Yun Zhang,Xi Jiang,Xinrui Guo,Shihua Ding,Tianxiu Song
The Mg 1- x Li x ZrTa 2 O 8- x /2 ( x = 0–0.07) ceramics were fabricated to investigate the effects of Li + heterovalent substitution for Mg 2+ . The site occupancy and existence of the Li + were examined based on Vienna Ab-initio Simulation Package (VASP) and X-ray photoelectron spectroscopy (XPS), respectively. Single phase solid solution with wolframite structure was identified in the whole region. Li + substitution reduced the densification temperature from 1475 °C to 1400 °C. Notably, the Q × f exhibited an obvious increase by up to 74 % from 41,844 GHz at x = 0–72,782 GHz at x = 0.05. The Q × f for x < 0.01 was largely dependent on the intrinsic loss originating from packing fraction, while as 0.01 ≤ x ≤ 0.05, it was dominated by extrinsic factors, especially the oxygen vacancy concentration. The corresponding dielectric constant ε r decreased in term of ionic polarizability. The variations of resonant frequency temperature coefficient τ f were explained from the views of [TaO 6 ] oxygen octahedral distortion and bond energy. A near-zero τ f was obtained for x = 0.05 composition, accompanied by ε r = 11.97, Q×f = 72,782 GH at 1450 °C. Li substitution was effective and inexpensive for MgZrTa 2 O 8 ceramic to realize dielectric properties enhancement and densification temperature optimization simultaneously.
Herein, Mg 1− x Ni x ZrTa 2 O 8 ( x = 0, 0.1, 0.3, 0.5, 0.8, 1) ceramics are synthesized using a solid-phase reaction method. Ni 2+ replacing Mg 2+ improves the drawback of high sintering temperature and poor temperature stability. The samples have a relative density above 95% at the optimal sintering temperature. The sintering temperature of Mg 1− x Ni x ZrTa 2 O 8 reduces from 1475 to 1400 °C. And single-phase Mg 1− x Ni x ZrTa 2 O 8 ceramics with monoclinic wolframite structures are analyzed. According to the Rietveld refinement results, with the doping of Ni 2+ , the transformation of unit cell volume leads to the shift of the main peak in the X-Ray diffraction pattern to a higher angle in 0 ≤ x ≤ 0.3 and a lower angle in 0.5 ≤ x ≤ 1. The ε r of Mg 1− x Ni x ZrTa 2 O 8 is comparable to MgZrTa 2 O 8 . At 1400 °C, the Q × f is increased from 10 598 GHz in the undoped sample to 43 041 GHz in Mg 0.9 Ni 0.1 ZrTa 2 O 8 ceramic. The absolute values of τ f of ceramic samples decrease with the excessive Ni 2+ . The desirable dielectric properties ( ε r = 20.79, Q × f = 43 041 GHz, τ f = −33.04 ppm °C −1 ) are obtained at 1400 °C. Ni substitution is effective for MgZrTa 2 O 8 ceramic to optimize densification temperature and retain the high Q × f value.
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