溶胶-凝胶技术在光学制造中的应用

溶胶－凝胶技术是一种超结构材料处理技术，与浇铸工艺相结合形成了一种全新的光学制造技术。本文简要回顾和比较了基于溶胶－凝胶处理技术制造ＳｉＯ２单片光学元件和玻璃的多种处理方法，重点描述在常压下用水解硅醇盐方法制造ＳｉＯ２单片凝胶七个处理步骤的化学、物理过程。概述了这种方法的特点和制得光学元件的特性，列举和讨论其在制造折射、衍射、微型光学元件和复合系统中的应用及前景。     

Mathematical modeling of ceramics metallization technology

Metallization of ceramics by an incoherent light beam has been simulated. A ceramic substrate with a metal-containing paste deposited on its surface is considered as an infinite planar two-layer medium. The Stefan problem with phase transitions is formulated and solved by the quasi-stationary Leibenzon method.     

陶瓷加工中的激光技术应用研究

工程陶瓷作为２１世纪的三大应用材料之一，其有效的加工方法已成为国内外专家学者探索的热点。而激光技术由于是非接触式加工，没有切削力，加工速度快，能加工一些特殊型面等优点，在陶瓷的加工方面得到了一定的应用，并取得了一定的进展。本文介绍了应用激光技术加工陶瓷和激光技术在陶瓷磨削中的辅助应用，并阐述了各种技术的基本原理和加工质量评价，同时展望各种激光技术在陶瓷加工中的发展趋势。尽早开发应用于陶瓷加工的各种激光新技术，对推动陶瓷的应用发展和激光技术的发展应用都有着深远的意义。     

基于压电陶瓷的激光频率调谐技术

在充分研究各种激光调谐方法优缺点的基础上,针对其调谐范围、调谐速率、调谐线性等方面存在的不足,提出一种基于压电陶瓷的激光频率调谐技术。该方法将压电陶瓷与光纤光栅激光器的布拉格光栅进行粘结,通过调整压电陶瓷的驱动电压来带动布拉格光栅的伸缩,实现波长（即频率）的调谐。同时,利用虚拟仪器中的计算机软件拟合技术,校正压电陶瓷输入电压与输出位移之间的非线性,使系统呈线性频率调谐,以提高测量精度。实验结果表明,当压电陶瓷的驱动电压变化126 V时,可实现0.8 nm（即100 GHz）的调谐范围。     

Photoluminescence emissions of ceramics: a marker of production technology

Photoluminescence (PL) measurements have been performed on different sets of ceramic shards; the results obtained from the ceramic bodies of lustred majolicas produced in Iraq, Italy and Spain are reported. Non-lustred samples are analysed for comparison. The spectra obtained from all the lustred samples show PL emission bands that are attributed to the presence of α-quartz and diopside. The photoluminescence of the lustre-decorated ceramics is possibly a fingerprint of the particular production technology of lustre. PACS 81.05.Mh; 78.55.Hx     

Relaxor behaviour in PZN-PT-BT ferroelectric ceramics

The relaxor behaviour of the (Pb_0.8Ba_0.2)[(Zn_1/3Nb_2/3)_0.7Ti_0.3]O₃ ferroelectric ceramic is presented. A strong dispersion of the maximum of the dielectric permittivity (ε′) below the transition temperature (T_m) is observed, which shift towards higher temperatures with increasing frequency. There is a strong deviation from the Curie-Weiss law. The results are fitted by using the Volger-Fulcher relationship, showing typical behaviour of a spin glass system. The hysteresis loops suggest relaxor behaviour.     

Luminescence centers in bismuth-containing oxytungstate ceramics

We have studied the photoexcitation and luminescence spectra of Bi₂WO₆, Y₂WO₆ and Y₂WO₆:Bi ceramics. We used the Alentsev-Fock method to decompose the spectra into elementary components. The emission bands with maximum at 2.93 eV in the luminescence spectrum of Bi₂WO₆, 3.02 eV in the luminescence spectrum of Y₂WO₆, and 2.95 eV in the luminescence spectrum of Y₂WO₆:Bi are assigned to luminescence of self-localized Frenkel excitons. The bands with maxima at 2.35 eV and 1.90 eV in the spectrum of Bi₂WO₆, 2.25 eV and 1.75 eV in the spectrum of Y₂WO₆, and 2.35 eV and 1.85 eV in the spectrum of Y₂WO₆:Bi are connected with oxygen vacancies. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 688–691, September–October, 2007.     

Electron-stimulated processes in boron nitride-based ceramics

The low-energy electron radiation resistance of boron nitride-based BN + Si₃N₄ and BN + SiO₂ ceramics proposed as a construction material for ion space engines was studied. It was shown that a reduced silicon phase is formed on the BN + Si₃N₄ ceramics surface in the high-temperature region (～900 K), which results from material thermal destruction. The BN + SiO₂ ceramics exhibits high thermal stability and is not prone to destruction due to electron-stimulated oxygen desorption (the cross section of this process does not exceed 10^?20 cm²). It is preferable to use such ceramics as a construction material. Based on the results obtained, some models were proposed that explain variations in the erosion rate of ceramic units of ion engines under electron and ion irradiation.     

Light-induced intrinsic defects in PLZT ceramics

This paper reports on an EPR study of a ferroelectric, 1.8/65/35, and an antiferroelectric, 2/95/5, of optically transparent Pb_1?y La_yZr_1?x Ti_xO₃ (PLZT) ceramics within a broad temperature range (20–300 K) after illumination at a wavelength of 365–725 nm. Illumination with ultraviolet light, whose photon energy corresponds to the band gap of these materials, at T<50 K creates a number of photoinduced centers: Ti³⁺, Pb⁺, and Pb³⁺. It is shown that these centers are generated near a lanthanum impurity, which substitutes for both the Pb²⁺ and, partially, Ti⁴⁺ ions through carrier trapping from the conduction or valence band into lattice sites. The temperature ranges of the stability of these centers are measured, and the position of their local energy levels in the band gap is determined. The most shallow center is Ti³⁺, with its energy level lying 47 meV below the conduction band bottom. The Pb³⁺ and Pb⁺ centers produce deeper local levels and remain stable in the 2/95/5 PLZT ceramics up to room temperature. The migration of localized carriers is studied for both ceramic compositions. It is demonstrated that, under exposure to increased temperature or red light, the electrons ionized into the conduction band from Ti³⁺ are retrapped by the deeper Pb⁺ centers, thus hampering the carrier drift in the band and the onset of photoconduction. The part played by localized charges in the electrooptic phenomena occurring in the PLZT ceramics is discussed.     

Dielectric relaxation in LuFeO3 ceramics

LuFeO₃ ceramics were prepared, and the dielectric characteristics were investigated together with the structure. A giant dielectric constant step (8000 at 10 kHz, 7200 at 100 kHz, and 4000 at 1 MHz) very similar to that in LuFe₂O₄ was observed. The dielectric constant dropped quickly when the temperature decreased through a critical temperature which increased significantly when the frequency increased. A very high relaxor-like dielectric peak with strong frequency dispersion was also observed in a higher temperature range. Two obvious corresponding dielectric relaxation peaks were observed on the curve of dielectric loss vs temperature, and all these dielectric relaxations followed the Arrhenius law. The Fe²⁺/Fe³⁺ mixed-valence structure and the oxygen vacancy primarily governed these relxor-like dielectric behaviors. However, the present ceramics are not relaxor ferroelectric.     

Local polarization in Lead-Calcium Titanate ceramics

Calcium-modified Lead Titanate ceramics Pb_0.76 Ca_0.24 [(Co_0.5 W_0.5)_0.05 Ti_0.95O₃] are studied by means of dielectric and thermal expansion measurements as a function of temperature. Results show that the phase transition has a diffusive character with an exponent τ that decreases when temperature increases. Above the transition temperature, Tc, the electrical conductivity of samples affects the thermal behaviour of the dielectric constant. From thermal expansion measurements, results show that the thermal elongation and thermal expansion coefficient separate from their normal behaviour at a temperature Td = 460°C, much higher than Tc and close to the phase transition temperature of PbTiO₃. The difference Δα = α − α₀ (anomalous part of the thermal expansion coefficient) decreases to zero when the temperature increases from Tc, following a logarithmic law Δα = F(logΔT). Thermal expansion results may be ascribed to some type of local polarization in the crystalline grains, which disappears near the phase transition temperature of PbTiO₃.     

Electrical phenomena in barium titanate ceramics

We have studied the current voltage curves of BaTiO₃ ceramics, with fired silver electrodes, between 110–400°C. We have observed a current controlled negative resistance between low conductivity and high conductivity states. The passage from the low to the high conductivity state is accompanied by a brown coloration starting from the anode. Once the high conductivity state is reached it is impossible to return to the low conductivity state unless one anneals the sample for several hours at 400°C. We have measured the voltage distribution along the sample and the short circuited currents after applying a voltage. We interpret our results, assuming carriers injection, by means of the Rose and Lampert models of injection. In the low conductivity state the conduction is ohmic or due to electron injection, and in the high conductivity state there is a double injection regime. However the agreement of the results with the theoretical prediction is good only qualitatively. We think that a more elaborate model is necessary.     

Schottky-type grain boundaries in CCTO ceramics

In this work we studied electrical barriers existing at CaCu₃Ti₄O₁₂ (CCTO) ceramics using dc electrical measurements. CCTO pellets were produced by solid state reaction method and X-ray diffractograms showed which single phase polycrystalline samples were obtained. The samples were electrically characterized by dc and ac measurements as a function of temperature, and semiconductor theory was applied to analyze the barrier at grain boundaries. The ac results showed the sample’s permittivity is almost constant (10⁴) as function of temperature at low frequencies and it changes from 100 to 10⁴ as the temperature increases at high frequencies. Using dc measurements as a function of temperature, the behavior of barriers was studied in detail. Comparison between Schottky and Poole-Frenkel models was performed, and results prove that CCTO barriers are more influenced by temperature than by electric field (Schottky barriers). Besides, the behavior of barrier width as function of temperature was also studied and experimental results confirm the theoretical assumptions.     

Retrospective dosimetry with ceramics

This paper reviews recent progress that has been made in the application of luminescence techniques with ceramic materials to the problem of dose reconstruction at: Hiroshima and Nagasaki; areas downwind of the Nevada Test Site; regions of Belarus, the Russian Federation and the Ukraine contaminated by fallout from Chernobyl; settlements along the Techa River affected by releases from the Mayak facility at Chelyabinsk; Kiisa, Estonia where a stolen ¹³⁷Cs irradiator source was discovered. Luminescence has an increasingly important role in radiological health studies at such sites because of the ability of the method to measure dose retrospectively in areas where radiation monitoring was lacking or sporadic following the incident. Commonly produced ceramics such as brick, tile and porcelain fittings and artefacts have been used to determine the integrated external gamma radiation dose (the transient dose). Evaluation of the reliability and accuracy with which the transient dose can be estimated when it approaches and drops below the level of the integrated natural background dose (<50 mGy for ceramic samples of age 20–30 y) is the subject of current research. In addition to various improvements in the performance of the experimental techniques and understanding of the luminescence properties of the minerals, other developments include improved integration with the requirements of dose reconstruction by the combined use of luminescence and computational modelling.     

Role of vanadium in Bi-2223 ceramics

The effect of substitution of vanadium in Bi_1.6Pb_0.4Sr₂Ca₂Cu₃ O_δ ceramic at Bi and Cu sites has been investigated for the resistivity, a.c. susceptibility, XRD and a.c. magnetization studies. Enhancement in T _c for the smaller concentrations of V at either of the two sites was observed which followed an expected lattice distortion and decrease in T _c for higher V concentrations. V plays a role of substituting element more than just a sintering agent as reflected in the more rapid decrease in T _c at Cu-site and further in the magnetization values that are higher compared to the values at the Bi-site. It indicates higher magnetization scattering at the Cu site. The results were explained keeping in mind the liquid phase mechanism behind the formation of the 2223 phase and the possibility of magnetic scattering.     

Colossal dielectric behavior in TbFeO3 ceramics

TbFeO₃(TFO) ceramic samples were prepared via the traditional solid-state reaction route. The low-temperature (100–300 K) dielectric properties of TFO have been systematically investigated in the frequency range from 100 Hz to 5 MHz. Colossal dielectric behavior contributed from two thermally activated dielectric relaxations was found in the sample. The low-temperature relaxation with the activation energy of 0.22 eV was confirmed to be a polaron relaxation caused by electrons hopping between the mixed-valent states of Fe ions. The high-temperature relaxation shows the activation energy of 0.59 eV and can be enhanced by annealing treatment in oxygen and weakened in nitrogen. This relaxation was ascribed to be a Maxwell–Wagner relaxation due to surface-layer effect.