Middle Temperature Region Research Articles

Exploring the framework of small pore zeolites for passive NOx adsorption

The use of a passive NOx adsorber (PNA) for NOx storage during the cold-start period is crucial for eliminating NOx emissions from the diesel exhaust. Palladium (Pd)-loaded zeolites have garnered significant attention owing to their high potential as PNA. In this study, small-pore zeolites with different frameworks were synthesized, and their structural characteristics and passive NOx adsorption abilities were investigated. Five zeolites, with CHA, AEI, AFX (low- and high- Si/Al ratio), and LEV framework structures, were synthesized by the hydrothermal conversion of faujasite-type (FAU) zeolite. Pd (1 wt%) was impregnated in combination with hydrothermal treatment. NOx adsorption and desorption behaviors were studied based on breakthrough NO adsorption and subsequent temperature-programmed desorption (TPD) experiments. Pd-loaded CHA, AEI, and LEV zeolites and high-silica AFX zeolites adsorbed approximately 93.88, 107.93, 97.57 and 106.53 μmol-NO g−1, respectively. On the other hand, the incorporation of Pd into AFX zeolite with a low Si/Al ratio led to significant hydrothermal damage and resulted in lower levels of adsorption and desorption capabilities. This demonstrates that hydrothermally stable small-pore zeolites on Pd-incorporation have great potential for passive NOx adsorption. The zeolite with different framework exhibited the different temperature range of NOx release. CHA, AEI, and high-silica AFX zeolites desorbed NOx in the low-temperature region (T < 250 °C), whereas the LEV zeolite showed a relatively high NOx desorption amount in the middle-temperature region (250 °C < T < 350 °C).

Nucleation and growth of graphene at different temperatures by plasma enhanced chemical vapor deposition

Temperature plays an important role in the synthesis of graphene by chemical vapor deposition. In the low-temperature plasma-enhanced chemical vapor deposition (PECVD) process, although the cleavage of hydrocarbon precursor no longer depends on growth temperature, which can affect the nucleation and growth of graphene by the diffusion and adsorption of active species, the catalytic ability of substrate and other factors, and ultimately lead to various growth of graphene. In this paper, the nucleation and growth of graphene at different temperatures by PECVD were studied. We noted that the nucleation and growth of graphene varied greatly in different temperature zones. Graphene could not be grown at low temperatures (< 600 ℃). At the medium and low temperature (650 ∼ 800 ℃), high nucleation density of graphene with nanometer size were produced. In the middle and high temperature region (850 ∼ 900 ℃), larger grains of graphene grown, and in the high temperature region (> 950 ℃), the coexistence of large and small grains was finally formed. In addition, the models of the nucleation and growth of graphene in different temperature zones were proposed.

Development of a reduced kinetic chemical mechanism of gasoline surrogate containing alcohols and pentenes

Based on the methods of DRGEP, FSSA, reaction path analysis, and sensitivity analysis, the detailed mechanisms of 1-pentene, 2-pentene, and 2-methyl-2-butene are comprehensively reduced and combined with the mechanisms of TRF, methanol, ethanol, and n-butanol. A reduced mechanism of multi-component is proposed to predict gasoline combustion in the engine. It is rare in previous study about mechanisms of surrogate fuels that used pentene to represent unsaturated hydrocarbons, which is the most abundant olefin in actual gasoline in China. JSR experiments were carried out on the mixed fuels of isooctane/methanol/n-butanol and isooctane/1-pentene with different equivalence ratios under the condition of 1.03 atm and temperature range from 500 K∼1000 K, and the component concentration curves were obtained. The accuracy of the multi-component reduced mechanism is verified by JSR experimental data. The results showed that this mechanism could well fit the concentration curve of reactants and final reaction products, and showed the same change trend for intermediate products. The accuracy of the pentene mechanism is verified by the experimental data of ignition delay time and laminar flame velocity. The prediction of the ignition delay time of pentenes is accurate in the middle and high temperature regions, and the prediction in the low temperature region is slightly lower than that of the detailed mechanism, but it can predict the trend of ignition delay time and reflect the low temperature reaction properties of them. The simulated values of 1-pentene can well fit the experimental data of laminar flame velocity under various working conditions, and the difference in laminar flame velocity of three isomerides is also well reflected in this mechanism. The ignition delay time of multi-component mixtures was verified under different pressures by using the mixed fuels of G-A, G-B, and G-C. The results show that this mechanism can predict the ignition delay time of G-A and G-B more accurately under pressure of 20 atm, but the predicted values are lower under 50 atm pressure. Under the pressure of 10 atm, 30 atm, and 50 atm, the ignition delay time of this mechanism can well fit the experimental data of G-C fuel, while the predicted value at low temperature is slightly higher.

Nascent structure memory erased in polymer stretching.

Stretching of semicrystalline polymer materials is fundamentally important in their mechanical performance and industrial processing. By means of dynamic Monte Carlo simulations, we compared the parallel stretching processes between the initially bulk amorphous and semicrystalline polymers at various temperatures. In the early stage of stretching, semicrystalline polymers perform local and global melting-recrystallization behaviors at low and high temperatures, while the memory effects occur upon global melting-recrystallization at middle temperatures. However, the final crystallinities, crystalline bond orientations, chain-folding probabilities, residual stresses, and crystallite morphologies at high enough strains appear as the same at each temperature, irrelevant to the initially amorphous and semicrystalline polymers, indicating that the common post-growth melting-reorganization processes determine the final products. In addition, both final products harvest the highest crystallinities in the middle temperature region because the postgrowth stage yields the vast nuclei followed with less extent of crystal growth in the low temperature region and few nuclei followed with large extent of crystal growth in the high temperature region. Our observations imply that a large enough strain can effectively remove the thermal history of polymers, similar to the thermal treatment at a high enough temperature; therefore, the fracture strength of semicrystalline polymers depends upon their final structures in stretching, not related to their nascent semicrystalline structures.

Flow condensation pressure drop characteristics of zeotropic mixtures of tetrafluoromethane/ethane: Experimental and analytical investigation

Mixed-refrigerant Joule-Thomson (MRJT) refrigeration systems have distinct advantages in the temperature range from 80 to 230 K. Tetrafluoromethane (R14) and ethane (R170) are important components to improve the recuperation process in the middle and low temperature regions of a MRJT refrigeration system. In this paper, an experimental investigation on condensation pressure drop characteristics of R14/R170 mixtures (0.193/0.807, 0.437/0.563, 0.632/0.368 and 0.799/0.201 by mole) in a horizontal smooth tube was carried out. Experiments were implemented at mass fluxes from 100 to 350 kg m−2 s−1, saturation pressures from 1.5 to 2.5 MPa and heat flux from 8.4 to 44.22 kW m−2 over the entire range of vapor quality. The different contributions of frictional pressure drop and deceleration pressure drop were calculated and compared. The effects of concentration, mass flux, saturation pressure, heat flux and vapor quality were analyzed and discussed. Finally, a new flow pattern-based pressure drop correlation was developed with a good predictive ability for the present experimental data with a MARD of 8.63%, and 14.20% for the literature data. The model considers the combined effects of the individual pressure drops of liquid and vapor as well as the frictional pressure drop due to the liquid-vapor interaction.

Experimental and analytical investigation on flow condensation heat transfer of zeotropic mixtures of tetrafluoromethane/ethane

Tetrafluoromethane (R14) and ethane (R170) are important components to improve the recuperation process in the middle and low temperature regions of a mixed-refrigerant Joule-Thomson (MRJT) refrigeration system. In this paper, an experimental investigation on flow condensation heat transfer characteristics of R14/R170 mixtures (0.193/0.807, 0.437/0.563, 0.632/0.368 and 0.799/0.201 by mole) in a horizontal smooth tube was carried out. Experiments were implemented at mass fluxes from 100 to 350 kg m−2 s−1 and saturation pressures from 1.5 to 2.5 MPa over the entire range of vapor quality. The effects of concentration, mass flux, saturation pressure and vapor quality were analyzed and discussed. Compared with linear interpolation values of pure fluids, heat transfer degradation of mixtures increases with the rise of vapor quality. In addition, all experimental data were compared with sixteen well-known correlations combined with equilibrium method. It is found that all models underestimate the current experimental data. This should be due to the neglect of the intensifying effect of vapour-liquid interface fluctuation. Finally, an improved heat transfer correlation for zeotropic mixtures was proposed and predicted the experimental results well with a mean absolute relative deviation of 13.91%.

Systematic evaluation of a series of pectic polysaccharides extracted from apple pomace by regulation of subcritical water conditions

To investigate the influences of different subcritical water conditions on apple pomace pectic polysaccharides (APP) extraction, 20 samples were successfully prepared and systematically analyzed. At low temperature region (100–120 °C), extraction effect was predominant and extracted APP was high molecular weight, esterification degree and galacturonic acid content as well as light color. At middle temperature region (140 °C), the balance of extraction and degradation effects was reached and led to the highest APP yield (14.89%). At high temperature region (160–180 °C), degradation effect was predominant and led to serious degradation of APP and more extraction of co-extracts, which endowed the APP with low viscosity and good antioxidant activities in vitro. Overall, the relationship between different subcritical water conditions and APP properties are preliminarily illuminated, which not only provides a promising way for directed extraction of specific APP, but also promotes the potential application of subcritical water to commercial pectin.

Grain size-induced enhancement of thermoelectric performance of Cu3Sb1−xInxSe4 materials

The ternary Cu3SbSe4 thermoelectric material with diamond-like structure exhibits good thermoelectric performance in the middle temperature region. In this study, the Cu3Sb[Formula: see text]In[Formula: see text]Se4 ([Formula: see text]= 0–0.03) materials were fabricated by melting-annealing-ball milling-hot pressing process. The influences of In-doping and grain size on the microstructure and thermoelectric properties of Cu3Sb[Formula: see text]In[Formula: see text]Se4 were evaluated. It is found that the electricity conduction at room temperature for Cu3Sb[Formula: see text]In[Formula: see text]Se4 sample was significantly increased to 6.1 × 103Sm[Formula: see text] due to the In-doping. Additionally, during the powder processing, a relatively low thermal conductivity for the refinement Cu3Sb[Formula: see text]In[Formula: see text]Se4-BM-72H sample was obtained. Benefiting from the enhanced electrical conductivity and the decreased thermal conductivity, the maximum zT value of 0.75 was achieved for Cu3Sb[Formula: see text]In[Formula: see text]Se4-BM-36H at 650 K.

Grain size and compositional gradient dependence of thermoelectric performance for Cu3−xNixSbSe4 materials

The ternary Cu3SbSe4 thermoelectric material with diamond-like structure exhibit good thermoelectric performance in the middle temperature region. In this study, the Cu3−xNixSbSe4 (x = 0–0.03) materials were fabricated by melting-ball milling-hot pressing process. The influences of grain size and compositional gradient on the microstructure and thermoelectric properties of Cu3−xNixSbSe4 were evaluated. It is found that the electrical conductivity of Cu2.97Ni0.03SbSe4 was about 33% higher than that of Cu3SbSe4 sample, and the lattice thermal conductivity was reduced from 3.3 Wm−1K−1 to 2.4 Wm−1K−1 at room temperature due to the grain refinement and more VSe defects. The maximum zT value for the Cu2.97Ni0.03SbSe4 sample was 0.62 at 650 K.

Phase transition induced morphotropic phase boundary behavior and the electric properties in strontium modulated Bi4.5Na0.5Ti4O15 lead-free ceramics

Lead-free (Bi0.5Na0.5)1-xSrxBi4Ti4O15 ceramics (x = 0–0.9) are fabricated by solid state reaction process. XRD analysis shows the symmetry divergence from tetragonal to orthorhombic phase accompanied by morphotropic phase boundary with increasing strontium content. Raman spectra confirm the incorporation of strontium into (Bi2.5Na0.5Ti4O13)2- layers. SEM graphs exhibit the typical plate-like morphology with regular variation of grain size and crystallization as strontium increases. Multistage ferroelectric transition is observed with x = 0.2–0.4. Piezoelectric performance measurements present the well thermal stability at x = 0.4. The dielectric properties display a shifting of Curie temperature towards low temperature with increasing strontium ions. It can be due to the crystal lattice distortion by larger radius of strontium and the increasing tolerance factor. ac conductivity and impedance measurements suggest that electron hopping mainly contributes to the low temperature region. Ionization conductivity by oxygen vacancy migration including first-ionization and double-ionization plays the dominating role in the middle and high temperature region. The controllable properties indicate the potential applications for electric devices of (Bi0.5Na0.5)1-xSrxBi4Ti4O15 ceramic.

Sintering mechanism of copper nanoparticle sphere-plate of crystal misalignment: A study by molecular dynamics simulations

Sintering mechanisms of copper nanoparticles on copper plates are investigated based on molecular dynamics (MD) simulation. In this paper, the roles of tilt grain boundaries (GBs) and Σ5 twisted GBs on sintering kinetics are considered. Regardless of the diameter of the nanoparticle sphere, it is observed that stacking faults and dislocations are formed near the GBs by crystal analysis at the early stage of sintering and throughout the entire sintering process. The stacking faults are overflowed to the surface during the constant temperature stage. The tilt GBs disappears rapidly at the initial stage of sintering, while the Σ5 GBs is instantly eliminated in the middle temperature region of sintering by the rigid rotation of the nanoparticle sphere. Moreover, besides the contribution of atom diffusion in sintering neck growth, misalignment between the two particles brings about enhanced GBs diffusion. These observations provide insights into the tailored high-angle GBs applicable to nano-scale sphere-plate samples.

Low temperature carrier transport mechanism and photo-conductivity of WSe2

This work reports the electrical-transport and temperature-dependent photoconductivity in tungsten diselenide (WSe2) thin films. Temperature-dependent electrical conductivity has been demonstrated using different models. At lower temperatures (< 190 K), carriers become localized to small regions in the film due to Mott’s hopping mechanism. The middle-temperature region (190–273 K) follows Seto’s parameters and obtained low barrier height (0.0873 eV) may be responsible for the improved carrier mobility. At higher temperature (> 273 K) region, thermally activated conduction is dominated with two activation energies of ~138 meV and 98 meV. The peaks obtained in photoluminescent analysis attributes to the presence of mid-bandgap states or defect states which play an important role in the photoconductivity of WSe2. The transient photoconductivity measurements show consistent temperature-dependent behaviour. The effect of light intensity and wavelength variation on the photoconductivity of WSe2 thin films is also discussed. The photo-current is 1.19 * 10−5 A at 125 K, while at 350 K was observed to be 3.12 * 10−4 A. The light-on/off current cycles show that the current can recover to its initial state after various cycles, which points to the WSe2 thin-film device's stable and reversible properties that can be used in optoelectronic applications.

Optimization of thermoelectric properties of CoSb3 materials by increasing the complexity of chemical structure

As a representative thermoelectric material in the middle temperature region, skutterudite CoSb3 exhibits stable structure and excellent thermoelectric properties. Because of the special cage structure of CoSb3, it has been recognized that the thermoelectric performance of CoSb3 can be optimized by filling or doping. In this paper, we have studied the effects of doping alone, doping and filling phase combination, and the increase of atomic filling ratio on the thermoelectric properties of CoSb3 materials. In addition, under high temperature and high pressure (HPHT), the synthesizing cycle of CoSb3 material is only half an hour. The results show that the thermal conductivity of the samples decreases with the increase of the type and proportion of filled atoms. Finally, the maximum thermoelectric figure of merit ZT value of In0.2Ba0.2Co4Sb11.3Te0.7 reached 0.91 at 773K.

Investigations on electrochemical performance of La2NiO4+δ cathode material doped at A site for solid oxide fuel cells

In this work, the perovskite compounds La1.5A0.5NiO4+δ (A = Ca, Sr or Ba), which is one kind of mixed ionic electronic conductors (MIEC) with perovskite structure, was explored as cathode materials for proton conducting solid oxide fuel cells (H-SOFCs). In order to understand how the size of A-site cations affects material properties, the electrical conductivity and the electrochemical performances of La1.5A0.5NiO4+δ (A = Ca, Sr or Ba) were compared. The results are more interesting. In the middle temperature region (600 °C–700 °C), Ca doping is the best choice among the three alkaline earth elements. In the low temperature region (300 °C–600 °C), Sr-doped single cells Sr-doped single cells have better performance. However, Ba doping easily generates BaCO3 impurities, which reduces the ion transmission capacity and increases the interface resistance, resulting in poor performance of the single cell. This work shows that La1.5A0.5NiO4+δ (A = Ca and Sr) is the main cathode candidate material for H-SOFC.

Effect of different reaction conditions on catalytic activity of La(Mn, Fe)O3+λ catalyst for methane combustion

The La(Mn, Fe)O3+λ catalyst, a novel rare earth perovskite composite oxide catalyst for catalytic combustion, was prepared by sol-gel method, which was at natural gas automobile exhaust catalytic purification, smoke gas catalytically desulfurization and other fields had broad application prospects [–]. The newly developed catalysts were characterized by several complementary techniques. The effects of different reaction conditions such as calcination temperature, citric acid (CA) content and ethylene glycol (EG) content on the catalytic performance of the catalyst were investigated by using low concentration methane catalytic combustion as the probe reaction. The results showed that a stable single-phase rare earth perovskite La(Mn, Fe)O3+λ catalyst could be formed after calcination at 600 °C for 3 h in air atmosphere. When EG/M = 1/2, CA/M = 1/2, the catalyst exhibited good methane catalytic combustion activity in the middle and low temperature regions. The light-off temperature (T10) and complete conversion temperature (T90) were 363 °C and 493 °C, respectively, the surface activation energy was 60 ∼ 80 kJ mol−1, and the catalytic activity was significantly improved. The O2-TPD and XPS characterization results showed that La(Mn, Fe)O3+λ had higher surface Mn4+, Fe3+ and more active oxygen species, which significantly improved the catalytic activity.

Relaxation and conduction mechanism of BNT-KNN ceramics by impedance study

ABSTRACTImpedance spectra of 0.93BNT-0.07KNN revealed positive and negative temperature coefficient of resistance behaviour up to 400°C and above 400°C. With the increase of temperature, decreased radii and displaced centres of semicircles from the real axis of impedance Cole-Cole plots suggested non-Debye type relaxation mechanism. Three dielectric relaxation regions: within (i) grain, (ii) grain- grain boundary and (iii) grain boundary were prominent in lower, middle and high temperature regions, respectively. The activation energy ranging from ∼0.79 to 1.2 eV, suggested that the conduction carriers were cation and cation, oxygen vacancies in lower and higher temperature regions, respectively.

Crystallization kinetics of ethylene-co-propylene rubber/isotactic polypropylene blend investigated via chip-calorimeter measurement

Differential scanning calorimetry measurement via fast-scan chip-calorimeter Flash DSC1 has been widely applied to investigate the crystallization kinetics of homopolymer and random copolymers. We expanded the measurement to the crystallization kinetics of isotactic polypropylene (iPP) blending with 29.3wt% ethylene-co-propylene rubber (EPR) that contains 39.5wt% ethylene monomers (toughening iPP). The temperature dependence of half-crystallization times of the blend show three regions of deviations from the parallel results of pure iPP, reflecting the effect of interplay between phase separation and crystallization. We attributed faster crystallization in the high-temperature region to crystal nucleation accelerated by the interfaces of two incompatible components, attributed slower crystallization in the middle-temperature region to crystal nucleation retarded by the crystallization of ethylene sequences in the minor EPR component, and attributed faster crystallization in the low-temperature region to crystal nucleation accelerated by the crystallization of propylene sequences in the minor EPR component. The suppression of crystallization on fast cooling as well as the variation of Avrami indexes support the scenario of crystallization competition in the middle temperature region.

Tuning the property of Mn-Ce composite oxides by titanate nanotubes to improve the activity, selectivity and SO2/H2O tolerance in middle temperature NH3-SCR reaction

Mn-Ce composite oxides were regarded as a promising alternative to substitute V-based catalysts for the abatement of NOx. However, the obstacles in activity, selectivity and SO2/H2O tolerance still remained. In this paper, titanate nanotubes (TNTs) were used to tune the property of Mn-Ce composite oxides and led to an outstanding catalytic performance in selective catalytic reduction of NO by NH3 within middle temperature region. Compared with conventional MnCe/TiO2 and V2O5/TiO2 nanoparticle catalysts, a higher NO conversion with good SO2/H2O tolerance could be observed at 200–300°C over the prepared MnCe/TNTs catalyst. More interestingly, much lower N2O concentration was detected in the SCR reaction over this catalyst. Characterization results revealed that MnCe/TNTs had larger BET surface area and greatly enhanced NH3 adsorption, which were beneficial to SCR activity. Moreover, the growth of MnO2 was improved and the redox property of the catalyst was tuned in the nano-confined space of TNTs. Thus the oxidative abstraction of hydrogen from ammonia was suppressed, resulting in less N2O production.

Temperature effect on the low cycle fatigue behavior of a directionally solidified nickel-base superalloy

The effect of temperature on low cycle fatigue behavior of a directionally solidified nickel-base superalloy has been investigated from 550 to 850°C. The fatigue life reached maximum at about 700°C. The fracture modes can be divided into three types: crystallographic plane facets fracture (shear fracture) at low temperature region (<600°C), mixed fracture mode at middle temperature region (600–700°C) and non-crystallographic fracture (I type fracture) at higher temperature region (>700°C). The dominant reason of the fracture mode transition with temperature is the deformation of the γ′ precipitated phase at different temperatures. The precipitated phase is sheared by dislocations which are restricted to the primary {111} slip planes at 600°C. As the temperature increases, the deformation of γ′ precipitated phase becomes anomaly and some secondary precipitates can be observed in the matrix.

Investigation of the structural, electrical, and optical properties of MnAl2Se4 layers grown using the hot-wall deposition technique

MnAl2Se4 layers were grown using the hot-wall deposition technique with an attached reservoir tail. Precise control of the vapor pressure in the reservoir was thought to play an important role in the grown of a stoichiometric layer. From the relation between the reciprocal temperature and the carrier concentration, we extracted the dominant trap level as 96.1 meV in the high-temperature region and 13.9 meV in the middle-temperature region. Thus, from a log-log plot between the mobility and the temperature, the mobility showed the different temperature-dependent decreases of the mobility at temperatures above 100 K: T−1/2 in the temperature range of 100 225 K. The mobility decreased in proportion to T1 in the low-temperature range of T < 100 K. By analyzing the optical absorption results, the bandgap variation matched Eg(T) = Eg(0) − 3.19 × 10−3T2/(T + 488) well, where Eg(0) is estimated to be 3.5616 eV. Consequently, low-temperature growth of MnAl2Se4 layers was achieved by using the hot-wall deposition technique.