Separate Temperature Regions Research Articles

4D Printing of Triple-Shape Memory Cyanate Composites Based on Interpenetrating Polymer Network Structures.

The triple-shape memory polymer (TSMP) can be programmed into two temporary shapes (S1 and S2) and shows an ordinal recovery from S2 to S1 and eventually to the permanent shape upon heating, which realizes more complex stimulus-response motions. We introduced a novel strategy for forming triple-shape memory cyanate ester (TSMCE) resins with high strength and fracture toughness via three-step curing, including four-dimensional (4D) printing, UV post-curing, and thermal curing. The obtained TSMCE resins presented two separated glass transition temperature (Tg) regions due to the formation of an interpenetrating polymer network (IPN), which successfully endowed the polymers with the triple-shape memory effect. The two Tg increased with the increasing cyanate ester (CE) prepolymer content; their ranges were 82.7-102.1 °C and 164.4-229.0 °C, respectively. The fracture strain of the IPN CE resin was up to 10.9%. Moreover, the cooperation of short carbon fibers (CFs) and glass fibers (GFs) with the polymer-accelerated phase separation resulted in two well-separated Tg peaks exhibiting better excellent triple-shape memory behaviors and fracture toughness. The strategy for combining the IPN structure and 4D printing provides insight into the preparation of shape memory polymers integrating high strength and toughness, multiple-shape memory effect, and multifunctionality.

The structural and optical properties of GO: Temperature-dependent analysis of the electrical properties of Al/GO/p-type Si semiconductor structures

A spin-coating process has been used to generate graphene oxide (GO) thin films on p-type silicon. The temperature-dependent basic electrical characteristics of Al/GO/p-type Si metal–semiconductor structures have been examined through current–voltage (I–V) measurements in the temperature range 80–420 K by increments of 20 K based on thermionic emission (TE) theory. At each temperature, the main electrical parameters, such as ideality factor (n), rectification ratio (RR), saturation current (I0), barrier height (Φbo), and series resistance (RS), were determined. With increasing temperature, the values of n, I0, RR, and RS decreased, whereas the value of Φbo increased. These unusual temperature fluctuations in Φbo and n may be rationalized in terms of twofold Gaussian distributions at 80–200 K and 220–420 K. For the high- and low-temperature regions, evaluation of these two Gaussian distributions of the I–V curves for the Al/GO/p-type Si structure yielded mean barrier heights of 1.676 and 0.555 eV with standard deviations (σ0) of 166 mV and 76 mV, respectively. Modified ln(I0/T2)−(q2σs2)/(2k2T2) vs. q/(kT) plots, which correspond to these two separate temperature regions, further corroborated these barrier height values. The value of the Richardson constant (A*), 3.567 × 10−6 A K−2 cm−2, is substantially lower than the known value for p-Si. However, for the distribution in the 220–420 K region, the Richardson constant of 48.069 A K−2 cm−2 is near to the known theoretical value of 32 A K−2 cm−2 for p-type Si.

Modeling crystallization kinetics for selective laser sintering of polyamide 12

AbstractSelective laser sintering (SLS) of polymers represents a widely used additive manufacturing process, where the part quality depends highly on the present thermal conditions. One distinct feature of SLS is the existence of separate temperature regions for melting and crystallization (solidification) and that the process optimally operates within said regions. Typically a crystallization model, such as the Nakamura model, is used to predict the degree of crystallization as a function of temperature and time. One limitation of this model is the inability to compute negative rates of the crystallization degree during remelting. As we will show in this work, such an extension is necessary, considering the varying temperature fields appearing in SLS. To this end, an extension is proposed and analyzed in detail. Furthermore, a dependency of the temperature and crystallization fields on the size of geometrical features is presented.

Thermogravimetric insight in the reduction of xCuO – (1-x)MoO3 oxide system (0.1 ≤ x ≤ 0.9) by hydrogen

The oxide mixtures xCuO-(1-x) MoO3 were synthesized by gel-combustion procedure. The existence of phase mixture CuO + Cu3Mo2O9 and MoO3 + CuMoO4 in CuO-rich and MoO3 -rich composition region, respectively, were evidenced. The constant heating rate thermogravimetry in hydrogen atmosphere revealed that the reduction reactions proceed within the two clearly separated temperature regions. On the basis of mass changes, the mechanism of reduction processes was discussed. The measurements revealed considerable inhibition of CuO reduction by MoO3, and huge acceleration of MoO3 → MoO2 reduction step by copper. The particularities found in this system were commented in relation to our similar studies in NiO-MoO3 and CuO-WO3 systems. For particular composition, x = 0.5, existing preferably in form of a-CuMoO4, kinetic parameters of reduction were determined. The composition of oxide mixture influenced the particle size and morphology of resulting metallic Cu-Mo composites.

Solvent Switched Weak Interaction of a 4-Quinazolinone with a Cavitand Derivative.

Interaction of 4-quinazolinone with tetrakis (3,5-dicarboxylatophenoxy)-cavitand derivative has been studied in methanol and dimethylformamide media using fluorescence spectroscopy and molecular modeling methods. Results show temperature dependent complex formation: either the entropy gain or the high enthalpy changes are responsible for the formation of stable complexes in two separated temperature regions. However, different thermodynamic parameters are associated to different conformations of the complexes: while the high entropy gain associated to the formation of deeply included guest in methanol, the high entropy gain is associated with the formation of weakly included guest in dimethylformamide solvent. This finding highlights the importance of dynamic properties of the species interacted in different solvents.

Martensite Reversion Duality Behavior in a Cold-Rolled High Mn Transformation-Induced Plasticity Steel

The martensite reversion behavior of a cold-rolled high Mn transformation-induced plasticity (TRIP) steel was investigated through an elaborate continuous annealing procedure. The dilatometry analysis was employed to ensure a precise heating rate (50 °C/s) control during annealing along with acquiring the critical temperature range of the martensite reversion. The magnetic measurement was used for $$ \alpha^{\prime} $$-martensite quantification through annealing, and electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) analyses were also performed to characterize the microstructure. The dilatometry results revealed the dual temperature ranges for the $$ \alpha^{\prime} $$-martensite reversion in the experimental TRIP steel; the reversion occurred in two separate temperature regions of 540 °C to 730 °C and 730 °C to 850 °C. Although the inhomogeneity in the morphology of $$ \alpha^{\prime} $$-martensite (lath type and dislocation cell type), which was detected in the ECCI image of the cold-rolled structure, might be responsible for the reversion temperature range broadening, the discontinuity of the reversion was mainly attributed to the partitioning of Mn from $$ \alpha^{\prime} $$-martensite to austenite through annealing. The microstructural characterizations indicated that the duality would also exist in the case of the martensite reversion mechanism. Accordingly, the reversion of lath-type martensite appeared to occur merely through an athermal mechanism, while the dislocation cell-type martensite might be reversed to austenite via both shear and diffusional mechanisms.

Pyrolysis Kinetics Equation of Larch Bark

The larch bark was examined by non-isothermal means to determine the mass loss kinetics of the thermal decomposition with linear temperature programming in nitrogen atmosphere. In this work, mechanism equation of = was used for Coats-Redfern integral method at the different heating rates. The apparent activation energy, pre-exponential factor and the pyrolysis kinetic equations at the different heating rates were obtained. The pyrolysis temperature area was divided into two separate temperature regions for the pyrolysis kinetic equation and the two components were decomposed respectively at the two separate temperature regions. The global mass loss rate of the bark is considered as controlled respectively by the reactions of the two components respectively during the lower and higher temperature ranges. The kinetics of the two components are found to abide by the mechanism equation of =, which gave the best fits to the experimental data. The obtained kinetic equations of the bark at the different heating rates were additionally validated by the reasonable agreement between the experimental and calculated results.

Silica nanowires synthesized from gas by-product of SiC synthesis from alkoxide precursors

We synthesised silica nanowires (NWs) without catalysts from the otherwise waste SiO gas by-product of SiC synthesis from alkoxide precursors (phenyltrimethoxysilane and tetraethyl orthosilicate). Separate temperature regions were locally defined in a reactor tube for conversion of alkoxide (1500 °C) and deposition of NWs (1130 °C). Thermodynamic calculations prove that SiO gas abruptly condenses and deposits as silica NWs in the range 1160 to 1000 °C. The factors determining the growth of the NWs in this temperature range were fully elucidated. The photoluminescence spectra revealed differences in oxygen defects in the NWs synthesized under growth conditions with different stability. We expect that this strategy could be extended to synthesize various silica nanostructures from the waste silicon- and oxygen-containing gas by-products of other syntheses.

Reaction of Hydrogen Atoms with Hydroxide Ions in High-Temperature and High-Pressure Water

The rate constant for the reaction of hydrogen atoms (H(*)) with hydroxide ions (OH(-)) in aqueous solution has been measured from 100 to 300 degrees C by direct measurement of the hydrated electron ((e(-))(aq)) product growth rate. In combining these measurements with previous results, the reaction is observed to display Arrhenius behavior in two separate temperature regions, 3-100 and 100-330 degrees C, where the data above 100 degrees C show an obvious decrease in activation energy from 38.2 +/- 0.6 to 25.4 +/- 0.8 kJ mol(-1). The value of the rate constant is smaller than that estimated previously in the 200-300 degrees C range. The very unusual activation energy behavior of the forward and backward reactions is discussed in the context of transition state theory.

Superconductivity and spin gap in the spin-ladder cuprate, [formula omitted

Pressure-induced superconductivity in a spin-ladder cuprate Sr 2 Ca 12 Cu 24 O 41 was investigated on the microscopic level by using 63 Cu nuclear magnetic resonance (NMR) under pressure of 3.5 GPa. We observed both spin and superconducting gaps in separated temperature regions. We found that the superconducting state possesses a full gap and is quite stable even at high fields close to the conventional Pauli limit.

Dielectric Study of Dynamical Heterogeneity in Blends of Polystyrene and Low Mass Compounds

We investigated the dielectric relaxation in blends of 4-n-pentyl-4’-cyanobiphenyl (5CB) and polystyrene (PS) in order to examine the size effect of low mass molecules on the cooperativity of local dynamics. To obtain the dielectric data in the isotropic state, the phase diagram was produced. The temperature (T) dependence of the dielectric loss factor e” of PS/5CB blends containing 20 to 30 wt% of 5CB exhibited a bimodal peak near the calorimetric glass transition temperature Tg. The bimodal peak was resolved into two peaks termed α and β. The α process locates above Tg and can be assigned to cooperative motions of the PS segments and the 5CB molecules. The β process is responsible to independent reorientation of the 5CB molecules in the glassy PS matrix. The α and β loss peaks tend to merge with increasing T. This behavior contrasts to the dielectric behavior of the PS/toluene system which exhibits loss peaks due to segmental motions of PS and rotation of the toluene molecules in well separated temperature regions. The toluene molecule has a size of ca. 1/3 of the 5CB molecule and does not move cooperatively with PS but the 5CB molecule does. From the intensity of the α and β peaks, the fraction of the 5CB molecules moving cooperatively with PS has been estimated to be 0.25 and 0.63 for the blends containing 30 and 20 wt% of 5CB, respectively.

Kinetic Compensation Effect In The Thermal Decomposition Of Biomass In Air Atmosphere

The wood and leaf samples of eight species are examined by non-isothermal means to determine the mass loss kinetics of the thermal decomposition with linear temperature programming in air atmosphere. A simple kinetic description is developed based on the experimental results and integral analysis method. In the model, the mass loss process consists of three steps. The first step corresponds to the water evaporation, and the subsequent two mass loss steps are mainly due to two major pseudo components. The two pseudo components decompose respectively at two separate temperature regions. Under this kinetic scheme, the kinetic compensation effect is analyzed and it is found that the kinetic parameters E and A resulted from the variation of the species and the variation of model functions exhibit kinetic compensation effect. Quantitative and statistical criterion to distinguish between real and false compensation effect is discussed in detail.

Kinetic modeling of thermal decomposition of natural cellulosic materials in air atmosphere

The wood and leaf samples of eight species are examined by non-isothermal means to determine the mass loss kinetics of the thermal decomposition with linear temperature programming in air atmosphere. A simple kinetic description, named in this work as ‘First Order Pseudo Bi-component Separate-stage Model (PBSM-O1)’, is developed based on the experimental results and integral analysis method. The model assumes that the mass loss process of any wood or leaf sample consists of three steps. The first step corresponds to the water evaporation, and the subsequent two mass loss steps are mainly due to two major pseudo components. The two pseudo components decompose respectively at two separate temperature regions, other than at the global temperature region as used in the previous developed models by other authors. The global mass loss rate of the sample is looked on as controlled respectively by the reactions of the two components respectively during the lower and higher temperature ranges. The kinetics of the two components are found to abide by the first order equation, which gives the best fits to the experimental data compared with other model functions. The advantages of PBSM-O1 are discussed by comparing it with other kinetic models. PBSM-O1 is additionally validated by the reasonable agreement between the experimental and calculated results.

Pressure-induced amorphization and disordering on cooling in semi-crystalline polymers: calorimetric evidence for inverse melting in one component system

We report some unusual phase behaviour, of general implication for condensed matter, on the polymer poly-4-methyl pentene-1 (P4MP1) induced by changes in pressure (P) and temperature (T), as observed by in-situ X-ray diffraction and high pressure DSC. Upon increasing pressure beyond a threshold value, the polymer, crystalline at ambient conditions, looses its crystalline order isothermally. The process is reversible. This behaviour is observed in two widely separated temperature regions, one below the glass transition temperature (< 50°C) and one close to the melting temperature (250°C), thus showing solid state amorphization and inversion in the melting temperature with increasing pressure. This further suggests inverse melting, i.e. re-entrant of the two widely separated liquid and amorphous phases along the T-axis at fixed P. This is confirmed experimentally as disordering in the crystalline structure on cooling. The inverse melting in P4MP1 raises the possibility of exothermic melting and endothermic crystallization as anticipated by Tammann (1903), see reference 1. The anticipated exothermic melting and endothermic crystallization is confirmed experimentally in the one component system P4MP1. We are observing similar features in a range of polymers.

Colossal Magnetoresistance in the Transition-Metal Zintl Compound Eu14MnSb11

Magnetoresistance measurements have been carried out on single-crystal sample of the transition-metal Zintl compound Eu14MnSb11. Eu14MnSb11 is the first compound of the Ca14AlSb11 structure type in which magnetoresistive effects have been measured. Colossal magnetoresistance (CMR) at 92 K and 5 T with Δρ/ρ(0) = −36% and at 5 K and 5 T with Δρ/ρ(0) = −42% were observed. CMR is observed at the ferromagnetic−paramagnetic phase transition (92 K). A large negative magnetoresistance is observed below the second magnetic transition (15 K) and is attributed to a field-induced antiferromagnetic−ferromagnetic ordering of the Eu2+ lattice. The MR is highly anisotropic with respect to the crystallographic axes. A sign change in the curvatures of ρ(H) curves below and above 92 K has been observed, indicating that two different mechanisms are responsible for the magnetic scattering of the carriers in two separate temperature regions.

Pressure-Induced Amorphization and Disordering on Cooling in Semi-Crystalline Polymers; calorimetrie evidence for inverse melting in one component system

ABSTRACTWe report some unusual phase behaviour, of general implication for condensed matter, on the polymer poly-4-methyl pentene-1 (P4MP1) induced by changes in pressure (P) and temperature (T), as observed by in-situ X-ray diffraction and high pressure DSC. Upon increasing pressure beyond a threshold value, the polymer, crystalline at ambient conditions, looses its crystalline order isothermally. The process is reversible. This behaviour is observed in two widely separated temperature regions, one below the glass transition temperature (&lt; 50°C) and one close to the melting temperature (250°C), thus showing solid state amorphization and inversion in the melting temperature with increasing pressure. This further suggests inverse melting, i.e. re-entrant of the two widely separated liquid and amorphous phases along the T-axis at fixed P. This is confirmed experimentally as disordering in the crystalline structure on cooling. The inverse melting in P4MP1 raises the possibility of exothermic melting and endothermic crystallization as anticipated by Tammann (1903). The anticipated exothermic melting and endothermic crystallization is confirmed experimentally in the one component system P4MP1. We are observing similar features in a range of polymers.

Giant magnetoresistance at 300 K in single crystals of La0.65(PbCa)0.35MnO3

Giant magnetoresistance (GMR) at 300 K and 5.5 T with ΔR/R(0)=74% was observed in single crystals of La0.65(PbCa)0.35MnO3. The maximum GMR occurs exactly at the ferromagnetic–paramagnetic phase transition temperature (Tc). For T&amp;lt;Tc, the R(T) curve is closely related to the M(T) curve. A sign change in the curvatures of R(H) curves below and above Tc has been observed, indicating that two different mechanisms are responsible for the magnetic scatterings of the carriers in two separate temperature regions.

Unusual pressure‐induced phase behavior in crystalline poly‐4‐methyl‐pentene‐1

AbstractWe report some highly unusual phase behavior, of general implication for condensed matter, on the polymer poly(4‐methyl‐pentene‐1) (P4MP1) induced by changes in pressure and temperature, as observed in situ by x‐ray diffraction. Upon increasing pressure beyond a threhold, the polymer, crystalline under ambient conditions, loses its crystalline order isothermally, passing through a continuously varying sequence of mesomorphic states, the process being reversible. This behavior is observed in two widely separated temperature regions, suggesting, for the first time in a single component system, the possibility of reentrant liquid‐crystal and amorphous phases. At the upper temperature region (ca. 250°C) there is a consecutive increase and decrease of melting point with pressure. In the lower temperature region (room temperature) the pressure converts the crystal into an amorphous‐like glass obviating the need for going through the melt first, and this in a reversible manner. The latter pressure‐induced disordered phase converts into crystal on raising the temperature, and reverts to the glassy, disordered phase on lowering the temperature. Some aspects of this behavior have been found quite recently in water‐ice and silica but the process of “melting on cooling” has no precedent in any known system. Other unexpected findings include a new pressure‐induced modification of P4MP1 with a one‐way only entry with temperature, but full reversibility with pressure leading to a triple point in the PT phase diagram. The above highly uncommon results are putting several prevailing preconceptions to test which are being scrutinized. In the course of it some early expectations on general phase behavior, allowing among others for reentrant phases in one component systems, are being invoked as potentially appropriate for certain polymeric systems, if not for condensed matter in general. © 1993 John Wiley &amp; Sons, Inc.

The mechanism of kinetic oscillations in the NO + CO reaction on Pt(100)

Under constant flow conditions the reaction of NO and CO to the products N2 and CO2 on a Pt(100) surface exhibits multiple steady states and kinetic oscillations. Using Video LEED, measurement of the work function change and of the reaction rate the kinetic oscillations were investigateed in the 10−7 mbar range. Kinetic oscillations were found to occur in two separate temperature regions around 400 K. In the upper temperature range, the oscillations are coupled to the 1 × 1 ⇌ hex phase transition, while those in the lower temperature range proceed entirely on a 1 × 1 surface. Based on a mathematical model consisting of three coupled differential equations it could be shown that the 1 × 1 ⇌ hex phase transition is not essential for the oscillating mechanism. The main role in the mechanism can instead be ascribed to the autocatalytic behavior of the surface reaction between COad and NOad.

Pulse radiolysis study of the reaction of OH radicals with C2H4 over the temperature range 343–1173 K

AbstractThe reaction of OH and OD radicals with ethylene in the presence of 1 atm argon and 6 Torr water vapor was studied in the temperature range 343–1173 K. The results reveal three kinetically separate temperature regions: (1) 343–563 K, where the disappearance of OH radical is dominated by the addition of OH to the double bond of ethylene; (2) 563–748 K, where concurrent reactions of addition, the reverse reaction of addition and H‐atom abstraction is dominant; and (3) 748–1173 K, where H‐atom abstraction is likely the main reaction. The rate for hydrogen abstraction is 2.4 × 10−11 exp[(−2104 ± 125)/T] cm3/molec‐s (for OD 2.1 × 10−11 exp[(−2130 ± 172)/T] cm3/molec‐s). There was no obvious pyrolysis of ethylene below 1073 K. The study of OD radical with ethylene shows a small isotope effect.