Showing papers in "Materials Science & Engineering R-reports in 2005"

Dispersion and alignment of carbon nanotubes in polymer matrix: A review

Abstract: Polymer/carbon nanotube (CNT) composites are expected to have good processability characteristics of the polymer and excellent functional properties of the CNTs. The critical challenge, however, is how to enhance dispersion and alignment of CNTs in the matrix. Here, we review recent progress and advances that have been made on: (a) dispersion of CNTs in a polymer matrix, including optimum blending, in situ polymerization and chemical functionalization; and (b) alignment of CNTs in the matrix enhanced by ex situ techniques, force and magnetic fields, electrospinning and liquid crystalline phase-induced methods. In addition, discussions on mechanical, thermal, electrical, electrochemical, optical and super-hydrophobic properties; and applications of polymer/CNT composites are included. Enhanced dispersion and alignment of CNTs in the polymer matrix will promote and extend the applications and developments of polymer/CNT nanocomposites.

Interfacial reactions between lead-free solders and common base materials

Abstract: The objective of this review is to study interfacial reactions between pure Sn or Sn-rich solders, and common base metals used in Pb-free electronics production. In particular, the reasons leading to the observed interfacial reactions products and their metallurgical evolution have been analyzed. Results presented in the literature have been critically evaluated with the help of combined thermodynamic–kinetic approach based on the concept of local equilibrium and microstructural knowledge. The following conclusions have been reached: Firstly, the formations of intermetallic compounds in solid/liquid reaction couples are primarily controlled by the dissolution processes of base metals. Other factors that need be considered are the thermodynamic driving force for the formation of intermetallic compounds, their structures and concentration profiles in liquid. Secondly, annealing of solder interconnections in solid state can drastically change the microstructures formed in the solid/liquid reactions, especially if only one of the components in the solder takes part in the interfacial reactions. Thirdly, additional elements can have three major effects on the binary reactions between a base metal and Sn: (i) they can increase or decrease the reaction/growth rates, (ii) the additives can change the physical properties of the phases formed, and (iii) they can form additional reaction products or displace the binary equilibrium phases by forming new reaction products. Finally, if the local stable or metastable equilibrium is established at the interface, stability information together with kinetic considerations can provide a feasible approach to analyze interfacial reactions, which can have significant impact on the reliability of soldered electronics assemblies.

Patterning with block copolymer thin films

Abstract: The nanometer-scale architectures in thin films of self-assembling block copolymers have inspired a variety of new applications. For example, the uniformly sized and shaped nanodomains formed in the films have been used for nanolithography, nanoparticle synthesis, and high-density information storage media. Imperative to all of these applications, however, is a high degree of control over orientation of the nanodomains relative to the surface of the film as well as control over order in the plane of the film. Induced fields including electric, shear, and surface fields have been demonstrated to influence orientation. Both heteroepitaxy and graphoepitaxy can induce positional order on the nanodomains in the plane of the film. This article will briefly review a variety of mechanisms for gaining control over block copolymer order as well as many of the applications of these materials. Particular attention is paid to the potential of perfecting long-range two-dimensional order over a broader range of length scales and the extension of these concepts to functional materials and more complex architectures.

Wet etching of GaN, AlN, and SiC : a review

Abstract: The wet etching of GaN, AlN, and SiC is reviewed including conventional etching in aqueous solutions, electrochemical etching in electrolytes and defect-selective chemical etching in molten salts. The mechanism of each etching process is discussed. Etching parameters leading to highly anisotropic etching, dopant-type/bandgap selective etching, defect-selective etching, as well as isotropic etching are discussed. The etch pit shapes and their origins are discussed. The applications of wet etching techniques to characterize crystal polarity and defect density/distribution are reviewed. Additional applications of wet etching for device fabrication, such as producing crystallographic etch profiles, are also reviewed.

Novel cold cathode materials and applications

Abstract: Field emission (FE) is based on the physical phenomenon of quantum tunneling, in which electrons are injected from the surface of materials into vacuum under the influence of an applied electric field. A variety of field emission cold cathode materials have been developed to date. In this review, we shall focus on several kinds of novel cold cathode materials that have been developed in the past decade. These include materials for microfabricated field-emitter arrays, diamond and related films, carbon nanotubes, other quasi one-dimensional nanomaterials and printable composite materials. In addition, cold cathode materials have a wide range of applications such as in flat panel displays, high-power vacuum electronic devices, microwave-generation devices, vacuum microelectronic devices and vacuum nanoelectronic devices. Applications are in consumer goods, military industries and also space technology. A comprehensive overview of the various applications is presented. Recently, recognizing the strong possibility that vacuum nanoelectronic devices using quasi one-dimensional nanomaterials, such as carbon nanotubes may emit electrons with driving voltages comparable to that of a solid-state device, there is a growing interest in novel applications of such devices. With such exciting opportunities, there is now a flurry of activities to explore applications far beyond those considered for the conventional hot cathodes that operate on thermionic emission. We shall discuss the details of a number of fascinating potential applications.

Shearography: An optical measurement technique and applications

Abstract: This article reviews shearography and its applications. Shearography is a laser-based technique for full-field, non-contacting measurement of surface deformation (displacement or strain). It was developed to overcome several limitations of holography by eliminating the reference beam. It does not require special vibration isolation; hence, it is a practical tool that can be used in a field/factory environment. Shearography has already received considerable industrial acceptance, in particular, for non-destructive testing. In non-destructive testing, shearography reveals defects in an object by identifying defect-induced deformation anomalies. Other applications of shearography include strain measurement, material characterization, residual stress evaluation, leak detection, vibration studies and 3-D shape measurement.

Thermodynamics of metastable phase nucleation at the nanoscale

Abstract: Chemical and physical routes under conditions of moderate not extreme temperatures and pressures are generally used to synthesize nanocrystals and nanostructures with metastable phases. However, the corresponding bulk materials with the same metastable structures are prepared under conditions of high temperatures or high pressures. The size effect of nanocrystals and nanostructures may be responsible for the formation of these metastable phases at the nanometer size. To date, there has not been a clear and detailed understanding of the effects causing the formation of the metastable structures from the viewpoint of thermodynamics. There is no a clear insight into which chemical and physical origins leading to the tendency of the metastable phases emerging at the nanoscale. We have proposed universal thermodynamic approach on nanoscale to elucidate the formation of the metastable phases taking place in the microphase growth. In this review, we first introduce the fundamental concepts and methods of the thermodynamic approach on nanoscale (so-called nanothermodynamics). Note that our nanothermodynamics, by taking into account the size-dependence of the surface tension of nanocrystals, differs from the thermodynamics of small systems proposed by Hill [T.L. Hill, J. Chem. Phys. 36 (1962) 3182; T.L. Hill, Proc. Natl. Acad. Sci. U.S.A. 93 (1996) 14328; T.L. Hill, R.V. Chamberlin, Proc. Natl. Acad. Sci. U.S.A. 95 (1998) 12779; T.L. Hill, J. Chem. Phys. 34 (1961) 1974; T.L. Hill, J. Chem. Phys. 35 (1961) 303; T.L. Hill, Nano Lett. 1 (2001) 273; T.L. Hill, R.V. Chamberlin, Nano Lett. 2 (2002) 609; T.L. Hill, Nano Lett. 1 (2001) 159]. Our thermodynamic theory emphasizes the size effect of the surface tension of nanocrystals on the stable and metastable equilibrium states during the microphase growth. Then, taking the syntheses of diamond and cubic boron nitride (c-BN) nanocrystals as examples, we summarize the applications of the nanothermodynamics to elucidate the nucleation of diamond and related materials nanocrystals in various moderate environments. Firstly, we studied diamond nucleation upon chemical vapor deposition (CVD), and found out that the capillary effect of the nanosized curvature of diamond critical nuclei could drive the metastable phase region of the nucleation of CVD diamond into a new stable phase region in the carbon thermodynamic equilibrium diagram. Consequently, the diamond nucleation is preferable to the graphite phase formation in the competing growth of diamond and graphite upon CVD. Similarly, c-BN nucleation upon CVD has been investigated. Secondly, we investigated the c-BN nucleation taking place in the high-pressure and high-temperature supercritical-fluids systems under conditions of the low-threshold-pressures (<3.0 GPa) and low-temperatures (<1500 K), and predicted the threshold pressure of the formation of c-BN in the high-pressure and high-temperature supercritical-fluids system. Thirdly, to gain a clear insight into the diamond nucleation upon the hydrothermal synthesis and the reduction of carbide (HSRC), we have performed the thermodynamic approach on nanoscale, in which the diamond nucleation is preferable to the graphite phase formation in the competing growth between diamond and graphite upon HSRC. We theoretically predicted that the pressure of 400 MPa should be the threshold pressure for the diamond synthesis by HSRC in the metastable phase region of diamond in the carbon phase diagram. More importantly, these theoretical results above are consistent with the experimental data. Additionally, the developed nanothermodynamics was used to study the theory of nucleation and growth of diamond nanowires inside nanotubes. Accordingly, the thermodynamic

Development of nanophosphors—A review

Abstract: Nanophosphors have been extensively investigated during the last decade due to their application potential for various high-performance displays and devices. These act as a strategic component in almost all displays. Synthesis of nanophosphors can be accomplished in two ways namely, chemical and physical methods. Under chemical methods, different routes such as colloidal, capping, cluster formation, sol–gel, electrochemical, etc., are being followed. Physical methods widely used are molecular beam epitaxy, ionised cluster beam, liquid metal ion source, consolidation, sputtering and gas aggregation of monomers. Chemical precipitation in presence of capping agents, reaction in microemulsions, sol–gel reaction and autocombustion are commonly used techniques for synthesis of nanophosphors. However, the particle size has to be restricted to 3–5 nm to get the real advantage of quantum confinement. In other words, the particle size must be less than twice of Bohr radii of exciton as quantum confinement regime is limited to that size. A brief review of different synthesis techniques employed all over the world for the development of industrially important nanophosphors and extent of particle size reduction achieved is discussed.

Adhesion quantification methods for wafer bonding

Abstract: Integrated circuit technology and its later development, microsystem technology, make good use of wafer bonding. An increased interest in bond adhesion quantification can be anticipated when wafer bonding is optimized for complex microelectro-mechanical systems where the bonding process must take every other component into consideration when it comes to cost, temperature budget and process compatibility. Adhesion quantification methods for evaluation of bonded brittle material, especially direct wafer bonded, are reviewed in this paper. The most commonly utilized methods, namely the double cantilever beam, tensile, chevron and blister test methods, are thoroughly covered and miscellaneous techniques are mentioned. The physics background and modeling presented by different authors are examined. Based on models and experiences made in neighboring research fields improvements are suggested. Practical capabilities and limitations and origin and mitigation of measurement errors are addressed. Experimental foundation on fundamental knowledge in solid mechanics and the statistical nature of brittle fracture behavior is emphasized. The methods’ adequacy are compared and ranked for three types of uses: general understanding, bonding process optimization and quality control. It is shown that the quality of all commonly used methods for adhesion quantification of wafer bonding can be dramatically improved by small means.

Analysis of interactions in multicomponent polymeric systems: The key-role of inverse gas chromatography

Abstract: The properties of a polymeric system are a consequence of the interactions that occur between the various components of these complex systems. These components may vary significantly in terms of chemical nature (e.g. organic/inorganic), physical properties (e.g. particle size, surface area, molecular weight), structural characteristics and proportion in the formulations composition. This review paper addresses the major approaches in use regarding the analysis of the interactions that occur between the polymeric system components and the use of such approaches in the interpretation of the chemical, physical and thermodynamic properties of these systems. Special attention is given to the technique of inverse gas chromatography. A case study is presented, where use was made of inverse gas chromatography to characterize thermodynamically the surface of the major components of pigmented PC/PBT blends. The concept of Lewis acidity/basicity was used in the interpretation of the intermolecular interactions nature and potential in these blends, as encountered in phase separation and phase preferences phenomena and as expressed in the morphology, the physical and the mechanical properties of these commercially important composites.