Effect of Copper (II) Oxide on the Microstructure and Phase Transformations of Bismuth Orthoniobate
- PDF / 943,109 Bytes
- 5 Pages / 594 x 792 pts Page_size
- 73 Downloads / 215 Views
666.3:546.562-31:544.012:544.344.016.5:544.344.015.4-17
EFFECT OF COPPER (II) OXIDE ON THE MICROSTRUCTURE AND PHASE TRANSFORMATIONS OF BISMUTH ORTHONIOBATE N. A. Zhuk,1, 4 Ya. A. Busargina,1 V. A. Belyi,2 and B. A. Makeev3 Translated from Steklo i Keramika, No. 5, pp. 14 – 19, May, 2020.
The microstructure and phase transformations of copper atom doped ceramic BiNbO4 synthesized at 950 and 1100°C were studied. According to XPA the samples in compact, pressed form crystallize in the a-BiNbO4 structure irrespective of the synthesis temperature and in spite of the phase transformation a ® b at 1040°C. The substances BiNb1–xO4–d × xCuO (x £ 0.04) are graphite-colored composites with visually expressed grain microstructure. Thermograms of the samples show near 900°C an endo effect associated with the decomposition of the copper (II) oxide and thermal effects due to reconstructive phase transformations of the type a ® g ® b in BiNbO4. It was determined that copper oxide impurity in bismuth orthoniobate ceramic acts as a heat sink, which increases the temperature of the phase transition a ® g on heating of compact samples. Key words: ceramics, phase transformations, copper oxide, thermostating effect.
The interest shown by researchers in bismuth orthoniobate BiNbO4 is due to the rich polymorphism of the compound and the discovery of its catalytic and micro- and radio wave dielectric properties [1 – 7]. Four polymorphic modifications (a, b, g, and HP) are known for bismuth orthoniobate [1 – 5, 8]. The crystalline structure of a-BiNbO4 is orthorhombic (Pnna, a = 0.567 nm, b = 1.171 nm, c = 0.498 nm) [1, 3]. b-BiNbO4 possesses a triclinic structure (P – 1, a = 0.761 nm, b = 0.553 nm, c = 0.792 nm, a = 89.9°, b = 77.4°, g = 87.2°) [2, 4]. High temperature g-BiNbO4 forms above 1040°C and is stable up to the melting temperature of bismuth orthoniobate [5]. At 1040°C a phase transition occurs from the orthorhombic a to high-temperature g modification of BiNbO4, which on cooling at 998°C transforms into b-BiNbO4 which is stable at room temperature [5, 9]. Owing to the low synthesis temperature, high permittivity (er ~ 43), and Q-factor (Qf ~ from 10 to 70 MHz) materials based on bismuth orthoniobate are promising as dielectric interlayers in monolithic capacitors and noise-suppressing filters for microwave apparatus [6, 10]. The low density 1 2 3 4
of the ceramic (85% of the theoretical value) and significant magnitude of the thermal resonance-frequency coefficient limit the practical applications of bismuth orthoniobate [6]. The optimization of the dielectric properties of bismuth orthoniobate is accomplished by chemical modification of its composition, replacing the bismuth or niobium atoms [11 – 16]. Doping by copper oxide CuO increases the density of the ceramic synthesized at 925°C (93.25% of the theoretical value, BiNBO4 + CuO (10% by weight)) owing to porosity reduction in the samples [16], the permittivity increases and reaches values er = 40 – 45 depending on the composition, and Qf = 4000 – 8000 (n = 5.2 GHz). In addi
Data Loading...
