Coupled strain rate and temperature effects on the tensile behavior of strain-hardening cement-based composites (SHCC) with PVA fibers

This paper reports the experimental findings on the tensile behavior of Strain-hardening cement-based composites (SHCC) subjected to elevated temperatures and different strain rates and to combinations of these parameters. Uniaxial tension tests with in-situ temperature control were performed at 22 °C, 60 °C, 100 °C and 150 °C. In addition, the effect of loading rate was investigated using the strain rates of 10^? 5 s^? 1, 3 × 10^? 4 s^? 1 and 10^? 2 s^? 1 at all four temperatures considered. It was shown that tensile strength decreases both with an increase in temperature and with a decrease in the strain rate. The strain capacity increases with decreasing strain rate at temperatures of 22 °C and 60 °C, but for the temperature of 100 °C this material property increases when the strain rate increases. At 150 °C the investigated SHCC loses its ductility and no noticeable strain rate effect can be observed. Furthermore, the residual properties of SHCC were evaluated using uniaxial tensile tests at room temperature on the specimens which were previously heated to 60 °C, 100 °C or 150 °C. The residual tests showed that the strength, strain capacity, and work-to-fracture decrease with increasing pre-treatment temperature. However, in comparison with the results of the in-situ tests with elevated in-situ temperatures, the residual tests on SHCC yielded higher tensile strength and lower ductility. These results and possible mechanisms leading to changes in mechanical performance are discussed on the basis of the observed crack patterns on the specimens' surfaces as well as the microscopic investigations of the condition of fibers on fracture surfaces.     

Mechanical properties of geopolymer concrete exposed to dynamic compression under elevated temperatures

Through adoption of a self-designed high temperature SHPB apparatus herein, an experimental study is made on the mechanical properties of geopolymer concrete (GC) exposed to dynamic compression under elevated temperatures. As the results have turned out, the weight loss is remarkable within temperature ranges from room temperature to 200 °C as well as from 600 °C to 800 °C. The dynamic compressive strength of GC grows higher at 200 °C than at room temperature, but suffers a dramatic drop at 800 °C. The critical strain is higher at elevated temperature than that at room temperature. At 200 °C and 600 °C, respectively, its energy absorption property is superior to that at room temperature. However, at 400 °C and 800 °C, respectively, it is inferior to that at room temperature. The strain rate effect of the dynamic increase factor (DIF) obtained from test data can reflect the inherent nature of GC. The DIF assumes a linear relationship with the logarithm of strain rate.     

High temperature mechanical properties of laminated ZrB2–SiC based ceramics

Two laminated ZrB₂-SiC based ceramics were prepared by tape casting and subsequent hot pressing, with BN (LZB) and graphite (LZG ) as interface layers. The LZB specimen presents flexural strength of 381 MPa at room temperature and 111 MPa at 1500 °C; while the LZG specimen shows flexural strength of 414 MPa at room temperature and 377 MPa at 1500 °C. In addition, the flexural strength of LZG specimen is always higher than that of the LZB specimen in the temperature range from room temperature to 1500 °C. Such higher strength is attributed to the healing of surface microcracks and pores by the SiO₂ glass phase, producing less glass phase in graphite interface layers at high temperature.     

Effect of annealing temperature on structural and optoelectronic properties of γ-CuI thin films prepared by the thermal evaporation method

High quality transparent conducting CuI thin films were deposited at room temperature via thermal evaporation technique followed by post deposition annealing at different temperatures. The samples were characterised by X-ray diffraction (XRD), UV–Vis spectrophotometry, Scanning electron microscopy and I-V measurements. The structural, morphological and optical properties were studied as a function of the annealing temperature from room temperature (RT) to 200 °C. XRD results revealed that the films were polycrystalline with zinc blende structure of cubic phase. Increasing the annealing temperature increased the crystallite size from 33 to 49 nm whilst the dislocation density and lattice strain shifted to lower values. High transmittance of about 70–80% was exhibited by all films in the entire visible spectral range. The as deposited film possesed the lowest resistivity of 3.0×10⁻³ Ω cm.     

Macroporous rubber gels as reusable sorbents for the removal of oil from surface waters

Macroporous organogels were prepared by solution crosslinking various rubbers in benzene at ?18 °C. Butyl rubber (PIB), cis-polybutadiene (CBR) and styrene–butadiene rubber (SBR) were used as the rubber components, while sulfur monochloride was the crosslinker in the gel preparation. The organogel networks consist of large pores of 10¹–10² μm in size caused by the benzene crystals acting as a template during gelation. The networks formed by CBR and SBR showed an aligned porous structure consisting of regular pores, whereas those derived from PIB had irregular pores with a broad pore size distribution due to the phase separation of PIB chains at low temperatures. All organogels were very tough and could be completely compressed without any crack development. Sorption tests showed that the organogels were efficient at removing crude oil, gasoline, diesel, fuel oil and olive oil. The organogels are reusable once they are squeezed, leading to continuous sorption capacities of CBR or SBR gels for crude oil and olive oil of 33–38 g/g and 24–27 g/g, respectively. These sorption capacities are two to three times the capacity of the gels derived from PIB.     

Brazing of silicon nitride ceramic composite to steel using SiC-particle-reinforced active brazing alloy

Silicon carbide particles have been introduced as reinforcements in a commercially available active metal braze filler alloy (Incusil ABA, Wesgo Metals) used for the joining of ceramic-to-metal. The effect of particle reinforcement of the braze filler on the flexural strength of ceramic to metal joints has been investigated at room temperature and at elevated temperatures. An average four point flexural strength of nearly 400 MPa is achieved at room temperature when using Incusil ABA + 30 vol.% SiC (sandwich foil system) compared to 330 MPa with Incusil ABA alone. At a test temperature of 250 °C relaxation of residual stresses in the joints results in an average flexural strength of approximately 520 MPa when using Incusil ABA + 10 vol.% SiC. These values compare with an average room temperature flexural strength of nearly 800 MPa for the ceramic composite. The reaction products of the braze alloy at the joint interface were identified by SEM.     

Characteristics of samaria-doped ceria nanoparticles prepared by spray pyrolysis

Samaria-doped ceria (SDC) nanoparticles were prepared by spray pyrolysis. The means sizes of the samaria-doped ceria nanoparticles were controlled from 21 to 150 nm by changing the calcination temperatures between 700 and 1200 °C. The pellets formed from the SDC particles calcined at temperatures between 700 and 1000 °C had similar grain sizes between 0.75 and 0.82 μm. However, pellet formed from the SDC particles calcined at a temperature of 1200 °C had large grain size of 1.22 μm. The pellet formed from the SDC particles calcined at a temperature of 1000 °C had slightly smaller resistance of grain-boundary than those of the pellets formed from the SDC particles calcined at temperatures between 700 and 900 °C. However, the pellet formed from the SDC particles calcined at a temperature of 1200 °C had low resistance of grain-boundary. The pellet formed from the SDC particles calcined at a temperature of 1200 °C had conductivity of 44.65 × 10^?3 S cm^?1 at a measuring temperature of 700 °C that more twice than those of the pellets formed from the SDC calcined below 1000 °C.     

Fracture characteristics of Al2O3/YAG composite at room temperature to 2023 K

The fracture behavior of the notched unidirectionally solidified eutectic Al₂O₃/YAG composite was investigated by tensile test at room temperature to 2023 K, and the results were analyzed by the finite element method. The stress–strain behavior, notched strength and the fracture toughness were strongly dependent on temperature and displacement speed. The specimens fractured in a brittle manner at low temperatures and at high displacement speeds but in a ductile manner accompanying plastic deformation at high temperatures and at low displacement speeds. The notched strength for a given displacement speed of 10⁻⁷ m/s increased with increasing temperature from 132 MPa at 1873 K to 153 MPa at 1923 K, and then decreased to 133 and 110 MPa at 1973 and 2023 K, respectively. Also the notched strength for a given temperature of 2023 K increased with decreasing displacement speed from 67 MPa at the displacement speed of 10⁻⁵ m/s to 124 MPa at 10⁻⁶ m/s, and then decreased to 110 and 72 MPa at 10⁻⁷ and 10⁻⁸ m/s, respectively. The stress distribution and the plastic zone size ahead of the notch were calculated by a finite element method using the dependence of flow stress on temperature and displacement speed. Based on the calculation result, the experimentally observed increase in the notched strength with increasing temperature and decreasing displacement speed up to the maximum value could be accounted for by the increase in plastic zone size ahead of the notch. Also, the observed decrease in notched strength with further increasing temperature and decreasing displacement speed could be accounted for by the decrease in the stress carrying capacity of the yielded ligament.     

Fabrication and properties of ultra highly porous silicon carbide by the gelation–freezing method

Silicon carbide (SiC) with ultra high porosity and unidirectionally oriented micrometer-sized cylindrical pores was prepared using a novel gelation–freezing (GF) method. Gelatin, water and silicon carbide powder were mixed and cooled at 7 °C. The obtained gels were frozen from ?10 to ?70 °C, dried using a vacuum freeze drier, degreased at 600 °C and then sintered at 1800 °C for 2 h. The gels could be easily formed into various shapes, such as cylinders, large pipes and honeycombs using molds. Scanning electron microscopy (SEM) observations of the sintered bodies showed a microstructure composed of ordered micrometer-sized cylindrical cells with unidirectional orientation. The cell size ranging from 34 to 147 μm could be modulated by changing the freezing temperatures. The numbers of cells for the samples frozen at ?10 and ?70 °C were 47 and 900 cells/mm², respectively, as determined from cross-sections of the sintered bodies. The resulting porous SiC with a total porosity of 86%, exhibited air permeability from 2.3 × 10^?11 to 1.0 × 10^?10 m², which was the same as the calculated ideal permeability, and high compressive strength of 16.6 MPa. The porosity, number of cells, air permeability and strength of the present porous SiC were significantly higher than that reported for other porous SiC ceramics.     

Freezing as a path to build macroporous structures: Superfast responsive polyacrylamide hydrogels

Macroporous polyacrylamide (PAAm) hydrogels were prepared from acrylamide monomer and N,N′-methylene(bis)acrylamide (BAAm) crosslinker in frozen aqueous solutions. It was found that the swelling properties and the elastic behavior of the hydrogels drastically change at a gel preparation temperature of −6 °C. The hydrogels prepared below −6 °C exhibit a heterogeneous morphology consisting of pores of sizes 10–70 μm, while those formed at higher temperatures have a non-porous structure. PAAm networks with largest pores were obtained at −18 °C. The pore size of the networks increased while the thickness of the pore walls decreased by decreasing the monomer concentration. The hydrogels formed below −6 °C exhibit superfast swelling and deswelling properties as well as reversible swelling–deswelling cycles in water and in acetone, respectively.     

Simultaneous improvements in the cryogenic tensile strength,ductility and impact strength of epoxy resins by a hyperbranched polymer

Improvements in mechanical properties at low temperatures are desirable for epoxy resins such as diglycidyl ether of bisphenol A (DGEBA) that are often used in cryogenic engineering applications. In this study, a hydroxyl functionalized hyperbranched polymer (H30) is employed to improve the mechanical properties of a DGEBA epoxy resin at liquid nitrogen temperature (77 K). The results show that the tensile strength, failure strain (ductility) and impact strength at 77 K are simultaneously improved by adding a proper content of H30. The maximum tensile strength at 77 K is increased by 17.7% from 98.2 MPa of pure epoxy resin to 115.6 MPa of modified epoxy system for the 10 wt% H30 content. The failure strain at 77 K increases consistently with the increase of H30 content. The maximum impact strength at 77 K is attained by introduction of 10 wt% H30 with an improvement of 26.3% over that of pure epoxy resin. For the purpose of comparison, the mechanical properties of modified epoxy resins at room temperature (RT) are also investigated. It is interesting to note that the impact strength is not lower at 77 K than that at RT for the modified systems. Moreover, the glass transition temperature (T_g) is not reduced by the addition of H30 in appropriate amounts.     

Preparation and characterisation of the magneto-electric xBiFeO3–(1 − x)BaTiO3 ceramics

The solid solutions of BiFeO₃–BaTiO₃ have been prepared via solid state with a view to obtaining magnetoelectric properties, i.e. ferroelectric and magnetic activity in the same range of temperatures. Optimum calcination and sintering strategy for obtaining pure perovskite phase, dense ceramics (>97% relative density) and homogeneous microstructures have been determined. The sample of composition 0.7BiFeO₃–0.3BaTiO₃ reported in the present work is pseudo-cubic at room temperature. The permittivity is ɛ_r ≈ 150 at the room temperature and shows a broad ferro-para phase transition at around 175 °C where ɛ_r ≈ 1600. This diffuse maximum of the permittivity, similar to that in relaxors, is due to the chemical inhomogeneity in both A and B sites of the perovskite unit cell ABO₃. Higher losses, tan δ > 1, appear above 200 °C and other different conduction mechanisms start to be active particularly at temperatures higher than 400 °C, when the ceramic becomes conductive. The magnetic properties show a succession of transitions from weak ferro/ferrimagnetism-to-antiferromagnetism and antiferromagnetism-to-paramagnetism at T_N1 ≈ 10 K and T_N2 ≈ 265 K. Below T_N2 the ceramic 0.7BiFeO₃–0.3BaTiO₃ can present magnetoelectric coupling, due to the fact that is simultaneously ferroelectric and antiferromagnetic.     

Examining structure property relationships in coatings based on substituted linear aromatic polycyanurates

Three series of halogenated and non-halogenated polycyanurates are prepared in good yield and purity, and fully characterised. Many of the resulting polymers, formed at room temperature using phase transfer catalysis, can be cast into films with good resilience and high thermal stability (some examples suffer no mass loss when held isothermally at 190 °C and only display appreciable losses when held continuously at 300 °C). Char yields of 35–65% are achieved in nitrogen depending on backbone structure. Some problems were encountered with solubility, particularly with heavily halogenated dichlorotriazines, which limited molecular weights (M_n = 2000–4000 g mol^?1 depending on backbone structure) although when the phase volume ratio was altered to 0.25 higher molecular weights (M_n = 10,000–30,000 g mol^?1) were possible. Best solubility was achieved by using aromatic diols with at least two equivalent phenylene units per dichlorotriazine unit. DSC reveals polymerisation exotherms with maxima at 190–260 °C (ΔH_p = 35–57 kJ/mol) followed by embrittlement and shrinkage (when heated to 300 °C). These phenomena may be due to the formation of poorly formed crystallites (activation energies span 155–380 kJ/mol) combined with thermal isomerisation.     

Structure and conductivity of NiO/YSZ composite prepared via modified glycine-nitrate process at varying sintering temperatures

Nickel oxide and Yttria-stabilized zirconia (NiO/YSZ) composite is one of the most promising mixed conducting electrode materials in both solid oxide electrolysis cell and solid oxide fuel cell applications. In this study, 50 wt% NiO and 50 wt% YSZ composite was synthesized via a modified glycine-nitrate combustion process (GNP) and the effect of sintering temperatures (1100 °C, 1300 °C and 1500 °C) on its microstructure and electrical properties were investigated. TG/DTA and in-situ high temperature XRD revealed the thermal property behavior and the structural changes of the as-combusted precursor material. For all the samples sintered at different temperatures, room temperature XRD patterns revealed a distinct cubic phases of both YSZ and NiO while SEM images showed a porous microstructure. The total conductivities at 700 °C are 9.87 × 10⁻³, 5.26 × 10⁻³, 4.02 × 10⁻³ S/cm for the 1100, 1300, and 1500 °C with activation energies of 0.1722, 0.3555, and 0.3768 eV, respectively. Conductivity measurements of the different sintered samples revealed that the total conductivities as well as the activation energies are greatly affected by different sintering temperatures.     

Nanocrystaline solid solution CeO2–Bi2O3

Nanocrystalline powders of solid solution CeO₂–Bi₂O₃ were synthesized by self-propagating room temperature reaction (SPRT) procedure with composition (Ce_1?xBi_xO_2?δ where the x = 0.1–0.5). X-ray diffraction analyses show that for x < 0.50 a solid solution with fluorite structure is formed. Rietveld's structure refinement method was applied to characterize prepared powders and its microstructure (size–strain). The lattice parameters increase according to Vegard's rule with increasing of Bi concentration. The average crystallite size is about 2–3 nm. Spectroscopic ellipsometry and Raman scattering measurements were used to characterize the samples at room temperature. The Raman measurements demonstrated electron molecular vibrational coupling and increase of oxygen vacancy concentration whereas increase of Bi content provokes a small decrease of optical absorption edge in comparison with pure ceria. Specific surface area of obtained powders was measured by Brunauer–Emmet–Teller (BET) method.     

Preparation of poly(N-isopropylacrylamide) hydrogel beads by circulation polymerization

Thermosensitive poly(N-isopropylacrylamide) hydrogel (NIPA hydrogel) beads have attracted much attention due to their applications in reaction and separation processes. This study focuses on the preparation of millimeter-sized, monodispersed NIPA hydrogel beads, for which a novel circulation polymerization technique is proposed. The method involves the drop-wise addition of a pre-gel aqueous solution into swirling silicone oil through a nozzle, and the subsequent conventional free-radical polymerization of the suspended pre-gel droplets, which drifts with the swirling oil. NIPA hydrogel beads that are 3.0 mm in diameter were successfully prepared with a very narrow distribution under fundamental conditions with no coalescence of the pre-gel droplets. The circulation polymerization technique improves the residence time and can be applied to a polymerization system that requires a long gelation time. The size of the resultant hydrogel beads corresponds to the size of the pre-gel droplets that are delivered from the tip of the nozzle, and can be controlled by adjusting the size of the nozzle.     

High temperature elasticity measurements on oxides by Brillouin spectroscopy with resistive and IR laser heating

Knowledge of single crystal and aggregate elastic moduli of materials at high temperature is important in the development of high-temperature structural ceramics as well as for other areas of material sciences. Sound velocities, and hence elastic moduli, can be readily measured on micro-crystals, polycrystalline aggregates and amorphous materials using Brillouin scattering. We have developed techniques for determining the elastic moduli at high temperatures, using both electric resistive heating (to 1800 K) and CO₂ laser heating (to T > 2500 K). The full set of elastic constants of transparent oxides at high temperatures can be measured on samples with dimensions of 100 × 100 × 20 μm or even smaller. Compact resistance heaters of our design were used to study the temperature dependence of the elastic moduli of a variety of crystalline oxides and glasses, and can be used to observe high-temperature phase transitions involving elastic softening. The combination of Brillouin scattering with CO₂ laser heating allows measurements of the elastic moduli of oxides at even higher temperatures, approaching the melting points of refractory materials. The acoustic velocities of single-crystal MgO were measured to a maximum temperature exceeding 2500 ± 100 K. Both Brillouin and Raman measurements were performed on CO₂ laser-heated samples of single-crystal α-Al₂O₃ to temperatures exceeding 2000 ± 100 K. Our results show that Brillouin scattering coupled with CO₂ laser heating is a viable means of performing sound velocity measurements at temperatures significantly higher than those readily made using resistance heating.     

Effect of polymer concentration on porosity and pore size characteristics of alumina membrane substrates prepared by gelcasting

The effect of urea–formaldehyde (UF) polymer concentration on porosity and average pore size of alumina membrane substrates prepared by gelcasting has been studied. The soluble UF oligomers formed in the initial stages of polymerization act as steric stabilizer for alumina particles in the suspension. The porosity and average pore size of the substrate samples decreased with both the decrease of amount of polymer in the gelcast body and the increase of sintering temperature. Membrane substrates obtained by sintering of gelcast bodies containing UF polymer concentrations from 24.3 to 15.6 wt% at temperatures from 1250 to 1450 °C showed porosity and average pore size of 62.5–27 vol% and 0.43–0.20 μm, respectively. The membrane substrates prepared by the gelcasting method had narrow pore size distribution.     

Densification and properties of superhard B6O materials with cobalt additions

The search for suitable additives for boron suboxide (B₆O) materials which could improve densification, reduce sintering temperature and tailor the microstructure has been productive. B₆O materials doped with 0–5 vol% cobalt addition were sintered at temperatures up to 1850 °C and pressure of 50 MPa for 20 min. Relationships between the formed phases, microstructures and mechanical properties of the sintered materials were investigated as a function of sintering conditions and added cobalt content. The hardness of the sintered B₆O materials increases with sintering temperature, while the fracture toughness increases with increasing cobalt content and reduces with increasing sintering temperature.     

Protein release behavior from carbonate apatite hydrogel

Incorporation of protein in poorly crystalline carbonate apatite hydrogel, the protein release behavior from the cake and the dissolution of the cake in aqueous solution were studied. Poorly crystalline apatite was prepared by precipitation of saturated solution of calcium and phosphate in air or under N₂ atmosphere. The carbonate content and particle size of obtained hydrogel depends on the maturation period and atmospheric condition. The mixture of apatite hydrogel and protein was dried for 4 days in air at 40% of relative humidity to yield cakes. The quantity of loaded cytochrome c in apatite hydrogel formed in air was 0.490 wt.%, while that in apatite hydrogel formed under N₂ atmosphere was 0.305 wt.%. But the quantity of loaded albumin in apatite hydrogel formed in air was less than that in apatite hydrogel formed under N₂ atmosphere. For the cakes containing protein immersed in aqueous solution, the protein release was less than the decrease in weight of apatite cake, which may be associated with apatite recrystallization and protein re-incorporation. The quantity of released protein may depend on the quantity of loaded protein in apatite hydrogel.