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This chapter introduces key concepts that are developed in this textbook. It describes the concept of microstructure and other features of materials that undergo interesting changes with temperature or pressure. These changes are motivated by the thermodynamic free energy, but require a kinetic mechanism for atoms to move. Chemical unmixing and ordering on a crystal lattice are described, and the kinetics of diffusion by vacancies is explained. The free energy is used to explain melting. A summary of essential aspects of thermodynamics and kinetics is given at the end of the chapter, including basic ideas of statistical mechanics and the kinetic master equation.
The structures and dynamics of surfaces affects the chemical reactivity and growth characteristics of materials. Chapter 11 describes atomistic structures of surfaces of crystalline materials, and describes how a crystal may grow by adding atoms to its surface. Most inorganic materials are polycrystalline aggregates, and their crystals of different orientation make contact at “grain boundaries.” Some features of atom arrangements at grain boundaries are explained, as are some aspects of the energetics and thermodynamics of grain boundaries. Grain boundaries alter both the internal energy and the entropy of materials. Surface energy varies with crystallographic orientation, and this affects the equilibrium shape of a crystal. The interaction of gas atoms with a surface, specifically the topic of gas physisorption, is presented.
Chapter 12 discusses the enthalpy and entropy of solid and liquid phases near the melting temperature Tm, and highlights rules of thumb, such as the tendency for the entropy of melting to be similar for different materials. Correlations between Tm and the amplitude of thermal displacements of atoms (“Lindemann rule”), and between Tm and the bulk modulus are presented, but these correlations are semiquantitative at best. Richard's rule for the entropy of melting is more robust. Interface behavior during melting is covered in more detail, including premelting. At a temperature well below Tm, a glass undergoes a type of melting called a “glass transition” which is discussed in more detail in this chapter.Some features of melting in two dimensions are described, which are quite different from melting in three dimensions.
There is a marked kinetic asymmetry between melting and solidification -- the two are quite different as phase transformations. Solidification can occur by different mechanisms that create very different solid microstructures. This chapter emphasizes processes at the solid-liquid interface during solidification, and the microstructure and solute distributions in the newly formed solid. During solidification, a solid-liquid interface moves forward as the liquid is consumed, and the velocity of the interface increases with the rate of heat extraction. Instabilities set in even at relatively small velocity, however, and a flat interface evolves into finger-like columns or tree-like dendrites of growing solid. This instability is driven by the release of latent heat and the partitioning of solute atoms at the solid-liquid interface. Finger-like solids have more surface area, so countering the instability is surface energy. Solidification involves the evolution of several coupled fields. Crystallographic orientation of the growing solid phase is also important for the growth rate and surface energy.
Chapter 15 develops further the concepts underlying precipitation phase transformations that were started in Chapter 14. Atoms move across an interface as one of the phases grows at the expense of the other. The interface, an essential feature of having two adjacent phases, has an atomic structure and chemical composition that are set by local thermodynamic equilibrium, but interface velocity constrains this equilibrium. Interactions of solute atoms with the interface can slow the interface velocity by "solute drag." When an interface moves at a high velocity, chemical equilibration by solute atoms does not occur in the short time when the interface passes by. These issues also pertain to rapid solidification, and extend the ideas of Chapter 13. Solid–solid phase transformations also require consideration of elastic energy and how it evolves during the phase transformation. The balance between surface energy, elastic energy, and chemical free energy is altered as a precipitate grows larger, so the optimal shape of the precipitate changes as it grows. The chapter ends with some discussion of the elastic energy of interstitial solid solutions and metal hydrides.
The giant gypsum crystals of Naica cave have fascinated scientists since their discovery in 2000. Human activity has changed the microclimate inside the cave, making scientists wonder about the potential environmental impact on the crystals. Over the last 9 years, we have studied approximately 70 samples. This paper reports on the detailed chemical–structural characterization of the impurities present at the surface of these crystals and the experimental simulations of their potential deterioration patterns. Selected samples were studied by petrography, optical and electronic microscopy, and laboratory X-ray diffraction. 2D grazing incidence X-ray diffraction, X-ray μ-fluorescence, and X-ray μ-absorption near-edge structure were used to identify the impurities and their associated phases. These impurities were deposited during the latest stage of the gypsum crystal formation and have afterward evolved with the natural high humidity. The simulations of the behavior of the crystals in microclimatic chambers produced crystal dissolution by 1–4% weight fraction under high CO2 concentration and permanent fog, and gypsum phase dehydration under air and CO2 gaseous environment. Our work suggests that most surface impurities are of natural origin; the most significant anthropogenic damage on the crystals is the extraction of water from the caves.
Tooth enamel, the outermost layer of human teeth, is a complex, hierarchically structured biocomposite. The details of this structure are important in multiple human health contexts, from understanding the progression of dental caries (tooth decay) to understanding the process of amelogenesis and related developmental defects. Enamel is composed primarily of long, nanoscale crystallites of hydroxyapatite that are bundled by the thousands to form micron-scale rods. Studies with transmission electron microscopy show the relationships between small groups of crystallites and X-ray diffraction characterize averages over many rods, but the direct measurement of variations in local crystallographic structure across and between enamel rods has been missing. Here, we describe a synchrotron X-ray-based experimental approach and a novel analysis method developed to address this gap in knowledge. A ~500-nm-wide beam of monochromatic X-rays in conjunction with a sample section only 1 μm in thickness enables 2D diffraction patterns to be collected from small well-separated volumes within the enamel microstructure but still probes enough crystallites (~300 per pattern) to extract population-level statistics on crystallographic features like lattice parameter, crystallite size, and orientation distributions. Furthermore, the development of a quantitative metric to characterize relative order and disorder based on the azimuthal autocorrelation of diffracted intensity enables these crystallographic measurements to be correlated with their location within the enamel microstructure (e.g., between rod and interrod regions). These methods represent a step forward in the characterization of human enamel and will elucidate the variation of the crystallographic structure across and between enamel rods for the first time.
The Ti–35Nb–7Zr–5Ta (TNZT) alloy is a promising alloy because of its biocompatibility, high specific strength, and low Young's modulus. This work aimed at investigating the viability to process the TNZT alloy by selective laser melting (SLM) and at optimizing the processing parameters to obtain densified bulk samples. The single-track approach was first used, and the optimized laser parameters were determined to produce bulk samples with a relative density of 99.0% when an energy input of 58.3 J/mm3 was used. The effect of porosity on mechanical properties was studied, and the as-built SLM samples presented slightly lower compressive strength than samples produced by Cu-mould suction casting, as a result of a columnar grain structure in the SLMed samples. Prototypes were manufactured, proving the feasibility of manufacturing parts of the TNZT alloy with complex geometry by SLM.
Stable superhydrophobic coatings were produced from aqueous suspensions of epoxy nanoparticles. The superhydrophobic coatings demonstrated excellent mechanical robustness and chemical resistance. Aqueous solutions of ionic surfactants, nonionic surfactants, and small organic molecules on superhydrophobic coatings could wet the superhydrophobic coatings. However, the superhydrophobicity can be recovered by rinsing the wet surface with water. It was also discovered that, although seemed wetted, the superhydrophobic surface was separated from the solution of ionic surfactant by a layer of ionic surfactant molecules. In contrast, nonionic and small organic molecules could not aggregate on the superhydrophobic surfaces; the coatings were exposed to the solutions.
Telomeres are repeating DNA sequences found on the ends of chromosomes, which shorten with age and are implicated in senescence. Cross-species analyses of telomere shortening rates (TSR) and telomere lengths are important for understanding mechanisms underlying senescence, lifespan and life-history strategies of different species. Whittemore et al. (2019) generated a new dataset on variation in TSR, lifespan and body mass. In phylogenetically uncorrected analyses they found that TSR negatively correlates with lifespan. We re-ran analyses of their dataset using appropriate phylogenetic corrections. We found a strong phylogenetic signal in the association between TSR and body mass. We were able to corroborate Whittemore et al.’s major findings, including while correcting for body mass in a multivariate analysis. Since laboratory mice have different telomere lengths and potentially different telomere dynamics than wild mice, we removed mice from the analysis, which attenuates most associations.
Dynamic nuclear polarization (DNP) is a technique in magic-angle spinning (MAS) nuclear magnetic resonance (NMR) which leads to sensitivity enhancement and helps to overcome the issue of low polarization in detected nuclei. Recent research showed, that methyl groups, which show active reorientation dynamics and cause heteronuclear cross relaxation at typical DNP temperatures around 100 K, may be used as a pinpoint source of polarization for selective and site-specific probing. In this study, we investigated the cross-relaxation behavior of methyl groups in nicotine and caffeine under DNP. These effects could be useful for investigating receptor/ligand binding.
Electrospun membranes have potential applications in the field of waterproof and breathable textile products. However, challenges still exist to improve the breathability, and waterproof and mechanical properties of these microporous membranes. In this paper, a novel hydrophobic microporous nanofiber membrane was prepared via side-by-side electrospinning of fluorosilane-modified silica nanoparticles (F–SiO2) blended with synthesized polyurethane (PU) solution and composited with the polyacrylonitrile (PAN) solution. To prepare F–SiO2, SiO2 nanoparticles were hydrophobically modified by fluorosilane. Composite nanofiber membranes with different blending ratios of PU(F–SiO2)/PAN were fabricated via side-by-side electrospinning by controlling the extruding speed of two spinnerets. Experimental results indicated that regarding F–SiO2 as hydrophobic inorganic particle can improve the hydrophobic properties of PU nanofiber membrane. The prepared PAN/(F–SiO2/PU) nanofiber microporous membranes exhibit relatively excellent waterproof and mechanical properties as that robust tensile strength (19.5 MPa), preferable water vapor permeability [10.3 kg/(m2 d)], favorable water contact angle (137.2°), and superior mechanical properties. It was believed that the reinforced PAN/(F–SiO2/PU) nanofibrous composite membranes have potential applications in chemical protective clothing, army combat uniforms, self-cleaning materials, and other medical products.
In this study, the glass forming ability, thermal stability, and room-temperature mechanical behavior of a high Zr-containing Zr71Cu11Ni10.5Al7Ti0.5 bulk glassy alloy were investigated. The glassy alloy exhibits a high glass-forming ability with a critical casting diameter of 5 mm using copper mold injection casting, and its critical cooling rate is estimated to be smaller than 40 K/s. A small kinetic fragility index m of 32 indicates its good thermodynamic stability and glass-forming ability. Compressive tests indicate that the glassy alloy displays a significant average plastic strain of 12.3%, a high fracture strength of 1592 MPa, and Young's modulus of 74.5 GPa. The good ductility is attributed to the introduction of more free volume and local compositional inhomogeneity with increasing Zr addition. This finding may provide useful guidelines for the development of novel high Zr-containing glassy alloys.
As electrode materials, metal-organic frameworks always have low electrical conductivity and poor structural stability, which limits its applications in electrochemical fields. Here, Ni-BPDC/GO composites are synthesized using graphene oxide (GO) as a substrate and 4,4′-biphenyldicarboxylic acid (BPDC) as an organic ligand via a hydrothermal approach. The growth mechanism of the Ni-BPDC and Ni-BPDC/GO composites is proposed. In the composites, highly dispersed Ni-BPDC macro-nanostrips are supported on the GO surface in parallel. The presence of GO does not affect the growth and crystalline structure of Ni-BPDC. Compared with the Ni-BPDC, Ni-BPDC/GO composites exhibit higher specific capacitance, rate capability, and operating current density through lowering intrinsic resistance, charge-transfer resistance, and ion diffusion impedance. Moreover, the assembled Ni-BPDC/GO-3//reduced graphene oxide (rGO) asymmetric supercapacitor has large specific capacitance, good cycling stability, and high energy density (16.5 W h/kg at 250 W/kg). Hence, Ni-BPDC/GO composites are a potential electrode material for supercapacitors.
In probing quantum materials, thermal transport is less appreciated than electrical transport. This article aims to show the pivotal role that thermal transport may play in understanding quantum materials—longitudinal thermal transport reflects itinerant quasiparticles, even in an electrical insulating phase, while transverse thermal transport such as the thermal Hall and Nernst effects is tightly linked to nontrivial topology. We discuss three examples—quantum spin liquids wherein thermal transport identifies its existence, superconductors wherein thermal transport reveals the superconducting gap structure, and topological Weyl semimetals where the anomalous Nernst effect is a consequence of nontrivial Berry curvature. We conclude with an outlook on the unique insights thermal transport may offer to probe a much broader category of quantum phenomena.
Gold nanoparticles (AuNPs) are one of the most versatile and accessible classes of nanomaterials. Their chemical stability, ease of colloidal synthesis, surface functionalization, and plasmonic resonance—tunable from the visible through the near-infrared—have made AuNPs the metal nanoparticle of choice for many applications. This article summarizes the chemical synthesis of AuNPs, particularly gold nanorods, with a focus on recent developments in shape control and surface modifications. Current applications using the photothermal properties of AuNPs, as well as AuNP connections to biology and the environmental sciences, will be discussed.