静电纺丝法制备La0.67Ba0.33MnO3微纳米纤维及其红外发射率研究

采用静电纺丝法结合溶胶-凝胶技术制备了钙钛矿型La_0.67Ba_0.33MnO₃微纳米纤维, 并利用差示扫描量热-热失重分析(DSC-TGA)、 X射线衍射(XRD)、 傅里叶变换红外光谱(FTIR)和扫描电子显微镜(SEM)等技术对产物进行了表征, 利用IR-2红外发射率测试仪测试了La_0.67Ba_0.33MnO₃在280~370 K范围内的红外发射率. 结果表明, La_0.67Ba_0.33MnO₃在600 ℃时已形成钙钛矿结构. 随着煅烧温度的升高, La_0.67Ba_0.33MnO₃的形貌由纤维状向三维网络状转变, 并最终失去纤维形态. 在280~370 K范围内, La_0.67Ba_0.33MnO₃微纳米纤维的红外发射率随温度升高而升高, 由0.564增加至0.689. 利用钙钛矿材料双交换理论解释了这一现象, 并进一步探讨了其在红外发射率可变材料中的应用前景.     

推广的Kantorovich算子在Ba空间中的逼近

利用Ditzian-Totik光滑模,研究了推广的Kantorovich算子在Ba空间中的逼近,得到逼近的正定理与等价定理.所得结果改进、推广和统一了一些作者的结果.     

Impact of O2 exposure on surface crystallinity of clean and Ba terminated Ge(1 0 0) surfaces

In analogy with the case of Sr on Si [Y. Liang, S. Gan, M. Engelhard, Appl. Phys. Lett. 79 (2001) 3591], we studied surface crystallinity and oxidation behaviour of clean and Ba terminated Ge(1 0 0) surfaces as a function of oxygen pressure and temperature. The structural and chemical changes in the Ge surface layer were monitored by LEED, XPS and real-time RHEED. In contrast to the oxidation retarding effect, observed for 1/2 monolayer of Sr on Si, the presence of a Ba termination layer leads to a pronounced increase in Ge oxidation rate with respect to clean Ge. In fact, while the Ge(1 0 0) surface terminated with 1/2 ML Ba amorphizes for a pO₂ of 10⁻² Torr, LEED indicates that clean Ge forms a thin (4.5 Å), 1 × 1 ordered oxide upon aggressive O₂ exposure (150 Torr, 200 °C, 30 min). We briefly discuss the origins for the difference in behaviour between Ba on Ge and Sr on Si.     

Photocontrolled mechanical phenomena in photochromic doped polymeric systems

This work examines volume changes at the sub-micron scale, induced photochemically in polymeric matrices doped with photochromic molecules. To achieve this, spiropyran is employed as a photochromic molecule embedded in polyethylmethacrylate-co-methylacrylate (PEMMA) matrices. Spiropyran can be reversibly interconverted to merocyanine, its coloured isomer, by irradiation at 248 nm and 532 nm, correspondingly. It is demonstrated that the interconversion between the two forms activates volume changes in the polymer matrix. To this end, off-axis reflection holographic interferometry is employed as a sensitive probe of the induced volume changes. This scheme provides a novel method for controlling sub-micron volume changes reversibly, as required in several microactuator designs. Received: 17 April 2002 / Accepted: 18 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +30-810/391-318, E-mail: nassia@iesl.forth.gr     

Transmission and scanning electron microscopy studies of single femtosecond- laser-pulse ablation of silicon

The final state of the material resulting from laser irradiation of silicon using 130 fs pulses at 790 nm was studied using a number of techniques including scanning and transmission electron microscopies, as well as atomic force microscopy. Structural details and the level of damage to the nearby solid following irradiation were characterized and are discussed in the context of recent dynamical studies. Received: 28 September 2001 / Accepted: 3 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Department of Engineering Physics, McMaster University, Hamilton, Ontario, L8S 4M1, Canada RID="**" ID="**"Corresponding author. Fax: +1-905/521-2773, E-mail: borowia@mcmaster.ca RID="***" ID="***"Present address: Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK RID="****" ID="****"Department of Materials Science and the CEMD, McMaster University, Hamilton, Ontario, L8S 4M1, Canada RID="*****" ID="*****"Departments of Engineering Physics, and Physics and Astronomy, and the CEMD, McMaster University, Hamilton, Ontario, L8S 4M1, Canada     

Efficiently enhanced energy storage performance of Ba2Bi4Ti5O18 film by co-doping Fe3+ and Ta5+ ion with larger radius

We present an efficient strategy, that is the co-substitution of Fe³⁺ and Ta⁵⁺ ions with large radius for Ti⁴⁺ ion, to enhance energy storage performance of Ba₂Bi₄Ti₅O₁₈ film. For the films co-doped with Fe³⁺ and Ta⁵⁺ ions, the maximum polarization under the same external electric field is improved because the radius of Fe³⁺ and Ta⁵⁺ ions is larger than that of Ti⁴⁺ ion. Moreover, due to the composition and chemical disorder, the relaxor properties are also slightly improved, which can not be achieved by the film doped with Fe³⁺ ions only. What is more, for the films doped with Fe³⁺ ion only, the leakage current density increases greatly due to the charge imbalance, resulting in a significant decrease in breakdown strength. It is worth mentioning that the breakdown strength of Fe³⁺ and Ta⁵⁺ ions co-doped film does not decrease due to the charge balance. Another important point is the recoverable energy storage density of the films co-doped with Fe³⁺ and Ta⁵⁺ ions has been greatly improved based on the fact that the maximum external electric field does not decrease and the maximum polarization under the same external electric field increases. On top of that, the hysteresis of the polarization has also been improved. Finally, the co-doped films with Fe³⁺ and Ta⁵⁺ ions have good frequency and temperature stability.     

Effect of cation doping on low-temperature specific heat of LaMnO3 manganite

We have investigated the thermodynamic properties of perovskite manganite LaMnO₃, the parent compound of colossal magnetoresistive manganites, with the Ca²⁺ doping at the A-site. As strong electron-phonon interactions are present in these compounds, the lattice part of the specific heat deserves proper attention. We have described the temperature dependence of the lattice contribution to the specific heat at constant volume (C_v(lattice)) of La_1−xCa_xMnO₃ (x=0.125, 0.175, 0.25, 0.35, 0.50, 0.67, 0.75) as a function of temperature (1 K–20 K) by means of a rigid ion model (RIM).The trends of specific heat variations with temperature are almost similar at all the composition. The Debye temperatures obtained from the lattice contributions are found to be in somewhat closer agreement with the experimental data. The specific heat values revealed by using RIM are in closer agreement with the available experimental data, particularly at low temperatures for some concentrations (x) of La_1−xCa_xMnO₃. The theoretical results at higher temperatures can be improved by including the effects of the charge ordering, van der Waals attraction and anharmonicity in the framework of RIM.     

LiMBO3 : Dy3+材料的发光特性

采用固相法制备了LiM(M=Ca, Sr, Ba)BO₃ : Dy³⁺材料,并研究了材料的发光特性。LiM(M=Ca, Sr, Ba)BO₃ : Dy³⁺材料的发射光谱均呈多峰发射,对应于Ca,Sr,Ba,其主发射峰分别是Dy³⁺的⁴F_9/2→⁶H_15/2(484,486,486 nm),⁶H_13/2(577,578,578 nm)和⁶H_11/2(668,668,666 nm)跃迁。监测黄色发射峰时,所得激发光 谱峰值位置相同,主激发峰分别为331,368, 397,433,462,478 nm,对应Dy³⁺的⁶H_15/2→ ⁴D_7/2,⁶P_7/2,⁶M_21/2,⁴G_11/2,⁴I_15/2和⁶F_9/2跃迁。研究了敏化剂Ce³⁺及电荷补偿剂Li⁺、Na⁺和K⁺对LiM(M=Ca, Sr, Ba)BO₃ : Dy³⁺材料发光强度的影响。结果显示:加入敏化剂Ce³⁺提高了材料的发光强度,发光强度最大处对应的Ce³⁺浓度为3%;加入电荷补偿剂Li⁺、Na⁺和K⁺后,材料的发光强度也得到了明显提高,但发光强度最大处对应的Li⁺、Na⁺和K⁺浓度不同,依次为4%、4%和3%。     

Ablation of polymer foils for backside opening of thin film layered flexible polymer substrates

For increasing the packing density of electronic devices and enabling 3D wiring, new concepts of interconnection for flexible circuit boards are required. The backside wiring is one innovative concept which, however, requires interconnections from the back to the front side by means of vias.Results on backside opening of polymer foils for exposing a thin metal film deposited at the front side are presented. For the experiments, a thin polyimide foil covered with a thin molybdenum metal film was used. By using mask projection of a pulsed UV-laser beam (248 nm, 20 ns) polymer foil was ablated. The laser ablation process must be adjusted in the manner to avoid damage of the thin metal film, to prevent cones formation at laser ablation, but still enabling the clean ablation of the polymer. The influence of process parameters on the backside opening is discussed and compared with theoretical estimations of the laser-induced temperatures. Using a two-step ablation process applying first high fluences to ablate the main part of the foil and finishing with low laser fluence turns out to be advantageous. This backside opening (BSO) can be used to perform an electrical contact from the backside.     

Growth and spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) photorefractive crystals

RE, Mn:YAP (RE=Yb and Ce) crystals with dimension of Φ 25×60 mm were successfully grown by the Czochralski method. The spectroscopic properties of RE, Mn:YAP (RE=Yb and Ce) crystals before and after γ-irradiation were investigated at room temperature. The results show that the content of Mn⁴⁺ ions was increased with the Yb³⁺ ions co-doping, but decreased by Ce³⁺ ions co-doping. Thermoluminescence (TL) spectra of the crystals indicate three steps of recombination, and the probable recombination processes were discussed.