Effect of electron beam irradiation on the structural,thermal and optical properties of poly(vinyl alcohol) thin film

Poly(vinyl alcohol) (PVA) polymer was prepared using the casting technique. The obtained PVA thin films have been irradiated with electron beam doses ranging from 20 to 300 kGy. The resultant effect of electron beam irradiation on the structural properties of PVA has been investigated using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), while the thermal properties have been investigated using thermo-gravimetric analysis and differential thermal analysis (DTA). The onset temperature of decomposition T ₀ and activation energy of thermal decomposition E _a were calculated, results indicate that the PVA thin film decomposes in one main weight loss stage. Also, the electron beam irradiation in dose range 95–210 kGy led to a more compact structure of the PVA polymer, which resulted in an improvement in its thermal stability with an increase in the activation energy of thermal decomposition. The variation of transition temperatures with electron beam dose has been determined using DTA. The PVA thermograms were characterized by the appearance of an endothermic peak due to melting. In addition, the transmission of the PVA samples and any color changes were studied. The color intensity Δ E was greatly increased with increasing electron beam dose, and was accompanied by a significant increase in the blue color component.     

Studies of mass and energy correlations in thermal neutron fission of U-235 accompanied by long range alpha particles

Several characteristics of fission accompanied by long range alpha particles (LRA) have been studied in the thermal neutron induced fission of²³⁵U. The kinetic energies of fission fragments and the LRA were measured with a back-to-back ionization chamber and semiconductor detectors respectively. The kinetic energies of the two fragments and the LRA in LRA fission, along with the energies of pair fragments in the normal binary fissions, were recorded event by event on a magnetic tape by means of a four-parameter data acquisition system. The data were analysed to study the dependence of different quantities in LRA fission on the fragment mass ratio, LRA energy and the total kinetic energy of the fission fragments. It is seen that the most probable energy of LRA increases significantly for near symmetric mass divisions. The total kinetic energy for all mass ratios in LRA fission is found to be (2.6±0.7) MeV larger than that in binary fission. The difference in the total kinetic energies in LRA and binary fissions is seen to be dependent on mass ratio. This result may suggest that the scission configuration in LRA fission is different for different mass ratios. Correlations between the fission fragment and LRA energies have been studied for several mass ratios. It is seen that the most probable fragment kinetic energyĒ _k varies nearly linearly with the LRA energyE _a for various mass divisions but the variation of the most probable LRA energyĒ _a with fragment kinetic energyE _k is found to deviate from linearity for several mass ratios. From a least square fit to the variation ofĒ _k withE _a it is found that the slope (dĒ _k/dE_a) increases with the increase in mass ratio. The present results are discussed to arrive at a better understanding of the scission configuration in the fission accompanied by LRA emission.     

Kinetic analysis of thermal and thermo-oxidative degradation of polyethylene (nano)composites

Thermal properties and degradation of polyethylene LDPE (nano)composites were investigated by isoconversional thermogravimetric analysis in air and nitrogen atmosphere by applying the Kissinger–Akahira–Sunose method. Low-density polyethylene (LDPE) composites containing 3 wt.% nanofiller Cloisite 20A and 4, 6, and 8 wt.% of natural zeolite were prepared using extrusion/injection moulding. The parameters of thermal stability of the samples were determined i.e. onset temperature of the degradation (T₉₀), which exhibit initial mass loss (10 mass %) and maximum loss rate temperature (T_max). Also, activation energy (E_a) of samples was calculated and interpreted in terms of thermal degradation mechanisms. Under nitrogen, the thermal degradation of LDPE (nano)composites follows a random scission pathway but it was retarded and slowed by the presence of the fillers. The results show that thermo-oxidative degradation of studied (nano)composites is induced at lower temperatures and appears as much more complex and multi-stage process.     

Dynamic-mechanical and nuclear magnetic resonance study of relaxation processes in ultra-high molecular weight polyethylene fibres

The relaxation processes (α, β, and γ) in UHMW PE fibres drawn to different draw ratios λ have been studied by dynamic-mechanical and nuclear magnetic resonance methods. The temperature dependences of tensile loss moduli E″ and spin-lattice relaxation times T ₁ have been analyzed assuming distribution of correlation times τ according to the Davidson—Cole function. The activation energies E _a and parameters ε characterizing widths of distribution, and asymptotic value τ ₀ for correlation times have been determined from experimental data for low-temperature γ-process and for high-temperature α-process. A weak β-process has been found by dynamic-mechanical method and quantitative analysis was made only for fibres with λ = 9. The temperature dependences of second moment M ₂ of the broad-line NMR spectra have been analyzed according to the Gutowsky—Pake formula, which includes only a single correlation time τ _c without distribution. In this case the activation energies E _a and values of τ ₀ have been determined.     

Thermal behavior of gamma-irradiated low-density polyethylene/paraffin wax blend

The thermal properties of low-density polyethylene (LDPE)/paraffin wax blends were studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and melt flow index (MFI). Blends of LDPE/wax in ratios of 100/0, 98/2, 96/4, 94/6, 92/8, 90/10 and 85/15 (w/w) were prepared by melt-mixing at the temperature of 150°C. It was found that increasing the wax content more than 15% leads to phase separation. DSC results showed that for all blends both the melting temperature (T_m) and the melting enthalpy (ΔH_m) decrease linearly with an increase in wax content. TGA analysis showed that the thermal stability of all blends decreases linearly with increasing wax content. No clear correlation was observed between the melting point and thermal stability. Horowitz and Metzger method was used to determine the thermal activation energy (E_a). MFI increased exponentially by increasing the wax content. The effect of gamma irradiation on the thermal behavior of the blends was also investigated at different gamma irradiation doses. Significant correlations were found between the thermal parameters (T_m, ΔH_m, T₅%, E_a and MFI) and the amount of wax content and gamma irradiation.     

Thermal Degradation Behavior and Flame Retardancy of Polycarbonate Containing Poly[(phenylsilsesquioxane)-co-(dimethylsiloxane)] and Potassium Diphenyl Sulfonate

The synergistic effect of poly[(phenylsilsesquioxane)-co-(dimethylsiloxane)] (PPSQDS) and potassium diphenyl sulfonate (KSS) on the thermal degradation and flame retardancy of polycarbonate (PC) was studied. The flame retardancy of PC was improved by the combination of PPSQDS and KSS, and a V-0 rating for 1.6 mm thickness sample was successfully obtained. The thermal degradation of the flame retarded PC was characterized by thermogravimetric analysis (TGA) and the degradation activation energy (E _a) was calculated according to the Kissinger and Flynn-Wall-Ozawa (F-W-O) methods. The E _a value of PC was decreased by the combination of PPSQDS and KSS, indicating that the synergistic effect of PPSQDS and KSS facilitates the thermal degradation of PC and accelerates char formation.     

Study on the relationships between Raman shifts and temperature range for a-plane GaN using temperature-dependent Raman scattering

In this paper, Raman shifts of a-plane GaN layers grown on r-plane sapphire substrates by low-pressure metal-organic chemical vapor deposition (LPMOCVD) are investigated. We compare the crystal qualities and study the relationships between Raman shift and temperature for conventional a-plane GaN epilayer and insertion AlN/AlGaN superlattice layers for a-plane GaN epilayer using temperature-dependent Raman scattering in a temperature range from 83 K to 503 K. The temperature-dependences of GaN phonon modes (A₁ (TO), E₂ (high), and E₁ (TO)) and the linewidths of E₂ (high) phonon peak are studied. The results indicate that there exist two mechanisms between phonon peaks in the whole temperature range, and the relationship can be fitted to the pseudo-Voigt function. From analytic results we find a critical temperature existing in the relationship, which can characterize the anharmonic effects of a-plane GaN in different temperature ranges. In the range of higher temperature, the relationship exhibits an approximately linear behavior, which is consistent with the analyzed results theoretically.     

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.     

Energy expression of the chemical bond between atoms in metal oxides

The chemical bond between atoms in metal oxides is expressed in an energy scale. Total energy is partitioned into the atomic energy densities of constituent elements in the metal oxide, using energy density analysis. The atomization energies, ΔE_M for metal atom and ΔE_O for O atom, are then evaluated by subtracting the atomic energy densities from the energy of the isolated neutral atom, M and O, respectively. In this study, a ΔE_O vs. ΔE_M diagram called atomization energy diagram is first proposed and used for the understanding of the nature of chemical bond in various metal oxides. Both ΔE_M and ΔE_O values reflect the average structure as well as the local structure. For example their values vary depending on the vertex, edge or face sharing of MO₆ octahedron, and also change with the overall density of binary metal oxides. For perovskite-type oxides it is shown that the ΔE_O value tends to increase by the phase transition from cubic to tetragonal phase, regardless of the tilting-type or the 〈1 0 0〉 displacement-type transition. The bond formation in spinel-type oxides is also understood with the aid of the atomization energies. The present approach based on the atomization energy concept will provide us a new clue to the design of metal oxides.     

NMR relaxation study of disorder in condensed matter – low temperature studies in the mixed system − BPI(1−x)BP x – transition from classical to quantum rotation

¹H NMR spin-lattice relaxation time (T₁ ) studies have been carried out in the mixed system, betaine phosphite–betaine phosphate (BPI_(1?x)BP _x ), to study the effects of disorder. The T₁ data in the temperature range 100 K to 4 K, at two Larmor frequencies 11.4 and 23.3 MHz, has been analysed following Lourens’ model, which shows a gradual transition from classical reorientations to quantum tunnelling motions. At lower temperatures, (when thermal motions become too slow) differences (due to disorder) in the local environments of the reorienting groups result in a distribution in the activation energy (E_a ) and the energy gap of the ground to the first excited torsional level (E₀₁ ). Below 50 K, the system moves into the quantum tunneling regime and the magnetisation recovery shows biexponential behaviour which is another signature of disorder. These results are compared with those of the parent compounds BP and BPI.     

Density functional study of the reaction of O2 with a single site on the zigzag edge of graphene

The reaction between molecular oxygen and an isolated zigzag graphene edge has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. The initial reaction forms a peroxide, ΔH = −135 kJ mol⁻¹. If the graphene edge is pre-oxidised, the dangling peroxy atom can (E_a = 91 kJ mol⁻¹) migrate across contiguous ketone groups until finding another vacant site and stabilizing as a ketone. However, if no further vacant sites are available, the peroxy oxygen has a number of other possibilities open to it, including desorption of an oxygen atom (E_a = 140 kJ mol⁻¹), migration via the basal plane to form a lactone (E_a = 147 kJ mol⁻¹), and direct interaction with an adjacent oxide to form the lactone or a carbonate (E_a = 146 kJ mol⁻¹). The combination of thermal energy and the heat released in the initial formation of the peroxy adduct is likely to be sufficient to overcome these secondary barriers at modest temperatures.Transfer of the dangling peroxy O to the basal plane produces an epoxide that is mobile on the basal surface (E_a = 40–80 kJ mol⁻¹) but that is transferred back to the edge upon coming into proximity of either a vacant edge site or ketone. The instability of the edge epoxide structure implies that it cannot play a significant role in carbon gasification through promoting the reactivity of ketones, contrary to earlier suggestions.The desorption of an oxygen atom creates a very active species capable of reacting with basal or edge sites as well as with oxygen complexes. The reaction of ketone + O has been reported to yield a five-membered ring + CO₂, leading to an overall stoichiometry which is consistent with the observed oxyreactivity of carbon surface oxides identified in isotopic labelling studies in which one O atom is gasified and the other forms a new surface oxide.     

Computational analysis of factors influencing thermal conductivity of nanofluids

Numerical investigations are conducted to study the effect of factors such as particle clustering and interfacial layer thickness on thermal conductivity of nanofluids. Based on this, parameters including Kapitza radius and fractal and chemical dimension which have received little attention by previous research are rigorously investigated. The degree of thermal enhancement is analyzed for increasing aggregate size, particle concentration, interfacial thermal resistance, and fractal and chemical dimensions. This analysis is conducted for water-based nanofluids of Alumina (Al₂O₃), CuO, and Titania (TiO₂) nanoparticles where the particle concentrations are varied up to 4 vol%. Results from the numerical work are validated using available experimental data. For the case of aggregate size, particle concentration, and interfacial thermal resistance, the aspect ratio (ratio of radius of gyration of aggregate to radius of primary particle, R _g/a) is varied from 2 to 60. It was found that the enhancement decreases with interfacial layer thickness. Also the rate of decrease is more significant after a given aggregate size. For a given interfacial resistance, the enhancement is mostly sensitive to R _g/a < 20 indicated by the steep gradients of data plots. Predicted and experimental data for thermal conductivity enhancement are in good agreement. On the influence of fractal and chemical dimensions (d _l and d _f) of Alumina–water nanofluid, the R _g/a was varied from 2 to 8, d _l from 1.2 to 1.8, and d _f from 1.75 to 2.5. For a given concentration, the enhancement increased with the reduction of d _l or d _f. It appears a distinctive sensitivity of the enhancement to d _f, in particular, in the range 2–2.25, for all values of R _g/a. However, the sensitivity of d _l was largely depended on the value of R _g/a. The information gathered from this study on the sensitivity of thermal conductivity enhancement to aggregate size, particle concentration, interfacial resistance, and fractal and chemical dimensions will be useful in manufacturing highly thermally conductive nanofluids. Further research on the refine cluster evolution dynamics as a function of particle-scale properties is underway.     

Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

Proton-induced changes in Bayfol CR 1-4 nuclear track detector

Samples from the polymeric material Bayfol CR 1-4 have been exposed to 1 MeV protons with fluencies in the range 10¹¹–10¹⁴ ions/cm ². The resultant effect of proton irradiation on the thermal properties of Bayfol CR 1-4 has been investigated using thermo-gravimetric analysis (TGA). The onset temperature of decomposition T ₀ and activation energy of thermal decomposition E _a were calculated, results indicated that the Bayfol detector decomposes in one weight loss stage. In addition, the structural modifications in the proton-irradiated Bayfol samples have been studied as a function of fluence using X-ray diffraction and intrinsic viscosity of the liquid samples. Furthermore, the refractive index was measured for the non-irradiated and irradiated Bayfol samples. The results indicated that the degradation is the dominant mechanism in the fluence range 1×10¹¹–5×10¹⁴ ions/cm ². These results have been compared with those obtained in our previous work for Bayfol CR 6-2.     

X-ray diffraction investigations of the crystallographic parameters and thermal expansion of β-BaB2O4 crystals

The lattice parameters a and c of β-BaB₂O₄ crystals have been measured in the temperature range 80–300 K by the x-ray diffraction method. The thermal expansion coefficients α are calculated from the measured values of the parameters. A substantial anisotropy of the thermal expansion is found. It is shown that the thermal expansion coefficient α _c along the c axis is an order of magnitude greater than the thermal expansion coefficient α _a in a plane perpendicular to this axis. It is established that α _a becomes negative in the temperature range 80–190 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1038–1040 (June 1997)     

DSC, TGA and dielectric properties of carboxymethyl cellulose/polyvinyl alcohol blends

Films with different compositions of polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC) blends have been prepared using the casting method. Differential scanning calorimetery (DSC), thermogravimetric analysis (TGA) and dielectric spectroscopy of all compositions have been investigated. It was found that PVA and CMC are compatible in the studied range of composition. With increasing CMC content, the thermal stability of PVA increases. Based on DSC and TGA data, the activation energies of all the investigated samples were calculated. The absorption edge (E_a) was also determined from Ultraviolet-visible (UV-vis) spectra.Dielectric permittivity, loss tangent and ac conductivity of all samples were studied as functions of temperature and frequency. The results show that the dielectric dispersion consists of both dipolar and interfacial polarization. The frequency dependence of the ac conductivity indicates that the correlated barrier hopping (CBH) is the most suitable mechanism for conduction. The polaron binding energy (U_M) was determined. Results of the present system are compared with those of similar materials.     

Characteristics of thin gate dielectric in a rapid thermal processing machine and temperature uniformity studies

Rapid Thermal oxidation (RTO) of silicon has been investigated in the temperature range 1000° to 1250°C for an oxidation time of 5 to 60 s. The fairly extensive kinetics data show that linear growth occurs with an activation energyE _a of 1.4 eV. Rapid thermal nitridation of SiO₂ (96 Å) has been performed at three different temperatures: 1150°, 1200°, and 1250°C for a nitridation time up to 150 s. The characteristics of both materials have been investigated by capacitance-voltage, current-voltage, high resolution transmission electron microscopy and Auger spectroscopy. The results will be discussed with special emphasis on breakdown field statistics. The influence of Rapid Thermal Annealing (RTA) on the characteristics of both oxide and nitrided oxide will also be presented. A simulation model of a rapid thermal processing machine is presented with particular attention to the formation of slip lines. The theoretical results are in good agreement with those obtained experimentally.     

B（N）掺杂单壁碳纳米管的Al原子吸附性能的第一性原理研究

采用基于密度泛函理论的平面波赝势方法和广义梯度近似,对未掺杂、掺B、掺N的碳纳米管(CNT)不同位置上Al原子的吸附进行了几何优化,计算了吸附Al、掺杂前后CNT的能带结构、态密度、差分电荷密度、电荷布居数和吸附能.计算结果表明,掺B使CNT形成缺电子状态,利于具有自由电子的Al原子的吸附结合,可显著提高Al在金属性的(5,5)CNT和半导性的(8,0)CNT外壁的吸附能;掺杂N形成多电子状态,在费米能级附近半满的施主能级也利于填充Al的价电子,改善Al在(5,5)CNT和(8,0)CNT外壁的吸附结合性 关键词： 密度泛函理论 单壁碳纳米管 B(N)掺杂 Al原子吸附     

Effect of the deformation of the double bond in chlorinated ethylene on the rate and mechanism of the reaction with ozone

The reactivity of the strained C=C bond of the 1-chloroethylene molecule in the reaction with ozone was studied using ab initio (MP2, CASSCT, MRMP2) and DFT (B3LYP) calculations in conjunction with the 6–31+G** basis set. The mechanisms of concerted and nonconcerted addition were examined. The strain ɛ was introduced into the problem by changing the length of the C=C bond and specifying it as a non-optimized coordinate. It is shown that, at least at ɛ ≤ 2%, the activation energy E _a decreases linearly with increasing strain. The sensitivities of both channels of the reaction to the strain are similar and only slightly dependent on the method of calculation. The results are analyzed within the framework of a previously developed approach, which makes it possible to obtain an analytical dependence of E _a on the force and to relate the change in E _a during deformation to the length and rigidity of the initial and transition states.