Here we will attempt a brief overview of recent synthetic efforts for micropore and lower-end mesopore membranes. We will not address the very important classes of nonporous membranes, such as dense metals and solid electrolytes with applications in H2 and O2 separations, or meso- and macroporous membranes, which find applications in food processing and water treatment. Microporous materials provide high permselectivities for molecules encountered in the chemical-processing industry but suffer from low intrinsic permeabilities. Therefore, in order to bring microporous membrane materials to commercial applications, functional composites with small effective thicknesses (in the micron or submicron range) must be developed. For example, to achieve economical membrane-reactor sizes, fluxes as high as 0.1 mol/(m2 s) are desirable. Approaches to microporous membranes include modification of mesoporous membranes by sol-gel and chemical-vapor-deposition (CVD) techniques, carbonization of polymers to form molecular-sieve carbon, and polycrystalline-film growth of zeolites and other molecular sieves.

Microporous carbon is widely used for liquid or gas purification because of its strong adsorptive properties and high surface area. It is also used for air separation by pressure swing adsorption (PSA), relying on its adsorptive and molecular-sieving properties. From the standpoint of applications, microporous carbons are classified into activated carbons with pore size 0.8–2 nm, and ultramicroporous carbons or carbon molecular sieves with pores 0.3–0.6 nm. Activated carbons are used because of their strong adsorption properties, while carbon molecular sieves are useful on account of their molecular-sieving as well as adsorption properties.

With materials of ever-increasing complexity becoming key elements of the technologies underpinning industrial and economic development, there is an ongoing need for tools that reveal the microscopic origins of physical, electrical, magnetic, chemical, and biological properties. Neutron scattering is one such powerful tool for the study of the structure and dynamics of materials. Neutrons are well suited to this purpose for several reasons:

∎ Neutrons are electrically neutral, leading to penetration depths of centimeters and thereby enabling in situ studies.

∎ Neutron cross sections exhibit no regular dependence on atomic number and are similar in magnitude across the periodic table, giving rise to sensitivity to light elements in the presence of heavier ones.

∎ Certain large differences in isotopic scattering cross sections (e.g., hydrogen to deuterium, H/D) make neutrons especially useful for the study of light atoms in materials.

∎ The range of momentum transfer available allows probing of a broad range of length scales (0.1–105 Å), important in many different materials and applications.

∎ Thermal and “cold” (longer-wavelength) neutrons cover a range of energies sufficient to probe a wide range of lattice or magnetic excitations, “slow” dynamic processes such as polymer chain reptation, and so forth.

∎ Neutrons have magnetic moments and are thus uniquely sensitive probes of magnetic interactions.

∎ Neutrons can be polarized, allowing the cross sections (magnetic and non-magnetic) to be separated.

∎ The simplicity of the magnetic and nuclear interactions makes interpretation of results straightforward.

Traditional batteries stored energy in thick plates made from heavy metals like lead, nickel, and zinc. They delivered from 15 to 20 Wh/kg. Lighter-weight lithium anodes were used in some military batteries. Then came the need for lightweight batteries for powering cellular telephones and laptop computers. Lithiumion batteries were developed, and the worldwide demand, just for use in laptop computers, has grown to 150 million units per year.

The need to reduce air pollution in downtown areas has created a market for battery-powered electric vehicles. Clara Ford chose to drive an electric car, even though her husband, Henry Ford, was making gasoline-powered cars. However, the cost of replacing worn-out leadacid batteries soon ended the electric-car age of the early 1900s. The need for lightweight, long-life batteries for zero-emission cars has produced unprecedented investments in battery technology. The lithium-battery technology used in laptop-computer batteries did not support the requirements of high power and long life for the charge/discharge cycling needed in electric cars. An executive of a lithium-battery manufacturer was asked what he was doing about the cycle life of his batteries. His answer: “The life of a laptop computer is nine months. Then a newer model makes it obsolete. We meet this requirement!”

X-rays have found a wide range of applications in chemistry, physics, and materials engineering since their discovery in 1895 by W. Roentgen. The materials science community uses x-ray-based techniques extensively for characterization of materials. In the 1970s a new tunable source of x-rays from the radiation produced by synchrotron accelerators emerged. Synchrotron radiation (SR) is an intense and forward-focused beam of radiation that is emitted when the path of an electron traveling at almost the speed of light is bent by a magnetic field. Figure 1 illustrates the evolution of radiation intensity provided by various x-ray sources. In situ SR techniques provide real-time observation of atomic arrangements with high spatial sensitivity and precision, which are important features not only in fundamental materials research, but also in the development of novel processing techniques and in the search for new exotic materials. A major advantage of SR is that it covers a wide range of wavelengths continuously from infrared to gamma rays. This feature is attractive since a wealth of detailed information on the electronic and structural properties of materials can be obtained by optimizing the wavelength of the radiation.

Since the establishment of “first generation” facilities in the early 1970s, the x-ray emittance from synchrotron storage rings, where electrons traveling at almost relativistic speed s are constrained by magnetic fields to follow curved trajectories, has shown dramatic improvements. See Table I for an extensive list of SR facilities presentiy in use throughout the world.

The elements of the lanthanide series, from Ce (atomic number 58) to Yb (atomic number 70), form a group of chemically similar elements that have in common a partially filled 4f shell. These so-called “rare earth” (RE) elements usually take on a 3+ ionic state (RE3+). Because the 4f electronic-energy levels of each lanthanide ion are shielded from external fields by 5s2 and 5p6 outer-shell electrons, RE3+ energy levels are predominantly independent of their surroundings.

The characteristic energy levels of 4f electrons of the trivalent RE elements have been investigated in detail by Gerhard Heinrich Dieke and co-workers and were reported approximately 30 years ago. The Dieke diagram showing RE3+ energy levels is a familiar tool of scientists and engineers working with RE elements. However, the history of RE elements goes back to the year 1787 in the small Swedish town of Ytterby near Stockholm and to the gifted amateur mineralogist and military man Lt. Carl Axel Arrhenius. Arrhenius discovered an unusual black mineral in Ytterby (perceived initially as much rarer in occurrence and in concentration than the common ores or earths of aluminum, calcium, etc.). Many new elements were discovered by various chemists upon analysis of this black stone and others like it. The names given to these elements are variations of the location where the first discovery was made: yttrium, ytterbium, terbium, and erbium. The history of RE elements is fascinating and involves many other famous names in science: Berzelius, Gadolin, Bunsen.

The properties of these elements and their multifaceted applications to science and industry are equally fascinating and have remained important to this day. Commercial applications of RE elements began after World War II, when their available quantity and purity were greatly enhanced by improved separation techniques developed as a part of the Manhattan Project. Until fairly recently, the main industrial application of RE elements has been in permanent magnets. The unpaired 4f electrons result in some RE elements having the highest magnetic moments of any element. The development and applications of RE magnets are reviewed in a very interesting article by Livingston3 in a previous MRS Bulletin issue. In this issue of MRS Bulletin, we have taken as our aim to review some of the properties and applications of RE elements relevant to photonics.

The electrochemical dissolution of clean, oxide-free metal surfaces in electrolyte solutions is a fundamental process in metal corrosion as well as in technological etching and metal-refinement processes. Due to fundamental interest and for industrial reasons, dissolution processes and the effect of organic corrosion inhibitors have been studied intensely, concentrating on the determination of macroscopic etch rates and the effect of varying chemical parameters upon the etch kinetics. In the 1920s, ideas for the microscopic mechanism of the dissolution and the reverse deposition reaction had already been put forward by Stranski, who suggested that dissolution (growth) is dominated by detachment (attachment) of atoms at low coordination sites of the crystal surface and at kink sites along monoatomically high steps (see the schematic model in Figure 1). These kink sites are preserved during the removal (or addition) of the basic building blocks of the crystal (e.g., single metal atoms for elemental metals) and hence can sustain a continuous dissolution reaction. However, the direct verification of these ideas was impossible for more than seven decades, due to the very complex (defect) structure of the surfaces of typical metal electrodes, the presumably dominating effects of these defects on the etch process, and the lack of structurally sensitive probes suitable for in situ structural characterization.

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.

Living cells are formidably complex systems that perform highly coordinated tasks which relate multiple biochemical and biophysical inputs to cell activities. Cell tasks may include not only adhesion and spreading, receptor-ligand mediated signal transduction, division, growth and programmed death, but also cell-type-dependent functions such as the environmental barrier provided by skin cells. These various cellular activities, often performed simultaneously or in a hierarchical order, involve hundreds of membrane, cytoplasmic, and extracellular proteins, ions, and small molecules, which interact with one another by means of regulated forces.

For instance, cell migration requires the coordination of membrane extension and retraction, cytoskeletal gelation-contraction-dissolution, the formation of focal adhesions at the front of the cell, and detachment of these adhesions at the rear of the cell. These processes involve not only cytoskeletal polymers and motor proteins (which provide the cell with the necessary motor forces and passive mechanical resistance to sustain both cell movement and cell integrity), but also specific membrane proteins (“cell receptors”) to promote intimate contact between the cell and its extracellular milieu. Nevertheless, despite the critical function of cell migration in wound healing, immune response, cancer metastasis, and embryogenesis, the fundamental mechanisms of this phenomenon are not well understood.

One of the steps limiting our understanding of cellular activities such as cell migration has been the lack of fundamental theory, backed by experimental methods, to monitor and characterize cellular processes quantitatively, noninvasively, and in real time.

An issue on novel applications of materials doped with rare-earth (RE) ions can scarcely fail to address lasers, but it need not address all RE-based lasers. Some Nd3+ -doped lasers, particularly Nd:YAG (Y3Al5O12, yttrium aluminum garnet), emitting light with a wavelength of 1064 nm, are very well-established commercial products—by no means novelties.1 Some other near-infrared (NIR) lasers, based on Er3+ or Tm3+, are also available commercially. That wavelength region is relatively easy for RE laser ions, involving energy spacings between initial and final energy levels small enough to give large stimulated emission cross sections for useful, long upper-state life-times, yet large enough to minimize thermal deexcitation mechanisms. On the other hand, RE-doped lasers for ultraviolet (UV) and visible wavelengths are quite novel, since efficient laser operation is more difficult to achieve in these spectral ranges. Intriguing progress on such devices has been made in recent years, driven by several important applications.

In this article, we begin by noting some of the alternative ways to obtain laser light at these wavelengths, including their advantages and drawbacks. We then discuss basic properties of RE-doped laser materials and how these can be advantageous. We then review a few of the most important and recent RE-doped laser materials and techniques for obtaining UV and visible output.

Magnesium can reversibly store about 7.7 wt% hydrogen, equivalent to more than twice the density of liquid hydrogen. This high storage capacity, coupled with a low price, suggests that magnesium and magnesium alloys could be advantageous for use in battery electrodes and gaseous-hydrogen storage systems. The use of a hydrogen-storage medium based on magnesium, combined with a fuel cell to convert the hydrogen into electrical energy, is an attractive proposition for a clean transportation system. However, the advent of such a system will require further research into magnesium-based alloys that form less stable hydrides and proton-conducting membranes that can raise the operating temperature of the current fuel cells.

Following the U.S. oil crisis of 1974, research into alternative energy-storage and distribution systems was vigorously pursued. The controlled oxidation of hydrogen to form water was proposed as a clean energy system, creating a need for light and safe hydrogen-storage media. Extensive research was done on inter-metallic alloys, which can store hydrogen at densities of about 1500 cm3-H2 gas/ cm3-hydride, higher than the storage density achieved in liquid hydrogen (784 cm3/cm3 at –273°C) or in pressure tanks (˜200 cm3/cm3 at 200 atm). The interest in metal hydrides accelerated following the development of portable electronic devices (video cameras, cellular phones, laptop computers, tools, etc.), which created a consumer market for compact, rechargeable batteries. Initially, nickel-cadmium batteries fulfilled this need, but their relatively low energy density and the toxicity of cadmium helped to drive the development of higher-energy-density, less toxic, rechargeable batteries.

A better understanding of membrane microstructure and its role in the mechanisms of different separation processes is important in the development of new and more complex types of membranes. This understanding is particularly significant in the case of inorganic membranes because the permeation and separation properties are determined by the porous properties of this microstructure, which in turn is controlled by the synthesis route. This review will cover three topics: first, the origin and development of porosity in inorganic materials; second, the general definitions and terminology used to describe porous media; and third, a brief description of several techniques that can provide pore-structure characteristics of membranes.

A distinction will be made between two techniques: static and dynamic methods. Dynamic methods are based on membrane permeation characteristics and are used routinely to assess membrane performance. This review, however, will be restricted to static methods, which can provide details of porous microstructure directly. These include the following: stereology (microscopic techniques such as scanning electron microscopy [SEM]); intrusive methods (e.g., mercury porosimetry, physisorption of gases, calorimetrie methods, and nuclear magnetic resonance [NMR] analysis); and nonintrusive methods (e.g., radiation scattering with neutrons and x-rays, wave propagation, ion-beam analysis, and positron lifetime spectroscopy). Finally, the suitability of these techniques in the characterization of membrane pore structure will be discussed.

Porous materials can be formed in several ways, although the following three are most important in the synthesis of membrane structures.

The following is an edited transcript of the presentation given by H. Eugene Stanley (Boston University), recipient of the MRS 1998 David Turnbull Lectureship, at the 1998 MRS Fall Meeting on December 2,1998, in Boston.

I would like to begin with a kind of kitchen experiment that any of you can do. Take your favorite drink. For some, this is ice water (Figure 1). Ask a simple question: “Whatshould this thermometer read?” And think for a minute of the implications of the answer to your question.

The title of this talk concerns water in two different states: the liquid State and the glass State. We'll organize this talk around three questions:

■ “What is the puzzle of liquid water?”

■ “Why do we care about this puzzle?”

■ “What do we actually do?”

The “we” in this case is a rather large number of individuals. TU focus this talk on very recent work that has been published or at least submitted this year. This work was done in collaboration with M.-C. Bellissent-Funel, S.V. Buldyrev, M. Canpolat, M. Meyer, O. Mishima, M.R. Sadr-Lahijany, A. Scala, and F.W. Starr. It's also based on earlier research a few years back with C.A. Angell, P.G. Debenedetti, A. Geiger, P.H. Poole, S. Sastry, F. Sciortino, and J. Teixeira. Any of these 15 valued collaborators could probably give this talk as well as, if not better than, I will.

We will organize the answer to our third and principal question in the following way: First, we will attempt to identify, in the spirit of Sherlock Holmes, some interesting clues.

The development and optimization of materials and processes are generally extremely time-consuming and costly Operations. As a result, significant delays are frequently encountered before important materials advances can be introduced in technological applications. For these reasons, thermodynamic calculations and simulations based on critically evaluated data are now finding wide and increasing use as basic tools in materials and process design. Commercial Software packages incorporating thermodynamic databases are already available for this purpose. Their use enables the number of direct measurements to be minimized, as Information on necessary process conditions can be obtained very rapidly and inexpensively to achieve a product of the required purity with the minimum waste of energy and materials.

Typical examples of materials development now being assisted and improved with the help of thermodynamic calculations are

■ selection of optimum melting and casting conditions for complex alloys,

■ optimization of deposition conditions in chemical-vapor-deposition and physical-vapor-deposition production of metal and oxide coatings,

■ definition of suitable compositions and heat-treatment conditions in the production of application-specific materials, and

■ prediction of energy requirements and environmental emissions associated with specific materials-processing Operations.

Equilibrium thermodynamic calculations alone can sometimes prove satisfac-tory for Simulation of high-temperature technological processes. However, for reliability of simulations at lower temperatures, kinetic factors cannot be neglected. For this reason, recent Software developments include descriptions of diffusion phenomena, or rates of reaction. Significant here is the more fundamental Information provided for nonequilibrium conditions.

This often gives new insight into the basis of compositional and phase changes in complex Systems under different process conditions. The reliability of thermodynamic simulations clearly depends upon the reliability of the data used. A significant drawback here is that many classic thermodynamic data compilations contain values for pure inorganic substances only (e.g., References 4–6). There are very few processes, however, for which the reactants and products can be regarded as simple stoichiometric Compounds. Even very small amounts of dissolved gases or other impurities in a product material can seriously impair its properties.