry via the addition of B₄C is described and is shown to result in fully dense structures via wetting techniques at high temperatures, without the need for pressure-assisted infiltration. The relationships between the surface area of boron carbide and alumina powders, the effectiveness of infiltration, the material chemistry following infiltration, and the resulting mechanical properties of Al₂O₃–B₄C–Al composites are evaluated. Additional approaches, including the incorporation of aluminum metal powder as an additional wetting agent before infiltration, are described in conjunction with a variation of both the surface areas and the volumetric ratios of inert Al₂O₃ to reactive B₄C phases. These methods can provide the means to achieve low-cost metal matrix composites in both vacuum and argon infiltration environments, and represent an approach that enables the generation of articles with complex geometries, requiring minimal secondary finishing treatment.     

Thermal and neutron shielding properties of <Superscript>10</Superscript>B<Subscript>2</Subscript>O<Subscript>3</Subscript>/polyimide hybrid materials

In this study, ¹⁰B₂O₃/polyimide (PI) hybrid materials were synthesized with the aim to improve their thermal stability and neutron shielding properties. 3,3′-Diaminodiphenyl sulfone (DADPS) reacted with 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) in N-methyl-2-pyrrolidone (NMP) and mixed with amine functionalized ¹⁰B₂O₃ to prepare a series of poly (amic acid), meanwhile, corresponding PIs were obtained via the thermal imidization procedures. The morphologies and structures of the prepared hybrid materials were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The thermooxidative and flame retardancy properties of the PI films were examined by thermogravimetric analysis (TGA) and limiting oxygen ındex (LOI). The experimental results showed that as the amount of functionalized ¹⁰B₂O₃ was increased, flame retardant properties of the hybrid films were increased. Hybrid materials were also irradiated with thermal neutrons. The neutron shielding properties increasing depends on the amount and the distribution of the ¹⁰B isotope.     

Thermal Expansion and Glass Transition Temperature of Calcium Borate and Calcium Aluminoborate Glasses

The thermal expansion (the temperature coefficients of linear expansion for solid glasses and the structural temperature coefficients of linear expansion) and the glass transition temperatures are studied for glasses in the CaO–B₂O₃ and CaO–B₂O₃–Al₂O₃ systems. The results obtained are compared with the available data for barium borate and barium aluminoborate glasses. The revealed dependences are interpreted within the concepts of the borate glass structure.