Dielectric behaviour of barium titanate-polyvinylidene fluoride composites

The dielectric behaviour of composites of barium titanate (BaTiO₃) and polyvinylidene fluoride (PVDF) has been studied by changing the weight fraction of BaTiO₃. The dielectric behaviour of the composites has a significant influence of PVDF up to 50wt% BaTiO₃ in the composite, but this effect is insignificant at higher weight fractions of BaTiO₃. X-ray diffraction studies on composites are also reported to correlate the observed dielectric changes with the corresponding structural changes.     

Calcite/barium fluoride ultraviolet polarizing prism

A calcite/barium fluoride polarizing prism gives improved ultraviolet transmittance compared with a calcite/calcite ultraviolet polarizing prism. Beam spatial distortion is transmitted because of variations in absorption (and path length) in the calcite half of the calcite/barium fluoride prism.     

Radiation damage test of barium fluoride scintillator

Small samples of BaF₂ scintillator have been irradiated in a high-energy proton beam with doses of up to 1.3 × 10⁷ rad. To the precision of our measurement there was no effect in the scintillation output and only a slight decrease in transmission was observed.     

Gamma-ray spectroscopy with a cooled barium fluoride crystal

Cooling a 1 l BaF₂ crystal to 243 K yielded the same or even better gamma-ray energy resolution as is obtained NaI(Tl) detectors of comparable efficiency.     

On quality requirements to the barium fluoride crystals

This report summarizes the quality requirements to the barium fluoride (BaF₂) crystals for constructing a high precision electromagnetic calorimeter at future hadron colliders. The basic property of BaF₂ crystals and the design and performance of a BaF₂ calorimeter are presented. The emphasis of the discussion is in the radiation resistance of the current production BaF₂ crystals. An approach to implement optical bleaching in situ is also presented. By using optical bleaching current production quality BaF₂ crystals could serve as an excellent candidate to construct a precision calorimeter at future hadron colliders.     

Large drift chambers for future detectors

The design and construction of simple and stable drift chambers, suited for production in large quantities and sizes, is presented. These chambers are under construction for the muon detector of the L3 experiment at LEP.     

Efficiency of barium fluoride scintillators for monoenergetic gamma rays

We discuss the calculated and measured efficiency of barium fluoride detectors for monoenergetic gamma rays.     

Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon carbide three-phase nanocomposites

Dielectric polymer composites with high dielectric constants and high thermal conductivity have many potential applications in modern electronic and electrical industry. In this study, three-phase composites comprising poly(vinylidene fluoride) (PVDF), barium titanate (BT) nanoparticles, and β-silicon carbide (β-SiC) whiskers were prepared. The superiority of this method is that, when compared with the two-phase PVDF/BT composites, three-phase composites not only show significantly increased dielectric constants but also have higher thermal conductivity. Our results show that the addition of 17.5 vol % β-SiC whiskers increases the dielectric constants of PVDF/BT nanocomposites from 39 to 325 at 1000 Hz, while the addition of 20.0 vol % β-SiC whiskers increases the thermal conductivity of PVDF/BT nanocomposites from 1.05 to 1.68 W m(-1) K(-1) at 25 °C. PVDF/β-SiC composites were also prepared for comparative research. It was found that PVDF/BT/β-SiC composites show much higher dielectric constants in comparison with the PVDF/β-SiC composites within 17.5 vol % β-SiC. The PVDF/β-SiC composites show dielectric constants comparable to those of the three-phase composites only when the β-SiC volume fraction is 20.0%, whereas the dielectric loss of the PVDF/β-SiC composites was much higher than that of the three-phase composites. The frequency dependence of the dielectric property for the composites was investigated by using broad-band (10(-2)-10(6) Hz) dielectric spectroscopy.     

纳米BaF2粉体的热稳定性

采用醇/水混合溶剂方法制备了不同粒径的纳米BaF2粉体,研究了粉体的热稳定性.结果表明,纳米BaF2粉体的粒径随着混合溶剂中乙醇含量的提高而减小,粉体的形貌和分散性也都随之改变.根据在不同升温速率下测量的每种粒径纳米BaF2粉体的DSC曲线,由Kissinger方程求出了各晶粒长大激活能.BaF2晶粒长大激活能随着粉体粒径的减小而线性地减小,纳米晶BaF2的热稳定性随着粒径的减小而降低.     

Applications of large barium fluoride scintillators to in-beam gamma-ray spectroscopy

Properties of large-volume BaF₂ scintillators have been studied and first experiences of applications to in-beam γ-ray spectroscopy are reported.Energy resolutions of 9.1% at 662 keV and 6.3% at 1332 keV γ-ray energies were obtained. A time resolution of 375 ps FWHM was measured using a ⁶⁰Co source. Gain variations of the phototubes from changes in count rate could be limited to a 2% level.Lifetimes have been measured for the first 2⁺ state of ¹⁵²Sm using a ¹⁵²Eu source and the 5₁⁻ state of ⁴⁰Ca at 4.49 MeV populated in the (α, α′) reaction.     

Photoelectret and electrophotographic properties of barium titanate-polyvinylidene fluoride composite film

Photoferroelectric barium titanate (BaTiO₃)-polyvinylidene fluoride (PVDF) composite films in the ratio 7030 are found to possess good photosensitivity in the visible region of the spectrum. Results on investigations of charge generation, trapping, recombination, discharge and retention properties of the composite film are presented in this paper. Photoinduced polarization is obtained in a BaTiO₃-PVDF composite film by the simultaneous application of a d.c. field and illumination from an argon-ion laser or by white light. The dark (photo) polarization and the dark (photo) depolarization current decay characteristics of the polarized films have been studied under different experimental conditions. The studies show that the composite film retains good polarization for a fairly long time in the dark condition, and the magnitude of the depolarization current is comparable with that of BaTiO₃ single crystal. Under illumination, the decay is fast enough to make use of the film for electro-photographic applications.     

Mechanical properties and dislocation mobility in barium fluoride single crystals

Conclusions The elastic limit temperature dependence between 50 and 1000°C consists of three regions differing in the mechanisms governing dislocation motion. The general form of the dependence, however, is similar to that for metals. Schmid's law is not obeyed between 50 and 250°C for compressional deformations along the [110] and [112] directions. The increased elastic limit for the [110] orientation is explained by an additional interaction between dislocations lying in the same slip plane but pertaining to different systems.The dislocation velocity dependence on applied shear stress is rapid at 150°C (m-12–18) but decreases quite strongly as the impurity content changes. According to calculations the activation area is (100–200)b² for all the studied BaF₂ crystals. This indicates that dislocation motion is not limited by Peierls' barriers in the stress region in question, but by barriers connected with point defects.Chernogolovka, Leningrad. Institute of Solid-State Physics, Academy of Sciences of the USSR, Moscow. S. I. Vavilov State Optical Institute. Translated from Problemy Prochnosti, No. 6, pp. 10–16, June, 1970.     

Measurements of the thermo-optic coefficient of a barium fluoride single crystal

The thermo-optic coefficient dn/dT of single-crystal barium fluoride is measured with a precision spectrometer. The values over the visible region in the 30-90 °C and 90-150 °C ranges are -18.8 and -19.7 × 10(-6)/°C, respectively.     

Gamma radiation induced damage effects in the transmission of barium fluoride and cesium fluoride fast crystal scintillators

Radiation damage effects in the optical transmission of bulk material due to ⁶⁰Co gamma irradiations were measured in samples of barium fluoride (BaF₂) and cesium fluoride (CsF). While confirming the exceptional radiation resistance of barium fluoride in the visible and ultraviolet regions of the transmission spectrum, at least up to a gamma dose of a few times 10⁷ rad, the results show that cesium fluoride, at least in the presently available samples, is very sensitive to radiation and that doses below 10⁴ rad are sufficient to make it completely unusable.     

Large area X-ray detectors based on amorphous silicon technology

The replacement of the radiographic film in medical imaging is a major issue. It requires an ∼40 cm wide detector to cover all examinations, an equivalent noise level of 1-5 X-ray quanta per pixel, and spatial resolution in the range of a few hundred microns. The need for entirely electronic imaging equipment has fostered the development of many X-ray detectors, most of them based on an array of amorphous silicon pixels, which is the only technology able to achieve such large areas. Essentially two concepts have been implemented: intermediate conversion of X-rays to light by a scintillator, detected by an array of light sensitive pixels, comprising a photodiode and a switching device, either a TFT or a diode conversion into electron hole pairs in a photoconductor, collected by an array of electrodes and switches. In both cases, charge amplifiers read the generated charges line by line. Scintillator and photoconductor based systems are now close to production. They achieve better image quality than the classic filmscreen combination, at lower X-ray doses and with a much broader dynamic range. Dynamic imaging up to 30 frames/s has been demonstrated. The technical challenges at the level of the a-Si array are the number of acceptable defects, the on/off ratio of the switches, the quantum efficiency of the photodiodes, the memory effects associated with traps in a-Si. Of course, long-term reliability is a major concern for medical components.     

Thickness-dependent leakage current of (polyvinylidene fluoride/lead titanate) pyroelectric detectors

The novel pyroelectric IR detectors have been fabricated using the Polyvinylidene Fluoride (PVDF)/Lead Titanate (PT) pyroelectric bilayer thin films, which were deposited onto Pt(111)/SiO/sub 2//Si(100) substrates by a sol-gel process. The ceramic/polymer structure was constructed of the randomly oriented polycrystalline PT film (/spl sim/1 /spl mu/m) heated at 700/spl deg/C for 1 h and the /spl beta/-phase PVDF film crystallized at 65/spl deg/C for 2 h. The effects of PVDF thin film thickness (100 /spl sim/ 580 nm) on the pyroelectric response of IR detectors were studied. The results show that the depositions of PVDF thin films onto the PT films will cause the leakage current (J) of the detectors decrease from 6.37/spl times/10/sup -7/ A/cm/sup 2/ to 3.86/spl times/10/sup -7/ A/cm/sup 2/. The specific detectivity (D*) measured at 100 Hz decreased from 2.72/spl times/10/sup 7/ cm/spl middot/Hz/sup 1/2//W for detector without PVDF to 1.71/spl times/10/sup 7/ cm/spl middot/Hz/sup 1/2//W for detector with PVDF thickness of 580 nm. By optimizing the ratio of the specific detectivity (D*) to leakage current, D*/J, the detector with PVDF thickness of 295 nm exhibits the best performance.     

Sequential electron beam evaporation of YBCO thin films through the barium fluoride route

Superconducting YBCO thin films have been fabricated by sequential electron beam evaporation of metallic constituents for yttrium and copper while barium is replaced with BaF2. The conversion of BaF2 BaO has been attained through wet oxygen annealing. Correlations of stoichiometry with microstructural, as well as superconducting, properties of the films have been examined. Results are given for YBa2Cu3O7– thin films on SrTiO3 (1 0 0) and YSZ (1 0 0) single crystal substrates. The usefulness of the BaF2 preparation route for the fabrication of mixed phase (Y123 + Y124) and pure Y124 is also demonstrated. Microstructural properties of Y123 thin films indicate different growth processes for the two substrates.     

Effect of fluoride doping on impedance spectra of barium strontium titanate glass ceramics

A system of barium strontium titanate glass ceramics with different fluoride concentrations were prepared by melt-annealing technique. The effect of fluoride doping on impedance spectra of the barium strontium titanate glass ceramics were investigated. According to the impedance spectroscopy studies, three (low, middle, and high frequency) electrical responses, which corresponds to glass phase, crystal–glass interface and crystal interior were identified. It is shown that with the increase of fluoride concentration, the resistivity of the glass phase passed through a minimum and then increased. In addition, the capacitance of the crystal–glass interface increased and the capacitance of the crystal phase decreased with the increase of the fluoride concentration. Moreover, as a result of ac conductivity calculation and analysis, it is believed that the dc conduction was affected by the glass and crystal–glass interface regions and ac regime was attributed to the crystal phase. Based on the results, a change of the compensation mechanism from electronic to ionic one with variation in fluoride concentration was proposed.     

BaF2纳米粉体的沉淀制备和成核机制

采用NH4F和Ba（NO3）2水溶液沉淀反应制备BaF2纳米粉体，在临界过饱和和快速混合过饱和反应条件下分别获得了10nm和80nm的BaF2沉淀粉体，对不同沉淀反应条件下的成核机制进行了分析。对于低浓度Ba（NO3）2溶液和NH4F固体快速混合反应情形，讨论了过饱和度的合理计算和扩散对形核的作用。     

Formation of three-dimensional lead telluride nanoislands on strained barium fluoride (111) surface

The nucleation and growth of three-dimensional lead telluride (PbTe) nanoislands from vapor phase on BaF₂(111) substrates strained by an external load under conditions close to thermodynamic equilibrium have been studied. Evolution of the shape of nanoislands was traced and their size distribution was statistically analyzed as dependent on the character of straining of the substrate. Homogeneous arrays of faceted PbTe nanoislands with an ultrahigh surface number density (above 10¹¹ cm^?2) and an average height of h ₁ ～ 3.5 nm (at a 8–9% rms deviation) are obtained for the first time on elastoplastically strained BaF₂(111) substrates under the conditions of small vapor supersaturation in the condensation zone. The formation of nanoisland arrays with such parameters is possible due to (i) the heterogeneous nucleation of PbTe nanoislands at the surface monatomic slip steps aligned in the 〈110〉 direction and (ii) the influence of straining on the kinetic processes on the substrate surface.