Showing papers in "Materials Science & Engineering R-reports in 2012"

Recent progress in cathode materials research for advanced lithium ion batteries

Abstract: New and improved materials for energy storage are urgently required to make more efficient use of our finite supply of fossil fuels, and to enable the effective use of renewable energy sources. Lithium ion batteries (LIB) are a key resource for mobile energy, and one of the most promising solutions for environment-friendly transportation such as plug-in hybrid electric vehicles (PHEVs). Among the three key components (cathode, anode and electrolyte) of LIB, cathode material is usually the most expensive one with highest weight in the battery, which justifies the intense research focus on this electrode. In this review, we present an overview of the breakthroughs in the past decade in developing high energy high power cathode materials for lithium ion batteries. Materials from six structural groups (layered oxides, spinel oxides, olivine compounds, silicate compounds, tavorite compounds, and borate compounds) are covered. We focus on their electrochemical performances and the related fundamental crystal structures, solid-state physics and chemistry are covered. The effect of modifications on both chemistry and morphology are discussed as well.

Thermomigration in solder joints

Abstract: In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10 μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1 °C across a micro-bump, the temperature gradient will be 1000 °C/cm, which can cause thermomigration at the device operation temperature around 100 °C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds.

Radiation-Induced Degradation of Stainless Steel Light Water Reactor Internals

Abstract: In order to provide a scientific basis for proposed life extension of current light water reactors, the radiation-induced degradation of stainless steel reactor internals will be discussed. A brief review of the basic radiation damage effects in stainless steels at LWR relevant conditions will be presented. It will be discussed how these basic effects result in the key degradation modes that have been identified by light water reactor experience to date, as well as the possibility for more severe degradation or new forms of degradation under extended service conditions. The forms of degradation that will be discussed include radiation hardening, embrittlement, dimensional stability (e.g., creep and swelling), radiation-induced segregation and precipitation, transmutation effects and irradiation-induced stress corrosion cracking.

Elastic modeling of bone at nanostructural level

Abstract: Bone is a connective tissue which gives body its support and stability. In mechanical terms, bone is a nanocomposite material with a complex hierarchical structure which contributes to bone's excellent mechanical properties, including high stiffness, strength and fracture toughness, and light weight. At nanoscale, cross-linked collagen molecules, hydroxyapatite (HA) nanocrystals, water, and a small amount of non-collagenous proteins (NCPs) form mineralized collagen fibrils (MCF). The MCF serves as the primary building block of bone, and, thus, its physical and mechanical characterization is critical for finding structure–property relations in bone and understanding bone's overall behavior. In this paper, we review the composition and structure of the MCF and summarize the existing models proposed in literature to predict its effective elastic response. These models can be classified into the following four categories: I. Models based on strength of materials approach which are mainly variants of Voigt and Reuss bounds. Most of such models were originally proposed for characterization of composite materials; however, they are also applicable to model a MCF as a collagen–HA composite. II. Models based on micromechanics theories. III. Computational models, involving mostly a finite element method (FEM). IV. Atomistic simulations using molecular dynamics (MD). Each of these types of models has some advantages and disadvantages. The strength of materials models are simpler mathematically but they involve approximate solutions, while the micromechanics approaches usually involve simpler geometrical models which are solved more rigorously. Computational models, based mainly on the finite element method, can account more precisely for the structural features of bone including collagen–HA arrangement, collagen cross-links, and collagen–HA interphase. MD simulations, conducted at the atomic level and over very small regions, provide insights into properties of collagen molecules and fibrils, the effect of collagen cross-linking, and collagen–HA interphase, and can serve as inputs for continuum-based models. In this paper, we outline some representative models of bone at nanoscale (mineralized collagen fibril) and discuss the assumptions, limitations, and drawbacks of these models, present their comparison, and offer recommendations on the future work in this area. Such discussion will help to develop more complete models of MCF addressing physical, mechanical, and biological aspects of bone's behavior at the nanoscale. Furthermore, it will shed light on designs of collagen–HA nanocomposites with desired mechanical properties which can be used as biomaterials for orthopedic applications such as surface coatings for implant materials, as bone substitutes, and as scaffolds for bone tissue regeneration.

Methodology of materials discovery in complex metal hydrides using experimental and computational tools

Abstract: We present a review of the experimental and theoretical methods used in the discovery of new metal–hydrogen materials systems for hydrogen storage applications. Rather than a comprehensive review of all new materials and methods used in the metal hydride community, we focus on a specific subset of successful methods utilizing theoretical crystal structure prediction methods, computational approaches for screening large numbers of compound classes, and medium-throughput experimental methods for the preparation of such materials. Monte Carlo techniques paired with a simplified empirical Hamiltonian provide crystal structure candidates that are refined using density functional theory. First-principle methods using high-quality structural candidates are further screened for an estimate of reaction energetics, decomposition enthalpies, and determination of reaction pathways. Experimental synthesis utilizes a compacted-pellet sintering technique under high-pressure hydrogen at elevated temperatures. Crystal structure determination follows from a combination of Rietveld refinements of diffraction patterns and first-principles computation of total energies and dynamical stability of competing structures. The methods presented within are general and applicable to a wide class of materials for energy storage.

Secondary ion mass spectrometry of dopant and impurity elements in wide bandgap semiconductors

Abstract: Based on secondary ion mass spectrometry (SIMS) measurements, we have compiled state-of-the-art data concerning dopant elements and natural impurities in the wide bandgap semiconductor materials diamond, SiC, ZnSe, GaN and AlN. Samples were prepared by ion implantation of different elements into these materials and post-implantation thermal annealing. SIMS depth profiling techniques were used to determine atomic depth profiles of implanted elements and subsequent changes produced by annealing. Range statistics and SIMS relative sensitivity factors were established for major dopant and impurity elements in these wide bandgap materials. Results of these studies are presented in tabular form along with representative depth profile figures.