Showing papers on "Transition temperature published in 2006"

Ferroelectric Phase Transition in Individual Single-Crystalline BaTiO3 Nanowires

Abstract: We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO3) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling. The diameter at which TC falls below room temperature is determined to be ∼3 nm, and extrapolation of the data indicates that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining ...

Phase Equilibria in the System BaO–TiO2

Abstract: The system BaO-TiO2 was investigated using quenching, strip-furnace, and thermal techniques. Five compounds were found to exist in the system: Ba2TiO4, BaTiO3, BaTi2O5, BaTi3O7, and BaTi4O9. Of these, only barium metatitanate (BaTiO3) melts congruently (at 1618°C.). The dititanate melts incongruently at 1322° C. to yield BaTiO3 and liquid; the trititanate melts at 1357°C. to yield BaTi4O9 and liquid; the tetra-titanate melts to TiO2 and liquid at 1428° C. The nature of melting of the orthotitanate could not be determined accurately because of the high temperature involved and the rapid reaction with platinum. The two eutectics in the system occur between Ba2TiO4 and BaTiO3 at 1563°C. and between BaTi2O5 and BaTi3O7 at 1317°C. The temperature of the cubic-hexagonal transition in barium metatitanate was determined as 1460°C. and the transition has been shown to be reversible. The transition temperature is raised sharply by the addition of a small percentage of TiO2 although the extent of solid solution is quite limited. Some applications to the manufacture of titanate bodies and to the growth of single crystals of barium metatitanate are discussed.

Electrical properties of transparent and conducting Ga doped ZnO

Abstract: In this paper, we report on the metal-semiconductor transition behavior observed in transparent and conducting ZnO:Ga films grown by pulsed-laser deposition. The electrical resistivity measurements were carried out on ZnO films with varying Ga concentration in the temperature range of 14to300K. The electrical properties were correlated with film structure, and detailed structural characterization was performed using x-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy. The room-temperature resistivity of these films was found to decrease with Ga concentration up to 5% Ga, and then increase. The lowest value of resistivity (1.4×10−4Ωcm) was found at 5% Ga. Temperature dependent resistivity measurements showed a metal-semiconductor transition, which is rationalized by localization of degenerate electrons. A linear variation of conductivity with T below the transition temperature suggests that the degenerate electrons are in a weak-localization regime. It was also found t...

Large magnetic entropy changes in the Ni45.4Mn41.5In13.1 ferromagnetic shape memory alloy

Abstract: The inverse magnetocaloric effect associated with the martensitic transition in the Ni45.4Mn41.5In13.1 Heusler alloy is reported. A large positive magnetic entropy change of 8J∕kgK under a low magnetic field of 10kOe is found near the martensitic transition temperature. This change originates from the first-order transition from a low-temperature weak-magnetic martensitic phase to a high-temperature ferromagnetic austenitic phase. The large low-field magnetic entropy change indicates a great potential of Ni–Mn–In alloys as working materials for magnetic refrigeration in a wide temperature range.

Unified equation for the strength of bulk metallic glasses

Abstract: In the present study, a conceptual approach to evaluate the strength of metallic glass systems is proposed from a free volume point of view. Based on the physical analogy between the plastic deformation and glass transition, the strength of amorphous structures was found to depend on both the localized shear mechanism and the atomic cohesive energy. Interestingly, we find that the strength at the ambient temperature (T0) can be determined by the glass transition temperature (Tg) and molar volume (V), and can be specifically predicted by a unified parameter of (Tg−T0)∕V. The predicted strength was unambiguously verified from experimental data reported for a number of metallic glass systems.

Hydrogen bonding, mobility, and structural transitions in aliphatic polyamides

Abstract: Some salient results in nylon research are reviewed to identify the fundamental principles that are applicable to other strongly interacting or hydrogen-bonded polymers, including proteins. The effects of hydrogen bonds on stress-, heat-, and solvent-induced changes in macroscopic properties are discussed. These data provide a window into the chain mobility and linkages between the crystalline and amorphous domains, both of which are important for any predictive model. The changes in the characteristics of the amorphous phase with the crystallinity and orientation require that it be modeled with at least two components: a rigid/immobile/anisotropic component and a soft/mobile/isotropic component. The deformation and shrinkage behavior of these polymers are discussed in terms of the relative contributions of the amorphous and crystalline domains and of the interactions between them. The premelting crystalline transition is accompanied by the merging of intersheet and intrasheet diffraction peaks in some nylons, as observed by Brill, and not in others even though the underlying mechanism that gives rise to these transitions, the onset of volume-increasing librational motion of the crystalline stems, is the same. Because the effects of the temperature, deformation, and solvent have a common origin associated with mobility, a fictive temperature can be associated with a given solvent activity or stress level. The magnitude of this fictive temperature is the amount by which the glass or Brill transition temperature is reduced in the presence of solvents (∼50 °C) or stress or by which the annealing temperature can be reduced in the presence of a solvent (or active stress) to achieve the same structural state as that of a dry (or static) polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1763–1782, 2006

Temperature dependence of Mott transition in VO_2 and programmable critical temperature sensor

Abstract: The temperature dependence of the Mott metal-insulator transition (MIT) is studied with a VO_2-based two-terminal device. When a constant voltage is applied to the device, an abrupt current jump is observed with temperature. With increasing applied voltages, the transition temperature of the MIT current jump decreases. We find a monoclinic and electronically correlated metal (MCM) phase between the abrupt current jump and the structural phase transition (SPT). After the transition from insulator to metal, a linear increase in current (or conductivity) is shown with temperature until the current becomes a constant maximum value above T_{SPT}=68^oC. The SPT is confirmed by micro-Raman spectroscopy measurements. Optical microscopy analysis reveals the absence of the local current path in micro scale in the VO_2 device. The current uniformly flows throughout the surface of the VO_2 film when the MIT occurs. This device can be used as a programmable critical temperature sensor.

Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature

Abstract: The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in doped ZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5?B and 3?B respectively. Furthermore a clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier-induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetization on carrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers. We observe a positive magnetoresistance but no anomalous Hall effect or anisotropic magnetoresistance in the ferromagnetic samples.

Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature

Abstract: The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in doped ZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5mB and 3mB respectively. Furthermore a clear correlation between the magnetisation per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetisation on carrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers.

Temperature variation of elastic constants of quartz across the α - β transition

Abstract: The α − β transition of quartz was successfully observed with using a single sample by means of the rectangular parallelepiped resonance (RPR) method. An oriented rectangular parallelepiped of α-quartz single crystal was prepared and the resonant frequencies of 30–11 vibrational modes were measured from room temperature to 700°C. The softening of quartz crystal was observed as the significant reduction of resonant frequencies near the α–β transition. The present study is the first application of the RPR method to the study of phase transition. The complete set of elastic constants of α- and β-quartz were determined as a function of temperature by the least-squares inversion of the measured frequency data obtained by a single run. This is a merit yielded by the RPR method. It is shown near the α − β transition in both α- and β-quartz that the elastic parameters decrease proportionally to |T−T 0|−n , where T is temperature and T 0 is the transition temperature, 573.0°C for α-quartz and 574.3°C for β-quartz. It was also seen that linear incompressibilities K 1 = (C 11 +C 12 +C 13)/3 and K 3 = (C 33 +2C 13)/3 decrease rapidly toward the transition, whereas, shear moduli C 44, C S1 = (C 11 +C 33 -2C 13)/4 and C S3 = (C 11 -C 12)/2 = C 66 decrease only slightly. The shear modulus C S3 = C 66 increased slightly in α-quartz. The elastic properties of isotropic aggregate of quartz were calculated, and it is shown that the longitudinal wave velocity significantly decreases at the α − β transition, whereas, the shear wave velocity decreases only slightly.

Classification of transition temperature behavior in ferroelectric PbTiO3-Bi(Me'Me )O3 solid solutions

Abstract: The ferroelectric transition temperature (Tc) behavior of perovskite solid solutions based on PbTiO3–Bi(Me′Me″)O3 (Me′=Fe3+, Zn2+, Sc3+, In3+, Mg2+, Ni2+, etc., and Me″=Ti4+, Nb5+, W6+) was considered. Trends in the Tc compositional dependence near the PbTiO3 end member could be described with a geometrical polynomial expression. Three main cases were observed: Case 1, a continued increase in transition temperature above the end member PbTiO3 (495°C); case 2, an increase and then decrease of the transition temperature; and case 3, a continuous decrease in the transition temperature with Bi(Me′Me″)O3 additions. It was noted that for all case 2 examples the enhancement of ΔTc=Tc(max)−Tc(PT) increased as the distribution of B-site ionic radii increased. A correlation was therefore proposed between the maximum enhancement in transition temperature and the spread of tolerance factor (Δt) and∕or variance in B-site ionic radius (σ2). Finally, it was proposed that these observations are consistent with random-fie...

Size effect on the dielectric properties of BaTiO 3 nanoceramics in a modified Ginsburg-Landau-Devonshire thermodynamic theory

Abstract: Grain size effects on the dielectric properties of BaTiO3 nanoceramics have been studied by using the modified Ginsburg-Landau-Devonshire (GLD) thermodynamic theory. Considering the existence of internal stresses, it is found that with decreasing grain size the transition temperature of cubic-tetragonal phase decreases, while those of tetragonal-orthorhombic and orthorhombic-rhombohedral phases increase. With further reducing grain size, our model predicts that the two ferroelectric structures of orthorhombic and tetragonal phases will become unstable and disappear at a critical size, leaving only one stable ferroelectric phase of rhombohedral structure. Consequently, a theoretical phase diagram of the transition temperature versus grain size is established wherein two triple points and a reentrance behavior are indicated. The results are compared with experimental data.

Structure of strontium barium niobate SrxBa1 − xNb2O6 (SBN) in the composition range 0.32 ≤ x ≤ 0.82

Abstract: The structure of strontium barium niobate crystals SrxBa1 − xNb2O6 is comprehensively studied in the whole range of the tetragonal tungsten bronze phase (x = 0.32–0.82) using both powder and single-crystal X-ray diffraction measurements. Unit-cell parameters, density, site-occupancy factors and interionic distances show an explicit composition dependence which can be consistently explained using simple model calculations. The temperature dependence of the unit-cell parameters exhibits a remarkable anisotropy in a broad temperature region below the phase transition temperature. This proves that the electrostrictive contribution to the thermal expansion plays an important role in strontium barium niobate.

Magnetic nanoparticles with enhanced γ-Fe 2 O 3 to α-Fe 2 O 3 phase transition temperature

Abstract: We report a simple method for producing magnetic nanoparticles with enhanced maghemite (?-Fe2O3) to haematite (?-Fe2O3) phase transition temperature. By controlling the properties of the solvent media, we have been able to tune the particle size from 2.3 to 6.5?nm. Nanoparticles with higher transition temperatures have been achieved by using NaOH as an alkali during the co-precipitation. The maghemite to haematite phase transition was complete at a temperature below 600??C for nanoparticles prepared using ammonia, whereas the phase transition was not complete until 750??C for the samples prepared with NaOH. The increase in average particle size after heat treatment at 600??C is attributed to coalescence of particles by solid state diffusion, where the system reduces its free energy by reducing the surface area. The final particle diameters of the haematite after the heat treatment were 35.4, 31.38 and 26.85?nm respectively for nanoparticles of initial diameters 5, 7 and 9.8?nm. Our studies on the effect of the initial particle size on the transition temperature show that the transition temperature decreases with decreasing particle size due to the reduced activation energy of the system.

Impedance and modulus spectroscopy studies on Ba0.1Sr0.81La0.06Bi2Nb2O9 ceramic

Abstract: The modulus and impedance spectroscopy studies on barium strontium lanthanum bismuth niobate (Ba 0.1 Sr 0.81 La 0.06 Bi 2 Nb 2 O 9 , BSLBN) were investigated in the frequency range, 45 Hz–5 MHz and in the temperature duration from room temperature (rt) to 570 °C. The dielectric anomaly with a broad peak was observed at 485 °C. Simultaneous substitution of Ba 2+ and La 3+ increases the transition temperature of SrBi 2 Nb 2 O 9 (SBN) from 392 to 485 °C. XRD studies in BSLBN revealed an orthorhombic structure with lattice parameters a = 5.5023 Ǻ, b / a = 1.000 and c = 25.0697 Ǻ. Impedance and modulus plots were used as tools to analyze the sample behavior as a function of frequency. Cole–Cole plots showed non-Debye relaxation. DC and AC conductivity measurements were performed on BSLBN. The electric modulus which describes the dielectric relaxation behavior is fitted to the Kohlrausch exponential function. Near the phase transition temperature, a stretched exponential parameter β indicating the degree of distribution of the relaxation time has a small value.

Magnetic and structural properties of GexMn1-x films: Precipitation of intermetallic nanomagnets

Abstract: We present a comprehensive study relating the nanostructure of Ge0.95Mn0.05 films to their magnetic properties. The formation of ferromagnetic nanometer-sized inclusions in a defect-free Ge matrix fabricated by low-temperature molecular beam epitaxy is observed down to substrate temperatures T-S as low as 70 degrees C. A combined transmission electron microscopy and electron energy-loss spectroscopy analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound Mn5Ge3. The volume and amount of these precipitates decrease with decreasing T-S. Magnetometry of the films containing precipitates reveals distinct temperature ranges: Between the characteristic ferromagnetic transition temperature of Mn5Ge3 at approximately room temperature and a lower, T-S-dependent blocking temperature T-B the magnetic properties are dominated by superparamagnetism of the Mn5Ge3 precipitates. Below T-B, the magnetic signature of ferromagnetic precipitates with blocked magnetic moments is observed. At the lowest temperatures, the films show features characteristic of a metastable state.

Transport and thermal properties of weakly ferromagnetic Sr2IrO4

Abstract: We have synthesized polycrystalline Sr2IrO4 and measured its magnetic susceptibility, electrical resistivity, specific heat, Seebeck coefficient, and thermal conductivity. The magnetic susceptibility χ(T) shows a ferromagnetic transition at 250 K while the behaviour above the transition temperature is well described by a Curie–Weiss fit with a small effective moment μeff = 0.33 μB and a paramagnetic Curie–Weiss temperature, θCW = +251 K, consistent with previous studies on this compound. However, specific heat, Seebeck coefficient, and thermal conductivity are all dominated by the phonon contribution and show no anomalies at the ferromagnetic transition. Electrical resistivity, unlike the single crystal, shows a huge increase, three orders of magnitude, with decreasing temperature. The temperature dependence of resistivity is logarithmic at high temperatures (210 K

Temperature dependence of the direct piezoelectric effect in relaxor-ferroelectric single crystals: Intrinsic and extrinsic contributions

Abstract: The direct piezoelectric response d33 of [001]C-poled 0.955Pb(Zn1∕3Nb2∕3)O3–0.045PbTiO3 [PZN-4.5PT] and 0.98Pb(Zn1∕3Nb2∕3)O3–0.08PbTiO3 [PZN-8PT] has been investigated as a function of temperature upon heating above 40°C to the paraelectric phase. Using a Rayleigh-law based analysis, it is shown that both the reversible/intrinsic and irreversible (extrinsic) contributions to the response increase in both compositions as the phase transition to a tetragonal phase is approached. The latter is likely due to an increased domain wall mobility close to the first order transition temperature, which also gives rise to an increased frequency dispersion. Large reversible direct piezoelectric responses d33>1600pm∕V are observed for both compositions, which increase dramatically close to the transition temperature. Most importantly, the reversible contribution is always much larger than the irreversible part in the low temperature, domain-engineered phase, the latter accounting for around 20% of the response in PZN-8...

Crystal Pattern Formation and Transitions of PEO Monolayers on Solid Substrates from Nonequilibrium to near Equilibrium

Abstract: The crystal pattern formation and transition of the monolayers of a low-molecular-weight poly(ethylene oxide) (Mn = 5000 g/mol) on the surface of silicon wafer were studied using atomic force microscopy with a hot stage in the temperature range 25 ≤ Tc ≤ 59 °C. Our observations show that the crystal patterns are greatly dependent on the crystallization temperature, Tc. At 25 ≤ Tc < 39 °C a dendrite pattern with a fractal dimension Dp = 1.672 ± 0.027 was found. At 39 < Tc ≤ 48 °C the crystal pattern become seaweed with the same fractal dimension. The difference between dendrite and seaweed patterns can be quantitatively described by the angle between main and side branches. The dendrite-to-seaweed transition occurred at Tc = 39 °C. At 54 ≤ Tc ≤ 59 °C the crystals had a square shape. A dramatic change occurred at the transition temperature from 49 to 52 °C. In this temperature range the crystal pattern alters from seaweed to compact structure and finally to the square shape of single crystals. The possible...

Martensitic transition and superelasticity of Co–Ni–Al ferromagnetic shape memory alloys with β + γ two-phase structure

Abstract: The martensitic and magnetic phase transitions of Co–Ni–Al β single and β + γ two-phase alloys have been systematically investigated. The Curie temperature T C and the martensitic transition temperature of the β single-phase alloys are sensitive to the Ni and Al compositions. The value of T C increases with decreasing both the Ni and Al contents, while the martensitic transition starting temperature M s decreases with decreasing the Ni content and with increasing the Al content. T C and M s of the β + γ two-phase alloy depend on the annealing temperature due to the variation of the chemical composition of β phase equilibrated with γ phase, increasing by the increase of annealing temperature. In addition, the superelasticity (SE) in tensile loading of the Co–Ni–Al polycrystal β + γ alloys have been investigated. It was found that the SE properties in the β + γ two-phase alloys strongly depend on the average β grain size as well as the volume fraction of γ phase. Consequently, the maximum recovery SE strain of about 6.3% was achieved in a sheet specimen with the large β grains.

Sintering temperature effect on electrical transport and magnetoresistance of nanophasic La0.7Sr0.3MnO3

Abstract: Nanophasic La0.7Sr0.3MnO3 (LSMO) samples were prepared by the sol–gel method. The samples were sintered at different temperatures ranging from 600 to 1000 °C. It is shown that the transport and magnetoresistive properties of LSMO samples strongly depend on the sintering temperature (Ts). A substantial decrease in the insulator–metal transition temperature (TIM) and an enhancement in resistivity are found on lowering the sintering temperature. Furthermore, a reduction in magnetization and a slight decrease in paramagnetic–ferromagnetic (PM–FM) transition temperatures (Tc) have been observed as the sintering temperature decreases. The magnetoresistance (MR) at T

Hot hardness and indentation creep study on Al–5% Mg alloy matrix–B4C particle reinforced composites

Abstract: Al–5% Mg matrix composites reinforced with 10 and 20% B 4 C were prepared by stir casting method. Microhardness of unreinforced alloy and composites were measured using a Nikon hot hardness tester. It was observed that hardness values decrease gradually with increase in temperature. Transition temperature increases marginally from 262 to 274 °C due to addition of 20% B 4 C. Stress exponent for all the materials varies from 4.3 to 4.5 at temperatures in the range 310 and 370 °C. Apparent activation energy for creep increases with increase in B 4 C content. At temperatures below transition temperature, deformation is attributed to the increased interatomic spacing due to increase in temperature and the unpinning of attractive junction between glide and forest dislocations. At temperatures above the transition temperature, deformation is diffusion assisted phenomenon such as dislocation glide and dislocation climb.

Strain-induced phase separation in La0.7Sr0.3MnO3 thin films

Abstract: La0.7Sr0.3MnO3 thin films having different thicknesses were grown by pulsed laser deposition with in situ reflection high energy electron diffraction diagnostics on LaAlO3 substrates. The mismatch between film and substrate gives rise to an in-plane compressive biaxial strain, which partially relaxes in films thicker than 30 unit cells. Accordingly, the ratio between the out-of-plane and the in-plane lattice parameter (c/a) varies between 1.06 (fully strained) and 1.03 (partially relaxed). In-plane compressive strain favors the stabilization of the 3z(2)-r(2) orbitals (chain-type antiferromagnetic phase), thus giving rise to a sizeable x-ray absorption linear dichroism signal. The shape of the linear dichroism depends weakly on the c/a ratio, while its intensity strongly increases with c/a. At the same time, the metal-insulator transition temperature shifts from about 360 K towards lower temperatures with decreasing thickness, eventually reaching an insulating state for the 30 unit cells film. Low-temperature nuclear magnetic resonance spectra show a decrease of the Mn-DE double-exchange metallic contribution with decreasing the thickness, which becomes negligible in the 30 unit cells thick film. The experimental results demonstrate a strain driven competition between two stable phases: the orbital ordered chain-type insulating antiferromagnetic and the orbital disordered metallic ferromagnetic. For intermediate values of the epitaxial strain the local minimum state of the system lies in a gap region between the two stable phases. Such a region has glassy characteristics with coexisting clusters of the two phases. The strain is used as a driving force to span the glassy region.

3d transition metal intercalates of the niobium and trantalum dichalcogenides

Abstract: Optical reflectivity spectra of the first-row transition metal intercalation complexes, M1/3NbS2 and M1/3TaS2 (M = V, Cr, Mn, Fe, Co, Ni), Mn1/4 TaS2 and Fe1/4 NbSe2 have been measured at room temperature and compared with those for the host compounds, 2H NbS2, 2H TaS2 and 2H NbSe2. It is found that a simple rigid-band model is inconsistent with the data and that it is necessary to include both broadening of the valence and the lowest conduction band through increased interlayer interactions and broadening of the 3d energy levels on the intercalate ions through interaction with the Nb or Ta d-band conduction electrons. Quantitative analysis of the free-carrier reflectivity edge is performed by fitting to a simple Drude model from which the free-carrier effective mass and scattering time can be deduced.

Two-stage melting of Au-Pd nanoparticles.

Abstract: Several series of molecular dynamics runs were performed to simulate the melting transition of bimetallic cuboctahedral nanoparticles of gold−palladium at different relative concentrations to study their structural properties before, in, and after the transition. The simulations were made in the canonical ensemble, each series covering a range of temperatures from 300 to 980 K, using the Rafii−Tabar version of the Sutton and Chen interatomic potential for metallic alloys. We found that the melting transition temperature has a strong dependence on the relative concentrations of the atomic species. We also found that, previous to the melting transition, the outer layer of the nanoparticle gets disordered in what can be thought as a premelting stage, where Au atoms near the surface migrate to the surface and remain there after the particle melts as a whole. The melting of the surface below Tm is consistent with studies of the interaction of a TEM electron beam with Au and Au−Pd nanoparticles.

Superstructure Evolution in Poly(ethylene terephthalate) during Uniaxial Deformation above Glass Transition Temperature

Abstract: The evolution of superstructure and its relationship with the phase transition during uniaxial deformation of poly(ethylene terephthalate) (PET) at temperatures (90 and 100 °C) above its glass transition temperature were investigated by in-situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). It appears that deformation at lower temperatures enhances the metastability of mesophase but narrows the strain window for phase transition. Very similar superstructure evolution pathways were observed at both temperatures. In zone I (the plastic deformation zone), WAXD did not show any crystal diffraction peak; however, SAXS exhibited an equatorial streak at the later stage, indicating the formation of a microfibrillar structure. Strain hardening took place in zone II, which could be categorized in two substages. In zone II-a, SAXS showed an X-shaped pattern that coincided with the appearance of crystal diffraction peaks in WAXD. The initial X-shaped patterns possessed strong intensity n...