BAS Glass Research Articles

Preparation of monoclinic celsian glass-ceramic by a solid-state reaction of the BaO–Al2O3–SiO2 eutectic glass, BaAl2O4 and SrAl2O4

Abstract A simple, rapid and efficient approach for the monoclinic celsian glass-ceramic fabrication is proposed here. The first step was preparing the amorphous BaO–Al 2 O 3 –SiO 2 glass powder (BAS, with a eutectic temperature of 1400 °C). The second step was fabricating glass-ceramics by the solid-state reaction of the amorphous BAS glass, BaAl 2 O 4 and SrAl 2 O 4 powders. The formation mechanism of monolithic celsian is analyzed via differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The monolithic celsian rapidly formed at 1350 °C with 1.5 h due to the presence of SrAl 2 O 4 . And the complete monolithic celsian glass-ceramic is obtained as soon as the content of SrAl 2 O 4 exceed 0.2 mol. The coefficient of thermal expansion (CTE) is determined from ambient temperature to 1200 °C, and the result indicates that with an increase in the SrAl 2 O 4 content, the CTE of glass-ceramic decreases first and then increases. And the monolithic celsian glass-ceramic containing 0.2 mol SrAl 2 O 4 has the lowest CTE of 4.927 × 10 −6 / o C (20–1200 °C). Besides, the prepared monolithic celsian glass-ceramic has an excellent phase stability at 1600 °C, which is higher than the temperature of the phase transition from monolithic into hexagonal occurring.

Effect of silica sol content on the structure and properties of MoSi2-BaO-Al2O3-SiO2 coating

ABSTRACTThe MoSi2-BaO-Al2O3-SiO2 (BAS) coating with BAS glass as binder, MoSi2 as high emissive agent and silica sol as annexing agent have been prepared on porous fibrous insulation. The addition of silica sol affects the structure and properties of the coating by promoting the fusion of it during sintering. The MoSi2-BAS coatings gradually change from a porous structure (with 0 wt%–5 wt% silica sol) into a dense structure (with 5 wt%–10 wt% silica sol) with obviously decreasing porosity when the silica sol content increases. In addition, the porous coatings show better thermal insulation property (0.18 ± 0.01 W·mK−1 thermal conductivity) and interface bonding strength (0.58 ± 0.01 MPa) than the dense coatings (0.27 ± 0.01 W·mK−1 thermal conductivity, 0.42 ± 0.02 MPa). However, the flexural strength of substrates with dense coatings (6.12 ± 0.12 MPa) is higher than that of substrates with porous coatings (4.97 ±0.17 MPa).

Effect of glass phase content on structure and properties of gradient MoSi2–BaO–Al2O3–SiO2 coating for porous fibrous insulations

The gradient MoSi2–BaO–Al2O3–SiO2 (MoSi2-BAS) coating with MoSi2 as high emissive agent, BAS glass as binder and B2O3 as annexing agent has been prepared on porous fibrous insulation, and the influence of B2O3 content on their structure and properties has been investigated. The addition amount of B2O3 affects the formation of liquid glass phase during the sintering. As the B2O3 content increases, the MoSi2-BAS coatings gradually change from a porous structure into a dense structure with obviously decreasing porosity. A desired gradient coating consisting of a porous bonding-layer and a dense surface-layer has been obtained by adding 2 wt% or 4 wt% B2O3, while the porous coating has been obtained without B2O3 and the dense coating with 6 wt% or 8 wt% of the B2O3. Then the interface bonding strength and thermal shock resistance of the MoSi2-BAS coatings were measured. The results show that the gradient coatings have much better interface bonding strength (1.4 ± 0.1 MPa) and thermal shock resistance (75 cycles without cracking) than the porous coating (0.9 ± 0.1 MPa, 60 cycles with cracking) and the dense coating (0.7 ± 0.1 MPa, 30 cycles with cracking).

Crystallization and microstructural characterization of B 2O 3–Al 2O 3–SiO 2 glass

Glass containing B 2O 3–Al 2O 3–SiO 2–Li 2O–K 2O was prepared by conventional melting and annealing technique, and converted to transparent glass–ceramics by controlled nucleation and crystallization process. The nucleation and crystallization temperatures were determined by differential thermal analysis. X-ray diffraction was used to determine the crystal phases. Scanning electron microscopy was used to study the glass–ceramics morphology, the grain size and distribution in the residual glass. Infrared spectroscopy was used to confirm the reorganization of amorphous solid. X-ray photoelectron spectroscopy was used to determine the valence state of element. The transmittance was measured by UV–Vis–NIR scanning spectrophotometer. Two main crystal phases including Al 4B 2O 9 and Al 20B 4O 36 were observed in all samples. The change in B 2O 3/Al 2O 3 ratio has little influence on the crystal types of the B 2O 3–Al 2O 3–SiO 2 glass–ceramics. B atoms existed in the form of [BO 3] and [BO 4]. Al existed in the forms of [AlO 4] and [AlO 6], and forming network together with [SiO 4] tetrahedron. The transmittance of BAS glass–ceramics reached 90% at visible and infrared wavelength, and cut-on absorption at ultraviolet and cut-off absorption at infrared wavelength were observed.

Celsian/yttrium silicate protective coating prepared by microwave sintering for C/SiC composites against oxidation

A novel celsian/yttrium silicate coating was prepared by microwave sintering on the surface of C/SiC composites. During sintering process, celsian crystallized from BaO–Al 2O 3–SiO 2 (BAS) glass in the coating and yttrium silicate was formed through the reaction of yttrium oxide and molten BAS glass. The composition and microstructure of the coatings were investigated and the efficiency of the composite coating against oxidation was characterized. The result showed the coating was dense and pore-free. The celsian/yttrium silicate coating can protect C/SiC composites for no less than 90 min at 1773 K in air.

Double layer oxidation resistant coating for carbon fiber reinforced silicon carbide matrix composites

Double layer coatings, with celsian–Y 2SiO 5 as inner layer and Y 2Si 2O 7 as outer layer, were prepared by microwave sintering on the surface of carbon fiber reinforced silicon carbide matrix composite. Both celsian, Y 2SiO 5 and Y 2Si 2O 7 were synthesized by in situ method using BAS glass, Y 2O 3 and SiO 2 as staring materials. The sintering temperature was 1500 °C, and little damage was induced to the composite. The composition and micrograph of the fired coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The oxidation and thermal shock resistance of samples with doubled-layered coating were characterized at 1400 °C in air. After 150 min oxidation and thermal cycling between 1400 °C and room temperature for 15 times, the weight loss of double layer-coated sample was 1.22% and there were no cracks in the coating.

Preparation and laser induced damage properties of rare earth doped BAS glass

Rare earth doped B 2O 3-Al 2O 3-SiO 2 glass (RE x BAS, x=5, 10, 20; RE=La, Sm) were prepared by solid state reaction method. Optical transmission spectra of such glass were characterized by ultraviolet spectrometers, and 1064 nm laser induced damage performance was investigated through the method of “1-on-1”. The results indicated that there was a strong absorptive peak near 1064 nm in Sm x BAS glass, the peak was enhanced with increasing x. While La x BAS glass was transparent to 1064 nm laser, at the same time, the results of laser induced damage showed that the anti-laser induced damage performance of such glass was strengthened with the addition of rare earth oxide. Furthermore, the laser induced damage threshold (LIDT) of Sm x BAS glass was significantly higher than that of La x BAS glass. Consequently, Sm 3+ doping was favor in the improvement of anti-laser induced damage performance for BAS glass.

Fabrication and mechanical properties of carbon short fiber reinforced barium aluminosilicate glass–ceramic matrix composites

Dense BaSi 2Al 2O 8 (BAS) and Ba 0.75Sr 0.25Si 2Al 2O 8 (BSAS) glass–ceramic matrix composites reinforced with carbon short fibers (C sf) were fabricated by hot pressing technique. The microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning and transmission electron microscopies, and three-point bend tests. The carbon fibers had a good chemical compatibility with the glass–ceramic matrices and can effectively reinforce the BAS (or BSAS) glass–ceramic because of associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects. Doping of BAS with 25 mol% SrSi 2Al 2O 8 (SAS) can accelerate the hexacelsian to celsian transformation and result in the formation of pure monoclinic celsian in C sf/BSAS composites, which can avoid the undesirable reversible hexacelsian to orthorhombic transformation at ∼300 °C and reduce the thermal expansion mismatch between the fiber and matrix.

Synthesis of 30 wt%BAS/Si 3N 4 composite by spark plasma sintering

30 wt%BAS/Si 3N 4 composite was fabricated via spark plasma sintering (SPS) process. The as-sintered specimens were subsequently heat-treated at 1800 °C to further optimize the microstructure and mechanical properties. The results show that SPS could effectively promote the reaction synthesis of BAS glass and α- to β-Si 3N 4 phase transformation, resulting in a dense 30 wt%BAS/Si 3N 4 composite with fine elongated β-Si 3N 4 grains dispersed in the continuous BAS glass matrix. Post heat-treatment could realize the complete crystallization of BAS and microstructural optimization. The obtained composite exhibits excellent mechanical properties compared to unreinforced BAS glass–ceramic. The flexural strength and fracture toughness could reach 944 MPa and 8.9 MPa m 1/2, respectively, which reach the level of conventional Si 3N 4 composites sintered by traditional sintering process. SPS followed by heat-treatment provides a feasible way to control microstructure, thus optimizes the mechanical performance of glass–ceramics matrix composites.

BAS, CMAS and CZAS glass coatings deposited by plasma spraying

In this study, three different industrial frits BaO–Al 2O 3–SiO 2 (BAS), CaO–MgO–Al 2O 3–SiO 2 (CMAS), CaO–ZrO 2–Al 2O 3–SiO 2 (CZAS) have been deposited on porcelainized stoneware tiles by plasma spraying. In the as-sprayed conditions, the microstructure of the coatings is defective because of pores, microcracks and low intersplat cohesion. Hot stage microscope and differential thermal analysis measurements made on the glass powders allowed to characterize the frits thermal behaviour. Post process thermal treatments have been arranged, following these indications as well as preliminary tests, in order to achieve the lowest porosity and the highest resistance to abrasion. At the chosen temperatures, a microstructural improvement has been induced, but in the BAS specimens, an optimal sintering has not been accomplished because of the unavoidable full overlapping of the sintering and crystallization processes.

Synthesis of silicon nitride-barium aluminosilicate self-reinforced ceramic composite by a two-step pressureless sintering

Dense 40wt%BAS/Si 3N 4 self-reinforced composite was synthesized by a two-step pressureless sintering process. The results showed that the BAS glass–ceramic not only served as an effective liquid phase sintering aid for attaining full densification and completing the α- to β-Si 3N 4 phase transformation, but also remained as a structural matrix. A bimodal microstructure could be obtained via this two-step process without the addition of special β-Si 3N 4 seeds. It is due that the first step could suppress the α- to β-Si 3N 4 phase transformation while allowing densification, and then at the higher temperature second step, the continued transformation could be used to facilitate the growth of the β-nuclei formed in the first step. The obtained composite exhibits excellent mechanical properties compared to unreinforced BAS matrix. The flexural strength and fracture toughness could reach 565 MPa and 7.4 MPa m 1/2, respectively. The crack deflection, grain bridging and pullout are considered as major toughening mechanisms in this composite.

Microstructure and mechanical properties of SiC platelet reinforced BaOAl 2O 32SiO 2 (BAS) composites

BaOAl 2O 32SiO 2 (BAS) glass–ceramic powders were prepared by sol–gel technique. SiC platelet reinforced BAS glass–ceramic matrix composites with high density and uniform microstructure were fabricated by hot-pressing. The effect of additional crystalline seeds on hexagonal to monoclinic phase transformation of Barium aluminosilicate was studied. The effects of SiC platelet content on the microstructure and mechanical properties of the composites were also investigated. The results showed that the flexural strength and fracture toughness of the BAS glass–ceramic matrix composites can be effectively improved by the addition of silicon carbide platelets. The main toughening mechanism was crack deflection, platelets' pull-out and bridging. The increased value of flexural strength is contributed to the load transition from the matrix to SiC platelets.