Experimental investigation and modeling of the creep behavior of ceramic fiber-reinforced SiO2 aerogel

Ambient creep tests at constant stresses of 0.05 and 0.1 MPa were conducted on a ceramic fiber-reinforced SiO₂ aerogel for more than 240 h. Experimental result shows that there are creep deformations even at very low stress level, and the creep strains finally reach constant values. High temperature creep experiments were also investigated at constant stresses of 0.05 and 0.2 MPa for 1800 s. The creep strains in 1800 s at 900 °C are 20–69 times higher than that at 300 °C. The creep behavior could be well described by the power-law creep model. Scanning Electron Microscope (SEM) analysis was also conducted to understand the micro mechanism of the creep behavior. Thermal softening of aerogel nanoparticles to generate larger ones was found to be the main reason for the change in mechanical properties at high temperature.     

Plastic Creep Features of Sapphire at Near-Melting Point Temperatures

Crystallography Reports - The anomalous plastic creep of sapphire crystals in the temperature range of 1890–2050°С, which includes their melting point, has been studied. A method...     

Influence of selenization temperature on synthesis,morphology, and properties of nanostructured CoSe2 films

Nanostructured cobalt selenide (CoSe₂) thin films were deposited on a glass substrate using the selenization of Co films at different selenization temperatures (300 °C, 400 °C, and 500 °C) in a pure Se vapor for two hours. The morphology and structure of the as‐deposited films shows that the film morphology and crystallinity are affected by the selenization temperature. Increasing the selenization temperature from 300 °C to 400 °C and 500 °C results in a change in the surface and cross sectional morphology. At 300 °C, the Co films have an almost amorphous structure, while at temperatures of 400 and 500 °C, the Co films have a crystalline nanostructure with bilayered morphology. Optical analyses of the CoSe₂ films at 500 °C show a large absorption (α > 1.0 × 10⁵ cm⁻¹) and a direct band gap (∼1.0 eV).     

Morphology of ice droxtals grown from supercooled water droplets

This study describes the growth modes of polyhedral ice crystals grown from spherical ice crystals formed by freezing of supercooled water droplets. Frozen water droplets were grown on a thin glass plate at -15 and -7°C and at humidities above and below water saturation. In the case of growth at -15°C and at a humidity above water saturation, a frozen water droplet grows into an 8-faced ice crystal through a 20-faced ice crystal, while in the case of growth at -15°C and at a humidity between the ice saturation and the water saturation, it grows into an 8-faced ice crystal with truncated edges and corners through an 8-faced ice crystal with curved and flat surfaces. The growth modes at -7°C and at humidities above and below water saturation are almost the same as those at -15°C, except that the axial ratios c/a of the droxtals at -15°C are about 0.95 but those at -7°C are about 1.04.     

Effect of indium content and rapid solidification on microhardness and micro‐creep of Sn‐Zn eutectic lead free solder alloy

The Sn‐Zn alloys have been considered as lead‐free solders. In this paper, the effect of 0.0, 0.5, 1.0, 1.5 and 2.0 wt.% Indium as ternary additions on melting temperature, structure, microhardness and micro‐creep of the Sn‐9Zn lead‐free solders were investigated. It is shown that the alloying additions of Indium to the Sn‐Zn binary system result in a suppression of the melting point to 187.9 °C. From x‐ray diffraction analysis, a new intermetallic compound phase, designated β‐In₃Sn is detected. The formation of an intermetallic compound phase causes a pronounced increase in the electrical resistivity and mechanical strength. Also, an interesting connection between dynamic Young's modulus and the axial ratio (c/a) of the unit cell of the β‐Sn was found in which Young's modulus increases with increasing the axial ratio (c/a). The ternary Sn‐9Zn‐xIn exhibits creep resistance superior to Sn‐9Zn binary alloy. The better creep resistance of the ternary alloy is attributed to solid solution effect and precipitation of In₃Sn in the Sn matrix. The addition of small amounts of In is found to refine the effective grain size and consequently, improves hardness. The 89%Sn‐9%Zn‐2%In alloy is a lead‐free solder designed for possible drop‐in replacement of Pb‐Sn solders. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dielectric and thermal behaviour of holmium tartrate trihydrate crystals

Measurements on dielectric constant of holmium tartrate trihydrate crystals at frequencies of the applied a.c. in the range 1 kHz to 1 MHz and at temperature in the range 30°C to 330°C are reported. The dielectric constant ε′ increases with temperature at all frequencies, attains a peak near 250°C, and then decreases as the temperature goes beyond 250°C. The anomalous dielectric behaviour at near about 250°C is attributed to be as a result of crystallographic/polymorphic phase transition brought about in the material. The results on the dielectric behaviour of the material are supplemented by results of thermal analysis viz., TG and DTA. Thermogravimetric and differential thermal analytic techniques have been used to study thermal behaviour of the material. It is shown that the material is thermally stable up to 220°C beyond which it decomposes through three stages till the formation of holmium oxide at 1200°C. The non‐isothermal kinetic parameters e.g., activation energy and the frequency factor have been evaluated for first two stages of thermal decomposition by using the integral method of Coats and Redfern. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Novel Cinnamate Esters—Synthesis and Mesomorphic Properties in Relation to Molecular Structure

A novel homologous series 4-[4′-n-alkoxy cinnamoyloxy] benzyl cinnamates consisting of 11 homologs was synthesized and studied with a view to understanding and establishing the relation between molecular structure and mesomorphic behavior of a substance. Mesomorphic behavior of the series commences from the third homolog and then continues until the last hexadecyloxy homolog. The first and second members of a series are non-mesomorphic. Nematogenic mesophase formation is observed from the propoxy homolog to the hexadecyloxy homolog, but the smectogenic mesophase formation is observed from the hexyloxy homolog to the tetradecyloxy homolog. All mesomorphic transitions are enantiotropic in nature. Transition temperatures were determined by an optical polarizing microscopy equipped with a heating stage. The textures of the nematic mesophase are threaded or Schlieren in type and that of the smectic mesophases are focal conic fan shaped of the smectic A or smectic C type. A phase diagram of the series shows a normal behavior of the transition curves with a minor abnormality of the last three homologs for the nematic-isotropic transition curve. The average thermal stability for smectic and nematic mesophases is 176.0°C and 219.3°C, respectively. Analytical and spectral data agree with the molecular structures of homologs. Mesomorphic phase length varies between 22°C and 76°C. Smectic phase length varies from 6°C to 33°C and nematic phase length varies from 22°C to 54°C. Thus, the novel present series is predominantly nematogenic and partly smectogenic. Mesomorphic properties of the novel series are compared with structurally similar other known homologous series.     

Thermo-mechanical behavior of uniaxially drawn and crystallized poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films

Thermo-mechanical properties of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films in the amorphous state, uniaxially stretched and thermally crystallized were compared and show different microstructure–property relationships. The glass transition temperature shifts to higher temperatures when films are stretched above T_g at 160 °C, in relation with a crystallinity increase and subsequent confinement effects of the amorphous phase, but films stretched below T_g at 100 °C show an opposite trend while drawing. This is discussed in relation with the highly mobile structural state arising from plastic deformation of the amorphous phase. The confinement effects were particularly assessed by the estimation of a ‘rigid amorphous fraction’ (RAF). Crystallinities and RAF were estimated from DSC data for the uniaxially drawn and thermally crystallized films respectively. It was observed that the RAF was higher in thermally crystallized films than in uniaxially drawn samples in relation to a lamellar semi-crystalline morphology. In addition to this detailed thermo-mechanical characterizations, low stress creep measurements were carried out during heating scans. They can be related to the microstructure and thermal transitions of the films. In particular, for the uniaxially stretched films, the measured shrinkage decreases when the draw ratio increases because the crystalline entities developed during the draw process are locking the chain motions.     

Electron microscopy of biomaterials based on hydroxyapatite

Three types of biomaterials based on hydroxyapatite are synthesized and investigated. Hydroxyapatite nanocrystals or microcrystals precipitated from low-temperature aqueous solutions serve as the initial material used for preparing spherical porous granules approximately 300–500 μm in diameter. Sintering of hydroxyapatite crystals at a temperature of 870°C for 2 h or at 1000°C (for 3 h) + 1200°C (for 2 h) brings about the formation of solid ceramics with different internal structures. According to the electron microscopic data, the ceramic material prepared at 870°C is formed by agglomerated hydroxyapatite nanocrystals, whereas the ceramics sintered at 1200°C (with a bending strength of the order of 100 MPa) are composed of crystal blocks as large as 2 μm. It is established that all the biomaterials have a single-phase composition and consist of the hydroxyapatite with a structure retained up to a temperature of 1200°C.     

Dimensional changes of silica-, borosilicate- and germania-glasses and quartz under irradiation

Length changes of crystalline SiO₂, of two SiO₂ glasses (HERASIL1, SUPRASIL1), of two SiO₂–B₂O₃ glasses (DURAN, VYCOR) and of vitreous GeO₂ were measured during proton, electron and UV irradiations under various applied tensile stresses at temperatures up to 400 °C. No change was observed under UV irradiation, while proton and electron irradiation initially caused transient compaction and subsequent irradiation induced creep with linear stress dependence (viscous flow). Asymptotic compaction strain is reduced at elevated temperatures due to recovery processes, while creep rate is virtually independent of temperature. Compaction tends to decrease while creep rate increases with decreasing SiO₂ content of the glasses. A comparison of flow rates to electronic and nuclear stopping power gives no safe discrimination of the underlying mechanisms. Quartz shows continuous dilatation under proton irradiation, with a very low stress dependence, which allows only determination of an upper limit of viscous flow compliance.     

The Effect of Additions of Mn and Fe on the Structure of an Al-2 at.% Zn-1 at.% Mg Alloy

The structure of an Al-1.9 at.% Zn-1.3-at.% Mg mother alloy with additions of Mn and Fe, respectively, were investigated by TEM and microprobe analysis for two different heat treatments, namely direct quench from 490 °C to 160 °C and a two-step ageing treatment (preageing at RT for 7d before storing at 160 °C), to explain the differences in the course of the hardness obtained during the ageing at 160 °C. The differences of the structure observed are explained in terms of the tendency of the additives to form intermetallic phases, particularly in the grain boundaries.     

Mesomorphism dependence on lateral substitution of functional groups and their position in series: 4(4′-n-alkoxy benzoyloxy)-3-chloro phenyl azo-2″-methoxy benzenes

Eleven homologues of the title series were synthesized. The methyl-to-pentyl derivatives are nonmesomorphic. The nematic mesophase commences from the sixth member of the series without any smectic phase. An odd–even effect in the nematic–isotropic transition curve is not observed. The nematic mesophase appeared as a threaded or a Schlieren-type texture as observed through a hot-stage polarizing microscope. The nematic–isotropic transition temperatures are between 89°C and 127°C with the mesomorphic range varying from 9°C to 44°C at the hexyl and tetradecyl derivative of the series, respectively. The nematic–isotropic transition curve initially rises and then falls in a normal manner as the series is ascended, but it abnormally rises beyond the 10th homologue. The series is enantiotropic nematic with a middle-ordered melting type. Analytical data support the structures of the molecules. The thermal stability and some other mesomorphic characteristics are compared with structurally similar homologous series. The average nematic–isotropic thermal stability is 105.4°C.     

The Mesogenic Behavior of Isomeric and Nonisomeric Series of Novel Azoesters. 4-(4′-n-Alkoxy Benzoyloxy) Phenylazo-2″-Bromobenzenes

A novel homologous series: 4-(4′-n-alkoxy benzoyloxy) phenylazo-2^″-bromobenzenes is synthesized and studied with a view to understanding and establishing the effects of molecular structure on mesogenic behavior in a series. The mesogenic property commences from fifth homologue to the last homologue. The transition temperatures of the series are relatively low ranging between 68°C and 143°C. The mesogenic range varies between 6°C and 22°C. The novel azoester series is nematogenic without exhibition of any smectogenic property and an average thermal stability of 89.4°C. The mesogenic behavior of the novel series is compared with structurally similar isomeric/nonisomeric other known series.     

Thermal Analysis of LiGaO2 and NaGaO2

The high-temperature thermal properties of the ternary oxides LiGaO₂ and NaGaO₂ are studied by simultaneous differential thermal analysis and thermogravimetry between room temperature and about 1700 °C. For the melting temperature of LiGaO₂ a value of 1595 ± 10 °C is determined. NaGaO₂ undergoes a solid state phase transition at 1280 ± 10 °C and melts at 1395 ± 10 °C.     

Dependence of Mesomorphism on Molecular Rigidity with Reference to Lateral Substitution and Central Bridge

A novel mesogenic ester homologous series is synthesized and studied with a view to understanding and establishing the effect of laterally substituted -OCH₃ on mesomorphic behavior. The series consists of twelve members. C₁ to C₄ members are nonmesogenic, C₆ to C₁₂ are smectogenic in addition to nematogenic, and C₁₄ to C₁₆ are only smectogenic. The textures of smectic and nematic phases are A or C type and threaded or Schlieren, respectively. The transition curves in a phase diagram exhibit an odd–even effect and behave in normal manner except for the C₁₄ and C₁₆ derivatives in Sm-I transitions. Thermometric data were determined by an optical polarizing microscope equipped with a heating stage. Average thermal stability for smectic and nematic are 107.7°C and 121.0°C, respectively. Smectogenic and nematogenic mesophase length ranges from 11.0 to 44.0°C and 12.0°C to 39.0°C, respectively. The mesomorphic properties of present series are compared with structurally similar other known series. Thus, present series is predominantly smectogenic and partly nematogenic of middle ordered melting type.     

Impedance Measurements on Sintered Discs of Sodium Metavanadate

Impedance measurements of sintered discs of sodium metavanadate (NaVO₃) are investigated in the frequency range 10 KHz to 1 MHz and in the temperature region from room temperature (≈ 30 °C) to 450 °C. A.C. conductivity is calculated from the data. D.C. Conductivity in measured in the temperature range 150 to 450 °C. Debye type of relaxation effects are observed in the loss measurements. The activation energy of the dipoles involved in the relaxation is estimated to be 0.76 eV. The activation energy for A.C. conduction in the temperature range 380 to 450 °C is about 0.78 eV. The relaxation phenomenon observed is explained in terms of space charge polarisation due to defects.     

Synchrotron X-ray diffraction study of the structure of oral stratum corneum model lipid membranes

Membranes modeling the lipid fraction of the mucous membrane of the human oral cavity have been studied by X-ray synchrotron diffraction. Ternary systems, composed of a mixture of sphingomyelin, dipalmitoylphosphatidylcholine, and dipalmitoylphosphatidylethanolamine with component weight ratios of 1: 2: 2 and 1: 2: 1, have lamellar structures in the gel phase at the physiological temperature (37°C). An inverted hexagonal phase is formed in the system with 40% dipalmitoylphosphatidylethanolamine in the temperature range of 60–70°C. This phase coexists with the liquid-crystalline lamellar phase in a narrow temperature range and completely surpresses the lamellar phase with an increase in temperature to 80°C. Multi-component oral stratum corneum membranes are characterized by several lamellar phases at 20–37°C and the coexistence of one or several lamellar phases with inverted hexagonal phase at 80–90°C.     

Crystal Growth in Gels at Elevated Pressures: The Upper Limit of Temperature for Metastable Formation of Aragonite

Both rapid precipitation and diffusion controlled gel growth were applied to crystallize calcium carbonate at temperatures in the range of 100 °C to 270 °C. The amount of aragonite was determined by means of X-ray diffraction data. The morphology of the aragonite crystals are described. Metastable formation of aragonite was observed only at temperatures below 270 °C.     

Mesomorphism Dependence on the Position of Substitution of a Pair of Esters

A novel homologous series of ethylene derivatives of thermotropic liquid crystals has been synthesized. The methoxy to octyloxy derivatives are nematogenic, the decyloxy to tetradecyloxy derivatives are smectogenic, in addition to nematogenic, and the hexadecyloxy homologue is smectogenic only. All the members of the series are enantiotropically mesogenic. Thermotropic behavior was determined by an optical polarizing microscope equipped with a heating stage and Differential Scanning Calorimetry (DSC) study. Analytical and spectral data confirm the molecular structures of homologues (infrared, nuclear magnetic resonance, mass spectra, X-ray, and DSC data). Textures of the nematic phase are threaded or Schlieren and that of smectic phase are focal conic fan-shaped of smectic A or C. Transition curves of the phase diagram behave in a normal manner except one or two deviations from the normal trend. The mesophase range (Sm+N) varies from 3°C to 44°C. The average thermal stability for smectic is 93°C and that for nematic 117.4°C. The LC behavior of the novel series is compared with a structurally similar known series.