Electrical conductivity of Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> single crystals doped with Os,Re, Ru,and Rh

Bi₁₂ SiO₂₀ crystals doped with Os, Re, Ru, and Rh were grown by the Czochralski technique, and their conductivity (real and imaginary parts) was measured as a function of temperature and frequency. The results are interpreted as evidence that the charge transport in the crystals is due to hopping along chains of localized states.Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 197–200.Original Russian Text Copyright © 2005 by Milenov, Veleva, Petrova, Gospodinov, Skorikov, Egorysheva, Kargin, Vasilev.     

Electric conductivity of a bulk composite based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> core-shell nanoparticles

Composite Bi₂Te₃/SiO₂ nanoparticles of the core-shell type have been synthesized for the first time with a view to creating bulk composites possessing high thermoelectric figure of merit (conversion efficiency). It is suggested that bulk composited based on Bi₂Te₃/SiO₂ nanoparticles will provide a combination of low lattice heat conduction due to SiO₂ insulator and rather high electric conduction due to charge-carrier tunneling via dielectric spacers between adjacent Bi₂Te₃ semiconductor grains. The electric resistance of the composite increases with increasing temperature in the range of 130–300 K. This temperature dependence can be described in terms of a tunneling conduction model.     

Optical excitation of space-charge waves involving interband transitions in Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> Crystals

The excitation of space-charge waves in a semiconductor (B₁₂GeO₂₀) crystal using light with photon energy close to the bandgap width has been studied. The sample crystal has been optically excited by exposure to an oscillating interference pattern. The existing model notions are consistent with the experimental results.     

Ethylenediaminetetraacetic acid-assisted synthesis of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> nanostructures with unique edge sites

Nanomaterials with unique edge sites have received increasing attention due to their superior performance in various applications. Herein, we employed an effective ethylenediaminetetraacetic acid (EDTA)-assisted method to synthesize a series of exotic Bi₂Se₃ nanostructures with distinct edge sites. It was found that the products changed from smooth nanoplates to half-plate-containing and crown-like nanoplates upon increasing the molar ratio of EDTA to Bi³⁺. Mechanistic studies indicated that, when a dislocation source and relatively high supersaturation exist, the step edges in the initially formed seeds can serve as supporting sites for the growth of epilayers, leading to the formation of half-plate-containing nanoplates. In contrast, when the dislocation source and a suitably low supersaturation are simultaneously present in the system, the dislocation-driven growth mode dominates the process, in which the step edges form at the later stage of the growth responsible for the formation of crown-like nanoplates.

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Electrical conduction properties of Sr-doped Bi<Subscript>4</Subscript>(SiO<Subscript>4</Subscript>)<Subscript>3</Subscript> with the eulytite-type structure

Electrical conduction in 1 mol% Sr-doped Bi₄(SiO₄)₃ with the eulytite-type structure at elevated temperatures was investigated with conductivity measurements. Conductivity of the material under wet condition was higher than that under dry condition, and were 1.2 × 10⁻⁶ – 9.7 × 10⁻⁵ S cm⁻¹ at 500–850 ^°C. From H/D isotope effects and p(O₂)-dependencies of the conductivity, it was found that the Sr-doped Bi₄(SiO₄)₃ exhibited protonic conduction at all the temperatures investigated while contribution of p-type conduction became significant with increasing p(O₂) and/or temperature. Protonic and p-type conductions in the material were discussed in terms of defect equilibria.     

Novel Bi<Subscript>12</Subscript>TiO<Subscript>20</Subscript>/g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> composite with enhanced photocatalytic performance through Z-scheme mechanism

Novel Bi₁₂TiO₂₀/g-C₃N₄ composite was successfully prepared with Bi₁₂TiO₂₀ nanoparticles embedded within the fluffy crumpled g-C₃N₄ nanosheets. Bi₁₂TiO₂₀/g-C₃N₄ composites exhibit superior photoactivity and stability. As compared with g-C₃N₄ and Bi₁₂TiO₂₀, the photocatalytic efficiency of Bi₁₂TiO₂₀/g-C₃N₄ is effectively enhanced about 1.8- and 4.9-fold, respectively. Based on the trapping experiment, ^·OH and ^·O₂^? radicals are the dominant reactive oxygen species involved in the photocatalytic process. The proposed Z-scheme mechanism of charge transfer markedly promotes the carriers’ migration and separation, leading to the enhanced photocatalytic performance.     

Bridgman growth and defects of Nd<Superscript>3+</Superscript> : Sr<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> laser crystals

Nd³⁺ : Sr₃Ga₂Ge₄O₁₄ crystals have been grown by the modified Bridgman method. The growth defects, such as striations, scattering particles and dislocations were investigated. Some featherlike striations were observed in as-grown crystals. EPMA analysis suggested that these inclusions were caused by the segregation of Nd₂O₃ from the melt. Chemical etching results showed that the dislocation density was in the range of 10³ ∼ 10⁵/cm².     

Effects of sodium oleate on synthesis and photocatalytic activity of Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript>/Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>@RGO

In this study, the effects of sodium oleate on synthesis of Bi₂WO₆/Bi₂O₃ loaded reduced graphene oxide photocatalyst was studied. The as-prepared composites were characterized by X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy, UV–visible diffuse reflectance and photoluminescence spectroscopy. The results suggested that addition of sodium oleate not only promoted synthesis of Bi₂O₃, but also enhanced the reduction of GO to graphene. When the amount of sodium oleate was 4 mol (Bi:SO?=?1:1), Bi₂WO₆/Bi₂O₃@RGO to the best visible-light photocatalytic activity can be synthesized by a facile one-step solvothermal process without further reduction reaction. Hence, it indicated that sodium oleate could affect the synthesis of the as-prepared composites and the photocatalytic activity for degradation of RhB. This study did provide not only a facile method to synthesize Bi₂WO₆/Bi₂O₃@RGO, but also a method to reduce graphene oxide to graphene.     

Influence of Primary Milling on Structural and Electrical Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Ceramics Obtained by Sintering Process

In this article, the influence of primary mechanical milling of precursors on the microstructure and dielectric properties of Bi₄Ti₃O₁₂ ceramics was studied. Precursor material (mixture of Bi₂O₃ and TiO₂ powders) was ground by a high-energy attritorial mill for (1, 3, 5, and 10) h. Bi₄Ti₃O₁₂ ceramics were obtained by a solid-state reaction process, synthesized at an intermediate temperature (800 °C) and finally sintered at high temperature (1140 °C). Structure studies were performed by X-ray diffraction (XRD) and scanning electron microscopy techniques. XRD patterns were analyzed by the Rietveld method using the DBWS 9807a program. The thermal properties of the studied materials were measured using differential thermal analysis and thermal gravimetric techniques. These studies indicate that one-, three-, and five-hour primary high-energy ball milling followed by sintering is a promising technique for pure Bi₄Ti₃O₁₂ ferroic ceramics preparation. The investigation of Bi₄Ti₃O₁₂ shows that ceramics obtained from a precursor and milled for 5 h have the best dielectric properties.     

Scaling behavior of dynamic hysteresis in Bi<Subscript>3.15</Subscript>Nd<Subscript>0.85</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics

The scaling behavior of dynamic hysteresis was investigated in Bi_3.15Nd_0.85Ti₃O₁₂ bulk ceramics at a frequency of 1–1000 Hz and an external electric field amplitude of 79–221 kV/cm. The scaling behavior at low amplitude (E ₀ ≤ 114 kV/cm) takes the form of \(\langle A \rangle \propto f^{ - 0.013} E_{0}^{0.7}\) for low frequency (f ≤ 200 Hz) and \(\langle A \rangle \propto f^{ - 0.013} E_{0}^{0.22}\) for high frequency (f > 200 Hz), where \(\langle A \rangle\) is the area of hysteresis loop and f and E ₀ are frequency and amplitude of external electric field, respectively. At high amplitude (E ₀ > 114 kV/cm), we obtain \(\langle A \rangle \propto f^{0.011} E_{0}^{1.163}\) at low frequency and \(\langle A \rangle \propto f^{ - 0.015} E_{0}^{0.7}\) at high frequency. At low E ₀, the contribution to the scaling relation mainly results from reversible domain switching, while at high E ₀ reversible and irreversible domain switching concurrently contribute to the scaling relation.     

Influence of C<Subscript>4</Subscript>F<Subscript>8</Subscript>/Ar/O<Subscript>2</Subscript> plasma etching on SiO<Subscript>2</Subscript> surface chemistry

The plasma assisted etching of SiO₂ in a commercial RF reactor with a variety of C₄F₈/Ar/O₂ chemistries has been studied by XPS, SIMS and FTIR. A simple model of surface reactions is proposed. In particular, the role of oxygen in the etch process has been investigated. According to our experiments, oxygen inhibits the formation of CFH based polymeric films on the surface. As the etching process is due to an exchange reaction between the oxygen in the SiO₂ and the gaseous fluorine species in the plasma, the presence of oxygen in the etch hinders this process by occupying adsorption sites on the surface. The results would confirm that argon does not participate in chemical reactions with the SiO₂ substrate but provides energy for reactions in which F, C and O are involved. The results also indicate that a thick fluorocarbon layer only forms on the surface in the absence of oxygen, regardless of the oxygen source. Consequently, only when the SiO₂ layer has been substantially removed does this film form.     

Relaxor behavior and ferroelectric properties of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-K<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-KNbO<Subscript>3</Subscript> lead-free ceramics

Lead-free ferroelectric ceramics of (1−x) [0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃]-x KNbO₃(x = 0, 0.02, 0.04, and 0.06) were prepared by the conventional ceramic fabrication technique. The crystal structure, dielectric properties and P-E hysteresis loops were investigated. XRD data showed that all compositions could form pure perovskite structure. Temperature dependence of dielectric constant ε _r and dissipation factor tanδ measurement between room temperature and 500^∘C revealed that the compounds experience phase transitions that from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric in the range of x = 0–0.04. The frequency dependent dielectric constant showed these compounds were relaxor ferroelectric. At low frequency and high temperature, dielectric constant and dissipation factor increased sharply attributed to the superparaelectric clusters after the KNbO₃ doped.     

On contamination of Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> single crystals with <Superscript>207</Superscript>Bi isotope

Two versions of generation of ²⁰⁷Bi radionuclide are known from the literature: natural phenomena (cosmic rays) and technogenic factors (high-power nuclear weapons tests). The amounts of the generated radionuclide are so large that it can be detected even in Bi₄Ge₃O₁₂ (BGO) scintillator. Previously we demonstrated that natural phenomena cannot generate these amounts of ²⁰⁷Bi. The identification of ²⁰⁷Bi of cosmic origin in BGO is doubtful. BGO crystals are inevitably contaminated with isotopes from natural radioactive series, which are significant components of the background radiation of the Earth. Contamination with ²²⁷Ас due to several factors is possible. The decay of ^211+211mPo to the energy levels of ^207mPb can be misinterpreted as the ²⁰⁷Bi decay.     

A Study of the Crystal Structure of Co<Subscript>40</Subscript>Fe<Subscript>40</Subscript>B<Subscript>20</Subscript> Epitaxial Films on a Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Topological Insulator

Laser molecular-beam epitaxy has been used to form Co₄₀Fe₄₀B₂₀ layers on Bi₂Te₃ topological insulator substrates, and their growth conditions have been studied. The possibility of growing epitaxial ferromagnetic layers on the surface of a topological insulator is demonstrated for the first time. The CoFeB layers have a body-centered cubic crystal structure with the (111) crystal plane parallel to the (0001) plane of Bi₂Te₃. 3D mapping in the reciprocal space of high-energy electron-diffraction patterns made it possible to determine the epitaxial relationships between the film and the substrate.     

The Development of Microstructure and Ferroelectric Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3.96</Subscript>Nb<Subscript>0.04</Subscript>O<Subscript>12</Subscript> Thin Films

Bi₄Ti_3.96Nb_0.04O₁₂ thin films were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a sol–gel method and rapid thermal annealing. The effects of Nb-substitution and annealing temperature (500–800°C) on the microstructure and ferroelectric properties of bismuth titanate thin films were investigated. X-ray diffraction analysis reveals that the intensities of (117) peaks are relatively broad and weak at annealing temperatures smaller than 700°C. With the increase of annealing temperature from 500°C to 800°C, the grain size of Bi₄Ti_3.96Nb_0.04O₁₂ thin films increases. The Bi₄Ti_3.96Nb_0.04O₁₂ thin films annealed at 700°C exhibit the highest remanent polarization (2P _r), 36 μC/cm² and lowest coercive field (2E _c), 110 kV/cm. The improved ferroelectric properties can be attributed to the substitution of Nb⁵⁺ to Ti⁴⁺ in Bi₄Ti₃O₁₂ assisting the elimination of defects such as oxygen vacancy and vacancy complexes.     

Crystallization of amorphous Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> films

The crystallization kinetics of amorphous Bi₂S₃ films have been studied by kinematic electron diffraction. The results have been used to evaluate the activation energies for activation energy and crystal growth.     

Fabrication and properties of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> composites

Composites in the form of precipitated powders, hybrid xerogels, and SiO₂ core/TiO₂ shell particles have been produced via hydrolysis of precursors (alkoxides and inorganic derivatives of titanium and silicon) and have been characterized by differential thermal analysis, X-ray diffraction, adsorption measurements, and macroelectrophoresis. The results demonstrate that heat treatment of the composites leads to crystallization of the titanium-containing component and, accordingly, reduces their specific surface area. Hydrothermal treatment enables the fabrication of materials in which TiO₂ nanocrystals are evenly distributed over an amorphous SiO₂ matrix.     

Electric properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>(BIT)–CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> (CCTO) composite substrates for high dielectric constant devices

A study of the effect of the presence of BIT (Bi₄Ti₃O₁₂) in the dielectric and optical properties of the CaCu₃Ti₄O₁₂ (CCTO) is presented. The samples were prepared by the solid state procedure. Mechanical alloying followed by the solid state procedure has been used successfully to produce powders of CaCu₃Ti₄O₁₂ (CCTO) and BIT (Bi₄Ti₃O₁₂) to be used in the composites. We also look at the effect of the grain size of the BIT and CCTO in the final properties of the composite. The samples were studied using X-Ray diffraction, scanning electron microscopy (SEM), Raman and infrared spectroscopy. We also did a study of the dielectric function K and dielectric loss of the samples. The role played by the grain size of CCTO and BIT in the dielectric constant and structural properties of the substrates are discussed. For frequencies below 10 MHz the K value presented by the CCTO100 sample is always higher than the K value presented by the BIT100 sample. At 100 Hz the value of K 1900 for the CCTO100 sample and 288 for the BIT100 sample. However for the composite sample one has an unexpected result. The dielectric constant is higher for all the frequencies under study. At 100 Hz the value of the K is around 10.000 for the BIT10 sample. Which is more than one order bigger compared to the CCTO100 value for the same frequency. Therefore, these measurements confirm the potential use of such materials for small high dielectric planar devices. These composites are also attractive for capacitor applications and certainly for microelectronics, microwave devices (cell mobile phones for example), where the miniaturization of the devices is crucial.     

Influence of sintering temperature on thermoelectric properties of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/Polythiophene composite materials

Bi₂Te₃/Polythiophene (PTH) thermoelectric bulk composite materials were prepared by a two-step method. Firstly, Bi₂Te₃ and PTH nanopowders were prepared by hydrothermal synthesis and chemical oxidative polymerization, respectively. Secondly, the mixture of the Bi₂Te₃ and PTH nanopowders (50:50 wt) was pressed under vacuum at 80 MPa and 298, 473, or 623 K. For comparison, Bi₂Te₃ powders were hot pressed at 623 K. The bulk materials were analyzed by conventional methods, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA) and field emission scanning electron microscopy equipped with electron dispersive X-ray spectroscopy. The XRD and TGA results showed that the PTH decomposed when the hot pressing temperature exceeded 473 K, and Bi₂Te₂S phase was formed. The thermoelectric properties of the bulk composite materials were investigated. The composite pressed at 623 K showed a higher power factor, ~2.54 μ Wm⁻¹ K⁻² at 473 K, which is as ~20 times as that of the composite pressed at 473 K, although, it is still much lower than that of the pressed Bi₂Te₃ material (~1,266 μ Wm⁻¹ K⁻² at 348 K).