Amorphous Borosilicate Research Articles

On the high temperature oxidation of MoSi2 particles with boron addition

Boron doped MoSi2 particles have been envisioned as sacrificial particles for self-healing thermal barrier coatings (TBCs) but their oxidation behaviour is yet not well understood. In this work, oxidation of MoSi2 based particle is studied in the temperature range of 1050–1200 °C. The oxidation proceeds from a transient to a steady-state oxidation stage. The kinetics during steady-state oxidation is captured with a thermal diffusion-based model. As compared to the oxidation of pure MoSi2 particles, the addition of boron strongly enhances the silica formation. Also, a finer dispersion of MoBx in the MoSi2 matrix accelerates the formation of silica. The oxide growth rate constant increases proportional with the boron content of the MoSi2 particles. This enhanced oxidation is related to the microstructure of the oxide scale. Upon oxidation, boron yields B2O3, which promptly merge with SiO2 to form amorphous borosilicate, hindering the formation of crystalline SiO2. Consequently, the migration of oxygen in the borosilicate oxide scale is faster than in the silica oxide scale on pure MoSi2 particles.

Improvement of “Pest” Resistance of MoSi2 Intermetallic Compound at 500 °C and 600 °C via Addition of B Fabricated by Spark Plasma Sintering

Mo–Si–B alloys were obtained by spark plasma sintering (SPS) using fine Mo–Si–B alloy powders with a particle size of 2.5 μm. The SPS Mo–Si–B alloys consisted of fine MoSi2/Mo2Bi5(Mo5Si3)/MoB/SiO2 phases. Under isothermal conditions, the weight-gain kinetics of the Mo–Si–B alloys obtained by SPS exhibited parabolic time dependence and these Mo–Si–B alloys showed better oxidation resistance at moderate temperatures of 500 °C and 600 °C with no “pest” failure. The oxide scale on the sample surface consisted of MoO3, amorphous borosilicate and crystalloid SiO2 phases. An increase in the oxidation temperature from 500 to 600 °C caused a significantly higher fluidity and a shorter incubation stage for the formation of a continuously protective borosilicate layer, so that the mass gain was smaller than 1 mg/cm2 for oxidation at 600 °C for 100 h, which corresponds to the excellent oxidation resistance.

Hot corrosion behaviour of Mo-62Si-5B (at. %) alloy in different molten salts at 900°C

Hot corrosion behaviour of SPS-processed Mo-62Si-5B (at. %) alloy in pure Na2SO4 and mixture of Na2SO4 + NaCl melts at 900 °C was studied. The results show that the damage process of Mo-62Si-5B alloy is deteriorated in presence of Na2SO4 and Na2SO4 + NaCl. After hot corrosion at 900 °C for 100 h, the mass changes of the alloy in Na2SO4 and mixture of Na2SO4 + NaCl are −0.24 mg/cm2 and −0.50 mg/cm2, respectively. Oxides mainly contained SiO2 and amorphous borosilicate. Hot corrosion mechanisms of the alloy in presence of Na2SO4 and mixture of Na2SO4 + NaCl deposits are discussed.

Toughening effect of multi-walled boron nitride nanotubes and their influence on the sintering behaviour of 3Y-TZP zirconia ceramics

Abstract Different amounts (0.5, 1, 2.5 and 5 wt%) of hollow “cylindrical” and “bamboo-like” boron nitride nanotubes (BNNTs) have been used to reinforce 3Y-TZP zirconia ceramics via spark plasma sintering. No significant influence of different morphologies of BNNTs on the mechanical properties at the macro-scale (elastic modulus, hardness, and fracture toughness) has been observed. The fracture toughness increased continuously with the increasing amount of the BN nanotubes up to 2.5%, resulted in the improvement of ∼100% compared to the reference ZrO 2 . A direct influence of BNNTs on the toughening of ZrO 2 has been recognized. The BNNTs strengthen the zirconia grain boundaries resulting in the alteration in fracture mode from inter- to trans-granular. The BNNTs also promoted the transformation toughening of zirconia. Their influence on the bridging and pull out has been confirmed by the investigation of the composites with the amorphous borosilicate matrix.

Effect of Si addition on mechanical properties and high temperature oxidation resistance of Ti–B–Si hard coatings

In this work the effect of Si addition on the mechanical properties and the high temperature oxidation resistance of TiB2 coatings is investigated. Nanocrystalline Ti–B–Si coatings were prepared by magnetron sputtering using co-deposition from TiB2 and Si targets. Si addition causes decrease of hardness and effective Young's elastic modulus of the coating. Annealing in high vacuum increases the content of crystalline TiB2 phase that in turn leads to enhanced hardness. High resolution thermogravimetry together with x-ray diffraction (XRD), electron probe microanalysis (EPMA), x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) is used to investigate the processes associated with exposure to air at elevated temperatures. The formation of nanocrystalline Ti–Si–O oxide scale after exposure to air at 500°C is related to B2O3 release. Increased Si content suppresses the growth of crystal phase and contributes to the high temperature oxidation resistance. At 800°C the oxidized scale formed the layered structure of globular TiO2, columnar TiO2 and amorphous borosilicate.

In-situ characterization of femtosecond laser-induced crystallization in borosilicate glass using time-resolved surface third-harmonic generation

Coherent phonon dynamics in condensed-phase medium are responsible for important material properties including thermal and electrical conductivities. We report a structural dynamics technique, time-resolved surface third-harmonic generation (TRSTHG) spectroscopy, to capture transient phonon propagation near the surface of polycrystalline CaF2 and amorphous borosilicate (BK7) glass. Our approach time-resolves the background-free, high-sensitivity third harmonic generation (THG) signal in between the two crossing near-IR pulses. Pronounced intensity quantum beats reveal the impulsively excited low-frequency Raman mode evolution on the femtosecond to picosecond timescale. After amplified laser irradiation, danburite-crystal-like structure units form at the glass surface. This versatile TRSTHG setup paves the way to mechanistically study and design advanced thermoelectrics and photovoltaics.

On the enhancement of the spark-plasma sintering kinetics of ZrB2–SiC powder mixtures subjected to high-energy co-ball-milling

The spark-plasma sintering (SPS) kinetics of ZrB 2 –SiC powder mixtures was investigated as a function of the degree of high-energy co-ball-milling and of the SiC content (5, 17.5, or 30 vol%). As in ZrB 2 without SiC, it was found that the crystal size refinement induced by the continued milling progressively enhances the SPS kinetics of ZrB 2 –SiC, again only moderately if the refinement is to the ultra-fine range, but very marked if the refinement is to the nanoscale. It was also found that the SiC addition further enhances the SPS kinetics of ZrB 2 , although the improvement did not scale directly with the SiC content, and became less relevant with the refinement of the ZrB 2 crystal sizes to the nanoscale. The improved kinetics induced by the SiC addition was identified as being due to the formation of amorphous borosilicate from the oxide passivating layers on the ZrB 2 and SiC particles. This not only speeds up the interparticle diffusion, but also it is segregated under the application of pressure into the multi-grain joints, filling pores. The enhanced kinetics induced by the progressive milling is due to the continuous reduction of the diffusion distances and to the development of a greater density of grain boundaries available as faster diffusion paths, together with the greater formation of amorphous borosilicate. Implications of interest for the ultra-high-temperature ceramics community are discussed.

Raman spectroscopy, vibrational analysis, and heating of buergerite tourmaline

Polarized Raman spectra were collected for single crystal buergerite (NaFe3Al6(BO3)3Si6O18(O0.92(OH)0.08)3F) from room temperature to near 1,375°C. Vibrational assignments to features in the room temperature spectra were determined by lattice dynamics calculations, where internal BO3 motions dominate modes near 1,300 cm−1, internal SiO4 displacements dominate modes between 900 and 1,200 cm−1, while less localized displacements within the isolated Si6O18 ring mix with motions within Na, Fe, Al, F, and BO3 environments for fundamental modes below 780 cm−1. At elevated temperatures, most buergerite Raman features broaden and shift to lower frequencies up to 900°C. Above this temperature, the lattice mode peaks evolve into broad bands, while OH stretch modes near 3,550 cm−1 disappear. According to Raman spectroscopy, X-ray diffraction, differential thermal analysis, and scanning electron microscopy, buergerite undergoes a complex transition that starts near 700°C and extends over a 310°C interval, where initially, Al and Fe probably become disordered within the Y- and Z-sites, and most F and all OH are later liberated. A reversible crystal-to-amorphous transition is seen by Raman for buergerite fragments heated as high as 930°C. Buergerite becomes permanently altered when heated to temperatures greater than 930°C; after cooling to room temperature, these altered fragments are comprised of mullite and Fe-oxide crystals suspended in an amorphous borosilicate matrix.

Oxidation of Amorphous Co75.26−xFe4.74(BSi)20+x Magnetic Alloy

The oxidation characteristics of the Co-rich amorphous magnetic alloy, $${\text{Co}}_{75.26 - x} {\text{Fe}}_{{\text{4}}{\text{.74}}} \left( {{\text{BSi}}} \right)_{20 + x}$$ were investigated. A TEM study of the microstructure revealed a complex oxidation behavior of the alloy depending on composition, especially the boron and silicon concentrations. It was determined that the critical concentration of the metalloid to be 21 at.% above, which a continuous layer of an amorphous borosilicate phase formed on the surface. Phase separation of the surface oxide was also observed when the composition is rich in boron. The metalloid (boron and silicon) concentration was critical in determining the surface-oxide morphology, which in turn, affected the subsurface microstructure. As the magnetization behavior of the Co-rich amorphous alloy depends upon the surface oxide and the internal-oxide precipitates, the guidelines are provided by which one can engineer the microstructure of the alloy to optimize the magnetic properties.

Magnetic field independent capacitance thermometers at very low temperatures

We have established the magnetic field independence up to 20 T of two capacitance thermometers based on different dielectric materials: amorphous borosilicate and the incommensurate ferroelectric (Pb 0·45Sn 0·55) 2P 2Se 6. The obtained sensitivity of the thermometers, dln C/dln T, is 8 × 10 −4 and 5 × 10 −3 for the borosilicate and the (Pb 0·45Sn 0·55) 2P 2Se 6 samples, respectively.

Thermal stability of [B]ZSM-5 molecular sieve

The thermal stability of [BJZSM-5 (boronsilicalite) has been examined by IR, XRD,11B MAS NMR and XPS techniques. [B]ZSM-5 and amorphous borosilicate were converted to α-cristobalite at high temperatures (=750C). However, Na-free amorphous borosilicate was not converted to a-cristobalite at 750C. Therefore, the presence of Na ions in [BJZSM-5 would determine whether this transition occurred or not. When the phase transition to α-cristobalite occurred, most of B atoms tetrahedrally coordinated were released from the ZSM-5 structure and migrated to the exterior surface as a boron compound having trigonal BO3 units and oxidation states of 3.