Kinetic study of silica gels by a new rheological ultrasonic investigation

The last decades have seen the development of sol-gel (SG) process currently used to develop new materials in a wide range of scientific applications. The SG process leads to an oxide macromolecular network through a sol (liquid phase) to gel transition. To optimize this process, the control of the kinetic of the chemical reaction is required. This kinetic can be deduced from the temporal evolution of the viscoelastic parameters. Upto date no complete investigation during the SG formation can be achieved by a unique non-destructive technique. In this paper, we present an ultrasonic technique to measure the viscoelastic parameters (storage G' and loss G' shear moduli) of the gel material during its formation. By using a suitable model which takes into account the mass loading on the surface, the viscoelastic parameters of these materials are accurately deduced. In order to study the efficiency of this technique, silica gels transition is monitored at various formation temperatures and for different initial hydrolysis molar ratio (h). In addition, the monitoring is performed at different oscillatory shear measurements in the 6-54 MHz frequency range to determine a new characteristic time t(vs) corresponding to the moment when the material is no more a newtonian liquid. This characteristic time is then compared to the gelation time t(g) determined by rheological or acoustic audible range methods. Thus the new characteristic time is also a good criterion to characterize earlier the SG matrix transition. Our AT-cut quartz technique using our model can also be used as a high frequency rheometer for the sol-gel materials.     

Confining medium and absorptive overlay: Their effects on a laser-induced shock wave

In this paper the characteristics (such as amplitude, width) of a laser-induced shock wave under confining conditions is studied. For engineering applications, a physical study of this method is useful in order to optimize this technique. We have first introduced a new pressure gauge – PVDF (polyvenyliden fluoride) gauge with short rise time and wide linear response range. Experimentally, by measuring the generated pressures under different confining materials, the relationship between the pressures and the acoustic impedance of confining materials, is illustrated, which somewhat agrees with the theoretical calculation. We have also found that under confining conditions laser-induced shock waves persist longer than a laser pulse. Then, the effects of black paint overlay (absorptive overlay) is studied. We experimentally point out that a black paint overlay placed before an irradiated target can greatly increase the generated pressure under any confining material in our experiments for its beneficial effect on the plasma-generating process. To our surprise, comparing the impulse ( ), which the shock wave induced under absorptive overlay executes on the target, to that induced under no black paint overlay, the increase ratio is approximately equal.     

Elastic properties of substances in the megabar pressure range: Inversion of shear rigidity

The behavior of elastic moduli of substances is analyzed in the megabar pressure range. A new effect—inversion of the shear moduli and mechanical properties upon compression—is predicted for various classes of substances. The melting-curve data for different materials confirm the predicted phenomenon. The materials traditionally considered the softest, such as rare gas solids and molecular substances, may become the hardest in the megabar range. This should be taken into account in developing the experimental high-pressure technique.     

Time-resolved two-color LII: size distributions of nano-particles from gas-to-particle synthesis

A two-color LII technique for in situ measurements of particle size distributions is described. The technique is based on the simultaneous detection of time-resolved LII signals at two different wavelengths with one-dimensional spatial resolution using a newly developed experimental setup. The ratio of both LII signals yields particle temperatures as a function of time and location. Measured particle temperature decays are numerically simulated based on a detailed cooling model for particle ensembles. Particle size distributions are obtained by fitting simulated particle temperature decays to measured ones using multi-dimensional non-linear regression. The two-color LII technique for particle sizing can be applied to a wide range of materials because it is independent of the optical properties of the particle material. Exemplarily, the measuring technique is applied to investigate the synthesis of nanoscaled metal oxide particle in a laser vaporization reactor.     

Effect of the Cu co-activator on the charge-carrier trapping efficiency in CaSO4:Tm,Cu

Rare earth doped CaSO₄ materials are well known thermoluminescent dosimeters with friendly properties. They are used in the practice for a long while. New phosphors based on CaSO₄:Tm were synthesized using a modified preparation method for adding Cu as co-dopant. The produced materials have several favorable new characteristics. Larger linear dose response range, simpler glow-curve structure and a small diminution in the sensitivity are the consequences of the copper addition. Trapping efficiency of samples containing different amounts of Tm and Cu was investigated as the function of the composition using a consecutive RL/TL measuring technique. It was cleared up that the Cu addition, depending on its amount, reduced this value. This plays the main role in the sensitivity loss and forms one of the factors in the explanation of the linear dose range widening.     

Aerodynamic levitation and laser heating:

Aerodynamic levitation is an effective way to suspend samples which can be heated with CO₂ lasers. The advantages of this containerless technique are the simplicity and compactness of the device, making it possible to integrate it easily in different kinds of experiments. In addition, all types of materials can be used, including metals and oxides. The integration of aerodynamic levitation at synchrotron and neutron sources provides powerful tools to study the structure and dynamics of molten materials. We present here an overview of the existing techniques and of the developments made at the CEMHTI in Orléans, as well as a few examples of experimental results already obtained.     

Continuous X-ray induced Auger microprobe analysis and microscopy: First results

X-ray induced Auger electron spectroscopy can be applied to the chemical analysis of thin samples with a spatial resolution in the 10 μm range. The first experimental results obtained with various anode materials and various samples are reported. The signal intensities (in the range of 10³ counts/s) are similar to those obtained by using a conventional electron spectrometer with a resolution in the millimeter range. Theoretical considerations together with the results obtained make it possible to evaluate the sensitivity of this technique and suggest how the spatial resolution can be improved further (20 μm at present, 2–3 μm in the near future). The radiation damage and sample thickness constraints of this new technique and of conventional Auger electron spectros- copy are compared. Application of X-ray induced Auger analysis to biological objects is suggested and the effect of the present results on scanning X-ray radiography and characteristic X-ray absorption microanalysis is also pointed out.     

State of art: Optically stimulated luminescence dosimetry – Frontiers of future research

Since the commercial adoption of the optically stimulated luminescence (OSL) technique in dosimetry, almost 20 years ago, we have seen major advances in the deployment of OSL dosimeters in different areas, including personal, medical, and space dosimetry. The objective of this paper is to provide a critical overlook at the OSL technique from three different points of view: strengths, challenges and opportunities. We discuss factors that made the OSL technique successful: its simplicity, accuracy, wide dynamic range of measured dose, ease for automation, re-read capability, ability to perform imaging, and the availability of diverse instruments and materials. We look into problems that were overcome and others that remain in several areas of new applications into which OSL has expanded in the past 10 years, such as medical, space, neutron and accident dosimetry. Finally, we discuss unexplored possibilities, new driving forces, and open questions. We hope the broad overview presented here will encourage more discussion and stimulate the research that will advance our fundamental understanding of the OSL process.     

Thickness measurement in composite materials using Lamb waves

Pulse propagation in plate elements is investigated with the identification of Lamb modes in the range 40–150 kHz. The possibilities of the application of this technique in the measurement of thickness in composites and coarse materials are evaluated, particularly in ferrocement materials.     

Laser optoacoustic spectroscopy of gold nanorods within a highly scattering medium

We describe a spectroscopic comparative analysis based on the optoacoustic technique over the wavelength range from 410nm to 1000nm using a Q-switched Nd:YAG pumped optical parametric oscillator tunable source on a gold nanostructure solution located within a highly scattering medium. The advantages of this method over standard spectroscopy techniques are the possibility to localize and monitor the spectroscopic response of absorbing materials located within turbid media. The operation is confirmed using a comparative analysis with the spectroscopic results obtained from a reference measurement scheme, based on a highly sensitive collimated optical transmission setup in parallel and under the same experimental conditions as the optoacoustic technique.     

Ultrashort high repetition rate exposure of dielectric materials: laser bonding of glasses analyzed by micro-Raman spectroscopy

We report on the joining of different glass types with dissimilar optical, thermal and mechanical properties by ultrashort laser welding at high repetition rates. Femtosecond laser pulses were focused at the interface of two optically contacted transparent samples. Using nonlinear absorption processes and heat accumulation of successive pulses, we achieved strong bonds between the samples. We used a three-point bending test to determine the breaking strength. With this technique, we achieved for instance for a borosilicate glass a breaking strength of up to 95 % of the bulk material. In addition, we even welded different material combinations. Although the welded glasses exhibit different thermal and mechanical properties, we obtained breaking strengths which are comparable to the utilized bulk materials. Using Raman spectroscopy we mapped the laser-processed material along the welded interface. Thereby, we determined that the welds consist of a mixture of both species, which is formed during the laser induced melting of the materials.     

Standardization of proton-induced x-ray emission technique for analysis of thick samples

This paper describes the standardization of the proton-induced x-ray emission(PIXE) technique for finding the elemental composition of thick samples. For the standardization, three different samples of standard reference materials(SRMs) were analyzed using this technique and the data were compared with the already known data of these certified SRMs. These samples were selected in order to cover the maximum range of elements in the periodic table. Each sample was irradiated for three different values of collected beam charges at three different times. A proton beam of 2.57 Me V obtained using 5UDH-II Pelletron accelerator was used for excitation of x-rays from the sample. The acquired experimental data were analyzed using the GUPIXWIN software. The results show that the SRM data and the data obtained using the PIXE technique are in good agreement.     

Data-fusion of high resolution X-ray CT,SEM and EDS for 3D and pseudo-3D chemical and structural characterization of sandstone

When dealing with the characterization of the structure and composition of natural stones, problems of representativeness and choice of analysis technique almost always occur. Since feature-sizes are typically spread over the nanometer to centimeter range, there is never one single technique that allows a rapid and complete characterization. Over the last few decades, high resolution X-ray CT (μ-CT) has become an invaluable tool for the 3D characterization of many materials, including natural stones. This technique has many important advantages, but there are also some limitations, including a tradeoff between resolution and sample size and a lack of chemical information. For geologists, this chemical information is of importance for the determination of minerals inside samples. We suggest a workflow for the complete chemical and structural characterization of a representative volume of a heterogeneous geological material. This workflow consists of combining information derived from CT scans at different spatial resolutions with information from scanning electron microscopy and energy-dispersive X-ray spectroscopy.     

A novel trilayer antireflection coating using dip-coating technique

We report a new structure for broadband antireflection coating by dip-coating technique, which has minimal cost and is compatible with large-scale manufacturing. The coatings are prepared by depositing SiO 2 sol-gel film on a glass substrate, subsequently depositing SiO 2 single-layer particle coating through electrostatic attraction, and depositing a final very thin SiO 2 sol-gel film to improve the mechanical strength of the whole coating structure. The refractive index of the structure changes gradually from the top to the substrate. The transmittance of a glass substrate has been experimentally found to be improved in the spectral range of 400 1 400 nm and in the incidence angle range from 0 to at least 45 . The mechanical strength is immensely improved because of the additional thin SiO 2 sol-gel layer. The surface texture can be applied to the substrates of different materials and shapes as an add-on coating.     

New calibration method for UV-VIS photothermal deflection spectroscopy set-up

Photothermal deflection spectroscopy has emerged as a useful technique for the determination of the absorption of materials with a small absorption coefficient. The technique offers relative values of the material absorptivity and, therefore, requires a calibration procedure in order to determine the absolute values. In this work, we present a new calibration method for a photothermal deflection spectroscopy set-up working in the UV-VIS, spectral range. The method is based on the use of reference samples with different levels of absorption. The samples, consisting of single thin films of amorphous carbon on transparent substrates, are optically characterized by means of spectrophotometric measurements. The accurate characterization of the samples enables the computation of their corresponding optical absorptivity in the PDS set-up. The calibration method is cross-checked by comparison of the measurements for the different reference samples and is finally applied to the study of the absorption of dielectric films in the UV.     

Thermal conductivity spectroscopy technique to measure phonon mean free paths

Size effects in heat conduction, which occur when phonon mean free paths (MFPs) are comparable to characteristic lengths, are being extensively explored in many nanoscale systems for energy applications. Knowledge of MFPs is essential to understanding size effects, yet MFPs are largely unknown for most materials. Here, we introduce the first experimental technique which can measure MFP distributions over a wide range of length scales and materials. Using this technique, we measure the MFP distribution of silicon for the first time and obtain good agreement with first-principles calculations.     

EDXRF微量元素分析在文物断源断代中的研究

能量色散X荧光分析（EDXRF）是重要的元素成分析方法之一,现已成为一种强有力的定性和精确定量的无损分析测试技术。能量色散X荧光分析（EDXRF）技术具有不破坏分析样品,并能快速进行钠（Z11）至铀（Z92）的多元素的同时分析,以及分析的浓度范围宽广,精度高等特点,因此,特别适合于进行文物材料的成分分析研究,尤其适合测试极其珍贵的古陶瓷完整文物样品分析与鉴定。文章运用该方法对选自杭州南宋官窑窑址、龙泉古窑址出土的14个样品的南宋官窑青瓷、南宋龙泉窑青瓷和现代仿古青瓷等进行了元素分析的对比研究,分析测定古陶瓷样品中胎、釉的主、次量及微量元素组成,寻找组成中的“指纹”特征元素,试图为古陶瓷文物的断源、断代研究和真伪鉴定提供科学依据。研究结果表明能量色散X荧光分析（EDXRF）是一种较理想的文物研究和鉴定的科学分析手段。     

慢正电子束流技术在金属/合金微观缺陷研究中的应用

正电子湮没谱学技术是研究材料微观结构非常有效的一种核谱学分析方法, 主要用于获取材料内部微观结构的分布信息, 特别是微观缺陷结构及其特性等传统表征方法难以获取的微观结构信息. 近年来, 在慢正电子束流技术快速发展的基础上, 正电子湮没谱学技术在薄膜材料表面和界面微观结构的研究中得到了广泛应用. 特别是该技术对空位型缺陷的高灵敏表征能力, 使其在金属/合金材料表面微观缺陷的形成机理、缺陷结构特性及其演化行为等研究方面具有独特的优势. 针对材料内部微观缺陷的形成、演化机理以及缺陷特性的研究, 如缺陷的微观结构、化学环境、电子密度和动量分布等, 正电子湮没谱学测量方法和表征分析技术已经发展成熟. 而能量连续可调的低能正电子束流, 进一步实现了薄膜材料表面微观结构深度分布信息的实验表征. 本文综述了慢正电子束流技术应用研究的最新进展, 主要围绕北京慢正电子束流装置在金属/合金材料微观缺陷的研究中对微观缺陷特性的表征和表面微观缺陷演化行为的应用研究成果展开论述.     

Disorder promotes ferromagnetism: rounding of the quantum phase transition in Sr(1-x)Ca(x)RuO3

The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr(1-x)Ca(x)RuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.