Influence of Accelerator Content on Cure Kinetics of Sulfur-vulcanized Natural Rubber

Based on the cure characteristics of the NR/S/TBBS system, a kinetic model with induction, curing, and post-cure periods was chosen to simulate the cure reaction of this system. Cure curves reflecting the evolution of crosslink density were recorded as a function of curing time for NR compounds using a rubber processing analyzer (RPA). The cure curves were then non-linear fitted and the kinetic parameters were determined. The results showed that the simulated curves fit well with the experimental curves. As the concentration of activated sulfurating agents, A_o, increased, the activity of crosslink precursors increased (K₂ increased), while the activity for crosslinks to degrade decreased (K₆ decreased) due to shortening of the sulfur chain both in crosslink precursors and in crosslinks, leading to the improved thermal stability of NR vulcanizate at elevated temperatures. Increased accelerator amount also increased the competition of the reaction to form crosslinks over the reaction to form dead by-products, which improved the efficiency of sulfur. Reaction activation energy E₂ and E₆, calculated from K₂ and K₆ through the Arrhenius equation, showed that E₆ is higher than E₂. The increase of accelerator dosage led to the decrease of E₂ and increase of E₆, which explained the phenomenon of improved anti-reversion.     

Influence of Zinc Oxide Amount on Cure Kinetics of Sulfur-Vulcanized Natural Rubber

A kinetic cure model with induction, curing, and postcure periods, based on cure characteristics of Natural rubber/sulfur/N-t-butylbenzothiazole-2-sulfenamide (NR/S/TBBS) system, was chosen to simulate the cure reaction of this system. Cure-curves reflecting the evolution of crosslink density were recorded as a function of curing time for NR compounds using a rubber processing analyzer (RPA). The data of experimental cure curves were nonlinear fitted with the chosen model and the kinetic parameters were determined. The results showed that the simulated curves fit well with the experimental curves. The incorporation of zinc oxide changed the cure mechanism of this system. The concentration of activated sulfurating agents, A ₀, increased with increased zinc oxide amount. With the increase of zinc oxide amount, the activity of crosslink precursors decreased slightly (k ₂ decreased) which had little influence on its activation energy. The activity of crosslink degradation decreased with the increase of zinc oxide amount from 1 phr to 2 phr. Higher zinc oxide load had no further contribution to crosslink density and reversion resistance. Parameter ψ, representing the competition between reactions of activated crosslinking precursors to form crosslinks and to form dead side-products, decreased significantly when zinc oxide increased from 1 phr to 2 phr. Higher zinc oxide load (from 2 ～ 4 phr) induced a slight increase of ψ, especially at higher cure temperature.     

Influence of Carbon Black on Crosslink Density of Natural Rubber

Vulcanization and reinforcement are two important factors contributing to the properties of vulcanized rubber. In order to investigate the influence of carbon black (CB) on chemical crosslinking, three groups of samples with different crosslink densities were prepared. In each group with the same crosslink density, different amounts of CB were introduced. Data fitting showed that delta torque (ΔM = M _H–M _L, the difference between the highest and lowest torques during curing) in the cure curves of each group had a good linear relationship with CB load and extrapolation of the fitting lines almost intercepted the x coordinate at the same value, which indicated that CB had no influence on the chemical crosslinking of the rubber. To verify the above result, a series of nonfilled natural rubber (NR) vulcanizates with different crosslink densities were studied using equilibrium swelling and the swelling ratios were compared with those of corresponding CB filled rubbers with the same sulfur and accelerator amount. The results of both the equilibrium swelling and NMR relaxation parameter measurements showed that CB filled vulcanizates had higher apparent crosslink densities than those of unfilled ones due to the strong interaction between rubber molecules and the surface of the CB particles. The swelling ratios of filled rubbers had a parallel relationship with those of the unfilled ones which indicated that CB had little influence on chemical crosslink density introduced by chemical vulcanization.     

Properties of Natural Rubber Vulcanizates/Nanosilica Composites Prepared Based on the Method of In-situ Generation and Coagulation

Using the characteristics of silica sol dispersing well in water and easy formation of silica gel when the silica sol is heated, by mixing a system of concentrated natural rubber latex and silica sol, the silica sol can in-situ generate SiO₂ particles when heated. After coagulation of the mixed system, natural rubber/nanosilica composites C(NR/nSiO₂) were obtained. The composites C(NR/nSiO₂) and their vulcanizates were studied using a rubber processing analyzer (RPA), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). The influence of silica contents on the C(NR/nSiO₂) vulcanizates mechanical properties, cross-linking degree, Payne effect, dissipation factor (tanδ), and the particle size and dispersion of SiO₂ in NR were investigated. The results obtained were compared with the NR/SiO₂ composites based on traditional dry mixing of bale natural rubber and precipitated silica (white carbon black). The results showed that when using a sulfur curing system with a silica coupling agent (Si69) in C(NR/nSiO₂), the vulcanizate had better mechanical properties, higher wet resistance, and lower rolling resistance than those without Si69. In the composites C(NR/nSiO₂) and their vulcanizates, the SiO₂ particles’ average grain diameter was 60 nm, and the good-dispersion of the in-situ generated SiO₂ in the rubber matrix were a significant contribution to the satisfactory properties of C(NR/nSiO₂) composites and their vulcanizates.     

Influence of Crosslink Density on Mechanical Properties of Natural Rubber Vulcanizates

Crosslink density is an important structural parameter for cured rubber. Natural rubber (NR) vulcanizates with different crosslink densities were obtained through using different sulfur and accelerator amounts and different accelerator types. The crosslink density was characterized by an ¹ H-NMR technique and its influence on mechanical properties, such as Shore A hardness, 300% modulus, tensile strength, and elongation at break, of NR vulcanizates was investigated. The results showed that both the sulfur amount and the accelerator type and amount had an influence on the crosslink density of the NR networks. The relationship between total crosslink density and mechanical properties was also studied. The results, by changing either the sulfur or the accelerator amount, showed that tensile strength of NR vulcanizates reached maximum value when the total crosslink density was around 13.5 × 10^?5 mol/cm³, equivalently the average molecular weight of the intercrosslink chains (Mc) was around 7000 g/mol. The maximum value of tensile strength came from the balance between contributions of crosslink joints and stretch-induced orientation and/or crystallization of intercrosslink chains. The study on influence of total crosslink density on Shore A hardness and 300% modulus of NR vulcanizates showed that they both increased linearly with the crosslink density, the slopes were 2.7 ～ 3.0 cm³/10^?5 mol and 0.27 ～ 0.31 MPa cm³/10^?5 mol for Shore A hardness and 300% modulus, respectively, whether the crosslink density was varied by sulfur or accelerator.     

Nanoindentation of hydrogenated amorphous silicon

Nanoindentation was carried out on thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition. The composite values of elastic (Young's) modulus, E _c, and hardness, H _c, of the film/substrate system were evaluated from the load–displacement curves using the Oliver–Pharr approach. The film-only parameters were obtained employing the extrapolation of the depth profiles of E _c and H _c. Scanning probe microscopy was employed to image the nanoindenter impressions and to estimate the effect of film roughness and material pile-up on the testing results. It was established that the elastic modulus of thin a-Si:H films is in the range 117–131 GPa, which is lower than for crystalline silicon. In contrast, the values of hardness are in the range 12.2–12.7 GPa, which is comparable to crystalline silicon and higher than for hydrogen-free amorphous silicon. It is suggested that the plastic deformation of a-Si:H proceeds through plastic flow and it is the presence of hydrogen in the amorphous matrix that leads to a higher hardness.     

Pre-exponential factor for non-isothermal crystallization of glassy Se85− x Te15Sb x (0 ≤ x ≤ 10) alloys

We report observations of the Meyer–Neldel rule for the non-isothermal crystallization of glassy Se_{85? x} Te₁₅Sb _x (x =?0, 2, 4, 6, 8, 10) alloys. We found a strong co-relation between the pre-exponential factor K ₀ of the rate constant K(T) for crystallization and the activation energy of crystallization E _c. This indicates the presence of a compensation effect for the non-isothermal crystallization process in this glassy system. The composition dependence of the crystallization temperature T _c and the activation energy for crystallization E _c is discussed.     

High-resolution infrared spectra of HCF3 in the ν6 (500 cm) and 2ν6 (1000 cm) regions: rovibrational analysis and accurate determination of the ground state constants C0 and DK

The high-resolution infrared spectrum of HCF₃ was studied in the ν₆ fundamental (near 500 cm⁻¹) and in the 2ν₆ overtones (near 1000 cm⁻¹) regions. The present study reports on the analysis of the hot bands in the ν₆ region, as well as the first observation and assignment of the 2ν₆² perpendicular band. Using ν₆, 2ν₆^±2–ν₆^±1 and 2ν₆² experimental wavenumbers, accurate coefficients C₀ and D_K⁰ of the K-dependent ground-state energy terms were obtained, using the so-called “loop method.” Ground-state energy differences Δ(K,J)=E₀(K,J)−E₀(K−3,J) were obtained for K=3–30. A least-squares fit of 81 such differences gave the following results (in cm⁻¹): C₀=0.1892550(15); D_K⁰=2.779(26) × 10⁻⁷.     

K0.5Na0.5NbO3-LiSbO3-BiFeO3/CuFe2O4复合陶瓷的制备与磁电性能研究

采用传统的固相法制备了(1-x)(K_0.5Na_0.5NbO₃-LiSbO₃-BiFeO₃)-xCuFe₂O₄ (x=0.1, 0.2, 0.3, 0.4) 磁电复合陶瓷, 并借助X射线衍射仪、扫描电镜和磁电耦合系数测试仪等对复合陶瓷的微结构和性能进行了分析. 结果表明, 复合陶瓷的K_0.5Na_0.5NbO₃-LiSbO₃-BiFeO₃和CuFe₂O₄物相之间发生了一定的离子相互扩散作用, 且两相的颗粒大小匹配性较好. 随着CuFe₂O₄含量增加, 复合陶瓷的压电系数从130 pC/N减小到30 pC/N, 饱和磁致伸缩系数从4.5×10^-6增加到12.4×10^-6左右, 磁电耦合系数表现出先增加后减小, 在x=0.3时获得最大的磁电耦合系数9.4 mV·cm^-1·Oe^-1. 关键词： 0.5Na_0.5NbO₃-LiSbO₃-BiFeO₃')" href="#">K_0.5Na_0.5NbO₃-LiSbO₃-BiFeO₃ 2O₄')" href="#">CuFe₂O₄ 磁电耦合     

Kinetics and thermodynamics of reversible disproportionation–comproportionation in redox triad oxoammonium cations – nitroxyl radicals – hydroxylamines

Kinetics and equilibrium of the acid‐catalyzed disproportionation of cyclic nitroxyl radicals R₂NO^? to oxoammonium cations R₂NO⁺ and hydroxylamines R₂NOH is defined by redox and acid–base properties of these compounds. In a recent work (J. Phys. Org. Chem. 2014, 27, 114‐120), we showed that the kinetic stability of R₂NO^? in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R₂NO⁺ and R₂NOH to R₂NO^? and found that increasing in –I‐effects of substituents greatly reduces the overall equilibrium constant of the reaction K₄. This occurs because of both the increase of acidity constants of hydroxyammonium cations K_3H+ and the difference between the reduction potentials of oxoammonium cations E_R2NO+/R2NO? and nitroxyl radicals E_R2NO?/R2NOH. pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E_1/3Σ and bond dissociation energies D(O–H) for hydroxylamines R₂NOH in water were determined. For a wide variety of piperidine‐ and pyrrolidine‐1‐oxyls values of pK_3H+ and E_R2NO+/R2NO? correlate with each other, as well as with the equilibrium constants K₄ and the inductive substituent constants σ_I. The correlations obtained allow prediction of the acid–base and redox characteristics of redox triads R₂NO^?–R₂NO⁺–R₂NOH. Copyright © 2014 John Wiley & Sons, Ltd.     

Driven dynamics of the Frenkel Kontorova Aubry model

The dynamics of the Frenkel-Kontorova model in the presence of an accelerating fieldE is studied analytically and numerically in the damping-free limit and for small periodic potential strengthK. ForE>K ^4/3, the velocityV(t) increases linearly with time while the internal vibration amplitude remains small. ForK ²<E<K ^4/3,V(t) stops increasing after a timet ₁=1/E, after which the whole power is absorbed by internal vibrations. These vibrations become (unphysically) huge, and stop increasing after a time which cannot exceedt ₂8.33/E. Then the velocity goes on increasing linearly with time. The effect of a weak damping is also considered.     

Concentrated Kondo systems

This review considers the experimental and theoretical studies of concentrated Kondo systems (CKS), Kondo lattices, substitutional solid solutions and their transition from Kondo impurity to Kondo lattice, and ‘intermediate valence compounds’ which are, in fact, high T _K CKS (T _K is the Kondo temperature). The anomalous low temperature properties of CKS are related to the formation of the narrow (～k _B T _K) high-amplitude Abrikosov-Suhl resonance E _R in the vicinity of the Fermi level E _F. This resonance is situated exactly at E _F in low T _K CKS with T _K < Δ_CF and near E _F in high T _K CKS with T _K > Δ_CF (Δ_CF is the crystal field splitting). In low T _K ‘j=1/2’ CKS the condition E _R=E _F leads to an increase of the density of states at E _F, which is large enough to induce heavy fermion superconductivity in CeCu₂Si₂, UBe₁₃. We demonstrate that the transition from low T _K (E _R=E _F) to high T _K CKS (E _R≠E _F) might be what was formerly considered as a ‘Kondo-lattice-intermediate valence state’ transition. It appears that in many cases the essentially non-integer valence state of the rare-earth elements in metallic compounds is thermodynamically unstable with respect to a transition to an almost integer valence state, because it realizes the maximum gain in free energy from the Kondo condensation.     

The total yields of K *(892)+, Σ+(1385), and Σ0 in neutrino-induced reactions at ? E v? ≈ 10 GeV

Using the data obtained with the SKAT bubble chamber, the total yields of K*(892)⁺, Σ⁺(1385), and Σ⁰ are estimated for the first time in neutrino-induced reactions at moderate energy 〈E _v〉 = 10.4 GeV. It is shown that the recently observed enhancement of the K ⁰ and Λ yields in vA interactions as compared to vN interactions is contributed only slightly by the K*(892)⁺ and Σ⁺(1385) production. The contribution of resonances to the K ⁰ and Λ yields is found to be in qualitative agreement with higher energy (〈E _v〉 ≳ 40 GeV) data. It is shown that the energy dependence of the K*(892)⁺ mean multiplicity in vN interactions is approximately linear in the range of 〈E _v〉 ≈ 10−60 GeV, while that for Σ⁰ in vA interactions (A = 20−21) is approximately logarithmic in the range of 〈E _v〉 ≈ = 10−150 GeV. The text was submitted by the authors in English.     

Resonances in the KSKSKL system produced in collisions of negative pions with a carbon target at a momentum of 40 GeV

The experimental spectrum of the K _S K _S K _L system from the reaction π ⁻C → K _S K _S K _L + Y at amomentum of 40 GeV was obtained experimentally with the aim of studying resonance states featuring open strangeness and occurring in the high-mass region. The experiment was performed at the 6-m spectrometer installed at the Institute for High Energy Physics (IHEP, Protvino). The spectrum displays, along with well-known resonances [K ₁(1640), K ₂(1770), K ₂(1820), K ₂(1980)], which fit in the quark-antiquark classification, the exotic resonances K ₂(2280) and K ₄(2500). The K ₂(2280) resonance exhibits the properties of a hybrid and has an exotic decay mode producing a triplet of known resonance states related to each other by quark-gluon mixing. The mechanism of the formation of the observed hybrid K ₂(2280) is dominated by the exchange of a natural spin-parity in the t channel of the reaction. The mode of decay into f ₀(980)K _L and the mode of decay into f ₂(1270)K _L are observed for the K ₂(1770) and the K ₂(1980) resonance, respectively. __________ Translated from Yadernaya Fizika, Vol. 66, No. 5, 2003, pp. 860–867. Original Russian Text Copyright ? 2003 by Tikhomirov, Erofeev, Erofeeva, Luzin.     

Li (i = 1, 2, 3) sub‐shell fluorescence yields for 79Au, 80Hg and 81Tl

The L _i (i = 1, 2, 3) sub‐shell x‐ray fluorescence yields (ω _i ) for ₇₉Au, ₈₀Hg and ₈₁Tl were deduced from the measured cross‐sections for the L _i (i = 1, 2, 3) sub‐shell x‐rays following ionization by 59.54 keV γ‐rays (B < E_inc < B_K) and for the L₃ sub‐shell x‐rays following ionization by Rb K x‐rays (B < E_Kα < B, B < E_Kβ < B_K), where B is the K shell/L _i sub‐shell ionization threshold of the target element. An energy‐dispersive x‐ray fluorescence setup, involving photon sources consisting of a ²⁴¹Am annular source in the direct and secondary excitation modes along with RbCl secondary exciter and an Si(Li) detector, was used for the measurements. The measured ω₂ and ω₃ values exhibit good agreement with those based on the relativistic Dirac–Hartree–Slater (RDHS) calculations, while the ω₁ values are found to be higher by ～25%. The present data indicate that the L₁–L₃ Coster–Kronig yield f₁₃ based on the RDHS calculations are overestimated. Copyright © 2004 John Wiley & Sons, Ltd.     

Dynamic Rebound Behavior of Silica/Natural Rubber Composites: Fly Ash Particles and Precipitated Silica

This article investigated the elastic response of natural rubber (NR) compounds filled with silica from fly ash particles (FASi) and commercial precipitated silica (PSi), through a dynamic rebound test. The effects of silica content and initial drop‐height on the height and number of rebounds, dynamic stiffness, and the energy loss were of interest. The results suggested that the unfilled NR vulcanizates exhibited a greater elastic response than the FASi and PSi‐filled vulcanized composites. For given silica contents, the NR compounds with FASi had better elastic response than those with PSi, where the elastic response decreased with an increase in silica content. The greater the silica contents, the higher the dynamic stiffness of the composites. The initial drop‐height had no effects on the elastic response change for the unfilled NR compound, but resulted in an increase in the energy loss for the silica‐filled NR composites. The differences in the elastic responses for the NR compounds filled with silica from FASi and PSi were associated with the differences in crosslink density and the filler–filler interaction influenced by content of bis(3‐triethoxysilylpropyl) tetrasulfane (designated as Si69) used.     

A theoretical study on the stability and detonation performance of 2,2,3,3‐tetranitroaziridine (TNAD)

In recent years, there has been a considerable interest in developing high oxygen compounds as oxidizers for, for example, composite explosives. 2,2,3,3‐Tetranitroaziridine (TNAD) is a new designed compound with high oxygen balance (25.11%) and is environmentally friendly. A synthesis route of TNAD was suggested in this study, and the thermodynamic possibilities of reactions were evaluated by the changes in the free energy obtained with the density functional theory (DFT). The strong strain energy (E_s = 292.28 kJ/mol) of TNAD leads the C–C bond in the ring more fragile than the C–NO₂ bond, and the activation energy (E_a) of pyrolysis of the C–C bond (119.14 kJ/mol at the B3LYP/6‐31G* level of DFT) is higher than that of 2,4,6‐trinitrotoluene (TNT) (113.00 kJ/mol). The topological analysis with the contour maps of electron density was used to show the changes of the electron density at the critical points (BCP) in the process of homolysis of the C–C bond. In addition, the energy gap between the frontier orbitals of TNAD (ΔE_g = 5.22 eV) is slightly higher than that of 1,3,3‐trinitroazetidine (TNAZ, 5.06 eV). And the HOMO → LUMO transition plays important roles in the UV spectrum. The noncovalent interactions in the TNAD/RDX composite were estimated to be stronger than that in TNAZ/RDX, that is, the former may have better compatibility than the latter. TNAD/RDX with the weight ratio of w_TNAD/w_RDX = 0.46/0.54 has the wonderful performance (D = 9.14 km/s, P = 37.30 GPa, and I_s = 285.47 s) which is better than that (D = 8.85 km/s, P = 35.09 GPa, and I_s = 272.33 s) of TNAZ/RDX with the same weight ratio. Copyright © 2014 John Wiley & Sons, Ltd.     

Dispersive optical constants of a-Se100−xSbx films

Thin films of amorphous Se_100−xSb_x (x=5,10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (E_o), dispersion energy (E_d) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300–2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, E_g^opt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.     

Reactive scattering of alkali dimers: chemi-ionization

Measurements of total cross sections Q_i for chemi-ionization in scattering of a K₂ dimer beam by a range of halogen containing molecules, at translational energies E ～ 7–11 kcal mol^-1, are reported. Substantial cross sections, Q_i ～ 2–10 Ų are exhibited by the halogen molecules Br₂, ICl, IBr, I₂. Distinctly lower values Q_i ～ 0·1–0.2 Ų are exhibited by BrCN and the mercuric and stannic halides, HgX₂, SnX₄. The results show a close correlation with the chemi-ionization exoergicities, particularly for formation of a K⁺,X^- ion pair. These chemi-ionization data and results from previous reactive scattering studies are compared, in order to estimate relative reaction yields for different reaction paths. The reaction dynamics for K₂ with halogen molecules and cyanohalides are rationalized in terms of the electronic structure of the potential surface, where the orientation of the K₂ dimer plays a crucial role.     

Signature dependence of 1/2+ bands in rare-earth nuclei: Rotation-alignment of g7/2 protons

The origin of the signature splitting of theK=1/2⁺ bands in odd Z rare-earth nuclei Ho, Tm and Lu, unexplained in previous models, is studied by angular momentum projection from deformed Hartree-Fock configurations. Energy spectra,E2 matrix elements and the rotation-aligned angular momenta of nucleons in i_{1 3/2}, h_{1 1/2} and g_7/2 orbits are calculated. Staggering in the rotation-alignment of protons in the g_7/2 orbit causes signature dependent effects in the 1/2⁺ bands.