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.     

Thermogravimetric study of alcohol adsorption on active charcoal

Interest of using the Mac Bain thermogravimetric balance in order to characterize alcohol adsorption or desorption on various active charcoal is developed. Precautions and peculiarities of proposed method are indicated. They result from very good precision and reproducibility in the thermal range of 273.15 to 313.5 K, and in the pressure range of 666.6 Nm⁻² to 26664 Nm⁻².Some results are given to illustrate the statics, and kinetics of adsorption desorption process of alcohol on various active charcoals.These results are important to develop and optimize refrigerating machines, or heat pumps, or physico-chemical storage using these systems.     

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.     

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.     

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 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.     

Production and characterization of beaded nanofibers from current heating of charcoal

A simple and low-cost method of current heating of charcoal was developed to fabricate nanofibers for being used as reinforcements in composite materials. Based on using an arc-discharge apparatus, a common stationery charcoal rod as the source material was heated by moderate current to produce masses of fibers. Morphology, particular structure and chemical compositions of the as-grown fibers were characterized using scanning electron microscopy, energy dispersive analysis of X-rays and X-ray diffraction. The results show that the products are beaded nanofibers with coexistent carbon nanotube and silicon carbide phases in the fibers and stoichiometric alumina and oxides of other trace elements of charcoal additionally in the beads.     

The effect of active carbon on the reduction of concentrated nitric acid by HCOOH

In nuclear industry, removal of nitric acid from solutions is required in the course of chemical separation and waste treatment procedure as well as in chemical conversion steps. The reduction of HNO3 by HCOOH to gaseous products such as nitrogen, nitrogen oxides, and carbon dioxide is an attractive way to accomplish the denitration. A typical problem for the denitration is the existence of the induction period. The induction period has been explained as the time necessary to increase the concentration of HNO2, which is an important reaction intermediate, above a threshold value. In this study, adsorption sites on the surface of active carbon were found to promote HNO2 formation and efficiently suppress the induction period. Induction time was shortened by increasing the amount of active carbon in the solution. When the solution contains 3 M HNO3 and 1 M HCOOH, 10 g/L of active carbon was enough to eliminate the induction period at 50 degrees C. The catalytic effect exhibited by active carbon was similar to that reported for Pt/SiO2. Therefore, on the surface of active carbon, there is a redox cycle, where HNO3 is reduced to HNO2 and then the oxidized surface site will be reduced by HCOOH.     

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.     

升温速率对生物质热解的影响

稻壳、稻秆及麦秆是中国主要的农业废弃物,如何综合、有效地利用这些农业废弃物进行资源化研究显得十分必要。热解是热化学转化中最为基本的过程,是气化、液化及燃烧过程的初始和伴生反应,对热解的分析有助于热化学转化过程控制及高效转化工艺的开发。目前,国内外对生物质及其组分的热解已有大量的研究,但对中国主要的农业废弃物稻壳、稻秆及麦秆的研究较少。本研究利用热重和红外联用技术深入研究了升温速率对三种典型生物质热解气体产物的影响,并对生物质的热解动力学及热解气体产物的析出规律进行实时在线分析。     

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.     

The use of charcoal and carbon cloth for the poison-protection of catalytic gas sensors

The properties of charcoal granules and granules and carbon cloths when used as poison filters in catalytic gas sensors are compared. Some forms of carbon cloth offer protection against poisoning similar to an equivalent weight of charcoal, while having less effect on response time to methane. However, the performance of the cloths is shown to be very variable, and some cloths themselves poison the sensing elements.     

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.     

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.