Phase relations relevant to hexagonal barium aluminates

Phase relations in the system Al₂O₃BaAl₂O₄ were established for the temperatures higher than 1600°C. In the place of conventionally referred “barium hexaaluminate” were confirmed to exist two distinctive compounds for which chemical compositions were determined. Directional solidification experiments as well as vaporization loss measurements were carried out for construction of a correct phase diagram to the best of the available knowledge.     

Combustion synthesis of fine-particle metal aluminates

Fine-particle metal aluminates, MAl₂O₄ where M=Mg, Ca, Sr, Ba, Mn, Co, Ni, Cu and Zn as well as 3CaO · Al₂O₃ (C₃A), CaAl₁₂O₁₉ (CA₆) and MgCeAl₁₁O₁₉ have been prepared by the combustion of mixtures of the respective metal nitrates (oxidizers) and urea or carbohydrazide (fuels) at 500 or 350 C, respectively, over a time of 5 min. The solid combustion products were identified by their characteristic X-ray powder diffraction patterns. The fine-particle nature of these metal aluminates was investigated using SEM, TEM, particle size analysis and surface area measurements. The surface areas of the as-prepared metal aluminates using carbohydrazide fuel were higher (45 to 85 m² g^–1) compared with urea (1 to 20 m² g^–1).     

Sur des gallates et aluminates mixtes de lanthanides

Solid solutions between different perovskite-type compounds A⁺⁺⁺ B⁺⁺⁺O₃ is described. An immiscibility gap, often narrow, exists between the pseudo-monoclinic and rhombohedral symmetries. Near this zone exists a solid solution with pseudocubic symmetry which differs from the pseudo-monoclinic one by a larger movement of cations.     

Crystal morphology of calcium aluminates hydrated for 14 days

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

纳米铝酸锶长余辉发光粉体的制备与性能表征

采用共沉淀法首先制得铝酸锶前驱体,经水热处理后煅烧获得了颗粒尺寸约为50nm的SrAl2O4:Eu2 ,Dy3 长余辉发光粉体.利用X射线衍射仪、透射电镜、荧光分光光度计和余辉测试仪等手段对粉体进行了表征.结果表明,在相同的煅烧条件下,与直接煅烧前驱体获得的粉体相比,经水热处理的粉体不仅颗粒尺寸小、粒度分布均匀而且发光性能(亮度和余辉)优异.     

Characterization of plasma-sprayed titanium aluminates by electron spin resonance spectroscopy

The structures of plasma-sprayed titanium aluminates containing 13 wt.% and 40 wt.% TiO₂ were investigated using electron spin resonance (ESR) and X-ray diffraction. These sprayed materials were identified as mixtures of α-Al₂O₃, γ-Al₂O₃, β-Al₂TiO₅ and rutile phases. An ESR peak observed near g = 1.9 at room temperature and 77 K was assigned to Ti³⁺ ions in octahedral environments. The relations between the halfwidths of the α-Al₂O₃, γ-Al₂O₃ and β-Al₂TiO₅, the linewidth and the g value of the ESR peak suggest that the Ti³⁺ ions are in the β-Al₂TiO₅ lattice. Further, the degree of asymmetry of the environments of the Ti³⁺ ions in the β-Al₂TiO₅ phase depends on the crystallinity of β-Al₂TiO₅.     

Electron emission and related properties of amorphous thin films of mixed barium and silicon oxides

Co-evaporated thin films of SiO and BaO were prepared and used as the insulator material in MIM and MIMIM structures, where the metal M was Al, Ag or Cu. Electroforming processes, resulting in an increase in the circulating current after the application of a direct forming voltage, were slow when Al electrodes were used but quite rapid with Ag or Cu electrodes. For devices formed with Cu or Ag electrodes the peak circulating current observed was at a voltage of the order of 2.6-3.2 V. At this applied voltage the electron emission current rose sharply and reached values comparable with those measured from MIM structures using SiO as the insulator. The probable existence of high field regions in the insulator was further tested by investigating triode structures and the forming processes were found to be quite complex. Although the polyfilamentary model can explain most of the diode properties, a refinement to this theory is needed to explain the forming effects observed with triode structures and this will involve a more careful consideration of the role of oxygen, as well as the usually considered metal and vacancy diffusion.     

铕掺杂铝酸锶基蓄光材料的制备与性能研究

采用高温固相合成法合成了蓄光性能良好的SrAl2O4基质蓄光材料,优化了制备的工艺条件,对合成样品的发光性能、形貌、物相组成等进行了测试与分析,对影响产品性能的主要因素进行了研究,结果表明,Eu2+掺杂量(氧化铕与碳酸锶的摩尔比)为0.02～0.03时发光效率较高.     

等离子喷涂制备磁铅石相结构铁氧体复合涂层研究

为解决热喷涂直接制备磁铅石结构铁氧体涂层问题,采用等离子喷涂工艺制备了铁氧体/聚乙烯复合涂层,研究了复合涂层的相结构和微观组织形貌。结果表明：等离子喷涂铁氧体/聚乙烯复合涂层厚度为1 mm,涂层中同时存在尖晶石相和磁铅石相,尖晶石相占55%,磁铅石相占45%,涂层中存在大量的未熔相,未发现典型的层状结构。通过牺牲聚乙烯保护磁铅石铁氧体,喷涂过程中低熔点聚乙烯首先熔融烧蚀,带走了等离子焰流的部分热量,使一部分磁铅石铁氧体吸热不足不能充分熔融,未发生熔化冷却结晶以及从尖晶石晶型向磁铅石晶型的转化过程,而是保留了原始的磁铅石相结构沉积到涂层中。通过该方法使等离子喷涂制备了具有磁铅石结构铁氧体的复合涂层。     

Effects of barium dissolution on dispersing aqueous barium titanate suspensions

Dissolution of barium ion and its effect on dispersion behavior of aqueous barium titanate suspensions at various pH values have been investigated. The amount of leached barium ion decreases with increasing pH value. The dissolution of barium ion also causes an increase in pH value of suspension, but the change decreases with increasing initial pH value. The iso-electric point (IEP) of leached barium titanate powder increases with increasing leaching pH value and solid loading as well. The dissolution of barium ion enhances the colloidal stability of aqueous barium titanate suspension, in agreement with zeta potential measurement.     

Semiconducting properties of barium vanadate glasses doped with barium chloride

Semiconducting barium vanadate glasses doped with BaCl₂ from 0 to 15 mol% were studied. Electrical d.c. conductivity measurements and an electron spin resonance investigation in the X-band were made. The concentration dependence of d.c. conductivity exhibits a minimum at 7 mol% BaCl₂. The activation energy increases with increasing BaCl₂ content up to 7% and decreases at higher concentrations of BaCl₂. The temperature dependence of the d.c. conductivity shows the effect of BaCl₂ as an oxidizing agent which controls the electrical conductivity, as a result of modification of the reduced vanadium valence ratio and a change in concentration of the paramagnetic ions. The plots of log versus 1 000/T are linear for all the BaCl₂-doped glasses. The data of the d.c. investigations were analyzed in terms of small polaron theory, and this confirms the applicability of the polaronic hopping model of electrical transport. The influence of BaCl₂ content on density indicates that in this ternary system the additivity role is not obeyed. The minimum occurs at the same concentration of 7 mol% BaCl₂ as noted for the electrical conductivity.     

Semiconducting properties of barium titanate and iron substituted barium titanate

Single crystals of barium titanate and members of the series BaTi_1?xFe_xO_3?xF_x have been grown from a potassium fluoride flux. These crystals were made conductive by heating in a sealed evacuated silica tube in the presence of freshly ground titanium. Electrical, optical and magnetic measurements were performed in order to characterize these crystals. Optical measurements indicated a decrease in the band edge from 3.2(1) eV for barium titanate to 2.8(1) eV for Bati_1?xFe_xO_3?xF_x(x = 0.02, 0.05, 0.07, 0.10). The photoelectrolytic behavior of reduced barium titanate was strongly quenched upon the substitution of iron and fluorine.     

Semiconducting barium titanate

Barium titanate, which is well known as a basic ferroelectric material, is also of interest when doped because of the interaction between semiconductivity and ferroelectricity. The resistance of blocking layers at surfaces and grain-boundaries is governed mainly by the ferroelectric properties, so that a resistance jump of four decades is observed on heating above the Curie temperature. A survey of the chemical and physical properties of such blocking layers both at surfaces and interfaces is presented. Doped titanates have been used as the basis of two new types of material:

1. Semiconductors with high positive temperature-coefficient of resistivity in special temperature ranges suitable for temperature-sensors and stabilisers.
2. Dielectrics with extremely high dielectric constant, used in so-called barrier-layer condensers.

     

Low-temperature synthesis and analysis of barium titanate nanoparticles with excess barium

BaTiO₃ nanoparticles with excess barium are directly synthesized from solution at 65 °C. It is noted that a small amount of BaCO₃ exists in the as-prepared samples. A series of standard mixtures with different proportion of BaTiO₃ and BaCO₃ are measured by XRD analysis to quantify the amount of BaCO₃ in our synthesized samples. After a deduction of the amount of BaCO₃, the concentration of excess barium in the lattice of BaTiO₃ nanoparticles can be determined and the results show that the excess barium can reach a considerable concentration. The influences of defects and surfaces on BaTiO₃ nanoparticles with excess barium are analyzed and two important reasons for the high concentration are also given.     

Radiation doses to patients undergoing barium meal and barium enema examinations

The radiation doses received by patients during 41 barium meal (BM) and 42 barium enema (BE) examinations in two Greek hospitals are presented. Radiation dose was measured in terms of the dose area product (DAP). The effective dose and doses to certain organs were estimated using the ODS-60 software. Mean total DAP values were found to be 25 +/- 11 Gy cm2 for BM and 60 +/- 35 Gy cm2 for BE examinations, whereas the estimated mean values of effective dose were 8.6 +/- 4.0 and 24 +/- 16 mSv respectively. DAP to effective dose conversion coefficients were estimated to be 0.34 mSv per Gy cm2 for BM and 0.41 mSv per Gy cm2 for BE.     

水热合成高纯四方相钛酸钡纳米粉末研究

以廉价易得氯化钡,四氯化钛为原料,在240℃下仅用12h水热合成得到四方相钛酸钡纳米粉末,从而为纳米粉末的产生化提供了简便条件。水热产物经XRD,SEM,XRF等手段特征,发现提高氢氧化钠过量的浓度,降低反应物四氯化钛浓度有利于四方相钛酸钡纳米粉末的生成,同时在水热产物中没有发现碳酸根和氯离子等残余的杂质表明产物具有较高的纯度。产物粒度小于100nm,比表面积为12m^2/g.     

Doped barium titanate nanoparticles

We have synthesized nickel (Ni) and iron (Fe) ion doped BaTiO₃ nanoparticles through a chemical route using polyvinyl alcohol (PVA). The concentration of dopant varies from 0 to 2 mole% in the specimens. The results from X-ray diffractograms and transmission electron micrographs show that the particle diameters in the specimen lie in the range 24–40 nm. It is seen that the dielectric permittivity in doped specimens is enhanced by an order of magnitude compared to undoped barium titanate ceramics. The dielectric permittivity shows maxima at 0.3 mole% doping of Fe ion and 0.6 mole% of Ni ion. The unusual dielectric behaviour of the specimens is explained in terms of the change in crystalline structure of the specimens.