Joining MgAl2O4 ceramics using ZnO–Al2O3–SiO2 glass ceramic with precipitated ZnAl2O4

In this work, crystallization, thermal expansion and wetting behavior of ZnO–Al₂O₃–SiO₂ (ZAS) glass were first investigated. The results showed that ZnAl₂O₄ was precipitated from ZAS glass after crystallization treatment. Crystallization increased the coefficient of thermal expansion (CTE) of ZAS glass ceramic due to the high CTE of ZnAl₂O₄. In addition, ZAS glass exhibited good wettability on the surface of MgAl₂O₄ substrate. On this basis, ZAS glass was used to join MgAl₂O₄ ceramic, and the microstructure and mechanical properties of joints obtained with different cooling methods were investigated. The flexural strength of joints was related to the content of ZnAl₂O₄ crystals in the brazing seams. Additional nucleation and crystallization treatment during cooling process improved the crystallinity of brazing seam, resulting in better matching of the CTE of brazing seam with that of MgAl₂O₄ ceramic. The maximum flexural strength of joints reached 201 MPa, which was equivalent to the strength of MgAl₂O₄ ceramic.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Effect of the use of mineral filler on the properties of concrete

Engineers of the concrete technology are increasingly concerned with the material passing through a sieve of the size under 0.149 mm. Materials called very fine aggregate or mineral filler may affect the performance of concrete in an either positive or a negative way. Discussions on aggregate containing very fine material are vitally important. Washing the aggregate residue has been the sole way to solve this matter to date. This is mainly based on the debatable opinion that materials of this kind are regarded as clay material. The goal of the study was to determine how the content of mineral filler might affect properties of concrete. Two types of aggregates with different amounts of cement and mineral filler were used. Basically, mineral filler replaced sand. The effect of applying different amounts of mineral filler on concrete was then determined. The addition of 7-10% of mineral filler to fine aggregate (0-2 mm) was found to considerably improve the properties of concrete.     

Joining of alumina using magnesium- or calcium-aluminosilicate glass–ceramic fillers

Magnesium- and calcium-aluminosilicate (MAS and CAS) glass–ceramics were used to join alumina with six different compositions. The fillers were applied onto the alumina by screen-printing, and then joining was performed slightly below and above the filler melting temperature (T_m). The evolution of various intermediate compounds upon heat treatment between the filler itself and at the joining interface was compared. MgAl₂O₄ and CaO·6Al₂O₃ was the main crystalline phase presented at the joining interface for the MAS and CAS system, respectively, while more intermediate compounds were observed when only filler was heat-treated. The formation of MgAl₂O₄ and CaO·6Al₂O₃ was attributed to the diffusion of Al ions from the alumina base, which is desirable for obtaining a sound joint due to the similar coefficient of thermal expansion to the base alumina. The maximum joint strength of 250 ± 41 and 301 ± 48 MPa was obtained for MAS and CAS filler system, respectively, after joining at T ≥ T_m due to complete interfacial wetting.     

Manufacturing of ceramic matrix composite using a hybrid process combining TiSi2 active filler infiltration and preceramic impregnation and pyrolysis

The manufacturing of silicon carbide reinforced ceramic matrix composites by a hybrid process is explored. Fibre preforms are infiltrated with TiSi₂ powders using the slurry method. Using TiSi₂ active filler leads to reduce the porosity by the subsequent formation of nitride phases after treatment under N₂ atmosphere at low temperatures (≤1100 °C). Taking into account the influence of the specific surface area of the powder on the nitridation rate, it is shown that it is possible to produce nitrides TiN and Si₃N₄ at 1100 °C with an interesting volume expansion inside the composite. To complete the densification of the composite, a polymer impregnation and pyrolysis (PIP) process are performed with a liquid polymeric precursor. Characterizations of the composites show that mechanical properties are improved with the presence of the TiN and Si₃N₄ phases, and the number of PIP cycles.     

Joining of Si3N4/Si3N4 with in-situ formed Si-Al-Yb oxynitride glasses interlayer

Si₃N₄ ceramic was successfully joined to itself with in-situ formed Yb-Si-Al oxynitride glass interlayer. The joints were composed of three parts: (I) Si₃N₄ matrix, (II) oxynitride glass interlayer in which hexagonal or fine elongated β-sialon grains and a few ball-like β-Si₃N₄ grains exist, and (III) diffusion zone in Si₃N₄ matrix containing a thin dark layer and a ~ 25?µm thick bright layer. The seam owned similar microstructure to matrix and was inosculated with the matrix as a whole. The strength of the joint tended to increase with the increase of bonding temperature and reached the value of 225?MPa, when the joints were prepared at 1600?°C for 30?min under a pressure of 1.5?MPa. The high-temperature strength remained 94.7% and 75.2% of R.T. strength when the joints were tested at 1000?°C and 1200?°C, respectively. It may be contributed to the high softening temperature of the Yb-Si-Al oxynitride glass phase formed in the seam. Even suffered to the air exposure for 10?h at 1200?°C, the residual strength of the joints was still 143?MPa, attributed to the existence of YbAG phase.     

Joining of Cf/SiC composites and Si3N4 ceramic with Y2O3–Al2O3–SiO2 glass filler for high-temperature applications

C_f/SiC composites and Si₃N₄ ceramics are candidate materials for applications in thermal protection system. This paper investigated the joining of C_f/SiC and Si₃N₄ using Y₂O₃–Al₂O₃–SiO₂ glass. The reliability of joints was evaluated by thermal shock tests. In this present work, the typical joint structure was C_f/SiC-glass-Si₃N₄. The results demonstrate that Direct bonding has been identified as the interfacial bonding mechanism at the SiC/glass and glass/Si₃N₄ interfaces. The maximum shear strength of the C_f/SiC–Si₃N₄ joint was ~34 MPa, which delivered an effective method to achieve strong, reliable bonding between the dissimilar materials. In addition, after thermal shock for 10 cycles, the residual strength remained ~13 MPa. Bubbles instead of microcracks formed in the glass filler, which was the main factor causing the degradation of the joint performance. It is suggested that improving the high temperature resistance of joining materials is the key to realize the application of this joint structure.     

Effects of temperature,time, atmosphere and sealing geometry on defects occurred in borosilicate glass-kovar alloy seal

Glass-metal sealing has wide applications, especially in chemical, optical, electronic, and energy industries. Sealing of borosilicate glass to Kovar alloy using an oxide interlayer was investigated considering the effects of sealing process variables i.e. temperature, time, and atmosphere on macroscopic soundness of seal specimens. The bubble formation tendency was decreased significantly by limiting the sealing temperature and time. In tubular butt joints, the soundness of the specimens showed an inverse correlation with the thickness of the metal tube and a direct correlation with the thickness of the glass tube. Some fracture tests were performed using tubular seals cooled with common rates (25–60 °C/min). The failures occurred inside the glass, often taking the form of a circle situated parallel to the seal interface. Structural studies based on SEM as well as EDS analysis showed that compositional segregation happened in the glass beside the interface and second phases, essentially made up of fayalite, nucleated on the interface.     

Study of chemical interaction at Al–ZERODUR interface

Effect of thermal annealing on bulk and interface properties of ZERODUR glass ceramics was investigated by Raman and X-ray spectroscopy as well as secondary ion mass spectrometry. For this purpose thin aluminium films were deposited onto the surface of ZERODUR samples prepared by mechanical polishing. Internal stresses in the bulk of investigated samples were revealed using modulation–polarization technique. The ability of chemical reactions in Al–ZERODUR system is briefly analyzed to explain experimental data. Contribution of amorphous and crystalline phases of ZERODUR to observed phenomena is discussed.     

Crystallization behavior of BaO–CaO–SiO2–B2O3 glass sealant and adjusting its thermal properties for oxygen transport membrane joining application

In this work, the thermal stability of a BaO–CaO–SiO₂–B₂O₃ glass sealant, named “H”, was investigated by differential scanning calorimetry (DSC). The crystallization behavior of glass H as the sealant matrix was investigated by a combination of experimental X-ray diffraction (XRD) analysis and thermodynamic simulation with the FactSage package. A good agreement was found between the Rietveld refinement of XRD experiments and the FactSage simulation. Particular attention was also given to the influence of the Sr₂SiO₄ filler added to the glass matrix “H” on the thermal expansion and microstructures of glass-Sr₂SiO₄ composites by means of dilatometry and scanning electron microscopy (SEM). The reinforced 20 wt% Sr₂SiO₄ composite (HS2S20) showed excellent properties and, thus, its joining performance was investigated using SrTi_0.75Fe_0.25O_3-δ (STF25) and Aluchrom as promising oxygen transport membrane (OTM) and counterpart, respectively. The joining behaviors were investigated by comparing different joining temperatures. 920 °C is the best joining temperature for HS2S20 sealant.     

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.