Low Temperature Relaxation Behavior Research Articles

Simultaneously achieving large energy storage density and high efficiency in the optimized Sr2NaNb5O15 system with excellent temperature stability at a low electric field

Lead-free capacitors operating at low electric fields with high-energy density and high efficiency are critical for advanced electronic systems and innovative pulsed power applications. In this work, single tetragonal phase dielectric ceramics with high-energy storage density and high efficiency are prepared using the solid-state method. The co-substitution of Ba2+, Sm3+, and Mn4+ on the Sr2NaNb5O15 substrate enhances low-temperature relaxation behavior and polarization modulation due to order/disorder distribution and polar nanoregions (PNRs) in the structural distortion system. Ferroelectric tests revealed that the optimized Sr1.83Ba0.16Sm0.01NaMn0.01Nb4.99O15 tungsten bronze system has large energy storage density (Wrec = 4.432 J cm−3) and high efficiency (η = 84.3 %) at a low electric field (250 kV cm−1). In addition, the temperature-dependent permittivity results for the Sr1.82Ba0.165Sm0.015NaMn0.015Nb4.985O15 and Sr1.81Ba0.17Sm0.02NaMn0.02Nb4.98O15 ceramic samples demonstrated that they are X9R capacitor-compatible. Finally, Sr1.83Ba0.16Sm0.01NaMn0.01Nb4.99O15 has a current density (CD) of ∼797.77 A cm−2 and power density (PD) of ∼87.75 MW cm−3 with a very fast discharge time (t0.9 < 60 ns). It is worthy to note that all the components display excellent temperature stability (ΔWdis < 10 %) in the over-damping test. Thus, this work provides an avenue to obtain dielectric ceramic materials with potential application value in innovative pulse power components.

Low-temperature relaxation behavior of a bulk metallic glass leading to improvement of both strength and plasticity

Low temperature relaxation and structural changes in a Cu36Zr48Al8Ag8 bulk glassy alloy well below the glass-transition temperature are examined by using differential scanning calorimetry, X-ray diffractometry, scanning and transmission electron microscopy. Although densification of the sample takes place owing to beginning of structural relaxation both room-temperature mechanical strength and plasticity values are increased. The studied samples exhibited an inverse relationship between compressive strength and hardness. However, no clear structural changes in terms of crystallization or phase separation are observed. The results are interpreted in terms of both structural relaxation and release of internal stresses.

Unusual low temperature relaxation behavior of crosslinked acrylonitrile-butadiene co-polymer

Conventional crosslinking of rubbers (using sulphur and peroxide vulcanization) usually leads to covalent rubber network junctions. The present article reports about a new unconventional crosslinking of acrylonitrile butadiene copolymer (NBR) by zinc chloride (ZnCl2). This crosslinked NBR displayed very interesting relaxation properties below the glass transition temperature. In this work, detailed chemical mechanistic pathways were suggested and correlated with the low-temperature relaxation dynamics of the crosslinked material. Fourier-transform infrared spectroscopy analysis clearly indicated a coordination effect between zinc and nitrile groups. At lower temperatures, beside the glass transition, another relaxation process (β relaxation) was detected. Interestingly, a hydrolyzed crosslinked sample showed even two other additional relaxation processes. The origin of such relaxations was critically discussed from the viewpoints of modification of the polymer chains as well as low molecular weight fragments of the polymer.

Magnetic-Field-Dependent Lifetimes of Hyperpolarized 13C Spins at Cryogenic Temperature.

Using a home-built cryogen-free dynamic nuclear polarization (DNP) system with a variable magnetic field capability, 13C spin-lattice T1 relaxation times of hyperpolarized [1-13C] carboxylates (sodium acetate, glycine, sodium pyruvate, and pyruvic acid) doped with trityl OX063 free radical were systematically measured for the first time at different field strengths up to 9 T at T = 1.8 K. Our data reveal that the 13C T1 values of these frozen hyperpolarized 13C samples vary drastically with the applied magnetic field B according to an apparent empirical power-law dependence (13C T1 ∝ Bα, 2.3 < α < 3.1), with relaxation values ranging from a few hundred seconds at 1 T to over 200,000 s at fields close to 9 T. This low temperature relaxation behavior can be ascribed approximately to a model that accounts for the combined effect of 13C-1H intramolecular dipolar interaction and the relaxation contribution from the paramagnetic impurities present in the DNP sample. Since the lifetime or T1 storage of the hyperpolarized state is intimately linked to DNP efficiency, these 13C relaxation data at cryogenic temperature have important theoretical and experimental implications as the DNP of 13C-labeled biomolecules is pushed to higher magnetic fields.

Origin of giant dielectric response in LiCuNb3O9 distorted perovskite ceramics

LiCuNb3O9 ceramics with the distorted cubic perovskite structure were prepared by a solid-state reaction method. The ceramic exhibited a very large value of permittivity (∼4.4×104 at 100kHz) at room temperature (∼300K) and a low-temperature dielectric relaxation behaviour following the Arrhenius law. The origin of the giant dielectric response of the LiCuNb3O9 ceramics was correlated with the structure of the ceramics. The barrier layers in the grain boundaries and the mixed-valent structure of Cu+/Cu2+ were found to contribute to the giant permittivity of the ceramics and confirmed by X-ray spectroscopy and complex impedance spectroscopy analyses.

バルク金属ガラス中の低温緩和挙動

Low temperature structural relaxation in Zr55Cu30Ni5Al10 bulk metallic glass (BMG) that evolved at about 100～150 K under glass transition temperature was investigated by electrical resistance relaxation and volume relaxation. The reversible isochronal change in resistance and the kinetics of relaxation was in agreement with low temperature relaxation behavior previously reported in many amorphous metals. It was checked whether or not the kinetic of relaxation examined obeys kinematical criterions deduced from dynamic mechanical analysis for non-organic glass, polymer glass as well as metallic glass. At least, with regard to a limited glass as pre-annealed Zr55Cu30Ni5Al10 BMG, the equivalency of low temperature relaxation to β-relaxation (Johari-Goldstein relaxation) was not verified.

Dielectric relaxation near 25 K in multiferroic BiFeO3 ceramics

Our investigations using low-temperature Raman spectroscopy, dielectric, and magnetodielectric studies of as-prepared and vacuum-annealed BiFeO3 ceramics revealed two dielectric anomalies near 25 and 281 K. Systematic studies were carried out to probe the anomaly near 25 K which exhibits clear frequency dependence and falls close to a previously observed magnetic transition. The low-temperature dielectric relaxation behavior yielded characteristic energy scales of ∼30 and ∼34 meV for the as-prepared and vacuum-annealed BiFeO3 samples, respectively. Raman spectra showed hardening of the E(TO6) and E(TO9) modes near 25 K, and the Raman shift in phonon frequencies is found to be broader in the vacuum-annealed BiFeO3 than in the as-prepared BiFeO3. We observed a linear magnetodielectric coupling in both samples at 25 K. These experimental results suggest that coupling among spin, lattice, and dielectric properties persist to low temperatures in BiFeO3.

Dynamical transport behavior in electron-doped manganites La0.4Ca0.6(Mn1−x Ru x )O3

Measurements of the dynamical electrical transport behavior are performed on electron-doped manganites La(0.4)Ca(0.6)(Mn(1-x) Ru (x) )O(3) (x=0 and 0.02). An undoped sample possesses a robust charge-ordered antiferromagnetic ground state, and only a positive resistivity relaxation can be observed. However, a low-temperature negative relaxation behavior arises after inducing a few ferromagnetic orders to the charge-ordered matrix by tiny Ru doping. We assigned this difference to the dynamical competition between ferromagnetic metallic and charge-ordered insulating phases. Consistently, for a doped sample, the crossover from positive to negative resistivity relaxation behavior ensues around T=115 K, which is just below the ferromagnetic Curie temperature.

Flux growth and low temperature dielectric relaxation in piezoelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals

AbstractSingle crystals of Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZNT 91/9) have been grown by flux method after modifications in temperature profile, flux ratio and addition of excess ZnO/B2O3 which resulted in enhanced perovskite yield (more than 95%). Only a few crystals showed the presence of pyrochlore phase/variation in composition. A comparative characterization of these crystals were carried out in respect of piezoelectric charge coefficient d33, dielectric constant, ac conductivity and hysteresis loop after cutting and poling the crystals along [001] direction. The total activation energy for conduction has been found to increase with Ti‐content in the sample. The effect of ZnO on growth behavior has been analyzed. A detailed analysis of PZNT (91:9) has been carried out at low temperature in respect of the various thermodynamic parameters related to the dielectric relaxation mechanism, like optical dielectric constant, static dielectric constant, free energy of activation for dipole relaxation, enthalpy of activation and relaxation time, have been calculated in the vicinity of transition temperature in the lower temperature region. The activation energy for relaxation at ‐10 and ‐49 °C have been found to be 0.09 and 0.02 eV respectively. The results were analyzed and a detailed dielectric analysis and low temperature relaxation behavior of PZNT crystals were interpreted. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Low frequency dielectric investigations into the relaxation behavior of frozen polyvinylpyrrolidone–water systems

The low frequency dielectric response of aqueous solutions containing 0, 1, 5, and 10% w/v polyvinylpyrrolidone (PVP) was studied to characterize the low temperature relaxation behavior of these systems. Complementary modulated temperature differential scanning calorimetry (MTDSC) studies allowed measurement of the glass transition temperature for these materials, corresponding to the behavior of the nonfrozen phase. Dielectric investigations in the frequency range of 106 to 10−2 Hz were performed on the systems in the liquid state, with a Maxwell–Wagner response noted for both the PVP solutions and water. The solid-phase responses were studied over a range of temperatures down to − 70°C, with a relaxation peak observed for the PVP systems in the kilohertz region. The spectra were modeled using the Havriliak–Negami equation and the corresponding relaxation times were calculated, with a satisfactory fit to the Arrhenius equation noted. The calculated activation energies were similar to literature values for the dielectric relaxation of water. It is suggested that the dielectric response is primarily a reflection of the relaxation behavior of the water molecules in the nonfrozen fraction, thereby indicating that the dielectric technique may yield insights into specific components of frozen aqueous systems. © 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90:157–164, 2001

Merging of the Dielectric α and β Relaxations in Glass-Forming Polymers

The merging of the main and the secondary dielectric relaxations in a series of six polymers has been investigated by using broadband dielectric spectroscopy (10-2−109 Hz). The data have been analyzed in two different ways. In one case, the whole relaxation process has been modeled by the simple addition of the relaxation functions corresponding to the main and secondary relaxations. In the other case, we have used the approach first proposed by Williams and Watts (Williams, G.; Watts, C.D. In NMR, Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld R., Eds.; Springer-Verlag, Berlin, 1971; Vol. 4, pp 271−285). This approach can be formally reduced to the addition of two terms, one corresponding to the main relaxation and the other to a combination of the functions corresponding to the main and secondary relaxations extrapolated from low temperatures. Both methods of analysis happen to describe the experimental data successfully in the merging region. In the framework of the Williams and Watts approach, the experimental behavior in the merging range is explained simply as a natural consequence of the extrapolation of the low-temperature relaxation behavior. However, using the simple addition method, this is not possible, which in this framework would be interpreted as due to a change of the secondary relaxation mechanism in the merging region. Therefore, the Williams and Watts approach provides the simplest physical picture for describing the merging of the α and β relaxations. Finally, the results obtained in this framework are interpreted using a free-energy landscape model.

Relaxation behaviour in blends of PEEK and PEI

Blends of poly(ether ether ketone) and poly(ether imide) have been produced in the composition range 2–20 wt% poly(ether ether ketone) and dynamic mechanical and dielectric behaviour of these materials have been investigated. A composition dependent α process was observed using both techniques, the temperature of which increased with increasing poly(ether imide) content. The low temperature relaxation behaviour of poly(ether imide) and the dielectric relaxation time in the glass transition region was found to be affected by the addition of poly(ether ether ketone). The variation of loss and storage permittivities showed that the asymmetric broadening was strongly dependent on the blend composition, indicating a high degree of interaction between the components.

Response to “Comment on ‘A 250 GHz ESR study of o-terphenyl dynamic cage effects above TC’ ” [J. Chem. Phys. 109, 10523 (1998)

We address the points raised by Giordano and Leporini (GL) and show that accounting properly for the nonexponential decay of the rotational correlation function leads to improved agreement with the Stokes–Einstein–Debye (SED) relation above the crossover temperature TC for those probes 3,3′-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL), and perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide) (MOTA) that are well-coupled to the viscous modes of o-terphenyl (OTP) when the average relaxation rate 16〈τ〉 is plotted versus 1/T. On the other hand, 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) shows simple Arrhenius behavior in this regime, because of weak coupling to the solvent cage, inconsistent with SED, which was clearly shown in our paper. We also suggest that the difference in chemical structure of the PDT probe, studied by us, compared to 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO), studied by GL, accounts for the difference in the low-temperature relaxation behavior of the two probes.

NON-EXPONENTIAL RELAXATION IN A RESONANT QUANTUM TUNNELING SYSTEM OF MAGNETIC MOLECULES

We have studied the low temperature relaxation behavior of the octanulcear iron molecular magnet, Fe8. For temperatures below 400mK we observed a crossover to a quantum tunneling regime. In this regime the relaxation is non-exponential and we argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is presented for a longer time range that is in good agreement with the data over 4 decades in time. Numerical calculations of the local field distribution indicate unusual correlation effects.

Scaling in small-particle systems: low-temperature behaviour

An alternative method for analyzing the magnetic relaxation data in small-particle systems based on the T ln(tτ0) scaling is presented. Two experimental systems have been studied using this procedure and the resulting master curves have been fitted using different functions for the energy barrier distribution to reproduce the low-temperature relaxation behaviour.

Persistent infrared spectral hole burning of Tb^3+ in the glasslike mixed crystal Ba_1−x_−yLaxTbyF_2+x_+y

Persistent IR spectral holes that exhibit glasslike characteristics—highly nonexponential low-temperature relaxation behavior and a broad distribution of barrier heights observable in thermal-cycling studies—have been produced in the inhomogeneously broadened 4.5-μm 7F6 → 7F5 electronic absorption band of the Tb3+ ion in the mixed crystal Ba1−x−yLaxTbyF2+x+y. Investigations of six compositions, ranging from x = 0, y = 0.005 to x = 0.30, y = 0.05, reveal no hole burning for the x = 0, y = 0.005 case and a hole-burning quantum efficiency of the order of 5 × 10−7 with a weak dependence of the lanthanide fraction for x + y ≥ 0.05. For all compositions with x + y ≥ 0.05 hole behavior is virtually identical, with both the widths and the peak positions of the distributions of relaxation rates and barrier heights for these mixed crystals being typical of those found for glasses.

Low temperature relaxation of DGEBA epoxy resins: A thermally stimulated discharge current (TSDC) study

AbstractThe effect of curing conditions on the low temperature relaxation behavior of catalyst‐cured epoxy systems based on digliycidyl ether of bisphenol A (DGEBA) has been characterized by the thermally stimulated discharge current (TSDC) technique. In these chemically relatively simple epoxy systems, five relaxation processes, designated as γ, β, β′, βOH, and β″, have been observed: Their molecular origins are discussed in detail. The results are in agreement with prior suggestions of an inhomogeneous cross‐link density morphology.

Mechanical relaxation properties of spiro-type epoxide resins cured with acid anhydrides

Low-temperature relaxation behaviour has been investigated for a bisphenol-A type and three spiro-type epoxide resins cured with acid anhydrides. In the spiro-type resin systems, one new relaxation, denoted here as the β′ relaxation, is clearly observed near room temperature. It is suggested that the β′ relaxation is related to the motion of p-phenylene group adjacent to the spiro ring. The impact strength of epoxide resin systems that show the β′ relaxation is considerably higher than that of other cured systems and is proportional to the intensity of the β′ relaxation. Consequently, it is concluded that the existence of the room-temperature relaxation such as the β′ relaxation is associated with increased toughness of cured epoxide resins.

Role of potential-energy scaling in the low-temperature relaxation behavior of amorphous materials.

Under the assumption that the Kohlrausch-Williams-Watts (KWW) function describes relaxation near the glass transition for amorphous substances, implications are explored for the character of the many-body potential-energy function. The analysis proceeds in several stages: (1) The configuration space is uniquely divided into minimum-containing cells by a steepest-descent construction on the potential hypersurface. (2) Attention is confined to the amorphous region of configuration space by projecting out all cells containing local crystalline patterns. (3) The potential is separated into a hard-core part ${\ensuremath{\Phi}}_{c}$ and a ``soft'' remainder ${\ensuremath{\Phi}}_{s}$. (4) ${\ensuremath{\Phi}}_{s}$ is coarse grained (smoothed) over a variable scale of lengths l. (5) A master equation is used to describe the relaxation spectrum subject to the interaction smoothed over any l. The demand that KWW relaxation emerge from the last constrains the statistical topography of ${\ensuremath{\Phi}}_{s}$. Specifically it requires that on widely separated length scales, multiply branched channels of relatively modest elevation change must exist in ${\ensuremath{\Phi}}_{s}$. Furthermore, the separating barriers between these channels tend to be largest in those portions of configuration space sampled by the system at the lowest temperatures, and to scale as lnl.

1H NMR relaxation studies of the hydrogen-bonded imino protons of poly(dA-dT).

Measurements on the thymine imino proton relaxation rates have been used to study various structural and dynamic properties of 53 +/- 15 base pair long poly(dA-dT). Below 10 degrees C, the relaxation is dominated by dipolar magnetic interactions. At 1 degrees C the relaxation of the transverse magnetization is exponential (R2 = 124 s-1), but the relaxation of longitudinal magnetization is highly nonexponential due to spin-diffusion effects (initial decay rate constant of 28 s-1 and a slower rate of approximately 2.5 s-1 after equilibration of spin polarization). Neither a rigid-rod model nor simple wormlike motions can account for the observed low-temperature relaxation behavior. However, when localized internal motions of the base pairs (three-state jump model) are allowed for, a good fit of the experimental data is obtained by using a correlation time for internal motion of 7 X 10(-10) s and an angular displacement of the bases of +/- 32 degrees relative to the helix axis. The observed R2/R1 ratio for the thymine imino proton yields a value of 1.14 +/- 0.08 A for the imino proton nitrogen distance. Nuclear Overhauser effect (NOE) measurements establish that the base pairing in poly(dA-dT) is Watson-Crick in solution and not Hoogsteen. Exchange of the T-imino protons with H2O dominates the longitudinal relaxation above 28 degrees C (activation energy of 17 +/- 2 kcal and an exchange rate of 5 +/- 2 s-1 at 300 K). Similar values have been reported for the A X T base pairs in DNA restriction fragments and for A X U base pairs in poly(A) X poly(U). These observations can be explained by a model in which exchange of T-imino protons occurs as a result of a single base pair opening, with a rate that is approximately independent of nearest-neighbor sequences and DNA length. Our observations appear to be inconsistent with a soliton model of proton exchange.