Mechanical and fracture properties of zircon–mullite composites obtained by direct sintering

Refractory materials based on zircon (ZrSiO₄) are applied in high temperature applications (1400–1500 °C). They are demonstrated to have an excellent chemical attack resistance, such as corrosion or degradation due to molten glass or metals. On the other hand mullite (3Al₂O₃2SiO₂) is important both in traditional and advanced ceramics. Although multi-phase ceramic materials were always used, nowadays composite materials have an important industrial and technological development, to enlarge the designing capability of the manufacturer in properties and behaviors. The objective of the present work is to study the influence of the starting composition on the mechanical and fracture properties of zircon–mullite composites obtained by direct sintering of consolidated samples by slip cast of concentrated aqueous suspensions in plaster molds. Zircon–mullite composites using 15–45 wt% mullite were prepared and compared with pure zircon material obtained in the same conditions. Flexural strength (σ_f), dynamic elastic modulus (E), toughness (K_IC) and initiation fracture surface energy (γ_NBT) were evaluated. The results were explained by microstructure and the XRD analysis. The presence of mullite increased the zircon thermal dissociation. The ZrO₂ was a product of this reaction and also influence the mechanical and fracture properties of these materials through several combined mechanisms.Zircon composites prepared with 45 wt% of mullite in the starting powder showed a higher fracture toughness and initiation energy than ceramics derived from pure zircon. Microstructure consisting in mullite as a continuous predominant phase in which zircon and zirconia grains were distributed improved almost all the mechanical and fracture properties.     

Dynamic mechanical analysis study of the curing of phenol‐formaldehyde novolac resins

The curing reaction of typical commercial phenol‐formaldehyde novolac resins with hexamethylentetraamine (HMTA) was followed by dynamic mechanical analysis. The evolution of the rheological parameters, such as storage modulus G′, loss modulus G″, and tanδ (G″/G′), as a function of time, for samples of the phenolic resins on cloth, was recorded. The curing reaction, leading to the formation of a crosslinked structure, is described by a third‐order phenomenological equation. This equation takes into account a self‐acceleration effect, as a consequence not only of the chemical reaction of crosslinking after the gel point but of phase segregation as well. This rheokinetic model of the curing of phenolic novolac resins permits the determination of the numerical values of the kinetic equation constants. The influence of the composition, structure, and physical treatment on the curing kinetics of the novolac resins is evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1902–1913, 2001     

Waterborne polyurethane/inorganic hybrid composites: preparation,morphology and properties

Abstract

A series of waterborne polyurethane/inorganic (WPU/TiO₂) hybrid composites were synthesised by a sol–gel process on the basis of isophorone diisocyanate, polyether polyol (GE-210), dimethylolpropionic acid, tetrabutyl titanate (TBT) and 3-glycidyloxypropyl trimethoxysilane as a coupling agent. The physical properties of the WPU and WPU/TiO₂ dispersions and hybrids were measured. Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy and X-ray diffraction were used to assess the fracture surface morphology and the dispersions of the WPU/TiO₂ hybrids. The scanning electron microscopy, transmission electron microscopy and atomic force microscopy results showed that the TiO₂ particles were dispersed homogeneously in the WPU matrix in nanoscale. The prepared hybrids showed good thermal stability and mechanical properties in comparison with pure WPU and showed tunable transparence with the TBT fraction in the film. Through suitable adjustment of TBT content, some thin hybrids have potential applications, such as coatings, leather finishing, adhesives, sealants, plastic coatings and wood finishes.     

Good temperature stability of K0.5Na0.5NbO3 based lead-free ceramics and their applications in buzzers

(K_0.5−xLi_x)Na_0.5(Nb_1−ySb_y)O₃ (KLNNSx–y, x = 0–4 mol% and y = 0–8 mol%) lead-free piezoelectric ceramics were prepared by the conventional mixed oxide method. The denser microstructure and better electrical properties of the ceramics were obtained as compared to the pure K_0.5Na_0.5NbO₃ ceramic. The temperature stability of the electrical properties of the ceramics was also investigated. The experimental results show that the KLNNS2.5–5 ceramic exhibits good electrical properties (k_p  49%, k₃₁  30% and , tan δ  0.019), and possesses good temperature stability in the temperature range of −40 to 85 °C. The related mechanisms for improved electrical properties and temperature stability were also discussed. Moreover, buzzers based on the KLNNS2.5–5 ceramic have been fabricated and their characterization is presented. These results show that the KLNNS2.5–5 ceramic is a promising lead-free material for practical application in buzzers.     

Solvent-free allylic oxidation of cycloolefins over mesoporous CrMCM-41 molecular sieve catalyst at 1 atm dioxygen

Oxidation of cycloolefins was effectively performed under aerobic (1 atm dioxygen) and mild solvent-free conditions using mesoporous CrMCM-41 molecular sieve catalyst. For instance, Cycloolefins (C₅–C₇) were selectively oxidized to the corresponding α,β-unsaturated ketones (for example, cyclohexene – 52.2% conversion and 71.2% selectivity) and cycloolefins (C₈–C₁₂) were oxidized to epoxides (for example, cis-cyclooctene – 50% conversion and >99% selectivity) at 323–353 K for 24 h. The catalytic activity over recycled catalyst remains nearly same.     

Structure formation of alkylammonium montmorillonites in organic media

Using adsorption, X-ray diffraction (XRD), rheological, electrokinetic, dielectric and calorimetric measurements, the mechanism for the gel formation of the alkylammonium derivatives of the Pyzhevsky montmorillonite (Ukraine) (OM) in organic liquids of different polarity was investigated. The influence of modification, type of the organic cations, length of the alkyl chain, type of organic liquid and polar additive as well as traces of water on the swelling volume, enthalpy of immersion, zeta potential and the Bingham yield stress of the organo-gels were evaluated.There exists a certain correlation between the swelling volume, the ability for gel formation and the basal spacing of OM (d₀₀₁), which depends on the size of the intercalated cation. Thermodynamical analysis indicates that the degree of swelling and the gel strength are determined by the energy of the layer cohesion, energy of the solvation, the electrostatic repulsion and entropy effects.The thickening of OM dispersions and gel formation is optimal when the surface coverage by alkylammonium ions reaches θ=0.85–1.0. The lyophilicity and zeta potential are then high but not too strong to cause complete peptization.The linear increase of rheological parameters of the organo-gels in benzene, toluene, p-xylene, o-xylene, chlorobenzene, benzonitrile and nitrobenzene is related to the increase of the polarization of the liquid molecules and the transition from positive to negative ζ-potentials and the enthalpy of immersion.The effect of protonic and aprotonic compounds on the rheological, electrokinetic and thermodynamic properties of the hydrocarbon gels of octadecyl benzyldimethylammonium montmorillonite was also studied. For homologue alcohols, this influence decreases with the increase of chain length, corresponding to the decrease of the adsorption and the dielectric permittivity (). The influence of the aprotonic compounds on the rheological properties and on the enthalpy of immersion does not correlate with , but this influence is larger when the polarization and electron donor ability of the polar molecules are higher.The maximal gel formation of the hydrocarbon-OM dispersions requires a certain amount of water, which corresponds to a monomolecular coverage of the polar surface centers. This effect is explained by the strong orientation of the adsorbed water molecules, which creates giant dipole moments on the particles and H-bonds between the particles.     

Synthesis, characterization, and platinum-catalyzed hydrosilation cross-linking of copoly(methylsilylene/1,4-phenylene/methylvinylsilylene): Properties of aromatic carbosilane thermosets

Low molecular weight copoly(methylsilylene/1,4-phenylene/methylvinylsilylene) (I) has been prepared by the reaction of a mixture of methyldichlorosilane and methylvinyldichlorosilane with the di-Grignard reagent prepared fromp-dibromobenzene.I has been characterized by¹H,¹³C, and²⁹Si NMR as well as FT-IR spectroscopy. Its molecular weight distribution has been determined by gel permeation chromatography (GPC), its thermal stability in nitrogen evaluated by thermogravimetric analysis (TGA), and its glass transition temperature (T _g) measured by differential scanning calorimetry (DSC). Platinum-catalyzed hydrosilation cross-linking ofI yields a thermoset material. The bending modulus (logE), tan·, andT _g of these cross-linked materials have been measured by dynamic mechanical thermal analysis (DMTA). Carbon fabric treated with catalytic amounts of platinum has been coated withI. Multilayers of coated carbon fabric have been consolidated (cured) to yield aromatic carbosilane/carbon composite materials. The interlaminar shear strength of these materials has been determined by short beam shear. Flexure strength and modulus of elasticity of these composite samples have been measured. The ultimate compressive strength and compression modulus as well as the tensile strength and elastic modulus of these have been determined.Presented at the XXVIth Silicon Symposium, Indiana University-Purdue University of Indianapolis, March 26–27, 1993.     

Crosslinking reaction and properties of two‐component waterborne polyurethane from terpene‐maleic ester type epoxy resin

A two‐component waterborne polyurethane (2K‐WPU) is prepared with the terpene‐maleic ester type epoxy resin‐based polyol dispersion and a hydrophilically modified hexamethylene diisocyanate tripolymer. Laser particle size analyzer and transmission electron microscopy are used to characterize the particle size distribution and the micromorphology of the 2K‐WPU. Crosslinking reaction kinetics of the 2K‐WPU is examined by fourier transform infrared spectrometry (FTIR) spectra. In the preliminary stage of the crosslinking reaction, it shows a very good fit with a second order reaction kinetics, and the apparent activation energy is 94.61 kJ mol^?1. It is also shown from the FTIR spectra that the complete crosslinking reaction of the 2K‐WPU needs 7 h at 70°C. The crosslinked products of the 2K‐WPU have good thermal resistant properties, with glass‐transition temperatures (T_g) in the range of 35–40°C and 10% weight loss temperatures (T_d) in the range of 275–287°C. The films obtained from the crosslinked products have good water‐resistance, antifouling, blocking resistance properties and impact strength of >50 cm, flexibility of 0.5 mm, adhesion of 1 grade, pencil hardness of HB‐2H. The pencil hardness and thermal‐resistant properties of the crosslinked products increase with the molar ratio of isocyanate (? NCO) group to hydroxyl (? OH) group. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Relaxation patterns of endlinking polydimethylsiloxane near the gel point

While the rheological behavior at the gel point (GP) itself is well known, many questions remain about the approach to the gel point (from either side). For studying this phenomenon, rheological experiments were performed on crosslinked polydimethylsiloxanes (PDMS) and characteristic patterns in the relaxation behavior were abstracted into a phenomenological model for the relaxation time spectrum. The well known power law with slope-n and cutoff at the longest relaxation time λ _max governs the relaxation near the gel point. Beyond the gel point, an additional box-like contribution appears in the spectrum. The relaxation exponent n decreases with increasing extent of crosslinking. The longest relaxation time λ _max and equilibrium modulus G_e show power-law scaling with the distance from the gel point, |p − p_c|. For samples with imbalanced stoichiometric ratios r ≥ 1, the function n (p − p_c) is independent of r. Samples with strong crosslinker deficiency (r = 0.5) exhibit a different functional form for n (p − p_c) from those with r ≥ 1. The power law relaxation with decreasing n in the terminal zone was found previously for a different crosslinking system. It seems to be a common pattern for partially crosslinked materials from polymeric precursors. Received: 11 December 1997/Revised version: 18 December 1997/Accepted: 18 December 1997     

Performances of mixed alcohols synthesis over potassium promoted molybdenum carbides

Molybdenum carbides, β-Mo₂C and α-MoC_1−X, promoted by K₂CO₃ were studied as catalysts for CO hydrogenation reactions at T = 573 K, P = 8.0 MPa, GHSV = 2000 h⁻¹, H₂/CO = 1.0. Unpromoted molybdenum carbides produced mainly light hydrocarbons under these conditions. Addition of K₂CO₃ as a promoter, however, both β-Mo₂C and α-MoC_1−X resulted in remarkable selectivity shift from hydrocarbons to alcohols. Moreover, the promoter of potassium enhanced the ability of chain propagation of β-Mo₂C and α-MoC_1−X with higher selectivity to C₂₊OH. Meanwhile, the investigation of the loadings of K₂CO₃ in β-Mo₂C and α-MoC_1−X revealed that the maximum of yield of alcohol was obtained at K/Mo (molar ratio) of 0.2 and 0.1, respectively. On K/β-Mo₂C catalysts, the distribution of hydrocarbons obeyed traditional linear A–S–F plot, while alcohols gave a unique linear A–S–F distribution with remarkable deviation of methanol compared with that of on β-Mo₂C catalyst. However, there was no significant difference between α-MoC_1−X and K/α-MoC_1−X catalysts about the distributions of alcohols and hydrocarbons; they both have similar linear A–S–F plots. Thus, the potassium promoter played a different role over β-Mo₂C and α-MoC_1−X catalysts upon the catalytic performance of mixed alcohols synthesis.     

Microstructural characterization and crystallization of (1 − x)TeO2–xCdF2 (x = 0.10, 0.15, 0.25 mol) glasses

The differential thermal analysis (DTA), X-ray diffractometer and scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) techniques were used to investigate the microstructural characterization and the thermal behavior of the three (1 − x)TeO₂–xCdF₂ (x = 0.10, 0.15, 0.25 mol) glasses. The effect of the heating rate, annealing temperature and CdF₂ content on the thermal and the microstructural properties of TeO₂–CdF₂ binary glasses were enquired. DTA analysis has shown that as the CdF₂ content in the glass composition increases, the value of the glass transition and the peak crystallization temperatures shift to higher values. SEM/EDS investigations have shown that the crystal formation of α-TeO₂, γ-TeO₂ and CdTe₂O₅ crystal phases were observed when the 0.90TeO₂–0.10CdF₂ and the 0.85TeO₂–0.15CdF₂ glass samples were annealed to temperatures above the crystallization temperatures. X-ray diffraction (XRD) results illustrated clearly the transformation of the metastable γ-TeO₂ phase to stable α-TeO₂ crystalline phase as the annealing temperature was increased from 385 to 425 °C for the 0.75TeO₂–0.25CdF₂ glass.     

Thermal properties of the nitrogen-rich Ca-α-Sialons

Nitrogen-rich Ca-α-Sialon (Ca_xSi_12−2xAl_2xN₁₆ with x = 0.2, 0.4, and 0.8, 1.2 and 1.6) ceramics were prepared from the mixtures of Si₃N₄, AlN and CaH₂ powders in a hot press at 1800 °C using a pressure of 35 MPa and a holding time of 4 h, and then were investigated with respect to reaction mechanism, phase stability and oxidation resistance. In addition the sample with x = 1.6 was prepared in the temperature range 600–1800 °C using a pressure of 35 MPa and a holding time of 2 h. The α-Sialon phase was first observed at 1400 °C but the α-Si₃N₄ and AlN phases were still present at 1700 °C. Phase pure Ca-α-Sialon ceramics could not be obtained until the sintering temperature reached 1800 °C. The phase pure nitrogen-rich Ca-α-Sialon exhibited no phase transformation in the temperature range 1400–1600 °C. In general, mixed α/β-Sialon showed better oxidation resistance than pure α-Sialon in the low temperature range (1250–1325 °C), while α-Sialons with compositions located at α/β-Sialon border-line showed significant weight gains over the entire temperature range tested (1250–1400 °C). The phases formed upon oxidation were characterized by X-ray, SEM and TEM studies.     

Crystallization controlled by layered silicates in nylon 6–clay nano-composite

To understand the effect of the montmorillonite (MMT) particles on the crystallization kinetics and crystalline morphology of nylon 6 upon nano-composite formation, we have characterized the crystallization behaviors by using light scattering, wide-angle X-ray diffraction (WAXD), transmission electron microscope (TEM) and rheological measurement. The correlation between the nucleating effect and the growth mechanism of the different polymorphism (γ-phase) of nylon 6 in the nano-composite (N6C3.7) was probed. N6C3.7 exhibited γ-phase crystal due to the nucleating effect of the dispersed MMT particles into the nylon 6 matrix throughout the whole T_c range (=150–215 °C). The lamellar growth of the γ-phase crystal took place on both sides of the dispersed MMT particles. In comparison between the temperature dependence of the characteristic relaxation time and the crystallization time, the lamellar growth of the γ-phase crystal has been discussed. The stable growth of the γ-phase was strongly disturbed at low T_c range (=160–190 °C) due to the lack of time for crystallization.     

Synthesis, structural characterization and third-order non-linear optical property of new three-dimensional metal-organic polymer [Fe2(μ10-btc)0.5(μ2-ox)0.5(μ2-O)1.5]n

One new metal-organic polymer formulated as [Fe₂(μ₁₀-btc)_0.5(μ₂-ox)_0.5(μ₂-O)_1.5]_n 1 (btc = 1,2,4,5-benzenetetracarboxylate, pyramellitate; ox = oxalate) has been synthesized by low-temperature solid-state reaction and characterized by single-crystal X-ray diffraction, elemental analyses, TGA, IR spectra and UV–visible spectra. Complex 1 presents the first 3D coordination network structure constructed by bridging btc, ox and O mixed ligands. In 1, carboxyl groups of btc are all deprotonated and they have a new type of μ₁₀-btc coordination mode. The third-order non-linear optical (NLO) properties of the title compound 1 were also investigated and they exhibit the reverse saturable absorption and self-defocusing performance with modulus of the hyperpolarizability (γ) 5.98 × 10⁻³⁰ esu for 1 in a 7.45 × 10⁻⁴ mol dm⁻³ DMF solution.     

The crystalline-to-amorphous transition in shock-loaded mullite Al2(Al2+2xSi2−2x)O10−x in the light of shear modulus anisotropy

We present experimental evidence for shock-wave induced amorphization in polycrystalline and single crystal mullite, Al₂^VI(Al_2+2x Si_2−2x)^IVO_10−x, at peak pressures above 35 GPa. The transition proceeds along with a network of very thin glass lamellae (planar deformation features (PDFs)) of mullite-normative composition extending parallel to low-index crystallographic planes including {1 2 0}, {2 3 0} and {1 1 0}. Cumulative microstructural evidence from the PDFs derived via analytical transmission electron microscopy suggests a shear-induced formation mechanism. Experimental PDFs match the relative minima of the calculated representation surfaces of the shear modulus suggesting that suitable PDF orientations can be derived from the elastic anisotropy of mullite. PDFs in mullite are in good agreement with those reported for naturally shocked sillimanite.Unlike the formation of shear-induced PDF-type glass lamellae in shocked mullite, the thermal decomposition of mullite following high post-shock temperatures results in a fine-grained phase assemblage consisting of corundum plus amorphous silica, and represents the most abundant transformation mechanism in the shock regime investigated (20–40 GPa). No stishovite was observed. At shock levels beyond 35 GPa thermal decomposition of mullite may occur along with PDFs within the same specimen.     

Contribution of soft segment entanglement on the tensile properties of silicone–urea copolymers with low hard segment contents

Novel, segmented thermoplastic silicone–urea (TPSU) copolymers based on rather high molecular weight aminopropyl terminated polydimethylsiloxane (PDMS) soft segments (<M_n> 10,800 and 31,500 g/mol), a cycloaliphatic diisocyanate (HMDI) and various diamine chain extenders were synthesized. Copolymers with very low urea hard segment contents of 1.43–14.4% by weight were prepared. In spite of very low hard segment contents, solution cast films showed very good microphase separation and displayed reasonable mechanical properties. Tensile strengths of TPSU copolymers showed a linear dependence on their urea hard segment contents, regardless of the structure of the diamine chain extender used. The modulus of silicone–urea copolymers is dependent on the urea concentration, but not on the extender type or PDMS molecular weight. When silicone–urea copolymers with identical urea hard segment contents were compared, copolymers based on PDMS-31,500 showed higher elongation at break values and ultimate tensile strengths than those based on PDMS-10,800. Since the critical entanglement molecular weight (M_e) of PDMS is about 24,500 g/mol, these results suggest there is a significant contribution from soft segment chain entanglement effects in the PDMS-31,500 system regarding the tensile properties and failure mechanisms of the silicone–urea copolymers.     

Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Phase Behavior and Mechanical Properties of Siloxane-Urethane Copolymer

Two series of siloxane-urethane copolymers were prepared from polydimethylsiloxane (PDMS) with a molecular weight of 1000 or 1800 which was used as a soft segment, 4,4′-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (1,4-BD). Differential scanning calorimetry (DSC) demonstrated that the position (T_gs) and breadth (ΔB) of soft-segment glass transition of copolymers remained constant as the hard-segment content increased. Heat capacities at soft-segment glass transition of the copolymer (ΔCp) were 0.195∼0.411 J/g^○C and heat capacities of pure PDMS (ΔCp₀) were 0.571∼0.647 J/g^○C, leading to the various ΔCp/ΔCp₀ ratios. The ΔCp/ΔCp₀ ratios decreased as the increasing of hard-segment content, showing poor phase separation. The FTIR spectrum confirmed the occurrence of hydrogen bonding in ether end-group of pure PDMS. The ether group of the soft segment led to interfacial mixing between soft and hard segments. The tan δ of the soft segment determined by dynamic mechanical testing (DMA) also identified the mixing of soft and hard segments. The mechanical properties of the copolymer were directly related to either the soft and hard segment contents or the chain lengths of soft and hard segments. The hard segment that reinforced the soft segment and interfacial thickness between soft and hard segment dominated the mechanical properties.     

On-Line Rheological Measurements and Mechanical Properties of Acrylonitrile-Butadiene-styrene/Corn Starch Composite

In this work, rheological and mechanical properties of acrylonitrile–butadiene–styrene/corn starch composites (ABS/starch) were studied. The composites were prepared using a laboratory-scale, single-screw extruder. Rheological properties were determined using the single-screw extruder, apparent shear rate (γ _a ), apparent shear stress (τ _a ), apparent viscosity (η _a ), non-Newtonian index (n), and flow activation energy at a constant shear rate (E _γ) and constant shear stress (E _τ). Mechanical properties in terms of tensile tests were performed using Testometric M350-10KN, stress at break, strain at break, and Young's modulus were determined. Rheological results showed that the composites are pseudo plastic in behavior, and the apparent viscosity of the composites increases with increasing starch content above the additive rule, which indicates a partial compatibility in the composite. It was also found that the flow activation energy of the composite increases with increasing starch content. The mechanical results showed that the strain at break of the composite decreases sharply by the presence of starch, whereas the Young's modulus increases with increasing starch content.     

An easy entry to novel early–late oligonuclear transition metal complexes containing π-conjugated systems

Heterotrinuclear Ti–Cu–Ru (5) and heterotetranuclear Ti–Cu–Pt–Fe (7) containing complexes are accessible by using {[Ti](CC^tBu)₂}CuMe (1) ([Ti]=(η⁵-C₅H₄SiMe₃)₂Ti) as key molecule; in 5 and 7, the corresponding early and late transition metal atoms are linked by π-conjugated organic moieties.