Effects of porosity on the measured fracture energy of brittle materials

Although it is known that growing cracks will interact with pores, it is unclear whether the magnitude of this effect is sufficient to influence the fracture energy. To study this, experiments have been carried out where cracks have been grown through simple distributions of pores in poly(methyl methacrylate). These show that the applied force required to grow the crack between two pores can be greater than that required to grow the crack in the pore-free material. Direct observation during crack growth shows that this increase in applied force is associated with the crack front becoming curved. Based on these observations, the effect of equiaxed pores on the fracture energy of brittle materials has been quantitatively described. The analysis predicts how the relative fracture energy should be influenced by the pore volume fraction, and that it should be independent of the size of the pores or the fracture energy of the matrix. These predictions give good agreement with experimental measurements in different ceramic materials, in which the microstructure of the matrix surrounding the pores does not change with pore volume fraction.     

Formation of three-dimensional arrays of magnetic clusters NiO,Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> by the matrix method

A method has been proposed for the formation of three-dimensional arrays of isolated magnetic clusters NiO, Co₃O₄, and NiCo₂O₄ in the sublattice of pores in the matrix of bulk synthetic opals through a single impregnation of the pores with melts of nickel and cobalt nitrate crystal hydrates and their thermal degradation. The method makes it possible to controllably vary the degree of filling of pores in the matrix with oxides within 10–70 vol %. The composition and structure of the synthesized materials, as well as the dependences of their static magnetic susceptibility on the magnetic field strength, have been investigated.     

Electronic Excitation Energy Transfer between Different Types of Dye Molecules in a Porous Glass Matrix

The processes of electronic excitation energy transfer (EEET) between different types of dye molecules inserted into the matrix of a porous glass have been investigated. An extreme character of the dependence of the EEET efficiency on the size of pores has been revealed. The dependence of the fractal dimensionality of the distribution of dye molecules on the sizes of the pores has been determined. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 446–449, July–August, 2005.     

Atomic mobility in a ternary liquid Ga–In–Sn alloy of the eutectic composition

The nuclear spin–lattice relaxation and Knight shift of ⁷¹Ga, ⁶⁹Ga, and ¹¹⁵In nuclei in a ternary liquid gallium–indium–tin alloy of the eutectic composition, which was introduced into pores of an opal matrix and porous glasses with pore sizes of 18 and 7 nm, have been investigated and compared with those for the bulk melt. It has been found that longitudinal relaxation is accelerated and the Knight shift is decreased, depending on the size of pores. The correlation time of the atomic motion has been calculated for the nanostructured melt in porous matrices. It has been shown that the atomic mobility in the melt decreases with decreasing size of pores in the glasses.

     

Properties of three-dimensional composites based on opal matrices and magnetic nanoparticles

Three-dimensional nanocomposites consisting of an opal matrix and a metal have been prepared by the interaction of salts and oxides of different elements (Ni, Co, Fe, etc.) embedded in an opal matrix with isopropanol in the range of supercritical state parameters of the alcohol. According to X-ray powder diffraction analysis and transmission electron microscopy data, the composites consist of an X-ray amorphous opal matrix with pores filled by nanoparticles of Co (or CoO _x ), metallic Ni, or Fe₃O₄ with a magnetite structure of various morphology. The sizes of the nanoparticles do not exceed the diameter of the pores in the opal matrix. A complex investigation of the nanocomposites has been performed using the electron magnetic resonance and vibrating magnetometry methods. All the studied samples at room temperature exhibit a ferromagnetic behavior. The coercive force of the samples lies in the range from 150 Oe for iron-containing nanocomposites to 565 Oe for cobalt-containing nanocomposites.     

Fracture of composites based on polyethylene and elastic particles

The composites based on low-density polyethylene with elastomer filling particles are studied. A fracture mechanism induced by the fracture of filler particles or their separation from the matrix polymer is revealed. The fracture of the composites is caused by the growth of formed rhombic pores. The natural relative elongation in a neck is shown to be an important characteristic of a polymer. If the relative elongation in a neck is lower than the strain of appearance of rhombic pores, they form at the stage of uniform tension after necking, and the composite remains plastic. If the relative elongation in a neck is higher than the strain of formation of rhombic pores, they nucleate during necking, and the material undergoes quasi-brittle fracture. Good adhesion between the matrix polymer and elastic particles hinders the appearance of rhombic pores in a neck and, thus, retains high deformation properties of the composites.     

Luminescence of Eu3+ and Tb3+ Ions in the Structure Microporous Xerogel/Mesoporous Anodic Aluminum Oxide

We have investigated the luminescence, luminescence excitation, and transmittance of europium- and terbium-doped xerogel films formed on smooth, nanotextured surfaces and in the pores of anodic aluminum oxide. Some factors responsible for enhancement of luminescence in the structure lanthanide-doped xerogel/mesoporous anodic aluminum oxide have been analyzed. It is assumed that the optical excitation of lanthanide ions can be realized directly, through a xerogel matrix, and due to the multiple scattering of exciting radiation by the matrix of mesoporous anodic aluminum oxide.     

Optical properties of a polymer host matrix containing mercuric iodide microcrystals

Microcrystals of mercuric iodide have been grown in a polymer matrix with pores 200 nm in diameter. The absorption and luminescence spectra indicate the formation of microcrystals of two (red and orange) modifications in the host matrix. The temperature dependence of the emission spectra and the evolution of absorption and emission spectra in the course of sample ageing have been studied. The evolution of the spectra is governed by the transition from the orange to red modification and the increase in size of microcrystals.     

Surface effect on the size evolution of surface plasmon resonances of Ag and Au nanoparticles dispersed within mesoporous silica

The optical absorption has been investigated for silver and gold nanoparticles dispersed within the pores of monolithic mesoporous silica after annealing at different temperatures. It has been shown that with reduction of the particle size, the surface plasmon resonance position blue-shifts first and then red-shifts for silver/silica samples, but only red-shifts for gold/silica samples. This size evolution of the resonance is completely different from that previously reported for fully embedded particles. Based on the interaction of the particle surface with ambient air and the porosity at the particle/matrix interface, we present a multi-layer core/shell model and assume that the chemical adsorption of gas molecules from the air on the free surface of nanoparticles within the pores is mainly responsible for the observed size evolution of the resonance.     

Thermal conductivity of opal filled with a LiIO<Subscript>3</Subscript> ionic conductor

The temperature dependences of the thermal conductivity of a synthetic opal and opal-based nano-composites prepared by introducing a LiIO₃ superionic conductor into pores of the opal matrix from an aqueous solution or melt are measured by the hot-wire technique in the temperature range 290–420 K. It is demonstrated that the thermal conductivity of pure opal increases with an increase in the diameter of the SiO₂ spheres forming a face-centered cubic lattice of an opal and is determined by the total thermal resistance of interfaces between the spheres. Filling of opal pores with the ionic conductor leads to an increase in the thermal conductivity. The behavior of the thermal conductivity and its magnitude in opal-based nanocomposites depend to a large extent on the method of filling the matrix pores.     

Features of proton transport in porous alumina with adsorber water

The water-ice phase transition in porous alumina films with pores of various sizes was detected using impedance spectroscopy. An electrical equivalent circuit of the structures under study was constructed. It was found that the activation energy of proton transport in water contained in alumina pores in the low-temperature region approximately four times exceeds that at high temperatures. The effect of pore sizes on proton transport in water contained in pores of the solidstate matrix was revealed.     

Theory of gas porosity produced as a result of diffusive decomposition of multicomponent systems

A complete set of equations, which describe the kinetics of formation of gas porosity, is derived. The large and small pores are defined and analyzed. The methods of solution for an arbitrary number of impurity atoms are given for the limiting cases of large and small pores. The filling of pores with a molecular gas containing matrix atoms is analyzed in detail. The obtained results are compared with the experimental data.     

Acoustic study of the melting and solidification of gallium incorporated in an opal matrix

An acoustic study is reported of the crystallization and melting of gallium embedded in an opal-like matrix. The variations of the velocity and absorption of longitudinal ultrasonic waves during phase transitions in the α and β modifications have been found to be hysteretic in nature. It is shown that acoustic methods do not detect gallium melting and crystallization in the tetragonal phase forming in a restricted geometry. Experimental evidence for heterogeneous crystallization of gallium in pores has been obtained.     

Effects of electron drag of gold in pores of anodic aluminum oxide: Reversible resistive switching in a chain of point contacts

The current–voltage characteristics of Au/AAO(Au)/probe structures based on anodic aluminum oxide with pores incompletely filled with gold have been studied. It has been found that an electric field initiates the mass transfer of a rear Au electrode and the subsequent growth of the metal in unfilled parts of pores of the oxide matrix in the form of chains of gold islands. It has been established that this transfer, which appears at a positive potential of the probe, is due primarily to the effect of electron drag of the metal (Au). Estimates have been obtained for the effective radius of Au islands (2 nm), the width of a gap between islands (0.5 nm), the height of potential barriers (100 meV), and the characteristic resistance of tunnel junctions (30 kΩ ~ h/e²), which is typical of point quantum contacts. The structures demonstrate reversible resistive switching between low- (~1 MΩ) and high-resistance (>100 GΩ) states.     

Size-quantization effects in the optical spectra of PbI2 and HgI2 nanocrystals

Size-quantization effects (short-wavelength shift of the structure and broadening) in low-temperature absorption spectra of PbI₂ and HgI₂ nanocrystals embedded in porous sodium borosilicate glass matrix with pores ranging in size from 2 to 50 nm have been observed and studied. The nanocrystal size was evaluated in the strong quantization approximation. It is shown that the size does not depend on the pore diameter of the matrices used, but is determined by the concentration of the solution introduced into the pores. The absorption and luminescence spectra of the systems thus obtained are shown to evolve as the substance emerges from the pores out onto the surface. The dynamics of formation and the phase composition of the PbI₂ and HgI₂ crystals on the surface are studied. Fiz. Tverd. Tela (St. Petersburg) 39, 468–473 (March 1997)     

Magnetic behaviour of a magnetite/silicon nanocomposite

Mesoporous silicon is utilized to infiltrate quite monodisperse iron oxide nanoparticles into the pores. This semiconducting matrix exhibits oriented pores, clearly separated from each other, with an average pore diameter of 55 nm. Iron oxide nanoparticles of 8 nm and 5 nm in size which are coated with a surfactant are prepared by high temperature decomposition in the presence of an organic precursor. The achieved nanocomposite consists of dispersed Fe₃O₄-nanoparticles within the pores and offers magnetic properties which are determined by the morphology of the silicon matrix as well as by the distribution of the particles within the individual pores. Thus, the change of regime between a superparamagnetic and a blocked state of the system can be tuned. Furthermore, magnetic anisotropy between the two magnetization directions, normal and parallel to the sample surface, is observed due to the oriented and separated pores of the template which are quasi-regular arranged. This porous silicon/magnetite composite with its adjustable magnetic properties is also of interest for possible applications in biomedicine due to the low toxicity of both materials.     

Influence of elastic strains on the adsorption process in porous materials. Thermodynamics and experiment

If we disregard the shape of the boundary hysteresis loop, H1 for SBA-15, MCM-41 and KIT-6, H2 for p⁺-type porous silicon and porous glass, the hysteretic features inside the loop are qualitatively the same for all these systems and show that none of them are composed of independent pores whether the pores are interconnected or not. We hence believe that the physical parameter which couples the pores is not the interconnectivity but the elastic deformation of the porous matrix. The thermodynamic approach we develop includes the elastic energy of the solid. We show that the variation of the surface free energy, which is proportional to the deformation of the porous matrix, is an important component of the total free energy. With porous silicon, we experimentally show that a stress external to the porous matrix exerted by the substrate on which it is supported significantly increases the total free energy and the adsorbed amount and decreases the condensation pressure compared to that of the same porous matrix detached from its substrate which is the relaxed state of the supported layer. This stress can be partly relaxed by making thicker porous layers due to the breaking of Si-Si bonds. This results in the shift of the isotherms towards that of the membrane. We propose a new interaction mechanism occurring through the pore wall elastic deformation in which the external mechanical stress is imposed on a given pore by its neighbours.     

Equilibrium partitioning of a simple fluid in a matrix-filled cylindrical pores with adsorbing walls: A grand canonical Monte Carlo study

We have studied a model of a hard sphere fluid adsorbed in a cylindrical pore filled with quenched disordered matrix of hard sphere particles using Grand canonical Monte Carlo simulations. The interactions between matrix species and pore walls are assumed of a hard sphere type. However, the pore walls exert a short-range attraction upon adsorbed fluid particles. We discuss the adsorption isotherms and the density profiles of fluid particles in pores with different microporosity for several values of the pore radius. We have observed that like in homogeneous microporous media the adsorption increases with increasing porosity. However, trends of behavior of the isotherms also reflect layering of adsorbed fluid. The data obtained in this study may serve as a benchmark for the development of the theory of confined quenched-annealed systems and for computer simulation investigation of models permitting phase transitions in pores. This project has been supported in parts by DGAPA of the UNAM under research grant IN111597, by the National Council for Science and Technology (CONACyT), grant No. 25301-E.