The effect of heating rate on the melting of polytetrafluoroethylene

When virgin polytetrafluoroethylene is heated at intermediate rates, two melting peaks are observed. As the heating rate is increased, the higher-temperature peak grows at the expense of the lower-temperature one without any significant change in the total heat of fusion. It is suggested that the higher-temperature peak represents a transition to a strained melt which subsequently changes to the more stable equilibrium melt. After recrystallization from the melt, there is only a single melting peak which occurs at a lower temperature than peaks for the virgin polymer. All of these transitions are subject to superheating.     

Effect of carbon nanostructures on the carbonization of polyacrylonitrile

Composite materials based on polyacrylonitrile with carbon nanofillers (technical-grade carbon, thermally expanded graphite, carbon nanotubes) were synthesized. A carbonization of film and fiber composite samples in the temperature range 20–1000°C provided a noticeable increase in the thermal stability of fibers and a rise in the electrical conductivity of the composite material. Dependences of the degree of carbonization on the concentration of nanostructures, type of material, and nature of modifier were determined. Differential-thermal and X-ray diffraction analyses revealed the formation of oriented nucleus structures of turbostratic carbon in the temperature range 450–550°C.     

Some demonstrations of the effect of the heating rate on thermoanalytical curves

Thermoanalytical curves of two parallel competitive reaction systems and of two systems in which two mutually independent reactions occur concurrently are made by simulation, as well as isothermal curves of the systems. From these curves, an effect of heating rate is illustratively shown; one peak is observed in a derivative thermoanalytical curve at one heating rate, while a shoulder or two peaks appear in another derivative curve of the same reaction system at another heating rate. An advantage of thermal analysis over isothermal experiments, and its limitations, are discussed.     

The determination of trace elements in active charcoal by INAA

Neutron activation followed by X-ray spectrometry with magnetic deflection of -rays was applied to the nondestructive determination of Co, Cu and Br in active charcoal.     

The effect of the simultaneous variation of sample mass and heating rate in DTA and DSC

Quantitative correlations between kinetic parameters (energy of activation, E), procedural factors (sample mass, m, and heating rate, φ) and the dependent variables (the temperature of inception of reaction, T_i, and temperature T_α at which a constant fraction α has decomposed) have been derived for differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The relations have the same form as those derived from earlier TG studies. The dependent variables T_i and T_α are related to m, φ and E by the equations

where C₃ and C₂ are constants, and

where C₄ and C₅ are constants for a given α. The variation of C₅ with α is given by C₅ = a + b α where a and b are constants. The equations are applicable to TG, DTA and DSC.     

The characterization of charcoal and high-density carbon pellets produced from Douglas-fir bark

High-density carbon pellets (HDCP) produced from Douglas-fir bark could provide an alternate source of carbon for industry. The production of HDCP in vertical retorts is discussed. Scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), carbon elemental analysis, and other analytical methods were used to characterize HDCP. HDCP produced from Douglas-fir bark in this work have 90% fixed carbon, an average density of 1.3 g/mL, and 1.18% average ash content with negligible metal impurities. These pellets should be particularly suited for use in the production of adsorbents, high-grade carbons, reductants, carbon black, carbon electrodes, and activated carbons.

     

The heating rate dependence of polymer melting points

The melting points (T_m) of small quantities (ca. 0.1 mg) of polyethylene (PE) single crystals in the dried state and in suspension were determined as a function of heating rate. Melting points were also obtained for large amounts (ca. 3 mg) of dried-down crystals. It has been reported previously that PE crystals of high molecular weight possess a single melting point and this T_m decreases with increasing heating rate. However, all samples in our study show multiple melting behavior. Moreover, suspension samples show little or no variation in the various peak temperatures with heating rate. There is some dependence of T_m on heating rate for small dry samples but the change in T_m is not nearly as great as previously reported. These small changes can be the result of the relatively poor resolution of small dry samples compared to suspension. Differences between the behavior reported here and those of other authors can be explained on the basis of the relative contributions to the various melting endotherms.     

One-step synthesis of novel biacidic carbon via hydrothermal carbonization

The novel biacidic carbon has been synthesized via one-step hydrothermal carbonization of glucose, citric acid, and hydroxyethylsulfonic acid at 180 °C for only 4 h. The novel carbon had an acidity of 1.7 mmol/g with the carbonyl to sulfonic acid groups molar ratio of 1:3, which was confirmed by IR, XPS, TPD, SEM, and BET analyses. The catalytic activities of the carbon were investigated through esterification and oxathioketalization. The results showed that the carbon owned the comparable activities to sulfuric acid, which indicated that the carbon holds great potential for the green processes.     

The effect of heating rate upon the coupling of complex reactions. I. Independent and competitive reactions

Theoretical curves of the rate of conversion vs. temperature at constant heating rate for first-order reactions with activation energies of 80, 160, 240 and 320 kJ mole^?1 are compared over a range of heating rates from 10^?9 to 10⁵ K s^?1 for independent and competitive reactions. Independent reactions with different activation energies may be separated from one another by either increasing or decreasing the rate of heating. The spectrum of derivative peaks for two competing reactions at various heating rates has a dispersion effect in the region of change from low to high activation energy reactions. The practical range of heating rates in thermal analytical experiments and the application of these model cases to the understanding of the kinetics of complex systems at high and low temperatures are discussed.     

Interactive effects of pore size control and carbonization temperatures on supercapacitive behaviors of porous carbon/carbon nanotube composites

Porous carbon-based electrodes were prepared by carbonization with poly(vinylidene fluoride) (PVDF)/carbon nanotube (CNT) composites to further increase the specific capacitance for supercapacitors. The specific capacitance, pore size distribution, and surface area of the PVDF/CNT composites were measured, and the effect of the carbonization temperatures was examined. The electrochemical properties were examined by cyclic voltammetry, impedance spectroscopy, and galvanostatic charge-discharge performance using a two-electrode system in TEABF(4) (tetraethylammonium tetrafluoroborate)/acetonitrile as a non-aqueous electrolyte. The highest specific capacitance of ～101 Fg(-1) was obtained for the samples carbonized at 600 °C. The pore size of the samples could be controlled to below 7 nm through the carbonization process. This suggests that micropores make a significant contribution to the specific capacitance due to improved charge transfer between the pores of the electrode materials and the electrolyte.     

Cyclization and carbonization of anionic polyacrylonitrile in the presence of carbon nanofibers

Composites based on anionic polyacrylonitrile and carbon nanofibers were studied by X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis, and electron microscopy. The effect of carbon nanofibers on the specific features of cyclization and carbonization of anionic polyacrylonitrile at temperatures of to 550°C was studied.     

The effect of fractal dimensionality and dispersion of catalysts on the rate of heterogeneous catalytic reactions involving carbon oxides

Analytical relationships describing the relation between the morphology of a heterogeneous catalyst and the rate constant of the controlling stage of a reaction taking place on its surface were obtained. It was shown that the structure of the catalyst surface, expressed in terms of fractal dimensionality, affects both the activation energy and the preexponential coefficient of the rate constant. The equations relating the rate constant of the catalytic reaction to the fractal dimensionality were used for both bulk and deposited catalysts. It was shown that the activation energy of the process increases with increase in the nonuniformity of the catalyst surface, characterized by the fractal dimensionality DF. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 5, pp. 267–274, September–October, 2007.     

Catalytic degradation of polystyrene in the presence of active charcoal

To obtain useful products from polystyrene degradation waste, the catalytic degradation of polystyrene over an active charcoal catalyst was carried out. By controlling the reaction conditions, the selective recovery of styrene dimer derivatives, a promising sourse of useful industrial additives, was investigated. Cis- and trans-l,3-diphenyl-2-butene. 1,3-diphenyl-1-butene and 1,3-diphenylbutane were detected as the main products. The yield of styrene dimer derivatives was about 15 wt.% of the total liquid products recovered. Also. selective recovery of the styrene dimer derivatives by the catalytic reaction of polystyrene in the presence of a catalyst was possible by controlling the contact time and activity of the catalyst.     

The effect of sample mass and heating rate on DSC results when studying the fractional composition and oxidative stability of lube base oils

The method of dynamic microdistillation instrumented through DSC has been further elaborated and validated using distillate and residual base oils as model systems. The two major experimental factors of the method—sample mass and heating rate were varied to determine the optimal (standard) experimental conditions for better fraction resolution and thus more reliable quality assessment of petroleum products (lube oils). If these are increased, the fraction resolution ability of the method is reduced—the lighter fractions evaporate at temperatures close to those of the heavier ones, and generally all the fractions evaporate/oxidize at higher temperatures. Two major types of reactions in the heated sample were identified—the one occurring on the metal surface of the crucible leading to a polymer (lacquer) film formation and the other the oxidative cracking in the bulk of the sample leading to the formation of gaseous products. The extent of the lacquer film formation on the crucible metal surface can be reduced by increasing the heating rate and/or the sample mass, while their reduction results in better fraction resolution.     

炭化温度对模板法合成介孔炭性能的影响

以蔗糖为炭源和硅酸钠为硅源,采用原位共聚法制备了炭/氧化硅复合体,除去氧化硅得到介孔炭材料.采用N2吸附-脱附、透射电子显微镜和红外光谱对不同炭化温度获得的样品进行表征.结果表明,随着炭化温度升高,所得的介孔炭比表面积和孔容均下降,650℃ ～ 950℃的炭化温度下获得的样品BET比表面积在586m2/g ～728 m2/g之间,孔容在0.549cm3/g～0.696cm3/g之间,孔径分布5-15nm之间.经红外光谱检测所得的介孔炭样品均含有氢和氧的功能团.     

Mesoporous graphitic carbon nanodisks fabricated via catalytic carbonization of coordination polymers

Mesoporous graphitic carbon nanodisks with hierarchical porous structure, facilely fabricated by catalytic carbonization of iron-based coordination polymer nanodisks, exhibit high capacitance even at high scan rates as electrode materials for electrochemical double layer capacitors.     

Structural effect of redox active sites on electron hopping rate in mixed conducting copolymers

The copolymers consisting of a redox monomer, vinylferrocene or 10-methyl-3-vinylphenothiazine, and an ion-conducting monomer, ω-methoxy-nona(ethylene oxide)-methacrylate, have been prepared. The copolymers, complexed with lithium perchlorate, exhibit reversible redox activity in the absence of low molecular solvents. The charge transport during the redox reactions is carried out by electron hopping reactions between the redox active centers, indicating that the copolymers can be regarded as intrinsic ion/electron mixed conductors. The electron hopping rates during the redox reactions have been evaluated by using an electrochemical technique under diffusion-controlled conditions. The electron diffusion coefficients in the vinylferrocene and 10-methyl-3-vinylphenothiazine copolymers are 6.8 × 10^?12 cm² sec^?1 and 3.2 × 10^?11 cm² sec^?1 at room temperature, respectively, and are larger in the latter copolymer. The electron hopping rate would be influenced by the slow and temperature-dependent dynamics of the polymer segment, judging from the relatively low electron diffusivity and from its quite large and non-Arrhenius temperature dependency.