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Contemporary Methods of Protection and Restoration of Components
Published in E. S. Gevorkyan, M. Rucki, V. P. Nerubatskyi, W. Żurowski, Z. Siemiątkowski, D. Morozow, A. G. Kharatyan, Remanufacturing and Advanced Machining Processes for New Materials and Components, 2022
E. S. Gevorkyan, M. Rucki, V. P. Nerubatskyi, W. Żurowski, Z. Siemiątkowski, D. Morozow, A. G. Kharatyan
A distinctive family of ceramic materials has come to be known as ultra-high-temperature ceramics (UHTCs). It includes ceramic borides, carbides and nitrides of early transition metals such as Zr, Hf, Nb, and Ta, especially hafnium diboride and zirconium diboride-based compositions, characterized by high melting points, chemical inertness and relatively good oxidation resistance in extreme environments (Gasch et al., 2005). Fahrenholtz and Hilmas (2017) point out that UHTCs are often defined as compounds that have melting points above 3000°C or, in the most pragmatic way, calling UHTCs ceramic materials that can be used for extended times at temperatures above 1650°C. However, none of these definitions captures the wide range of extreme conditions in which UHTCs may be used. The strong covalent bonds between the transition metals and B, C, or N produce UHTCs not only with melting temperature, but also with high hardness and stiffness, as well as higher electrical and thermal conductivities than that of oxide ceramics. The authors emphasize that the above-mentioned intriguing combination of metal-like and ceramic-like properties allows UHTCs to survive extreme temperatures, heat fluxes, radiation levels, mechanical loads, chemical reactivities, and other conditions that are beyond the capabilities of existing structural materials (Fahrenholtz and Hilmas, 2017).
Microheaters for Gas Sensor
Published in Sunipa Roy, Chandan Kumar Sarkar, MEMS and Nanotechnology for Gas Sensors, 2017
Sunipa Roy, Chandan Kumar Sarkar
Choosing a perfect material for the microheater is a challenge for the better performance of the gas sensor. The right choice of an appropriate heating material is prevalent; it depends on so many factors like high electrical resistivity, low thermal expansion coefficient, low thermal conductivity, high Young’s modulus, low cost, easy fabrication and, most importantly, compatibility to standard silicon fabrication technology. Various authors have reported mostly platinum and polysilicon as the heating element. The journey was started with Au [17] and Al [57] as heater elements. Gradually, it was found that these materials have so many limitations like low electrical resistivity making the length of the microheater longer, there is affinity towards oxidation even at room temperature, and there are poor contact formation and electromigration effect at a high temperature. The resistivity of polysilicon [60,61] is very high, making it suitable for a better heating element, but the problem is that it cannot be deposited by electron beam deposition technique. Rather, it has to be deposited using chemical vapour deposition technique, which is a quite expensive method. Platinum is a very popular high-temperature heating element, but the material itself is quite expensive, and electrical contact formation is also critical. Moreover, after 600°C, its resistivity changes [62–64]. Moreover, as Pt has a positive temperature coefficient, it produces amplifying effects on hot spots leading to a long-term reliability of the microheater structure. Researchers are continuously rendering their effort to find out some low-cost CMOS-compatible heating elements and ultimately to find out some alloys rather than the pure metal amalgamated with lots of positive features into a single material. Recently, nickel and nickel alloy–based microheater have also been reported [65,66]; they are now utilized for their lower TCE and low thermal conductivity. Alloys like NiCr [66], Dilver P1 [33], Mo [67] and tungsten [68] with some other semiconducting materials like doped silicon [58,69], silicon carbide [34], doped II–VI metal oxides like Sb-doped SnO2 [70], titanium [71], titanium nitride [72] and hafnium diboride [73] have also been implemented as good heater elements. Some extra features like high Young’s modulus, high yield, lower coefficient of thermal expansion, corrosion resistance, humidity resistance and nonmagnetic property make them eligible to be used as a new microheater element. Molybdenum (Mo) has been reported as a microheater element due to its linear nature of electrical resistivity at very high temperature (~700°C) [67] than that of platinum. Ali et al. [68] reported novel high-temperature tungsten (W) resistive heaters on the silicon-on-insulator microhotplates. Tungsten has a tendency of oxide formation above 300°C, which is its main drawback. The use of a specific heater material is application specific depending on the requirement; therefore, there is no thumb rule for it. Instead, there is a compromise to make a good choice of a heater element.
Long-term oxidation of MoSi2-modified HfB2–SiC–Si/SiC–Si coating at 1700°C
Published in Surface Engineering, 2023
Wuqing Ding, Lei Zhou, Jiaping Zhang, Qiangang Fu
Figure 2 displays the image of HfB2–SiC–Mo/SiC (HMS/S) pre-coated samples. From Figure 2(a), the diffraction peaks of hafnium diboride, silicon carbide and molybdenum could be observed. The porous surface of the pre-coated sample, induced by the decomposition of the phenolic resin [18], is given in Figure 2(b). Three types of coloured particles (white fine particles, black particles and grey particles) are evenly distributed on it. Spots 1, 2 and 3 are identified as hafnium diboride, silicon carbide and molybdenum, respectively, obtained by the EDS analysis (Figure 2(d)). A dual-layer coating could be found from Figure 2(c), including an inner layer containing SiC with free Si (50 μm) and a HfB2–SiC–Mo upper layer (250 μm), and there are no obvious cracks or delaminations to show that the two layers are compatible with each other.
Hafnium Diboride as a saturable absorber for Q-switched lasers
Published in Journal of Modern Optics, 2019
Ahasanul Haque, Monir Morshed, Haroldo T. Hattori
We then deposited (HfB2) particles onto the reduced cladding fibre section. Hafnium diboride in powder form (particles with size below 10 µm) was purchased from Sigma Aldrich company. Since Polyvinyl alcohol (PVA) solidifies quickly, it is used to keep HfB2 particles close to the fibre core (23). As the Polyvinyl alcohol (PVA) comes in powder form, we have prepared a solution mixed in boiling water for few minutes in the fume hood. We then added the particles of HfB2 into the PVA solution. We mixed the HfB2 particle with PVA solution with the help of a pipette. The mixed solution was then placed on top of the detached cladding on a microscope glass substrate, then the sample was left in the fume hood for several hours to form a dried PVA/HfB2 material on top of the fibre core. The solidified PVA keeps HfB2 bound to the thin cladding fibre and HfB2 would be accumulated on top of the cladding free sections of the optical fibre. We demonstrated that the total absorption (global linear attenuation) is 5.12 dB for a 10-cm section of a clad less fibre.