Toughening of dimethacrylate resins by addition of ultra high molecular weight polyethylene (UHMWPE) particles

The effects of the addition of UHMWPE particles, of nominal 〈80 μm〉 size, on the fracture toughness, flexural modulus and strength of composites made with dimethacrylate resins (60/40 wt/wt BisGMA-TEGMA) were investigated as a function of volume fraction of UHMWPE (0-60 vol%) and particle surface treatment. Interfacial shear strengths (τ) were measured via microbond shear strength tests using Spectra900™ (UHMWPE) fibers and BisGMA-TEGMA beads. τ increased by a factor of 4 compared with untreated UHMWPE, and surface treated particles improved the mechanical properties of the composite. Fracture toughness (K_IC) and flexural modulus (E) increased with increased volume fraction of UHMWPE, with maximum K_IC/E increases (at 60 vol%) of 238%/25% compared with the neat resin. SEM images showed debonding as well as yielding and fibrillation of the UHMWPE particles, suggesting that these were significant toughening mechanisms.     

Pressureless sintered SiC matrix toughened by in situ synthesized TiB2: Process conditions and fracture toughness

This paper reports the fabrication of SiC toughened by in situ synthesized TiB₂ based on pressure-less sintering technique using TiO₂, B₄C, C and SiC as starting materials. The process conditions were investigated in detail, including the pre-sintering temperatures, carbon contents, differently sized TiO₂ powders, TiB₂ volume contents, final sintering temperature and time. These conditions were found to have great influence on the TiB₂ toughened SiC in terms of relative density, TiB₂ particle size and fracture toughness. Homogeneous dispersion of in situ synthesized TiB₂ secondary phase was confirmed to enhance the K_IC of the SiC matrix. The K_IC of SiC toughened by in situ synthesized TiB₂ (15 vol%) reaches 6.3 MPa m^1/2, which is among the highest values reported so far on TiB₂ reinforced SiC composites based on the pressure-less sintering technique using TiO₂ as Ti source.     

Studies on the mechanical and mechanical interfacial properties of carbon–carbon composites impregnated with an oxidation inhibitor

In this work, the relationships between work of adhesion and fracture toughness parameters, such as work of fracture (W_f), the critical stress intensity factor (K_IC), and the specific fracture energy (G_IC), of carbon–carbon composites (C/C composites) were investigated. The impact properties of the composites were also studied in the context of differentiating between the initiation and propagation energies for failure behavior. Composites consisting of different contents of the oxidation inhibitor MoSi₂ displayed an increase of the work of adhesion between the fibers and the matrix, which improved both the fracture toughness and impact properties of the composites. The 12 wt% MoSi₂ composites exhibited the highest mechanical and mechanical interfacial properties. This was probably due to the improvement of the London dispersive component, W_A^L, of the work of adhesion, resulting in an increase in the interfacial adhesion force among the fibers, filler, and matrix in this system.     

Control of the functionality of graphene oxide for its application in epoxy nanocomposites

Amino- and epoxy-functionalized graphene oxide (GO) were synthesized separately through a wash-and-rebuild process utilizing two differently terminated silane coupling agents. The modified GO sheets were then incorporated into an epoxy resin to prepare nanocomposites. The addition of 0.2 wt% amino-functionalized GO (APTS-GO) yielded a 32% increase in Young's modulus (3.3 GPa) and 16% increase in tensile strength (81.2 MPa). Less reinforcement was observed with the epoxy-functionalized GO (GPTS-GO) but there was a more significant increase in ductility for GPTS-GO/epoxy, with the fracture toughness (critical intensity factor, K_IC) and fracture energy (critical strain energy release rate, G_IC) nearly doubling at 0.2 wt% loading (1.46 MPam^1/2 and 0.62 kJ/m² for K_IC and G_IC, respectively). Raman spectroscopy measurements revealed that the GPTS-GO was dispersed more uniformly than the APTS-GO in the epoxy matrix, and better interfacial stress transfer was found for the APTS-GO. Thus the wash-and-rebuild process affords a novel strategy for controlling the functionality of graphene in the quest to develop high-performance graphene-based nanocomposites.     

Preparation of polystyrene-based activated carbon spheres with high surface area and their adsorption to dibenzothiophene

Polystyrene-based activated carbon spheres (PACS_K) with high surface area were prepared through KOH activation. Effects of the carbonization temperature and the ratio of KOH to carbon spheres (CS) on the textural structure, hardness and yield of the resultant PACS_K were studied, and their adsorption to dibenzothiophene (DBT) were investigated. The as-prepared PACS_K exhibited a high surface area (up to 2022 m²/g), large total pore volume (≥ 0.78 cm³/g), superior mechanical hardness and high adsorption capacity (ca. 153 mg/g). With the increase of the KOH/CS ratio from 2:1 to 4:1, the surface area, total pore volume, volume of micropores, and volume of mesopores, increase, whereas the volume of small-micropores (< 0.8 nm) decreases from 0.36 to 0.31 cm³/g. The adsorption capacity has a good linear correlation with the volume of small-micropores rather than the surface area. In addition, the large quantity of acidic oxygen-containing groups of PACS_K may also be responsible for their higher adsorption capacity and selectivity of DBT. The PACS_K saturated by DBT can be regenerated by a washing process in a shaking bath or using ultrasonic with toluene at 80 °C.     

Preparation and characterization of trilobal activated carbon fibers

The objective of this research was to evaluate the effectiveness of several different methods for controlling the pore size and pore size distribution in activated carbon fibers. Variables studied included fiber shape, activation time, and the addition of small amounts of silver nitrate. Pure isotropic pitch and the same isotropic pitch containing 1 wt.% silver were melt spun to form fibers with round and trilobal cross sections. These fibers were then stabilized, carbonized, and activated in carbon dioxide. Field emission scanning electron microscopy (FE SEM), electron dispersive spectra (EDS), and wavelength dispersive spectra (WDS) were used to monitor the size and distribution of the silver particles in the fibers before and after activation. Each of these analyses showed that the distribution of silver particles was extremely uniform before and after activation. The fibers were also weighed before and after activation to determine the percent burn-off. The BET specific surface areas of the activated fibers were determined from N₂ adsorption isotherms measured at −196 °C. The results showed that round and trilobal fibers with equivalent cross-sectional areas yielded similar burn-off values and specific surface areas after activation. Also, activation rates were found to be independent of CO₂ flow rate. The porosity of the activated fibers depended on the total time of activation and the cross-sectional area of fibers. The N₂ adsorption measurements showed that the activated fibers had extremely high specific surface areas (greater than 3000 m²/g) and high degrees of meso- and macro-porosity. FE SEM was also used to investigate surface texture and size of pore openings on the surfaces of the activated fibers. The photos showed that silver particles generated surface macro- and mesopores, in agreement with the inferences from N₂ adsorption measurements.     

Investigation of fracture toughness of modified (KxNa1 − x)NbO3 lead-free piezoelectric ceramics

While most of the previous studies have focused on the processing and electrical properties of KNN-based ceramics, very little research has been carried out to evaluate their mechanical behavior. This work presents for the first time an examination of the fracture toughness, K_IC, of the most widely studied (K_xNa_1 ? x)NbO₃ (KNN)-based lead-free ceramics modified with lithium, tantalum and antimony. The samples were produced through the conventional mixed-oxide route and the K_IC values were measured using the single edge V-notched beam (SEVNB) method under four-point bending. The mean K_IC values were determined to be 0.48 ± 0.18 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)NbO₃, 0.8 ± 0.18 MPa m^1/2 for (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃, 0.86 ± 0.04 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.9Ta_0.1)O₃ and 1.06 ± 0.21 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ compositions. The microstructure, phase structure and dielectric constant values of the samples have been used to correlate the results of the K_IC values.     

Pyrolytically prepared carbon from fluorine-GIC

Formation mechanism, crystallinity, porosity and chemical reactivity were studied on the carbon prepared by pyrolysis of single phase, stage-1 fluorine-graphite intercalation compound (fluorine-GIC; C_xF). The stage-1 C_2.5F directly decomposes to fluorocarbon gases and carbon above 650 K, without forming higher stage compounds. The pyrocarbon prepared from C_2.5F gives hkl diffraction peaks indicating graphite-like stacking order of graphene layers. This carbon possesses average crystallite sizes along the c- and a-axes (L_c and L_a) of about 5 and 50 nm, respectively. The specific surface area of the pyrocarbon (about 40 m² g⁻¹) is only twice as large as that of the original crystalline graphite. Chemical behavior of the pyrocarbon as an intercalation host for sodium and potassium is similar to that of crystalline graphite, but it is easily fluorinated by elemental fluorine even at 573 K to give poly(carbon monofluoride) [(CF)_n] probably due to the small crystallite size and the mesopores formed by formation/decomposition processes of C_2.5F.     

Synthesizing activated carbons from resins by chemical activation with K2CO3

We prepared activated carbons from phenol-formaldehyde (PF) and urea-formaldehyde (UF) resins by chemical activation with K₂CO₃ with impregnation during the synthesis of the resins. The influence of carbonization temperature (773-1173 K) on the pore structure (specific surface area and pore volume) and the temperature range at which K₂CO₃ worked effectively as an activation reagent, were investigated. The specific surface area and micropore volume of PF-AC and UF-AC increased with an increase of carbonization temperature in the range of 773-1173 K. We prepared activated carbon with well-developed micropores from PF, and activated carbon with high specific surface area (>3000 m²/g) and large meso-pore volume from UF. We deduced the activation mechanism with thermogravimetry and X-ray diffraction. In preparing activated carbon from PF, K₂CO₃ was reduced by carbon in the PF char. The carbon was removed as CO gas resulting in increased specific surface area and pore volume above 1000 K. In preparing AC from UF, above 900 K the carbon in UF char was consumed during the K₂CO₃ reduction step.     

Exfoliated carbon fibers as an electrode for electric double layer capacitors in a 1 mol/dm H2SO4 electrolyte

Exfoliated carbon fibers (ExCFs) synthesized through the rapid heating of intercalation compounds of carbon fibers were examined as electrodes of electric double layer capacitors (EDLC). The measurement of EDLC was performed using a standard three-electrode cell with 1 mol/dm³ sulfuric acid electrolyte. The capacitance of as-prepared ExCFs reached 117 F/g, even though they had a relativity small surface area of about 330 m²/g. After air activation of ExCFs, the BET surface area increased slightly, but the capacitance of the EDLC increased up to 160 F/g. Capacitance of ExCFs strongly depended on their BET surface area, having a different dependence from that reported on activated carbon fibers.     

Synthesis and properties of (Hf1-xTax)C solid solution carbides

Thermodynamically stable (Hf_1–xTa_x)C (x?=?0.1–0.3) compositions were selected by First Principle Calculation and synthesized in nanopowders via high-energy ball milling and carbothermal reduction of commercial oxides at 1450?°C. The formation of a solid solution during powder synthesis was investigated. The solid solution carbide powders were sintered at 1900?°C by spark plasma sintering without a sintering aid. As a result, the (Hf_1–xTa_x)C solid solution carbides exhibited high densities, excellent hardness and fracture toughness (ρ: 98.7–100.0%, HVN: 19.69–19.98?GPa, K_IC: 5.09–5.15?MPa?m^1/2) compared with previously reported HfC and HfC–TaC solid solution carbides.     

Fabrication of dense ZrB2/B4C composites using pulsed electric current pressure sintering and evaluation of their high-temperature bending strength

ZrB₂/x·vol%B₄C (x = 30–90) composites were fabricated from ZrB₂ and amorphous B/C powders using pulsed electric current pressure sintering (PECPS) from 1600 °C to 1900 °C for 6.0 × 10² s (10 min) under 50 MPa in a vacuum, accompanied by self-propagating high-temperature synthesis (SHS). Since the B₄C phase was formed at 1600 °C, the relative density (D_r) was evaluated; the composites sintered at 1900 °C attained the highest D_r. Their D_r values increased gradually from 99.35% to 99.99% with increasing B₄C contents up to 60 vol% and showed a constant value above 60 vol%. At room temperature, the mechanical properties of Vickers hardness (H_v), fracture toughness (K_IC) and three-point bending strength (σ_b) were measured. H_v exhibited a monotonous increase from 20.3 to 32.7 GPa. On the other hand, K_IC and σ_b revealed the same behavior for each of the compositions; both exhibited the highest values, i.e., 10.2 MPa m^1/2 for K_IC and 870 MPa for σ_b, in the 60 vol%B₄C sample, and then the K_IC decreased gradually to 9.73 MPa m^1/2, and σ_b dropped suddenly from 850 MPa (70 vol%) to 340 MPa (80 vol%) and stayed as low σ_b in the 90 vol% B₄C sample. Next, the high-temperature σ_b values of the composites (40–70 vol%) were measured in Ar. The composites (40–60 vol%) revealed high σ_b (≥640 MPa) from R.T.~1600 °C; the maximum value of 803.5 MPa was observed for the 60 vol%B₄C composites at 1600 °C, and then the σ_b of all composites dropped to around 340 MPa at 1800 °C. From their stress-strain curves, elastic and plastic deformations were observed at 1600 °C and 1800 °C, respectively.     

Thermal runaway prediction for impregnated activated carbons from isothermal DSC measurements

Isothermal DSC measurements are used to extract the kinetics of the reaction between activated carbon, which has been impregnated with K₂CO₃, and air at elevated temperature. For the carbon studied here, the reaction takes place in two apparently independent processes. In the first process, the surface functional groups react with the carbon and with air, and in the second process the remaining carbon reacts with air. The impregnate causes the dramatic acceleration of the reaction compared to the reactivity of the pure base carbon in air. The reaction of the surface functional groups with air obeys the following reaction for the fractional degree of conversion, dα₁/dt=γ₁ exp(−E_a1/k_BT) α₁^m (1−α₁)ⁿ, with m and n approximately equal to −0.95 and 0.5, respectively. The reaction of the remaining carbon with air follows zero-order reaction kinetics. The obtained reaction kinetics can fit the results of isothermal DSC measurements over a wide range of temperature and scanning DSC measurements over a wide range of sweep rates, without any adjustment of parameters. The reaction kinetics, among other experimentally-derived parameters, were then used to calculate the temperature-time profiles for samples of carbon held in stainless steel mesh cylinders of various diameters within a heated constant-temperature oven. These calculations are shown to agree well with experiments at numerous temperatures for two cylinder radii using the same set of kinetic parameters used to fit the DSC results. Some of the temperatures and radii were selected such that thermal runaway of the carbon sample would occur, and the simulations modeled the time and temperature of this runaway accurately. Therefore, it should now be possible to simulate the temperature-time response of this particular impregnated carbon to conditions of high-temperature storage, such as those of the IATA oven exposure test or to storage within a shipping container exposed to a particular thermal history.     

Effect of soil organic carbon on sorption and desorption of perchloroethylene in soils

The objective of this study was to elucidate the sorption and desorption behaviors of PCE (Perchloroethylene, C₂Cl₄) in seven soils with different organic carbon (OC) content. Sorption/desorption kinetic and serial dilution desorption experiments were conducted in batch slurries. The sorption distribution coefficient (K _d ) of PCE ranged from 0.60 to 4.66 L kg^?1. K _d tended to increase as the soil OC increased, but K _oc tended to decrease, suggesting that adsorption into the mineral surface was not negligible in soils with low OC. Desorption kinetic data were analyzed by the two-site desorption model. The sorption/desorption of PCE was not reversible over short incubation times due to the presence of a non-desorbable site. The desorbable site fractions of PCE increased and non-desorbable site fractions decreased as the soil OC increased. It is suggested that partition of PCE into soil organic carbon is more reversible than adsorption on soil minerals.     

Refined measurements of indentation fracture resistance of alumina using powerful optical microscopy

A round-robin of the indentation fracture (IF) method using two alumina ceramics was performed in 12 laboratories to confirm the significantly improved reproducibility of indentation fracture resistance K_IFR, using powerful optical microscopy. Powerful optical microscopy with both an objective lens of 40× or 50× and a traveling stage was employed to reduce the error in reading crack length. Indentations at 98 N for the two samples had moderate between-laboratory standard deviations of 0.3 and 0.2 MPa m^1/2 for K_IFR of 4.3 and 3.6 MPa m^1/2, respectively, which indicates the effectiveness of this measurement technique to improve the reliability of the IF method. The deviations of the grand average K_IFR reported by the laboratories from those re-measured by the authors using the returned samples were only ca. 0.4 MPa m^1/2, which was attributed to the slight misreading of the crack length by the participant laboratories. Thus, the reliability of the IF method seems reasonable by this advanced approach because our recent round-robins, together with this study, have confirmed that the precision for the three major structural ceramics, SiC, Si₃N₄ and alumina, could meet the necessary condition of reproducibility.     

Polar and dispersion interactions at carbon surfaces: further development of the XPS-based model

Enthalpy of immersion (ΔH_i) in water has been measured for a series of ozone oxidised non-porous carbon blacks and, as in our previous studies been found to correlate directly with the total surface oxygen level [O]_T measured by X-ray photoelectron spectroscopy. An equation that allows calculation of either parameter from the other is given and shown to describe behaviour for a wide range of carbon black surfaces which contain ozone-generated or native oxygen functional groups. Using this approach, the surface polarity and the relative hydrophilic character of such surfaces can be predicted. A molar enthalpy for the polar interaction between water and surface oxygen atoms of 17 kJ mol⁻¹ is obtained by assuming a 1:1 co-ordination between water molecules and carbon surface oxygen atoms. The data lead to a predicted value of 37.5 mJ m⁻² for the immersion of oxygen-free carbon black external surface into water. This equates to a value of 2.5 kJ mol⁻¹ for the non-specific dispersion interaction between water and an oxygen-free carbon black surface when a molecular area of 10.5×10⁻²⁰ m² for water is assumed. The same carbon black when oxidised using nitric acid gives a different enthalpy of immersion to the ozone-treated and native oxide materials, this is attributed to differing chemistry of the two surface types, this aspect is discussed. The nitric acid treated carbons do, however, give the same value as the ozonated and native oxide carbons (37.5 mJ m⁻²) for the immersion of an oxygen-free carbon surface into water. A correlation between the point of zero charge (pH_PZC) of the carbons and ΔH_i or [O]_T is also presented. The results from these measurements show extremely good agreement with data from other groups who have used TPD to assess surface oxygen concentration. This gives a firm basis for confident prediction of the thermodynamic properties of carbon surfaces from single measurement techniques.     

Structure of K2Co3-loaded activated carbon fiber and its deodorization ability against H2S gas

Coal tar pitch containing finely dispersed KOH was spun centrifugally, followed by stabilization through heating to 330°C under a (1:1) mixture of air and CO₂ and carbonization/activation by heating to 850°C under CO₂. The activated carbon fiber obtained possessed of a specific surface area of 491 m²g⁻¹ and contained ca. 2% of K as K₂CO₃ over the peripheral region of fiber. The fiber showed high deodorization ability against 30 ppm of H₂S gas in air at ambient temperature. H₂S gas did not diffuse to the most interior parts of the fiber and was oxidized around outer regions of the fiber. Elemental sulphur was deposited in the fiber after H₂S absorption. The deodorization mechanism was discussed. The role and action of the K₂CO₃ supported was explained.     

Microstructure and mechanical properties of Al2O3–YSZ and Al2O3–YAG directionally solidified eutectic plates

Plates of Al₂O₃–YSZ and Al₂O₃–YAG eutectic composition with a thickness from 0.1 to 1 mm were prepared by directional solidification using a diode laser stack. The melt processed regions of plates exhibited colony microstructure consisting of finely dispersed phases. Due to the curved shape of the melted pool, the growth rate depends on the distance to the surface plate, decreasing from top to bottom. In this way, the microstructure characteristic length changes as a function of the distance to the plate surface. Vickers indentations and piezo-spectroscopy measurements were done on longitudinal and transverse cross-sections of the samples at different depths. From these measurements, we concluded that the Vickers hardness (H_V), indentation fracture toughness (K_IC) and residual stresses (σ_h) of the plates were mainly independent from the distance to the surface. The mean values that we obtained in the Al₂O₃–YSZ plates were H_V = 16 GPa, K_IC = 4.2 MPa m^1/2 and σ_h = −0.33 GPa, and in the Al₂O₃–YAG plates were H_V = 16 GPa, K_IC = 2.0 MPa m^1/2, and σ_h = −0.1 GPa. These values are similar to those found in directionally solidified eutectic rods.     

A small-angle neutron scattering study of activated carbon fibers

Phenolic resin based activated carbon fibers are widely used as filter materials in industry. We studied the fiber type ACFY-0204-3-18 which is applied to recover 2-propanol from air. The fibers were characterised by measuring the chemical composition, the apparent density, the wide angle X-ray diffraction data, and the sorption isotherms for nitrogen at 77.4 K and 2-propanol at 308.2 K. The porous structure was studied by small-angle neutron scattering. The two-dimensional contour patterns show an anisotropic intensity distribution I(q) at scattering vectors |q|<0.4 nm⁻¹. In this q-range I(q) is perpendicular to the fiber axis due to a refraction of the neutrons by the fibers, and I(q) increases abruptly with decreasing q. At |q|>0.4 nm⁻¹ scattering is isotropic and I(q) shows a weak interference peak due to the presence of micropores. The mean pore size of ∼2 nm was determined by fitting a monodisperse and a polydisperse Percus-Yevick hard sphere model to the experimental I(q) data. The microstructure of ACFY differs basically from the microstructure of polyacrylonitrile and pitch based carbon fibers.     

Fracture mechanics of dental adhesives supplemented with Polymethyl-vinyl-ether-co-maleic anhydride

The objective of this study was to determine the effect of Polymethyl-vinyl-ether-co-maleic anhydride (PVM/MA), called Gantrez AN, on interfacial fracture toughness (K_IC) of self-etch and etch-and-rinse dental adhesives. Sixty-five chevron-notched dentin-composite resin specimens were prepared. The following testing groups with different bonding agents were prepared and tested with a cross head speed of 0.1 mm/min: Clearfil SE (CF); Clearfil SE with Gantrez AN in primer (CFGp); Clearfil SE with Gantrez AN in bonding agent (CFG); Prime & Bond (PB); Prime & Bond with Gantrez AN (PBG). The K_IC values were determined and compared. The mode of failure was examined with light microscopy. The mean K_IC (standard deviation) of the Clearfil SE groups were 0.60 (0.09) MPa m^1/2 for CF, 0.64 (0.09) MPa m^1/2 for CFGp and 0.68 (0.16) MPa m^1/2 for CFG. The most common mode of fracture was cohesive. The mean K_IC (standard deviation) of the Prime & Bond groups were 0.63 (0.09) MPa m^1/2 for PB and 0.41 (0.11) MPa m^1/2 for PBG. Adhesive fracture most commonly occurred in the Prime & Bond groups. Gantrez AN did not adversely affect K_IC of the self-etch dental adhesive, but lowered K_IC of the etch-and-rinse adhesive. Addition of Gantrez AN to self-etch adhesive (CF) may be warranted to produce an antibacterial effect. Clinical studies of bacterial attachment and anti-bacterial effects to further justify the use of Gantrez AN in bonding agents are warranted.