Constant Temperature Cycles Research Articles

Low and intermediate temperatures fracture behavior of amorphous poly alpha olefin (APAO)-modified hot mix asphalt subjected to constant and variable temperatures

Low-temperature cracks (LTC) and intermediate-temperature cracks (ITC) are two important types of cracks in asphalt mixtures that cause damage to pavements and incur high maintenance costs on roads. On the other hand, temperature cycles, in addition to the traffic load on the asphalt mixtures, increase the possibility of structural failures. Therefore, it is necessary to study the solutions to deal with and control this type of cracking using appropriate methods. In this study, an attempt was made to evaluate the effect of amorphous poly alpha olefin (APAO) additive on the LTC and ITC behavior of Hot Mix Aspahlt (HMA) under loading conditions of modes I and II under constant temperature (CT) and variable temperature (VT) cycles. For this purpose, two geometries called symmetric specimen SCB (contains a vertical crack with an angle of 90°) and classical-modified specimen SCB-2 (contains an angled crack with an angle of 35°) were selected to simulate the loading conditions of mode I, as well as two geometries called modified specimen SCB-1 (contains a vertical crack with a 90-degree angle) and modified specimen SCB-2 (contains a vertical crack with a 90-degree angle) were selected to simulate mode II loading conditions. A total of four HMA mixtures were made, including one mixture without additives and three mixtures containing 3, 6, and 9 % APAO additives. Next, in order to simulate and match the road conditions with the laboratory samples, CT and VT conditions were applied to all samples. At temperatures of ± 15 °C, two fracture resistance indices called fracture toughness (KIC and KIIC) and fracture energy (GF) were studied to investigate the fracture behavior of HMA mixtures. Also, two other fracture indices, namely the Tensile Stiffness Index (TSI) and the Tensile Strength (TS), which represent the fracture behavior of the HMA mixture before the peak load, were evaluated. Finally, the flexibility of the mixtures was evaluated using brittleness indices such as the Flexibility Index (FI), Cracking Resistance Index (CRI), and Toughness Index (TI). The results showed that the fracture toughness and fracture energy of the HMA mixture containing 9 and 6 % APAO increased in mode I and II loading conditions under both CT and VT cycles compared to the base mixture. At temperatures of ± 15 °C, from the point of view of the fracture toughness index, the potential for creating angular cracks, and from the point of view of the fracture energy index, the full fracture potential of the mixture containing vertical cracks was higher (under CT and VT cycles). On the other hand, the reduction of the flexibility of the mixtures containing APAO percentages was concluded using FI, CRI, and TI indices. Also, the results showed that the addition of 3, 6, and 9 % of APAO additive to the HMA mixture increased the maximum preload indices, namely TSI and TS, and as a result, increased resistance to elastic deformation.

Preparation of a new Ti catalyst for improved performance of NaAlH4

Three effective Ti catalysts for NaAlH4 were made by stoichiometrically reacting TiCl3 with LiAlH4 in tetrahydrofuran (THF), NaAlH4 in THF, and LiAlH4 in diethyl ether (Et2O). The solid products produced after drying were named ex situ catalysts and designated respectively as Ti(Li)T, Ti(Na)T and Ti(Li)E. NaAlH4 was dry doped with 2 mol% of these ex situ catalysts, and for comparison, NaAlH4 was conventionally wet doped with 2 mol% TiCl3 in THF that made in situ catalyst (designated as TiCl3). All four doped samples were dry ball milled, and hydrogenation and dehydrogenation studies were carried out over five cycles. Temperature programmed desorption, constant temperature desorption, and constant temperature cycling curves showed that the effectiveness of these catalysts decreased as Ti(Li)T > Ti(Na)T > TiCl3 > Ti(Li)E. Ti(Li)T ex situ catalyst, being the best Ti catalyst, markedly decreased the dehydrogenation temperature, improved both the hydrogenation and dehydrogenation kinetics with sustained rates over cycling, and exhibited the least loss of hydrogen storage capacity over cycling. Ti(Li)T ex situ catalyst exhibited properties commensurate with some of the best NaAlH4 catalysts to date, such as CeCl3, ScCl3 and Ti nanocluster. It is easy to make, readily available and relatively inexpensive.

A novel integrated temperature investigation approach of sprayed coatings during APS process

Coating temperature is important for the coating reliability and reproducibility during the process of atmospheric plasma spray. A novel measurement and control approach for substrate and coating temperatures is presented. It is based on infrared (IR) pyrometry combined to specific robot spray trajectories in order to avoid harsh environment of spray workshops. The temperature evolution is continuously detected and recorded during preheating, spraying and cooling stages. The two specific factors, periodic average temperature and standard deviation were adopted to evaluate the temperature variation and the fluctuation of the thermal cycle relevant to one robot spray cycle based on the statistical method. These two factors are successful in describing the temperature variation during experimental processing sets. Finally, based on the monitoring system, the influence of Z-type robot spray trajectory parameters, including spray distance, scanning velocity and scanning step on coating temperature characterized by the two factors, is systemically investigated. Experimental results show that average temperature has no evident difference as a function of scanning velocity and no fixed relationship with scanning step, but just is dominated by the heating time of plasma jet and particle flux. Therefore, the selection of optimal scanning velocity just needs to take the temperature fluctuation into consideration. The temperature fluctuation decreases when scanning step increases, but both average temperature and fluctuation decrease with the increasing of spray distance and vice versa. Finally, on the basis of experimental results, a control experiment of sprayed coating temperature is presented to obtain constant temperature cycles by means of adjusting robot trajectories, cooling, among other considered operating parameters. Excellent control performance is observed.

Reversible hydrogen storage properties of NaAlH 4 catalyzed with scandium

A systematic study was carried out to compare the reversible hydrogen storage properties of Sc-doped NaAlH 4 with Ti-doped NaAlH 4. Uncycled and cycled (five times) samples of 1, 2 and 4 mol% Sc-doped NaAlH 4 and 4 mol% Ti-doped NaAlH 4 were examined. Temperature programmed desorption, constant temperature desorption, and constant temperature cycling results revealed the following: Sc-doped NaAlH 4 realized very fast hydrogenation kinetics, reasonably fast dehydrogenation kinetics and minimal hydrogen capacity losses during cycling over a broad range of temperatures without any additives or co-dopants. Sc-doped NaAlH 4 also exhibited comparable kinetics and a higher hydrogen storage capacity compared to Ti-doped NaAlH 4, depending on the temperature, metal dopant level, and cycling. Sc exhibited this superior performance over Ti perhaps because Sc is an earlier transition metal than Ti.

Increased cloning efficiency by temperature-cycle ligation.

Molecular cloning is a crucial, and often speed-limiting, step in many standard procedures of molecular biology. Ligation of cohesive DNA ends is normally carried out at 12–16 C to ensure a good balance between enzyme activity and stability of annealed DNA overhangs. Low temperatures generally reduce ligase activity, whereas too high temperatures may reduce cloning efficiencies by melting annealed DNA overhangs and increase overall molecular motion in the ligation reaction. Several procedures have been described to increase the efficiency ligation reactions, including the addition of condensing agents as polyethylene glycol (1) or hexamminedicobalt chloride (2). These approaches induce macromolecular crowding, and thus serve to mimic higher DNA concentrations in the ligation reaction. Other approaches seek to increase the efficiency of molecular cloning procedures by omitting the ligation step by generating long single-stranded DNA overhangs that can be annealed and transformed directly into an appropriate Escherichia coli host (3). Ligation of blunt-ended DNA fragments is normally carried out at room temperature using higher concentrations of T4 DNA ligase. We have devised a simple procedure in which high enzyme activity and DNA annealing is balanced by constant temperature cycling. We find temperature-cycle ligations (TCL) may increase the efficiency of staggered cut cloning ∼4–8-fold, while the efficiency of blunt-end clonings are increased ∼4–6-fold (see Table 1). The bacterial cloning vector pBluescript II KS+ was digested with AflIII and HindIII producing two cohesive end-fragments of 434 and 2526 bp respectively, or PvuII generating two blunt-end fragments of 448 and 2512 bp respectively. All enzymes were purchased from Amersham. Digested plasmid DNA was separated on 1% low-melting agarose (NuSieve) and purified as described by Sambrook et al. (1). For each digestion a ligation master mix was prepared containing 50 mM Tris–HCl (pH 7.6), 5 mM MgCl2, 5 mM DTT, 50 μg BSA/ml, 0.5 mM ATP, 200 ng DNA and 0.1 Weiss unit T4 DNA ligase/10 μl. The purified fragments were added in equimolar amounts. The master mix was divided into 10 μl ligation reactions and either incubated at 14 C, at room temperature (22 C), or subjected to temperature cycling. Temperature-cycle ligations were carried out for 12–16 h in a Perkin-Elmer-Cetus DNA Thermal Cycler 480 programmed indefinitely to cycle between 30 s at 10 C and 30 s at 30 C. In our setting, the machine performed 10 cycles/h. Competent XL1-Blue E.coli cells were transformed by electrotransformation according to manufacturer’s directions (BioRad), transferred to 1 ml LB medium and incubated at 37 C for 1 h. For enumeration of cloning efficiencies aliquots of 25–100 μl were spread onto LB plates containing 50 μg/ml ampicillin. The number of colonies appearing per transformation is shown in Table 1. We have tested the kinetics of temperature-cycle ligations over 12–16 h, and found them to be similar to those of ligations at constant temperatures resulting in a linear increase with time in the number of colonies appearing after transformation (data not shown). Furthermore, temperature-cycle ligation may be used together with hexamminedicobalt chloride in blunt-end ligations to give the combined effect.

Calculation of the Theoretical Efficiency of the Diesel Constant Temperature Cycle assuming that the Specific Heat is a Linear Function of the Temperature

Calculation of the Theoretical Efficiency of the Diesel Constant Temperature Cycle assuming that the Specific Heat is a Linear Function of the Temperature