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Capacitance-Based Humidity Sensors
Published in Ghenadii Korotcenkov, Handbook of Humidity Measurement, 2019
Extremely high sensitivity was also achieved for capacitive humidity sensors based on bismuth phosphates, such as cubic sillenite and monoclinic types synthesized through a hydrothermal method (see Figure 10.32). These sensors were investigated by Sheng et al. (2012). The sillenite family encompasses a variety of compounds having the general formula Bi12(Bi4/5−nxM5xn+)O19.2+nx (M = M2+, M3+, M4+ and M5+ [only V, As, and P]) in common. Cubic bismuth phosphate has mostly been accessed with solid-state reactions in the Bi2O3/BiPO4 system. It was established that sensors with the cubic bismuth phosphate structure revealed linear capacitance variations of four orders, namely from 1.1 to 12,908 pF over the RH range from 30% to 95%, while the monoclinic bismuth phosphate sample showed only three orders of magnitude change from 1.2 to 1,097 pF in a nonlinear fashion. Sheng et al. (2012) assumed that structurally flexible, sillenite-related cubic bismuth phosphate is a promising humidity sensor material due to a polarizable oxide framework of Bi3+ cations with neighboring oxygen atoms that can easily form hydrophilic OH− bonds and adsorb water molecules efficiently. Moreover, the high amount of polarizable Bi3+ in the sillenite structure enhances the number of active sites for the surface adsorption of water in comparison with monoclinic BiPO4.
Evaluation of mechanical properties of Bi12SiO20 sillenite using first principles and nanoindentation
Published in Philosophical Magazine, 2021
M. Isik, G. Surucu, A. Gencer, N.M. Gasanly
Fundamental properties and applicability to the device technology have made sillenite Bi12MO20 (where M is Si, Ge, Ti) materials important and investigations on their promising device characteristics have gained great momentum especially in solar energy and optoelectronics [1–6]. Photochromic, photorefractive, photoluminescent, photoconductive, electro-optic, piezoelectric and magneto-optic properties of the sillenites are employed in sensors, optical memories, optical detectors, holography and modulators [7–11]. Bi12SiO20 (BSO) is a member of this family and is a composition of Bi2O3 and SiO2 compounds with proper combination. BSO single crystals presenting properties of space group of I23 form a cubic structure with a lattice parameter a = 10.107 Å [12,13]. It is known as a wide band gap semiconductor material in which band-to-band transition takes place at 3.25 eV [14]. BSO can be used in applications such as optical data processing, image correction, real-time intensity inversion, light modulator, laser light control etc. [15–19]. BSO materials can be obtained in different forms: single crystals, ceramics, thin films, nanocrystalline powders etc. [14,20–22]. Single crystals of BSO were grown by mostly preferred techniques such as Czochralski, sol–gel process, laser-heated pedestal growth, Bridgman and hydrothermal crystallisation. BSO single crystals grown by the Czochralski method were investigated using Raman and Fourier transform infrared spectroscopy [23]. There were 19 Raman modes and 5 IR modes for the crystals. Temperature-dependent mobility of photo-excited electrons in BSO crystals was investigated in the range of 100–500 K [19]. It indicated that the mobility increased from 1.7 cm2 V–1 s–1 to 10 cm2 V–1 s–1 , but the temperature decreased from 500 K to 190 K. Photocatalytic activity of BSO film was explored by depositing the as-prepared BSO film on SiO2 photonic crystal in Ref. [22]. The resulting material exhibited better photocatalytic activity for the degradation of Rhodamine B under ultraviolet radiation than the as-prepared BSO film. Spectroscopic ellipsometry studies were done for BSO crystals in Ref. [14]. Spectral dependencies of dielectric function, refractive index and extinction coefficient were investigated and critical points were determined in the study. Refractive index changed between 2.37 and 2.58 eV in the energy region below band gap. Extinction coefficient varied between 0.16 and 1.34 in the energy region of 1.2–6 eV. Critical points were 3.54, 4.02, 4.82 and 5.58 eV.