MOCVD法生长SAWF用ZnO/Diamond/Si多层结构

使用等离子体辅助MOCVD系统在金刚石,硅衬底上成功地制备了氧化锌多层薄膜材料,通过两步生长法对薄膜质量进行了优化。XRD测试显示优化后的样品具有c轴的择优取向生长,PL谱测试表明样品经优化后不仅深能级发射峰消失,同时紫外发射峰增强。对优化后的样品的表面测试显示出较低的表面粗糙度。比较氧化锌多层薄膜结构的声表面波频散曲线,ZnO薄膜声表面滤波器受膜厚和衬底材料的影响较大。当ZnO薄膜较薄时,在它上面的传播速度将与衬底上的传播速度接近,与其他衬底上生长的薄膜相比,以金刚石这种快声速材料为衬底的ZnO多层薄膜结构,声表面波滤波器的中心频率将提高1倍左右。     

Multifrequency characterization of viscoelastic polymers and vapor sensing based on SAW oscillators

Simplified relations for the changes in SAW velocity and attenuation due to thin polymer coatings and vapor sorption are presented by making analytic approximations to the complex theoretical model developed earlier by Martin et al. [Anal. Chem. 66 (14) (1994) 2201–2219]. The approximate velocity relation is accurate within 4% for the film thicknesses up to 20% of the acoustic wavelength in the polymer film, and is useful for analyzing the mass loading, swelling and viscoelastic effects in SAW vapor sensors. The approximate attenuation relation is accurate within 20% for very thin films, (less than 2% of the acoustic wavelength in the film). Based on these relations, a new procedure for determination of polymer viscoelastic properties is described that exploits the frequency dependence of the velocity and attenuation perturbations, and employs multifrequency measurement on the same SAW platform. Expressions for individual contributions from the mass loading, film swelling and viscoelastic effects in SAW vapor sensors are derived, and their implications for the sensor design and operation are discussed. Also, a new SAW comb filter design is proposed that offers possibility for multimode SAW oscillator operation over a decade of frequency variation, and illustrates feasibility for experimental realization of wide bandwidth multifrequency SAW platforms.     

High frequency SAW stabilized oscillators for chemical agents sensors

The surface acoustic wave (SAW) chemical agents sensors usually operate in the oscillator feed-back configuration. It converts a molecular interactions between SAW surface and chemisensitive layer placed on it to relative easy to measurement electrical quantities (most often it is an operating frequency or phase change). Although in the SAW sensors the key role play chemisensitive coatings but nearly as important as the coatings are electronic circuits cooperating with SAW devices. The results of theoretical calculations show that the SAW sensors operating frequency increasing is profitable from the sensitivity point of view. Unfortunately, an advantageous sensitivity-frequency dependence is hard to apply because of decreasing of SAW device dimensions and thereby the area of the chemisensitive layer with the operating frequency. The smaller area of the layer, the smaller amount of detecting gas particles sorbed and the weakest response of the sensor. It is possible to avoid the problem using special constructions of SAW stabilised oscillators. In the paper such constructions have been described.     

X-Ray Study of Thermotropic Mesophases of an Adsorbed <Emphasis Type="Italic">n</Emphasis>-Triacontanol Film at the <Emphasis Type="Italic">n</Emphasis>-Hexadecane–Water Interface

Using synchrotron radiation with a photon energy of 15 keV, the molecular structure of an adsorbed n-triacontanol layer at the n-hexadecane–water interface in its different phase states has been studied by the diffuse X-ray scattering method. The analysis of the experimental data shows that a transition to the multilayer adsorption occurs at a temperature below the two-dimensional vapor–liquid transition at the interface. This transition has been attributed to a feature in the temperature dependence of the concentration of micelles in a surface layer 100–200 Å thick.     

Electrophysical response of thin-film titanium-containing nanocomposites based poly(<Emphasis Type="Italic">p</Emphasis>-xylylene) on change of the atmosphere

The dependence of the conductivity of poly(p-xylylene)-based titanium-containing nanocomposites on the extent of adsorption of polar molecules from the atmosphere is observed, while nonpolar molecules induce no such changes. A relationship between the dipole moment of the adsorbate molecule and the sensitivity of the composite conductivity to its vapor is revealed. A composite with filler content near the percolation threshold shows the highest sensitivity to polar gases. A model is proposed to explain the change in the resistance upon vapor adsorption by the formation of a surface dipole. Since the investigated vapors are adsorbed as donors, their adsorption reduces the electron work function of the nanoparticles, thereby increasing the conductivity. The dielectric spectrum of a nanocomposite containing 2 vol % titanium dioxide is very closely approximated by the Cole–Cole and Maxwell–Wagner equations, whereas the frequency dependence of the electric modulus is close to that predicted by the Debye law.     

由于表面粗糙引起的激光声表面波色散的实验和理论研究

表面粗糙是材料制造过程中必有的副产物, 粗糙表面会引起其中传播的声表面波的速度发生变化. 在利用激光声表面波对材料性质进行评估时, 常用宽带的激光声表面波速度频散特性对材料性质进行反演. 为了研究表面粗糙度是否能作为反演的特征参数之一, 本文建立了激光在表面粗糙样品中激发声表面波、聚偏氟乙烯换能器宽带接收声表面波的实验装置来研究不同粗糙度表面对声表面波速度的影响; 理论上建立了激光在粗糙表面中激发声表面波的计算模型, 利用有限元法得到声表面波的时域特征, 并进一步得到声表面波的速度色散曲线, 理论结果和实验结果能很好地拟合. 这为利用激光声表面波对表面粗糙的评估提供理论和实验依据. 关键词： 表面粗糙 激光声表面波 速度色散 聚偏氟乙烯传感器 有限元法     

Precursor effect during water vapor diffusion in a porous medium

In water vapor diffusing through a granular adsorbent layer into vacuum, water molecules overcoming an adsorption column without an appreciable delay as pulses, i.e., precursors of the main front, were detected. This effect is explained by the sluggishness of water molecule diffusion into adsorbent grains and the dependence of the adsorption capacity of molecules on their rotational states via the Stark effect in a surface electric field of an adsorbent.     

IR spectroscopic study of surface properties of amorphous water ice

The state of the surface of amorphous ice with a specific surface area of about 160 m²/g obtained by the condensation of water vapor at 77 K is studied by IR spectroscopy. As the temperature increases to 130–160 K, absorption bands of surface hydroxyl groups vanish, whereas changes in bands characteristic of hydroxyl groups in the bulk of ice are indicative of a phase transition of ice from amorphous to the polycrystalline structure. The surface sites of amorphous ice are characterized with low-temperature adsorption of carbon monoxide. It is shown that there are two types of CO adsorption sites, free hydroxyl groups and oxygen atoms of surface coordinately unsaturated water molecules. Upon adsorption of nitrogen, methane, and carbon monoxide, in addition to the perturbation of surface OH groups, reversible changes in the spectrum are observed in the region of vibrations of bulk hydroxyls, which indicate that the strength of hydrogen bonds between water molecules in the surface layer of icy particles increases approaching the strength of these bonds in the crystal and that the ice surface becomes less amorphous. These results indicate that the properties of the ice surface layer substantially depend on the presence of adsorbed molecules.     

Water vapor adsorption onto activated carbons prepared from cattle manure compost (CMC)

Activated carbons were prepared from cattle manure compost (CMC) using zinc chloride activation. The structural and surface chemical characteristics of CMC-based activated carbons were determined by N₂ adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the prepared activated carbons with various pore structure and surface nature were examined, and the mechanism of water adsorbed onto activated carbon was also discussed. The results show that the adsorption of water vapor on carbons begins at specific active sites at low relative humidity (RH), followed by micropore filling at medium RH through the formation of pentamer cluster of water molecules in the narrow micropores. The water vapor adsorption capacity of activated carbon is predominantly dependent on its pore volume and surface area. Although capillary condensation is not the mechanism for water adsorption onto activated carbon, water can adsorb on narrow mesopore to some extent.     

Graphene/LiNbO3 surface acoustic wave device based relative humidity sensor

This study described relative humidity (RH) sensing using a graphene/128° YX LiNbO₃ surface acoustic wave (SAW) device. The resonant frequency of the device decreased in a two-stage manner as the RH increased. For a low RH range (RH < 50%), a frequency downshift of 1.38 kHz per 1% RH change was observed. This was attributed to mass loading of the SAW propagation surface due to the adsorption of water molecules by the graphene surface. For a high RH range (RH > 50%), a frequency downshift of 2.6 kHz per 1% RH change was obtained, which was due to the change in elastic grapheme properties. The mass loading effect of the water layer was less effective at high temperature, resulting in a lower temperature coefficient of resonant frequency (TCF).     

Structure and composition of adsorbed layers formed by sequential exposure of Pt(100) and Pt(111) to pairs of compounds: Solvents and electrolytic substances

Studies are reported of the interaction of vapor of typical polar solvents and electrolytes at electrodes having Pt(111) or Pt(100) single-crystal surfaces: water, pyridine, acetonitrile, dimethyl-sulfoxide, hydrogen bromide, iodine, sulfur dioxide, acrylic acid, and ammonia. Exposure was extended from low pressures (about 10^?5 Torr) to pressures approaching the vapor pressure of the pure liquid. The results indicate that these typical electrochemical materials adsorb strongly to the clean Pt surface but once adsorbed tend not to react with each other. However, analysis of LEED patterns and Auger intensities suggests that exposure of an adsorbed layer of solvents such as dimethylsulfoxide to iodine results in adsorption of the halogen and molecular re-orientation of the adsorbed solvent.     

Investigation of titanium nitride coating by broadband laser ultrasonic spectroscopy

We present a laser ultrasonic method to investigate a titanium nitride(TiN) coating specimen.The technique is based on the principle of surface acoustic wave (SAW) dispersion during acoustic propagation on a half-space with the presence of a thin layer.Due to the high efficiency of laser line-source excitation,we have been able to generate and detect a SAW with an excellent signal-to-noise ratio in a wide frequency band.An inverse fitting algorithm was employed to extract simultaneously the thickness and the elastic parameters of the TiN coating from the experimental SAW velocity dispersion curve.     

基于气液混合层的吸附气体稳定性机理

实验表明溶解在水中的气体会在疏水表面吸附和集聚，学界普遍认为其有"纳米气泡"和"微气饼"两种存在形式。对于这两种形式，经典理论无法解释其稳定存在。本文采用分子动力学方法对气体吸附和积聚过程进行模拟，结果显示吸附气体表层覆盖着气液混合层，对该混合层的形状和特性进行分析和研究，发现该气液混合层具有较高的粘度并且对气体的扩散有明显的抑制作用。     

Effect of strong magnetic fields on gas adsorption

The effect of strong pulsed magnetic fields on gas adsorption at dielectric surfaces is demonstrated. We describe the experimental technique and the results indicating a considerable increase in the surface concentration of the adsorbed substance under the action of pulsed magnetic fields with an induction amplitude up to 50 T. The increase in the lifetime and stability of the adsorbed layer is observed. It is shown that the effect of magnetically induced sorption is also manifested in the interaction of iodine vapor with the surface of dielectrics.     

Performance of Non-Contact Linear Actuators Driven by Surface Acoustic Waves*

A new kind of non-contact linear actuator （motor） driven by surface acoustic waves （SAWs） is presented, in which the stators are made from SAW delay lines using 128° YX-LiNbO3 substrates. A fluid layer is introduced between the slider and the stator of the actuator, and the slider is a circular aluminum disk suspended on the surface of the liquid （water） layer. As the SAW is excited on the stator, the SAW is converted to a leaky wave in the interface of the stator and the liquid, and then propagates into the liquid. Owing to the nonlinear effect of wave propagation, acoustic streaming is generated, which pushes the slider to move. By the experiments, the relations between the slider velocity and the experimental parameters, such as the exciting voltage of the SAWs, the thickness and the kinematic viscosity of the liquid layer, are obtained.     

The special features of adsorption and the kinetics of decomposition of monosilane molecules on the epitaxial surface of silicon

Analytic equations relating the rate of the incorporation of silicon atoms into a growing crystal to the characteristic frequency of the pyrolysis of silane molecules on the surface of silicon were obtained over the temperature range corresponding to the epitaxial growth of silicon films. As distinct from the earlier works, it was assumed that adsorbed silicon atoms and monosilane molecules formed double bonds with the surface. The data of technological experiments for the most extensively used pyrolysis models obtained thus far were used to determine the region of the characteristic frequencies of the decomposition of hydride molecule radicals adsorbed on the surface of a silicon plate over the temperature range 450–700°C. The temperature dependence of the frequency of monosilane molecule decomposition was shown to be to a great extent determined by the form of the temperature dependence of the $ \tilde v_{SiH_2 }^0 $ \tilde v_{SiH_2 }^0 preexponential factor. It was also found that the characteristic frequency of the decomposition of silane molecules was sensitive to the stage of pyrolysis at which hydrogen atoms released from silane molecules were captured by the surface. Decomposition occurred at the highest rate if hydrogen molecules were adsorbed at the stage of the adsorption of monosilane. The lowest rate of decomposition was observed if hydrogen molecules were adsorbed at the stage of the decomposition of radicals already captured by the surface. The temperature dependence of the coefficient of adsorption of monosilane molecules was characterized by a negative activation energy of the process for almost all the most important system models over the temperature range of growth. At elevated growth temperatures, the adsorption of monosilane molecules by the surface of silicon proceeded via an intermediate state characterized by the difference of desorption and chemisorption energies on the order of 0.28 eV.     

Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics

A dual frequency mixing technique has been developed for measuring velocity changes caused by material nonlinearity. The technique is based on the parametric interaction between two surface acoustic waves (SAWs): The low frequency pump SAW generated by a transducer and the high frequency probe SAW generated and detected using laser ultrasonics. The pump SAW stresses the material under the probe SAW. The stress (typically <5 MPa) is controlled by varying the timing between the pump and probe waves. The nonlinear interaction is measured as a phase modulation of the probe SAW and equated to a velocity change. The velocity-stress relationship is used as a measure of material nonlinearity. Experiments were conducted to observe the pump-probe interaction by changing the pump frequency and compare the nonlinear response of aluminum and fused silica. Experiments showed these two materials had opposite nonlinear responses, consistent with previously published data. The technique could be applied to life-time predictions of engineered components by measuring changes in nonlinear response caused by fatigue.     

Adsorption of polyelectrolytes at an oppositely charged surface

We develop a scaling theory of polyelectrolyte adsorption at an oppositely charged surface. At low surface charge densities, the thickness of the adsorbed layer is determined by the balance between electrostatic attraction to the charged surface and chain entropy. At high surface charge densities, it is determined by the balance between electrostatic attraction and short-range monomer-monomer repulsion. These different stabilizing mechanisms result in the nonmonotonic dependence of the layer thickness on the surface charge density.     

Determination of the adsorbed layer thickness of polystyrene on glass surface by viscosity measurements to the extremely dilute concentration region

A newly developed theory regarding solute adsorption effect in viscometry of a polymer solution is reviewed and extended for determining the adsorbed layer thickness of polystyrene on a glass surface in viscosity measurements. This theory can adequately describe the concentration dependence of the viscosity of a polymer solution measured by glass capillary viscometer to the extremely dilute concentration region. Using this theory, both the free polymer chains in solution and the adsorbed polymer chain on the viscometer inner wall surface can be characterized. Examples are given for the characterization of six polystyrene samples with different molar masses by measuring their solution viscosity in benzene.     

Nonlinear frequency shift in surface acoustic wave resonator operating as a gas sensor

The shift in the resonance frequency of a two-port quartz surface acoustic wave (SAW) resonator operating as a gas sensor without a selective layer is studied versus the power of an SAW excited in the resonator. At working frequencies of the resonator (≈389 MHz) placed in the flow of moisture-containing nitrogen gas, an anomalously large positive shift of the resonance frequency is observed as the SAW power exceeds 1 mW. This shift is one order of magnitude larger than that due to the nonlinear amplitude-frequency effect, which is known for quartz SAW resonators. Possible physical mechanisms underlying this phenomenon are analyzed. Experimental data indicate that such a shift is associated with the influence of a powerful SAW on sorption processes taking place on the active surface of the resonator rather than being a direct consequence of heating of the SAW substrate by the powerful SAW.