Influence of brick pattern interface structure on mechanical properties of continuous carbon fiber reinforced lithium aluminosilicate glass-ceramics matrix composites

Continuous carbon fiber reinforced lithium aluminosilicate glass-ceramic matrix composites have been fabricated by sol-gel process and hot pressing technique. The results show that the Cf/β-eucryptite composites hot pressed at 1300 °C and Cf/β-spodumene composites hot pressed at 1400 °C form weak interface with brick pattern characteristics, leading to high mechanical performance. The maximum flexural strength and fracture toughness reach 571 ± 32 MPa and 9.8 ± 0.6 MPa m^1/2 for Cf/β-eucryptite composites and 640 ± 72 MPa and 19.9 ± 1.8 MPa m^1/2 for Cf/β-spodumene composites. On increasing the hot pressing temperature, the active chemical diffusion consumes brick pattern interface layer, which leads to the formation of strong bonding between carbon fiber and the matrix. As a result, the composites exhibit brittle fracture behavior and the mechanical properties decrease significantly.     

Macroscopic observation of thermal behavior of concentrated solution of coal extracts

The solvent extracts of Upper Freeport and Illinois No.6 coals were mixed with N-methyl-2-pyrolidinon (NMP) and annealed at 353 K to produce the gelatinous materials. Differential scanning calorimetric measurements revealed that the materials can hold significant amounts of nonfreezable NMP (as much as 3 g NMP per 1 g coal extracts) which disperse in the materials on a molecular scale, indicating the materials are not phase separated. The thermal behaviors were measured macroscopically as a function of the extract concentration using a needle penetrometer during heating from 223 to 360 K. The penetration-temperature curves were analyzed to estimate the apparent viscosity (η_a). During the penetrations, η_a was decreased very rapidly, approximately four orders of the magnitude by a temperature increase of 20 K, suggesting that the coal extracts-NMP mixtures undergoes a gel to sol transition. The heats of dissociation of crosslinks (ΔH_m) were estimated by applying Eldridge-Ferry equation. The ΔH_m of coal extracts-NMP mixtures was relatively small, i.e. approximately 10 kJ/mol, whereas the ΔH_m of polyvinyl alcohol-NMP gel in which the hydrogen bonds contribute the formation of the physical network structures, was about 65 kJ/mol. Not the specific interaction such as hydrogen bonds, but weak interactions such as van der Waals force were likely to contribute the formation of the coal extracts-NMP gel.     

Thermolysis of the polyyne C8H2 in hexane and methanol: Experimental and theoretical study

The mean lifetimes of polyyne C₈H₂ in hexane were determined at 50, 60, 80, and 100 °C and in methanol at 60 °C. The reactions are second order at all temperatures: ln k₂ = 20.5 ± 1.5-10303 ± 520T⁻¹ and the corresponding activation energy is 85.7 ± 6.3 kJ mol⁻¹ (7164 cm⁻¹). Extrapolation suggests that solutions at 1 mM concentration are significantly unstable at room temperature. Quantum chemical calculations show that polyynes C_mH₂ + C_nH₂ (m + n = 16) could be products, but these were not detected. Alternatively, C₁₆H₂ isomers could form. IR spectra of the solid residues from hexane and methanol solutions were obtained.     

Microstructure and mechanical properties of hydroxyapatite obtained by gel-casting process

In the present work, well-shaped HAp green bodies were obtained by the gel-casting process with 50 vol.% slurry. After drying, the microstructure and pore distribution of the green body were investigated. The density, compressive strength and flexural strength of the green body were 1.621 g/cm³, 32.6 ± 3.2 MPa and 13.8 ± 1.0 MPa, respectively. After pressureless sintering at the range of 1100–1300 °C for 2 h, the relative density of the final product ranges from 71.8 to 97.1% th. The maximum value of flexural strength, elastic modulus, hardness and fracture toughness were 84.6 ± 12.6 MPa, 138 ± 7 GPa, 4.45 ± 0.18 GPa and 0.95 ± 0.13 MPa m^1/2, respectively. SEM images show a compact and uniform microstructure; the average grain size was found by using the linear intercept method. XRD and FTIR determined the phase and the radical preserved after sintering.     

The breakdown of the Weibull behavior in dental zirconias

Here we attend to the controversy of the use of Weibull statistics for describing the strength distribution of dental brittle materials. Our approach is purely experimental by means of testing for the strength size effect, a requirement for Weibull materials. Zirconia materials of five important dental manufacturers were selected, each represented by two compositions, being one a 3 mol% Y₂O₃-stabilized zirconia, and the other being a “translucent” zirconia with 4 or 5 mol% stabilizer content. Specimens of increasing sizes were fractured, whether by using a biaxial flexure test in plates or a uniaxial bending test in beams, thereby sampling different ranges of effective surfaces and volumes. A systematic deviation from the Weibull behavior over the range of 1–40 mm² effective surface was demonstrated, regardless of manufacturer and Y₂O₃ content in the powder. Extensive testing using a wider range of specimen sizes narrowed down the threshold for the breakdown of the defect size distribution from the parent population to be located between 10 and 20 mm² effective surface. A comparable behavior was confirmed for the partly sintered white-bodies, with similar defect morphology to the fully sintered analogs, indicating a defect size distribution stemming from the pressing steps of manufacture. The defect shape related to open particle aggregate junctions, pointing to an association of their size distribution to that of the distribution of aggregate sizes in the source powders.     

Determination of the elastic constants of portlandite by Brillouin spectroscopy

The single crystal elastic constants C_ij and the shear and adiabatic bulk modulus of a natural portlandite (Ca(OH)₂) crystal were determined by Brillouin spectroscopy at ambient conditions. The elastic constants, expressed in GPa, are: C₁₁ = 102.0(± 2.0), C₁₂ = 32.1(± 1.0), C₁₃ = 8.4(± 0.4), C₁₄ = 4.5(± 0.2), C₃₃ = 33.6(± 0.7), C₄₄ = 12.0(± 0.3), C₆₆ = (C₁₁-C₁₂)/2 = 35.0(± 1.1), where the numbers in parentheses are 1σ standard deviations. The Reuss bounds of the adiabatic bulk and shear moduli are K_0S = 26.0(± 0.3) GPa and G₀ = 17.5(± 0.4) GPa, respectively, while the Voigt bounds of these moduli are K_0S = 37.3(± 0.4) GPa and G₀ = 24.4(± 0.3) GPa. The Reuss and Voigt bounds for the aggregate Young's modulus are 42.8(± 1.0) GPa and 60.0(± 0.8) GPa respectively, while the aggregate Poisson's ratio is equal to 0.23(± 0.01). Portlandite exhibits both large compressional elastic anisotropy with C₁₁/C₃₃ = 3.03(± 0.09) equivalent to that of the isostructural hydroxide brucite (Mg(OH)₂), and large shear anisotropy with C₆₆/C₄₄ = 2.92(± 0.12) which is 11% larger than brucite. The comparison between the bulk modulus of portlandite and that of lime (CaO) confirms a systematic linear relationship between the bulk moduli of brucite-type simple hydroxides and the corresponding NaCl-type oxides.     

A new test method for determining the fracture toughness of concrete materials

The Spiral Notch Torsion Test (SNTT) determines the intrinsic fracture toughness (K_IC) of structural materials by applying pure torsion to cylindrical specimens having a notch line that spirals around the specimen at a 45° pitch. K_IC values are obtained with the aid of a three-dimensional finite-element computer code, TOR3D-KIC. The SNTT method is suitable for testing a wide variety of materials used extensively in pressure vessel and piping structural components and weldments, as well as ceramic and graphite materials. One important characteristic of SNTT is that neither a fatigue precrack nor a deep notch is required for evaluation of brittle materials, significantly reducing the sample size requirement. Results are reported for a Portland cement-based mortar demonstrating applicability of the SNTT method to cementitious materials. The estimated K_IC of the tested mortar samples with compressive strength of 34.45 MPa was found to be 0.360 ± 0.017 MPa √m.     

Noniterative method for evaluation of the complex material constants of piezoelectric ceramics in the radial vibration mode

A new noniterative method, for determining the dielectric, piezoelectric and elastic constants, in complex form, for piezoceramic materials, in the radial mode, was proposed.This method uses the standard procedure to determine the elastic compliance and Poisson factor and the measurement of admittance at two frequencies to calculate the dielectric and piezoelectric constants, by solving a system of two equations.The accuracy of the new method was determined for materials with different planar coupling coefficients (k_p = 2.5-57%) and mechanical quality factors (Q_m = 20-3000). This method proved to be very accurate for all materials especially for those with large coupling factors. The accuracy of standard method was also evaluated for the same materials.     

Densification and grain growth during solid state sintering of LaPO4

This work is devoted to the kinetic study of densification and grain growth of LaPO₄ ceramics. By sintering at a temperature close to 1500 °C, densification rate can reach up to 98% of the theoretical density and grain growth can be controlled in the range 0.6–4 μm. Isothermal shrinkage measurements carried out by dilatometry revealed that densification occurs by lattice diffusion from the grain boundary to the neck. The activation energy for densification (E_D) is evaluated as 480 ± 4 kJ mol⁻¹. Grain growth is governed by lattice diffusion controlled pore drag and the activation energy (E_G) is found to be 603 ± 2 kJ mol⁻¹. The pore mobility is so low that grain growth only occurs for almost fully dense materials.     

Bimodal, templated mesoporous carbons for capacitor applications

Several high capacitance ordered mesoporous carbon (OMC) materials, containing a bimodal pore distribution, were synthesized directly using hexagonal mesoporous silicas (HMS) as the template material. The HMS templates were formed using amine surfactants (C_nH_2n+1NH₂) with hydrophobic chain lengths containing 8-16 carbons (n = 8-16). These HMS structures were found to have an interconnected wormhole structure, high textural mesoporosity, a surface area ranging from 910 to 1370 m²/g, and a total pore volume of 1.09-1.83 cm³/g. Also, evidence for a change in structure from hexagonally ordered to layered (for surfactants of chain length with n > 12) was found. The resulting OMCs, formed using sucrose as the carbon precursor, contain bimodal pores 1.6-1.8 and 3.3-3.9 nm in diameter and have a very high surface area (980-1650 m²/g). The OMCs were evaluated as electrode materials for electrochemical capacitors using cyclic voltammetry in 0.5 M H₂SO₄ solution, giving a tunable gravimetric capacitance that increased linearly with BET area (and surfactant chain length), up to 260 F/g, among the highest yet reported for ordered carbon formed from an HMS templated precursor. All OMCs studied in this work displayed a specific capacitance of ∼0.15 F/m².     

Biosorption properties of dried Neurospora crassa for the removal of Burazol Blue ED dye

Biosorption potential of dried Neurospora crassa for Burazol Blue ED was studied with respect to pH, equilibrium time, biomass concentration and temperature to determine equilibrium and kinetic model parameters. The most suitable pH, equilibrium time and biomass concentration were determined as 1 ± 0.2, 60 min and 1.6 g L^− 1, respectively, at 20 °C ± 1.0. The equilibrium data was best described by the Langmuir isotherm model. The maximum biosorption capacity (q_m) of biomass obtained from the Langmuir fit was 110.1 mg g^− 1 biomass at 30 °C. The overall biosorption process was best described by the pseudo-second-order kinetic model. The biosorption process was found to be favored at higher temperatures.     

Simultaneous synthesis and densification of α-Zr(N)/ZrB2 composites by self-propagating high-temperature combustion under high nitrogen pressure

Simultaneous synthesis and densification of α-Zr(N)/ZrB₂ composites from a 85 mol% Zr/15 mol% B mixed-powder compacts have been achieved by self-propagating high-temperature under a nitrogen pressure of 10 MPa. Composites consist of fine and short rodlike ZrB₂ grains (0.1 μm^?–0.5 μm^l) dispersed into α-Zr(N) matrix (3 μm). Dense composite materials (96.5% of theoretical) exhibit excellent mechanical properties, in which their bending strength and H_v are 560 MPa and 6.5 GPa, respectively. This bending strength is much superior to those (205 and 480 MPa) of dense equi-axial α-Zr(N) (10 μm) and dense ZrB₂ (6 μm). Fine and rodlike ZrB₂ grains greatly enhanced their mechanical properties.     

XPS and flow-cell EQCM study of albumin adsorption on passivated chromium surfaces: Influence of potential and pH

The adsorption of albumin (BSA: bovine serum albumin) on passivated chromium surfaces was studied in deaerated sulphate solutions as a function of potential (in the passive state) and pH (from 4 to 10). In situ switch-flow cell EQCM measurements were coupled to ex situ XPS analyses. EQCM results showed that (i) the initial adsorption rate is about 3.3 ng cm⁻² s⁻¹ which corresponds to 3 × 10¹⁰ molecules cm⁻² s⁻¹, irrespective of the passive potential and pH, and (ii) the passive potential as well as the pH have no influence on the amount of adsorbed BSA (Δm = 440 ± 70 ng cm⁻² on the adsorption plateau). From the XPS N 1s and C 1s signals, which provide a fingerprint for the protein, it can be concluded that BSA is adsorbed on the Cr surface and is chemically intact. The XPS results show that (i) when increasing the passive potential, the oxide layer thickness increases (mean value: d_ox = 2.2 ± 0.2 nm), and (ii) the passive film is not modified by the adsorption of protein. From combined EQCM and XPS data, a full coverage of the Cr surface by the adsorbed proteins (γ = 1) is demonstrated at pH 4 (whatever the passive potential). The thickness of the continuous BSA layer (h_BSA) is 3.3 ± 0.3 nm, which corresponds to one monolayer “side-on”, i.e. oriented parallel to the surface. At pH 5.5 and 10, the adsorbed proteins form islands. The surface coverage is much lower (γ ∼ 0.5), and the height of the protein islands is significantly higher (h_BSA ∼ 6.5 nm). The results suggest a strong interaction (partially covalent) between the protein and the passivated chromium surface.     

Low-swelling proton-conducting copoly(aryl ether nitrile)s containing naphthalene structure with sulfonic acid groups meta to the ether linkage

Wholly aromatic poly(aryl ether ether nitrile)s containing naphthalene structure with sulfonic acid groups meta to ether linkage (m-SPAEEN), intended for fuel cells applications as proton conducting membrane materials, were prepared via nucleophilic substitution polycondensation reactions. The incorporation of rigid naphthalene structure with meta-sulfonic acid groups was with the intent of improving the aggregation of hydrophilic and hydrophobic domains and to increase the acidity and conductivities. m-SPAEEN copolymers were readily synthesized by potassium carbonate mediated nucleophilic polycondensation reactions of commercially available monomers: 2,6-difluorobenzonitrile (2,6-DFBN), 2,8-dihydroxynaphthalene-6-sulfonate sodium salt (2,8-DHNS-6), and 4,4′-biphenol (4,4′-BP) in dimethylsulfoxide (DMSO) at 160-170 °C. The sulfonic acid group content (SC), expressed as a number per repeat unit of polymer, ranged from 0 to 0.6 and was readily controlled by changing the feed ratio of 2,8-DHNS-6 to 2,6-DFBN. High thermal stability of m-SPAEEN copolymers was indicated by observed glass transition temperatures (T_gs) ranging from 223 to 335 °C in sodium salt form and from 230 to 260 °C in acid form (m-SPAEENH) and decomposition temperatures (T_d)s over 250 °C in acid form and over 350 °C in sodium form in both nitrogen and air. All m-SPAEENH copolymers exhibited reasonable flexibility and tensile strength in the range of 39-78 MPa, indicating they were mechanically stronger than Nafion^®117, which had an approximate value of 10 MPa under the same test conditions. As expected, m-SPAEENH copolymers showed considerably reduced moisture absorption compared to previously prepared sulfonated hydroquinone based poly(aryl ether nitrile). m-SPAEENH copolymers also showed improved proton conductivities. Proton conductivity curves parallel to that of Nafion 117 were obtained with proton conductivity of 10⁻¹ S/cm at equivalent ion exchange capacities (IEC) of 1.6 and 1.9, comparable to Nafion^®117. The best compromise combining PEM mechanical strength, water swelling and proton conductivity, was achieved at SC of 0.5 and 0.6.     

Influence of silica nanoparticles added to an organosilane film on carbon steel electrochemical and tribological behaviour

The electrochemical behaviour and tribological properties of carbon steel coated with bis-[trimethoxysilylpropyl]amine (BTSPA) filled with SiO₂ were evaluated. The silane film filled with SiO₂ was prepared by adding different SiO₂ concentrations. The electrochemical behaviour of the coated steel was mainly evaluated by means of open-circuit potential (E_OC), electrochemical impedance spectroscopy (EIS) and polarization curves, in 0.1 mol L⁻¹ NaCl solution. Structural and morphological characterizations were made by optical, electron and atomic force microscopy (AFM). E_OC and EIS data showed that sample filled with 300 ppm SiO₂ presented the highest E_OC and total impedance value. AFM measurements showed a homogeneous particle distribution of SiO₂ particles. Nanohardness measurements showed SiO₂ promoted an increase of the hardness mean value (1.70 ± 0.11 GPa to non-filled BTSPA and 2.21 ± 0.05 GPa for sample filled with 300 ppm SiO₂). Silane films when filled with SiO₂ particles improved the corrosion resistance of the steel substrate. The optimum SiO₂ particles concentration in silane solution is 300 ppm SiO₂. Incorporation of an extra amount of silica into BTSPA film led to degradation of the corrosion protection of the film to the substrate.     

Uniaxially oriented carbon monoliths as supercapacitor electrodes

Cylindrical carbon monoliths of 7 mm in diameter and certain heights (1, 2, 3, 4 and 5 mm) are studied as model electrodes for supercapacitors. The monoliths show a narrow microporous structure with average micropore size of 0.73 nm and specific surface area of 1086 m² g⁻¹. The monoliths show straight walls and channels, both arranged along the cylinder axis. The former account for a remarkable electrical conductivity (6.5 S cm⁻¹ at room temperature). The latter allow a rapid ionic transport between the electrolyte bulk and the carbon walls and account for a high specific capacitance at high current density. The cell capacitance and resistance increase linearly with the monolith height according to C = (1.78 ± 0.06)h and ESR = (0.08 ± 0.01)h + (1.67 ± 0.04), respectively. The contribution of the electrolyte resistance, monolith resistance and monolith/collector resistance to ESR is discussed. The cell response time or constant time increases with the monolith height but according to a power dependence, τ = (4.5 ± 0.2)h^{(1.61 ± 0.03)}. The carbon of the monoliths show in KOH electrolyte a specific capacitance of 150 F g⁻¹ and a capacitance per surface area of 14 μF cm⁻².     

Mechanical properties at nanometric scale of alumina layers formed in sulphuric acid anodizing under burning conditions

A mechanical study (hardness, H and Young's modulus, E) at nanometric scale of the distribution of mechanical properties across the surface of porous anodic alumina (PAA) films formed during aluminium anodizing in sulphuric acid under burning conditions is presented. Statistical methods have been employed to extract the mechanical properties of the protruding oxide structures (POS) observed in the burning areas and the results have been compared with those obtained from the standard PAA (S-PAA) of the non-burning areas. The results indicated that H and E of the POS are 3.8 ± 0.3 and 80 ± 4 GPa, respectively, while those of the S-PAA are 6.8 ± 0.5 and 125 ± 8 GPa. Thus, the hardness and Young's modulus of POS are factors of about 1.6-1.8 times lower than those of S-PAA, indicating that POS have lower mechanical properties associated with their different chemical composition and structure.     

Cleaner production of ephedrine from Ephedra sinica Stapf by membrane separation technology

A promising cleaner approach, including chemical extraction, separation and purification by membranes separation technology, for producing ephedrine from Ephedra sinica Stapf was introduced. The extraction yield of ephedrine reached 92.45 ± 0.46%, increased by 28.25 ± 0.13% than that of the traditional process, at solid-to-liquid ratio of 1/10, extraction temperature of 80 °C, total extraction time of 20 h and reextraction for 3 times. In microfiltration, the transmissivity for ephedrine was up to 97.88 ± 1.06% and the retention rate of impurities reached 78.56 ± 0.96% when the membranes with pore size of 0.45 μm were employed at inlet and outlet operating pressure of 0.26 MPa and 0.14 MPa, respectively. The surface velocity of membrane channel was 3.5 m s⁻¹ and membrane flux was 207 ± 3.71 l m⁻² h⁻¹. Nanofiltration membranes with 160 Da molecular weight cut-off (MWCO) were adopted to separate the ephedrine from microfiltration permeate at a transmembrane pressure of 0.6 MPa wherein the retention rate of ephedrine reached 99.88 ± 0.23% and the membrane flux was 19.88 ± 1.12 l m⁻² h⁻¹. For this improved approach, the COD of nanofiltration permeate was only 110 ± 12.56 mg l⁻¹ which could be recycled to the extraction process, causing a decrease by 59.38 ± 1.67% of water consumption and 75.76 ± 1.89% of wastewater generation in comparison with those of the traditional process.     

Microstructure and mechanical properties of silicon carbide processed by Spark Plasma Sintering (SPS)

The unique combination of SiC properties opens the ways for a wide range of SiC-based industrial applications. Dense silicon carbide bodies (3.18±0.01 g/cm³) were obtained by an SPS treatment at 2050 °C for 10 min using a heating rate of 400 °C/min, under an applied pressure of 69 MPa. The microstructure consists of fine, equiaxed grains with an average grain size of 1.29±0.65 μm. TEM analysis showed the presence of nano-size particles at the grain boundaries and at the triple-junctions, formed mainly from the impurities present in the starting silicon carbide powder. The HRTEM examination revealed high angle and clean grain boundaries. The measured static mechanical properties (H_V=32 GPa, E=440 GPa, σ_b=490 MPa and K_C 6.8 MPa m^0.5) and the Hugoniot Elastic Limit (HEL=18 GPa) are higher than those of hot-pressed silicon carbide samples.