Substitution clustering in a non-stoichiometric celsian synthesized by the thermal transformation of barium exchanged zeolite X

The thermal transformation of Ba exchanged zeolite X to celsian has been studied by ²⁷Al and ²⁹Si MAS NMR spectroscopy. Evidence for the degradation of the zeolite framework is present in the ²⁹Si NMR spectra after thermal treatment at 850 °C. Confirmation is provided by the ²⁹Si NMR data that synthesis of celsian via the decomposition of Ba exchanged zeolite leads to a single defect phase. Clustering of the isomorphous replacement of aluminium by silicon must occur to explain the observed ²⁹Si chemical shifts. The ²⁷Al NMR data show distorted aluminium co-ordination sites upon the thermal transformation of Ba exchanged zeolite X. The distortions present in the amorphous matrix are greater than those present in the monoclinic and hexagonal crystalline phases of celsian.     

Li2O-Na2O-B2O3-SiO2烧结助剂对BaAl2Si2O8陶瓷结构与介电性能的影响

采用传统固相反应工艺,按化学计量比合成BaO-Al_2O3-SiO_2(BAS)-x%(w/w) Li_2O-Na_2O-B_2O3-SiO_2(LNBS)(x=0,1,2,3,4)陶瓷。研究不同LNBS烧结助剂添加量对BAS系微波介质陶瓷的结构和介电性能的影响。通过Clausius-Mossotti公式计算讨论了BAS理论与实验介电常数(εr)的差异。研究结果表明:LNBS烧结助剂中Li+进入钡长石Al3+位或单四元环(S4R)间隙,并产生了O_2-空位或Ba2+空位,从而促进BAS六方相向单斜相的转变。添加适量的LNBS烧结助剂后,BAS陶瓷的烧结温度从1 400℃降低到1 325℃,同时BAS陶瓷样品密度、品质因数(Qf)值以及频率温度系数(τf)得到改善。当x=1,烧结温度为1 325℃时,可获得综合性能相对较好的BAS陶瓷,其介电性能:Qf=35 199 GHz,εr=6.37,τf=-1.613×10-5℃-1。     

Effect of interlayer cations on high-temperature phases of vermiculite

Static and dynamic heating of vermiculite samples from Santa Olalla, Huelva, Spain, saturated with different cations, i.e. Na⁺, Cs⁺, NH₄ ⁺, Mg²⁺, Ca²⁺, Ba²⁺ and Al³⁺, have been studied. The characterization of the phases formed during heating has been carried out by X-ray diffraction. The phases formed depend on the cation present in the interlamellar position and the heating process. The phases identified in the vermiculite samples saturated with different cations and heated at different temperatures are the following: enstatite, forsterite, spinel, cordierite, anorthite, pollucite, nepheline, coesite, celsian and others various mixed silicates; also some dehydrated and amorphous phases have been observed. On static heating, at the maximum temperature reached in this work, the phases formed appear mixed with a glassy phase.     

Synthesis of monoclinic celsian from seeded alkoxide gels

Celsian gels seeded with fine monoclinic celsian seeds were prepared by an all-alkoxide gel route and the effect of seeding on crystallization of the gel-derived glass examined. Gels seeded with 10 wt.% of fine crystals sintered to greater than 93% and 95% density in the form of cold pressed gel powder pellets and as bulk gel respectively, when sintered at temperatures below 1050°C. XRD analysis indicated that seed facilitated the formation of monoclinic celsian in the densified celsian glass, and the amount of monoclinic celsian formed increased with increasing seed content. 92% of the glass crystallized into monoclinic celsian when 10% of 3.2 m seeds were introduced. Transmission Electron Microscopy revealed epitaxial growth of monoclinic celsian from the crystal seeds but homogeneous nucleation and growth of hexacelsian.