bluuebunny/crossref_metadata_2025 huggingface dataset:
Tribological Investigations Using Friction Force Microscopy
Authors: Overney R. , Meyer E.
Published: August 2016

Many attempts have been made in recent centuries to investigate friction, adhesion, lubrication, and wear. Most of the experimental approaches and theories were based on macroscopic experiments, such as tensile and indentation tests. For a long time, only the bulk properties of the materials were considered.

Late in this century a new term was created combining all of the above-mentioned properties which deal with the science of interacting material interfaces in relative motion: tribology. The state of the art of science today reveals that processing in nature depends strongly on interfaces that cannot be described only by bulk properties. Tribologists realize they must study the sliding surfaces by analytical surface-science tools. With the surface force apparatus developed by J.N. Israelachvili and D. Tabor, we have a surface analysis tool that provides new insight into the field of macroscopic sliding contact of lubricated systems.

After Amontons' laws were established as a first attempt to describe sliding friction analytically, theories were advanced over the course of this century. A classic discipline was developed: contact mechanics. More quantitative treatments of friction were developed by various authors. The energy dissipation in most processes in tribology induced the theorists to consider the sliding bodies as spring models creating phonon-phonon interactions. And with modern computer facilities, they started to perform computational experiments whenever classical experiments could not provide information on the submicron scale.
Interfacial Junctions and Cavitation
Authors: Landman Uzi , Luedtke W.D.
Published: August 2016

Understanding the atomistic mechanisms, energetics, structure, and dynamics underlying the interactions and physical processes that occur when two materials are brought together, separated, or rub against each other (hence the term tribology, from the Greektribos, meaning to rub) is fundamentally important to many basic and applied problems. Examples include adhesion, capillarity, contact formation, surface deformation, elastic and plastic response characteristics, hardness, micro- and nanoindentation, friction, lubrication, wear, fracture, atomic-scale probing, and modifications and manipulations of materials surfaces. These considerations have for over a century motivated extensive theoretical and experimental research into the above phenomena and their technological consequences.

Explorations of materials systems and phenomena in the nanoscale regime often require experimental probes and theoretical and computational methods that allow investigations with refined spatial, as well as temporal, resolution. Consequently, until recently most theoretical approaches to the above issues, with a few exceptions, have been anchored in continuum elasticity and contact mechanics. Experimental observations and measurements of surface forces and the consequent materials response to such interactions have been macroscopic in nature.
Simulation of Nanometer-Scale Deformation of Metallic and Ceramic Surfaces
Authors: Belak James , Boercker David B. , Stowers Irving F.
Published: August 2016

The precision machining of metal surfaces and the ductile-regime grinding of ceramic surfaces are examples of fundamental cutting processes used in fabricating high-tolerance parts. Components with dimensional tolerances of a few tens of nanometers are currently being produced by direct machining with single-point diamond tools. Despite the ability to fabricate these parts, little is understood of the basic deformation mechanisms that determine how material is removed and deformed, how a tool-tip interacts with a workpiece, how induced surface and subsurface damage occurs, and how cutting tools wear.

The key to solving these problems is a fundamental understanding of basic tribological processes such as surface indentation and scraping. Indentation experiments measure the mechanical response of a surface, the onset of plastic deformation, and material hardness. Macroscopic hardness measurements have been shown to correlate well with observed tensile yield strengths. Microscopic indentation studies, where the indentation size is smaller than the material grain size, show new and interesting phenomena. In the pioneering work of Gane and Bowden, no permanent penetration occurred until a critical load was achieved. They related this critical yielding to the theoretical shear strength in the metal, the strength required to create dislocations. Yielding of this sort has since been observed by many investigators.
Electroless Cu for VLSI
Authors: Cho James S.H. , Kang Ho-Kyu , Wong S. Simon , Shacham-Diamand Yosi
Published: August 2016

Interconnection technology is a key factor in the continual advancement of integrated systems. The rapid increase in device density and circuit complexity through scaling demands a similar increase in the interconnection density. Traditionally, this is achieved by reducing the metal pitch as well as gradually increasing the number of interconnection levels. As the width and spacing of interconnections are scaled down to submicron dimensions at the chip level and micron dimensions at the board level, signal delay, crosstalk, electromigration, and stress-induced migration become important concerns.

Cu holds promise as an alternative metallization material to Al alloy due to its low resistivity and ability to reliably carry high-current densities. Cu has a bulk resistivity of 1.68 μΩ-cm, whereas Al has a bulk resistivity of 2.65 μΩ-cm. The only metal with a resistivity lower than Cu is Ag. Since Cu has a melting point and atomic weight both higher than Al, it is expected to have better resistance to electromigration, although properties such as grain structure and resistance to corrosion at high temperatures may also affect electromigration characteristics.
New Developments in Atom Probe Techniques and Potential Applications to Materials Science
Authors: Blavette Didier , Menand Alain
Published: August 2016

The emergence of the field ion microscope, invented by E.W. Müller in 1951, allowed, for the first time, direct observation of metallic materials in real space on an atomic scale. Field ionization of rare gas atoms near the surface of the material allowed production of an atomic resolution image of the material; field evaporation of surface atoms allowed observation into the depth of the sample.

Field ion microscopy (FIM) has produced numerous impressive results, one of them being three-dimensional reconstruction of defect cascades produced by heavy ion irradiation in metals. FIM techniques have also contributed in a spectacular way to the knowledge of the crystallographic structure of grain boundaries. Surface diffusion as well as surface reconstruction phenomena are applications where, again, FIM gave surprisingly detailed information. This overview of applications of FIM in materials science emphasizes recent work. Detailed information and descriptions of earlier work can be found elsewhere.

During the development of FIM techniques, the basic shortcomings of the instrument for investigating metallic alloys were rapidly recognized. In Cu3Pd alloys, for instance, Cu atoms are darkly imaged and therefore appear as vacancies similar to Co atoms in PtCo and Pt3Co. It is thus impossible to distinguish vacancies from solute atoms. This clearly limited the quantitative use of FIM to study short-range

order in dilute alloys or to investigate long-range order (Figure 1). Similarly, in two-phase materials, precipitates often give rise to quite visible contrast. For instance, in nickel-based alloys, aluminum-enriched precipitates appear in bright contrast. It is therefore possible to find the size and also the number density of particles present in the material. However, FIM does not provide any composition data.
A Unified Approach to Solid-State Amorphization and Melting
Authors: Lam Nghi Q. , Okamoto Paul R.
Published: August 2016

The crystalline-to-amorphous (c-a) phase transformation can be induced by a variet of solid-state processes ranging from energetic particle irradiation, interface inter diffusion reactions, hydrogen charging and mechanical deformation to the application of high pressures. During the past decade, such transformations have become the focus of considerable research not only because of their potential technological applications, but also because of strong scientific interest in the relationship between the c-a transition and the melting process.

A common feature underlies all solid state amorphization processes: The atomic disorder created in the crystalline lattice in the form of static atomic displacement can induce volume change and elastic softening of the lattice. A particularly striking example of the softening effect is shown in Figure 1 for the case of radiation-induced amorphization of the intermetallic compound Zr3Al. The compound, which has the Ll2 (Cu3Au)-type superlattice structure, was irradiated with energetic ion at room temperature in a high-voltage electron microscope interfaced to a tandem ion accelerator. The rapid decrease in the intensities of both fundamental and superlattice reflections show that irradiation introduces antisite defects (chemical disorder) as well as static atomic displacements. The disordering of the long-range ordered structure, which occurs prior to the onset of amorphization, is accompanied by a volume expansion of about 2.5% and a ~25% decrease in the average velocity of sound. This decrease in sound velocity corresponds to a ~50% decrease in the average shear modulus, which is comparable to that observed for many metals during heating to melting. The volume dependence of this disorder-induced elastic softening is also similar to that associated with heating. In both cases, the shear modulus is a linearly decreasing function of volume expansion. However, for a given amount of expansion, the softening associated with static atomic displacements is nearly twice as large as that associated with increasing anharmonic lattice vibrations.
Microstructure and Processing of Ice Cream
Authors: Kilara Arun , Sharkasi Tawfik
Published: August 2016

Ice cream and frozen desserts are popular throughout the world, but the United States is the largest consumer of ice cream. In the United States it is estimated that 46.8 pints per capita of ice cream and frozen desserts were consumed in 1992.

Physically, ice cream is a foam in which the continuous phase is a partially frozen emulsion. Foams are also important for foods such as whipped toppings, souffle, mousse, meringue, and angel food cake. The structure of the foam affects the appearance, texture, consistency, lightness, and mouthfeel of these foods. A foam is mostly air and is characterized by high viscosity, low density, high surface area, and high surface energy. Food foams such as ice cream have an air content of 40-50% by volume. The continuous phase is a concentrated, unfrozen aqueous solution containing soluble milk salts, lactose, and added sugars. The dispersed solids (i.e., proteins, salts, stabilizer, and fat) are in the colloidal state and in the form of an emulsion. The ice crystals exist in the coarsely dispersed phase, occupying a major portion of the space between air cells.
Chemical Vapor Deposition of Copper for Advanced On-Chip Interconnects
Authors: Gelatos A.V. , Jain A. , Marsh R. , Mogab C.J.
Published: August 2016

Continued dimensional scaling of the elements of integrated circuits places significant restrictions on the width, density, and current carrying capability of metallic interconnects. It is expected that, by the year 2000, the transistor channel length will be at 0.18 μm, while microprocessors will pack more than 15 million transistors over an area ~700 mm. To conserve area, interconnects will continue to be stacked at an increasing number of levels (six by the year 2000, versus four in today's leading microprocessors), and the minimum spacing and width within an interconnect layer will shrink to 0.3 μm. In addition, it is expected that future interconnects will need to sustain increasingly higher current densities without electromigration failures.

Aluminum alloys are the conductors of choice in present-day interconnects, and much effort is focused on means to extend the usefulness of aluminum through improvements in reliability, either by new alloy formulations or by the development of complicated multimetal stacks. A more radical approach, which is gaining increased attention, is the replacement of aluminum altogether by copper. The bulk resistivity of copper is significantly lower than that of aluminum (1.7 μΩ cm for Cu versus 3.0 μΩ cm for Al-Cu), which is expected to translate to interconnects of higher performance because of reduction in signal propagation delay. In addition, the significantly higher melting temperature of copper (~1100°C versus ~600°C for Al-Cu alloys) and its higher atomic weight are expected to translate to improved resistance to electromigration.
Electroless Copper Deposition on Metals and Metal Silicides
Authors: Mak Cecilia Y.
Published: August 2016

Chemical plating techniques have been used in silicon processing for many years for junction delineation and ohmic contact formation. In recent years, interest in this area has been renewed because of the potential use of electroless copper deposition for ultra-large-scale integration (ULSI) metallization and for the formation of thin metal etch masks for deep-ultraviolet lithography. Good deposition selectivity, low operating temperature, high copper purity, good filling characteristics, and planar topography have been among the many advantageous attributes reported from early investigations.

In the June 1993 issue of theMRS Bulletinon Copper Metallization, Cho et al. gave an exposition on the use of electroless copper for VLSI. More comprehensive reviews of the electrochemical fundamentals can be found in References 8,9, and 10. This article summarizes the current understanding of various chemical and material aspects of this deposition method in an attempt to give an overview of the film growth characteristics for thin film applications. Because of length limitations, only selected topics are included. The emphasis is on the initiation conditions and the resultant microstructure and properties obtained. We also discuss special considerations for fine pattern formation.

The information presented here applies primarily to electroless copper deposition on metals and metal silicides since these are the typical substrate surfaces for metallization in contact vias of ULSI circuits. Strictly speaking, metallization of upper-level interconnects occurs mostly on a dielectric base. However, since copper systems usually require a diffusion barrier to shield the copper from diffusing into the silicon, we can treat the deposition process startingfrom this layer onward.
Why Crystal Engineering of Oxides?
Authors: Koinuma Hideomi
Published: August 2016

This issue of the MRS Bulletin, on the crystal engineering of high Tc and related oxide films, essentially responds to four questions. The first three are

∎ Why has a high Tc superconducting tunnel junction not yet been fabricated?

∎ Is it possible to fabricate a high Tc tunnel junction and if so, how?

∎ How can thin-film technology contribute to the progress of high Tc superconductor research beyond the fabrication of electronic devices?

Experimental efforts to answer these questions and solve associated problems must focus on atomic-scale control and characterization of surfaces and interfaces, which we designate here as crystal engineering of high Tc and related oxide films. Meanwhile, we have started to realize that the crystal engineering technology of oxides need not be limited to high Tc super-conductor research; it can be extended to electronic materials research, where it may have a somewhat more profound significance. In this light, I would like to pose the fourth question, one that may be especially relevant to this particular topic in the near future:

∎ Will we find or develop an electronic material superior to silicon?

Ever since the first success in superconducting La-Sr-Cu-O film deposition in early 1987, extensive studies have been conducted on the fabrication of high Tc oxide films and devices. Success has been achieved to some extent in relatively simple devices using plain superconducting films or grain boundary and step edge junctions. Nobody, however, has succeeded in fabricating an SIS (superconductor-insulator-superconductor) runnel junction that shows a clear hysteresis in I-V characteristics at a temperature above 77 K. Such a junction is presumably the key to the wide electronic device application of high Tc superconductors, for example, for switching devices expected to work much faster than conventional semiconductor transistors.
Up Close: Center for Electron Microscopy of Materials Science at the University of Antwerp
Authors: Van Tendeloo G. , Schryvers D. , Van Dyck D. , Van Landuyt J. , Amelinckx S.
Published: August 2016

The Center for Electron Microscopy of Materials Science (EMAT) at the University of Antwerp, Belgium, started in 1967 under the initiative of S. Amelinckx. While most materials science laboratories choose a particular research theme (e.g., superconducting materials) and study it through several available techniques, the EMAT group at Antwerp has taken a different approach. From the beginning, EMAT has concentrated on a single technique: transmission electron microscopy. Since the laboratory, as well as the university, is very small, we felt it would be better to be excellent in a single technique than to have limited proficiency in several techniques. Until recently, EMAT has been able to maintain this strategy, although the group has grown considerably; at present it consists of four academic staff members, four postdocs, and 12 PhD students, supported by a technical and administrative staff of eight people.

Instrumentation at EMAT

The range of electron microscopes and related instruments in the EMAT laboratory is quite extensive and includes several instruments dedicated to specific techniques. The instruments include:

∎ A 1,250 kV high-voltage electron microscope, used mainly for studying processed semiconductor devices and for in situ radiation experiments.

∎ A 400 kV and a 200 kV high-resolution electron microscope with resolving powers of 1.7 Å and 2.5 Å, respectively. Both instruments are equipped with a TV imaging system and a video camera.

∎ A 200 kV and a 100 kV analytical electron microscope with large tilting possibilities, cooling and heating facilities, and capability for energy dispersive x-ray analysis for measuring the local composition.
The Charisma of C60 Buckminsterfullerene
Authors: Kroto Harold W.
Published: August 2016

In 1990, almost exactly five years after C60 Buckminsterfullerene (see Figure 1) was discovered serendipitously during a series of graphite laser vaporization experiments designed to simulate the chemistry in a red giant carbon star, the molecule was isolated in macroscopic amounts. This breakthrough has triggered an explosion of research, and the molecule has already exhibited a wide range of novel phenomena which promise exciting applications. In addition to generating intense experimental interest in its physicochemical properties, the molecule holds a particular fascination for theoreticians; indeed the existence of the molecule was predicted in 1970, some 15 years before its discovery in 1985. It is interesting to consider the intrinsic aesthetic aspects of this molecule which are responsible for the extraordinary charisma it has for scientists and nonscientists alike.
Carbon Nanotubes
Authors: Iijima Sumio
Published: August 2016

Nanostructured materials have recently attracted the attention of some materials scientists. Because of their unique properties occurring in low-dimensional structures, nanostructured materials are sought for their possible industrial applications. This article introduces a specific nanostructured material, the carbon nanorube—an extremely thin filaments of graphite considered to be a quasi one-dimensional structure, with a simple well-understood atomic structure. Because of these qualities, the carbon nanorube has elicited great interest from diverse fields of basic and technological research. My discovery of carbon nanotubes was inspired by the discovery of C60 and its family and their mass production. The carbon nanotubes were serendipitously found during the examination of fullerene materials by a high-resolution transmission electron microscope (HRTEM). Since introducing this technique in 1971, I have been employing HRTEM to characterize the microscopic structural details of a variety of materials, including carbonaceous materials. So far, only nanotubes have been revealed with HRTEM.

Interest in the carbon nanorube is multifold. Academically the nanotube is an ideal model structure for a quasi one-dimensional structure since its known atomic structure makes computer simulations more reliable. It is worthwhile to study both rare structures of graphite—cylindrical forms with a helical arrangement of carbon atom hexagons and flexible graphitic sheets containing topological surface defects. These materials may find practical uses as tough graphite fibers, molecular wires, catalyst supports, molecular adsorbers, and so on.
New Horizons in Carbon Chemistry and Materials Science
Authors: Kroto H.W. , Hare J.P. , Sarkar A. , Hsu K. , Terrones M. , Abeysinghe J.R.
Published: August 2016

The discovery that C60 Buckminsterfullerene may be created spontaneously in high yield when carbon vapor condenses indicates that graphene sheet curvature and closure is a common occurrence during carbon nucleation to form extended networks. As a consequence, a net microscopic perspective on graphitelike carbonaceous materials has evolved. This perspective is summarized here because the net observations relate to various types of nonplanar graphitic structures that promise to be useful as viable nanoscale engineering materials.

C60 Buckminsterfullerene was discovered in 1985 among the products of chemical nucleation of carbon in the gas phase. The faint whisper, embedded in the helium wind that blew a laser-initiated carbon plasma into a mass spectrometer, was interpreted as the possible signature of the elegant soccer-ball-shaped molecule. That noticed signal, which indicated that a pure 60-carbon-atom molecule might be very stable, was quite unexpected and thus a truly serendipitous discovery. The original aim of the experiments was to simulate the conditions in a red giant carbon star and particularly to probe an earlier proposal that long carbon-chain molecules might have originated in such celestial objects and might have been subsequently ejected into interstellar space. The carbon chains themselves had been discovered to be abundant in certain regions of the interstellar medium by a radioastronomy search program in the late 1970s. The molecular rotational frequencies, which formed the basis for the radio search, resulted from a spectroscopic study of cyanopolyynes that had been synthesized in the laboratory.
Metal Container Materials for Nuclear Waste
Authors: McCright R. Daniel
Published: August 2016

Selecting materials to contain high-level nuclear waste presents an unparalleled challenge to materials scientists and engineers because of the very long times required for containing the waste and the high degree of certainty required when predicting a container's long-term performance. Material endurance without severe degradation is the primary property sought in container materials. There are, of course, additional factors related to mechanical properties, fabricability and weldability, and economics, but these factors are usually secondary to the chemical properties of the material. Metals and alloys have been given prime consideration as container materials because metal materials are ductile and tough, they can be easily fabricated in mass quantities and large container sizes, and they can be readily joined to create intact gas-tight seals between parts.

In selecting a suitable container material, the container's environment is the most important consideration. Several geological formations and resulting geochemical environments are under consideration as disposal sites in different countries. This short review will discuss the principles governing corrosion of the primary container materials being considered in various national nuclear waste disposal programs and will show how this background can guide materials selection. Important candidate materials are then considered along with examples of how these materials will be expected to perform under repository conditions existing in a range of national programs.
Actinide Colloids in Natural Aquifer Systems
Authors: Kim J.I.
Published: August 2016

Immobilization and geological isolation are the general goals of nuclear waste disposal. The appropriate isolation of immobilized long-lived radionuclides, mostly actinide elements, from interaction with water for a period of many thousands of years in any given geological formation is a scientific and technological challenge. This challenge can be met by considering a multibarrier concept that appraises engineered, geoengineered, and geological barriers designed to confine the migration of long-lived actinides. Once interaction with water occurs, one may expect many complex geochemical reactions of immobilized actinides; actinides will therefore be distributed into two kinds of chemical states, mobile and immobile species. Reflecting results in laboratory systems, the two species are expected to be chemical partners in equilibrium reactions. In multicomponent geochemical systems, however, the chemistry of the mobile and immobile species of actinides in trace concentration is, in general, masked by geochemical reactions of natural aquatic constituents, which are found in much higher concentrations. The mobile species are either complexed anions or colloidal species, whereas the immobile species are new solid phases of very low solubility, which are thermodynamically better stabilized than in the original host phases.
Functionally Gradient Materials
Authors: Rabin B.H. , Shiota I.
Published: August 2016

This issue of the MRS Bulletin provides an up-to-date look at ongoing research activities within the field of functionally gradient materials (FGM). The term FGM, now widely used by the materials community, originated in Japan in the late 1980s as a description for a class of engineering materials exhibiting spatially inhomogeneous microstructures and properties. Of course, gradient materials are not something new. It must be recognized that humans have extensively utilized materials containing microstructural gradients (either those found in nature or those created through processing) since the earliest days of craftsmanship and engineering construction. Indeed, there are examples of graded materials developed long ago, such as case-hardened steel, which are still in common use today. Contemporary examples of these materials serve in technologically significant applications, as, for example, in thermal-barrier coatings for gas turbines. Nevertheless, what is new and exciting about FGMs is the realization that gradients can be designed at the microstructural level to tailor a material for the specific functional and performance requirements of an intended application. In addition, recent advances in processing are opening the possibility for the extension of the gradient materials concept to new materials systems and engineering problems.

The recent resurgence of interest in gradient materials has been driven by the need for improved materials, capable of meeting the demanding performance requirements established by emerging technologies such as the aerospace plane, ceramic engines, and nuclear fusion.
Using Microstructure to Attack the Brittle Nature of Silicon Nitride Ceramics
Authors: Becher Paul F. , Hwang Shyh-Lung , Hsueh Chun-Hway
Published: August 2016

The evolution of silicon nitride ceramics over the last two decades has brought about the advancement of materials which were first fabricated by the application of mechanical pressure and temperature (i.e., hot pressing) resulting in high flexure strengths (e.g., 700–800 MPa) but rather poor resistance to creep at temperatures of ~1200°C. At the same time, these ceramics remained quite brittle with fracture-toughness values of 4–5 MPa m½, such that strengths were very sensitive to flaw or crack sizes. As a result, measured strengths exhibited considerable scatter, as reflected by a low Weibull modulus. In the ensuing years, approaches were sought to develop more economical methods of fabricating silicon nitride components by densifying to near-net shape. Methods were also sought for increasing the elevated-temperature reliability by minimizing the additives employed to promote densification and by utilizing additives that produced more stable and refractory grain boundary phases. The application of gas-pressure sintering methods, utilizing gaseous environments of 10–100 atmospheres, led to the ability to produce dense near-net shaped components with very high fracture strengths (e.g., ≥1000 MPa). At the same time, advances in processing and additive chemistry, sometimes combined with additional fabrication methods (e.g., hot isostatic pressing), have resulted in ceramics with excellent creep resistances at temperatures in excess of 1300°C. Some of these silicon nitride ceramics exceed the elevated-temperature capability of superalloys by 200°C. The initial desire for light-weight ceramic components that could sustain tensile loads for high-temperature applications is, indeed, beginning to bear fruit. One of the most impressive examples of the development of a complexly shaped lightweight component is the silicon nitride turbocharger rotor used in a number of Japanese automobiles, which is currently manufactured at a cost approaching that of the opposing superalloy rotor and provides exceptionally high mechanical reliability and production yields. Currently, there are also earnest efforts to incorporate silicon nitride valves for engines, as well as in a variety of other components (e.g., combustion swirl chambers, valve-lifter pads, etc.). The acceptance and use of this class of brittle materials, which were once considered prohibitively expensive for fabrication into complex shapes and not suited for such applications, is a remarkable testimony of the progress that has been made.
High-Temperature Properties of Si3N4Ceramics
Authors: Hoffmann M.J.
Published: August 2016

Silicon nitride is a highly covalent bonded compound which decomposes at 1877°C. Therefore, it is impossible to densify Si3N4without sintering additives. Densification is achieved by liquid-phase sintering usually using metal oxides such as MgO, Y2O3, A12O3, and most of the rare-earth oxides as sintering additives. The oxides react with SiO2—always present at the surface of Si3N4particles—to form an oxide melt and, with increasing temperature, an oxynitride melt by dissolution of Si3N4. The resulting microstructure consists of elongated Si3N4needles embedded in a matrix of smaller equiaxed Si3N4grains and a grain boundary phase, as shown in Figure 1. The amount and chemistry of the sintering aids determine the volume fraction of the grain boundary phase. The content required for complete densification depends on the sintering techniques: 2–5 vol% additives are sufficient if densification is supported by a high external pressure (hot pressing [HP] or hot isostatic pressing [HIP]); pressureless-sintered and gas-pressure-sintered (10-MPa nitrogen pressure) materials have additive contents of up to 15 vol%. Today, silicon nitride ceramics are regarded as a class of material comparable to steel. Different qualities depend on the size and shape of the silicon nitride grains and the amount and chemistry of the grain boundary phase. Materials with a high room-temperature strength exhibit a finegrained, elongated microstructure, while materials with a high fracture toughness are more coarse-grained. In both cases, a weak interface is required to induce transgranular fracture. (See Becher et al. in this issue.) Since all Si3N4grains are completely wetted by the grain boundary phase, the interface strength is determined by the additive composition. Nevertheless, a contradiction arises between the development of high-strength and high-toughness Si3N4ceramics and high-temperature resistant materials because the grain boundary phase is responsible for the excellent properties at low temperatures, but limits the properties at temperatures above its softening point.
Grain Growth in Si3 N4-Based Materials
Authors: L.|Kang Suk-Joong , Han Sang-Moo
Published: August 2016

The growth of Si3N4 grains in the usual sintering process occurs in an oxynitride liquid formed by reactions between sintering additives, impurity SiO2, and Si3N4. The shape of an Si3N4 grain in the liquid matrix is a hexagonal rod, although the aspect ratio (c/a) varies considerably, depending on the processing conditions and final crystalline forms of α or β. Figure 1 shows two types of microstructures observed in sintered Si3N4-based materials. The microstructure shown in Figure la is normal with unimodal grain-size distribution and that of Figure 1b is abnormal with a microstructure of exceptionally large elongated grains within fine matrix grains. When normal grain growth occurs, the microstructure varies little with sintering time, and the development may be described by a simple law. But, when abnormal grain growth occurs, a duplex grain structure with a bimodal grainsize distribution results; then no simple kinetic law can describe the microstructure development.

The grain growth in the microstructures shown in Figure 1 exemplifies the growth of faceted grains in a liquid matrix. The grain growth in a matrix occurs via growth of larger grains and dissolution of smaller ones by material transport through the liquid phase. The driving force of the material transport for an individual grain is determined by the difference between its size and the critical grain size, which is invariant at the moment of observation. Since the driving force is usually low, the volume change of each grain is relatively slow and analogous to the crystal growth or dissolution in a liquid matrix under low super- or undersaturation. Therefore, knowledge of the growth behavior of faceted crystals under low supersaturation in a liquid may provide fundamental understanding of the grain growth in Si3N4-based materials.
Dislocation-Based Deformation Mechanisms in Metallic Nanolaminates
Authors: Anderson P.M. , Foecke T. , Hazzledine P.M.
Published: August 2016

The appeal of nanolayered materials from a mechanical viewpoint is that, in principle, plastic deformation can be confined to small volumes of material by Controlling both the frequency and magnitude of obstacles to dislocation motion. As we shall see, the spacing of obstacles can be used to impart large plastic anisotropy and work hardening. However, how strong can such materials be made as layer thickness (and therefore obstacle spacing) is decreased to the nanoscale level? In perspective, large, micron-scale, polycrystalline materials generally display improved yield strength (and fracture toughness) as grain size is decreased. This behavior at the micron scale can be explained via modeis that are built on two assumptions: (1) the strength of obstacles to crystal slip is sufficiently large to require pileups of numerous dislocations in order to slip past them; and (2) the strength of such obstacles does not change, even if their spacing is decreased. The modeling presented here shows that these assumptions may break down at the nanometer scale. The result is that there is a critical layer thickness in the nanometer range, below which improvement in strength does not occur.

Our discussion to follow briefly outlines a more macroscopic, micron-scale approach to determine yield strength, and then contrasts that with a sequence of events leading up to yield in nanolayered materials. We also address whether nanoscale materials are expected to exhibit more uniform or coarse slip than micron-scale materials. Finally, a semi-quantitative model of yield strength is developed which requires, as input, the strength of an interface to crystal slip transmission across it. We discuss several contributions to the interfacial strength and apply the theory to demonstrate a peak in strength for a 50 vol% Cu-50 vol% Ni multilayered sample.
Applications of Synchrotron Surface X-Ray Scattering Studies of Electrochemical Interfaces
Authors: You Hoydoo , Nagy Zoltán
Published: August 2016

Aqueous-solution/solid interfaces are ubiquitous in modern manufacturing environments as well as in our living environment, and studies of such interfaces are an active area of science and engineering research. An important area is the study of liquid/solid interfaces under active electrochemical control, which has many immediate technological implications, for example, corrosion/passivation of metals and energy storage in batteries and ultracapacitors. The central phenomenon of electrochemistry is the charge transfer at the interface, and the region of interest is usually wider than a single atomic layer, ranging from a monolayer to thousands of angstroms, extending into both phases.

Despite the technological and environmental importance of liquid/solid interfaces, the atomic level understanding of such interfaces had been very much hampered by the absence of nondestructive, in situ experimental techniques. The situation has changed somewhat in recent decades with the development of the largely ex situ ultrahigh vacuum (UHV) surface science, modern spectroscopic techniques, and modern surface microscopy.

However in situ experiments of electrochemical interfaces are difficult, stemming from the special nature of these interfaces. These are so-called buried interfaces in which the solid electrode surface is covered by a relatively thick liquid layer. For this reason, the probe we use in the structural investigation must satisfy simultaneously two conditions: (1) the technique must be surface/interface sensitive, and (2) absorption of the probe in the liquid phase must be sufficiently small for penetration to and from the interface of interest without significant intensity loss.
Transport of Gases in Porous Membranes
Authors: Sotirchos Stratis V. , Burganos Vasilis N.
Published: August 2016

The capability of membranes to affect differently, both qualitatively and quantitatively, the transport rates of chemical species of dissimilar chemical structure through their interior space renders them attractive for use in many separation problems. Extensive research efforts have thus been undertaken on the preparation and characterization of membrane materials and the study of the transport processes involved in their use in separation applications. The study of the transport of gaseous species through the pore space of porous membranes and the analysis and understanding of the mechanisms that are involved in this process are a very important, if not the most important, element in the development of membranebased separation processes.

The resistance that a gaseous species encounters as it is transported through the pore space of a porous membrane is a function of its molecular properties, of its interaction with the material that makes up the walls of the pores, and of the membrane pore structure. Gaseous transport in pores can take place through various mechanisms, whose contribution to the overall transport rate of a particular species is, in general, determined by the strength of the interactions of the molecules of that species with the pore walls and by the relative magnitudes of three length scales that characterize the molecular size, the distance between pore walls, and the density of the fluid in the pore space.
Dense Palladium and Perovskite Membranes and Membrane Reactors
Authors: Ma Yi Hua
Published: August 2016

The development of high-temperature processes and tighter environmental regulations requires utilization of efficient gas-separation processes that will provide high fluxes, high selectivity of separation, and the ability to operate at elevated temperatures. Dense inorganic membranes and membrane reactors are especially well suited for high-temperature reactions and separations, due in part to their thermal stability and high separation selectivity (in theory, infinite). Furthermore, membrane reactors offer an inherent advantage of combining reaction, product concentration, and separation in a single-unit operation for the improvement of process economics and waste minimization.

The classification of membrane reactors can either be by membrane material and geometry or by the configuration of the reactor. Porous and dense membranes in both tubular and disk forms have been used for membrane reactors. The membrane can either be catalytically active (catalytic membrane reactor [CMR]) or simply act as a separation medium. In the latter case, the catalyst is packed in the reactor, whose walls are formed by the membrane (packed-bed membrane reactor [PBMR]). In addition, if the membrane is also catalytically active, the reactor is called a packed-bed catalytic membrane reactor (PBCMR).

The principal materials from which porous inorganic (ceramic) membranes are made are alumina, zirconia, and glass. Alumina and zirconia membranes are usually asymmetric and composite, with a porous support (0.5–2.0 mm thick) for mechanical strength and one or more thin layers for carrying out separations.

On the other hand, glass membranes, such as Vycor and microporous glass, have symmetric pores. Materials commonly used as the porous support are alumina, granular carbon, sintered metal, and silicon carbide.
Separation of Volatile Organic Compounds from Water by Pervaporation
Authors: Baker R.W.
Published: August 2016

Pervaporation is a membrane process used to separate liquid mixtures. Separation is achieved by a combination of evaporation and membrane permeation. As a result, the process offers the possibility of removing dissolved volatile organic compounds (VOCs) from water, dehydrating organic solvents, and separating mixtures of components with close boiling points or azeotropes that are difficult to separate by distillation or other means.

A schematic diagram of the pervaporation process is shown in Figure 1. In the example shown, the feed liquid is a solution of toluene in water which contacts one side of a membrane that is selectively permeable to toluene. The permeate, enriched in toluene, is removed as a vapor from the other side of the membrane. The driving force for the process is the difference in the partial vapor pressures of each component in the feed liquid and the permeate gas. This driving force can be increased by raising the temperature of the feed liquid to increase its vapor pressure or by decreasing the permeate gas pressure. The permeate gas pressure can be adjusted by using a vacuum pump, but industrially the most economical method is to cool and condense the vapor. Condensation spontaneously generates a vacuum. The permeate vapor pressure is then determined by the temperature of the permeate condenser and the composition of the permeate liquid generated by cooling and condensing the permeate vapor.

Pervaporation membranes are made by coating a thin layer of selective polymer material onto a microporous support.
Medical Materials for the Next Millennium
Authors: Hench Larry L.
Published: August 2016

The following article is an edited transcript of the Von Hippel Award address, given by recipient Larry L. Hench at the 1998 MRS Fall Meeting on December 3, 1998. Hench received the Materials Research Society's highest honor for “pioneering accomplishments in the field of glass and ceramics, including the demonstration of the first bioactive glass called Bioglass® and subsequent expansion of the field, demonstration of the feasibility of encapsulating nuclear waste products in glass/ceramic matrices, and development of sol-gel processing to produce ultrahigh-purity optical and dielectric materials with controlled microstructures.”

The problem our Society faces as we head into the next millennium is that our average age expectancy has increased to a very attractive number of years but our bodies begin to deteriorate long before then. With the use of antiseptics, anti-biotics, and vaccinations during this Century, we have increased our survival rate considerably, as shown in Figure 1. The average age expectancy has increased to the mid-70s for many countries. However, as we pass the age of 30, our bodies begin to deteriorate. The quality of our connective tissues, particularly our skeletal tissues that hold us upright, begins to decline. The volume of bone that Supports us decreases with time, and for at least some of us, the weight our bones have to bear increases with time. The net effect is that the strength of bone degrades with age, causing the probability of fracture to increase (Figure 2). lf we integrate the area under the curve in Figure 1 from 60 years onward to 90 years, multiply it by the couple of billion people on earth, and then multiply that by the fraction of people subject to bone failure (Figure 2), we have a catastrophic repair problem facing society.
Environment-Assisted Intergranular Cracking
Authors: Scully J.R.
Published: August 2016

Intergranular separation in polycrys-talline materials involves breaking metallic bonds along grain boundaries in response to stress. The surfaces created in this manner expose the grain facets on either side of the original boundary, as shown in Figure 1. This mode of fracture often occurs at much lower fracture stress and energy than cracking by ductile processes through the interior of grains. The exposure of specific materials to certain environments and stress can promote this low-energy, intergranular mode of separation, even when fracture of the same material in vacuum would occur along a ductile transgranu-lar path. Three types of environment-assisted intergranular cracking can occur in a wide variety of alloy/environment systems: intergranular stress-corrosion cracking (IGSCC), intergranular hydrogen embrittlement, and intergranular liquid-metal embrittlement.

Figure 1 shows an example of IGSCC. This type of cracking is a pervasive problem in many technological applications, leading to extensive repairs, loss of service function, and safety concerns. IGSCC occurs in the weld-heat-affected zones of stainless-steel pipes in high-purity primary coolant waters within nuclear power plants, and in nickel-based alloys utilized as heat-exchanger tubing when exposed to the high-purity primary as well as secondary coolant waters in power plants. It is also seen in Al-based alloys used for fuselage skins and structural components in military and commercial aircraft when exposed to humid atmospheric conditions. Ferrous alloys used in the oil and gas industry are also susceptible. For instance, IGSCC of mild steels used in buried gas-transmission pipelines is a widespread international problem, leading to explosions when leaking natural gas ignites.