Zezhen Yin,Yun Zhang,Shihua Ding,Tianxiu Song,Xiaoning Li,Jiaying Dan
Low permittivity LiAl 1- x Li x O 2- x ( x = 0–0.02) ceramics were prepared by conventional solid-state reaction method. XRD analysis indicated that the system remained single phase solid solution in the whole range of x examined. Li + heterovalent substitution for Al 3+ generated oxygen vacancy O vac , and enhanced the relative density from 94.33% to 97.13%. The dielectric constant ε r showed near linear variations versus sintering temperature and x value. Their relations mainly depended on the combined effects of relative density and dielectric polarizability . The quality factor Q×f was more than doubled from 15,647 GHz to 31,627 GHz, due to the packing fraction enlargement and covalent bond strengthening. The variation of temperature coefficient of resonance frequency τ f was explained from the view of oxygen bond valence. When x = 0.01, the ceramic exhibited the best microwave dielectric properties with ε r = 5.37, Q×f = 31,627 GHz and τ f = −104.2 ppm/°C when sintered at 1300 °C for 5 h.
Zezhen Yin,Yun Zhang,Shihua Ding,Tianxiu Song,Xiaoning Li,Jiaying Dan
The solid-phase method was utilized to synthesize LiAl 1- x Be x O 2- x /2 and LiAl 1- x Ga x O 2 ceramics. By Phillips-Van Vechten-Levine (P-V-L) theory, a strong correlation between chemical bond characteristics and microwave dielectric properties was established. X-ray diffraction (XRD) analysis confirmed that all components formed single-phase solid solution with tetragonal structure. Using Be 2+ and Ga 3+ as substitutes, the differences in ionic radii produced diametrically opposite changes in cell volume. The relative density increased to 97.61 % (98.57 %) with the addition of Be 2+ (Ga 3+ ). According to the P-V-L theory, the variation of ε r was influenced by bond ionization. The introduction of different ions caused bond lengths variations, which in turn affected the lattice energy fluctuations. Importantly, this change more than doubled Q×f from 16,268 GHz to 37,015 GHz. Furthermore, the evolution of τ f can be elucidated by the bond energy. Sintered at 1250 °C, both LiAl 0.96 Be 0.04 O 1.98 and LiAl 0.98 Ga 0.02 O 2 demonstrated outstanding microwave dielectric properties, characterized by ε r = 5.83–6.08, Q×f = 32,727∼37,015 GHz, and τ f = −134.8∼-112.4 ppm/°C.
The lack of cost-effective regeneration techniques for LiBH 4 limits its large-scale applications for supplying hydrogen via hydrolysis. Now, we report a facile method to regenerate LiBH 4 by ball milling its real hydrolytic byproduct (LiBO 2 · x H 2 O) with cheap Al based reducing agents, instead of traditional Mg. Effective strategies are developed to promote the formation of LiBH 4 by introducing Li 2 O to enable the dense Al 2 O 3 to convert into LiAlO 2 on Al surface and break up passivation layers, and by partially replacing Al with Al-RE alloy to improve grinding and contacting efficiency of raw materials and facilitate the conversion from initial H + in LiBO 2 ·2H 2 O to final H ¯ in LiBH 4 . A yield of 10.4 % can be obtained for a relatively short ball milling time (10 h) for the Al-Al 2 Ce-Li 2 O-LiBO 2 ·2H 2 O system. Another advantage of this method is that the utilization of H + source in LiBO 2 ·2H 2 O improves energy efficiency and reduces the cost. This work broadens the species of reducing agents for borohydride regeneration and enriches its regeneration methods.
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