Showing papers on "Thermal expansion published in 2010"

Pronounced Negative Thermal Expansion from a Simple Structure: Cubic ScF3

Abstract: Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ∼300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ∼1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.

Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene nanoelectromechanical systems resonators

Abstract: We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300 and 30 K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With a lowering of temperature, we find that the positively dispersing electromechanical modes evolve into negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature-compensated sensors.

Exceptionally large positive and negative anisotropic thermal expansion of an organic crystalline material.

Abstract: Occasionally, organic crystalline materials contract when heated (negative thermal expansion), and the mechanisms responsible for this phenomenon are poorly understood. The arrangement of dumbbell-shaped molecules in an organic material is shown to give rise to its negative thermal expansion. The packing and intermolecular interactions facilitate a cooperative mechanical response to temperature causing a decrease in lattice dimensions.

A first-principles approach to finite temperature elastic constants

Abstract: A first-principles approach to calculating the elastic stiffness coefficients at finite temperatures was proposed. It is based on the assumption that the temperature dependence of elastic stiffness coefficients mainly results from volume change as a function of temperature; it combines the first-principles calculations of elastic constants at 0 K and the first-principles phonon theory of thermal expansion. Its applications to elastic constants of Al, Cu, Ni, Mo, Ta, NiAl, and Ni₃Al from 0 K up to their respective melting points show excellent agreement between the predicted values and existing experimental measurements.

Elucidating Negative Thermal Expansion in MOF-5

Abstract: Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework.

Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene NEMS resonators

Abstract: We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 K to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300K and 30K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With lowering of temperature, we find that the positively dispersing electromechanical modes evolve to negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature compensated sensors.

Structure and Properties of La0.4Sr0.4TiO3 Ceramics for Use as Anode Materials in Solid Oxide Fuel Cells

Abstract: In this work, La0.4Sr0.4TiO3-based ceramics intended for use as anode materials in solid oxide fuel cells were investigated. The material was found to preserve its cubic structure throughout testing across a range of conditions, although slight signs of phase segregation were also identified under extreme conditions. The thermal expansion coefficient measured on differently processed samples predicts a good thermomechanical compatibility with yttria-stabilized zirconia (YSZ). Conductivity measurements indicated that the material exhibits a fast surface reduction, unlike the bulk reduction, which progresses very slowly. An increase of the prereduction temperature was found to significantly increase the extent of the reduction and, therefore, the conductivity. Samples sintered under a 5% H2/Ar flow, at 1400 °C, showed high conductivities of up to 96 S cm−1 at 880 °C and oxygen partial pressures of pO2 = 10−20 atm. Correlations between the oxygen deficiency, conductivity, porosity, reduction temperature, and...

Thermophysical Comparison of Five Commercial Paraffin Waxes as Latent Heat Storage Materials

Abstract: Thermophysical properties of phase change materials (PCM) are of utmost importance in latent heat thermal energy storage (LHTES) applications. Therefore, an experimental study is conducted in order to determine thermophysical properties of five technical grade paraffin waxes produced by major Croatian oil company, INA d.d. Rijeka. The temperatures and enthalpies of melting and solidification (latent heat capacity) and specific heat capacities of solid and liquid paraffin waxes were measured by differential scanning calorimetry (DSC). The thermal diffusivity of paraffin waxes was determined utilizing transient method. The importance of eliminating phase transformation interferences to thermophysical properties determination is addressed. The densities and the coefficient of thermal expansion were measured using Archimedes methods. A self-adopted simple and inexpensive laboratory procedure for the determination of liquid density as a temperature function is presented. Finally, the thermal conductivities have been calculated from measured densities, heat capacities and diffusivities. Based on results obtained, the investigated paraffin waxes were evaluated in regard to their applicability as PCM for LHTES.

Characterization of PTFE Using Advanced Thermal Analysis Techniques

Abstract: Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer used in numerous industrial applications. It is often referred to by its trademark name, Teflon. Thermal characterization of a PTFE material was carried out using various thermal analysis and thermophysical properties test techniques. The transformation energetics and specific heat were measured employing differential scanning calorimetry. The thermal expansion and the density changes were determined employing pushrod dilatometry. The viscoelastic properties (storage and loss modulus) were analyzed using dynamic mechanical analysis. The thermal diffusivity was measured using the laser flash technique. Combining thermal diffusivity data with specific heat and density allows calculation of the thermal conductivity of the polymer. Measurements were carried out from − 125 °C up to 150 °C. Additionally, measurements of the mechanical properties were carried out down to − 170 °C. The specific heat tests were conducted into the fully molten regions up to 370 °C.

Ground-state properties and high-pressure behavior of plutonium dioxide: Density functional theory calculations

Abstract: Plutonium dioxide is of high technological importance in nuclear fuel cycle and is particularly crucial in long-term storage of Pu-based radioactive waste. Using first-principles density-functional theory, in this paper we systematically study the structural, electronic, mechanical, thermodynamic properties, and pressure induced structural transition of PuO$_{2}$. To properly describe the strong correlation in the Pu $5f$ electrons, the local density approximation$+U$ and the generalized gradient approximation$+U$ theoretical formalisms have been employed. We optimize the $U$ parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic, ferromagnetic, and antiferromagnetic configurations for both ground state fluorite structure and high pressure cotunnite structure. The best agreement with experiments is obtained by tuning the effective Hubbard parameter $U$ at around 4 eV within the LDA$+U$ approach. After carefully testing the validity of the ground state, we further investigate the bonding nature, elastic constants, various moduli, Debye temperature, hardness, ideal tensile strength, and phonon dispersion for fluorite PuO$_{2}$. Some thermodynamic properties, e.g., the Gibbs free energy, volume thermal expansion, and specific heat, are also calculated. As for cotunnite phase, besides the elastic constants, various moduli, and Debye temperature at 0 GPa, we have further presented our calculated electronic, structural, and magnetic properties for PuO$_{2}$ under pressure up to 280 GPa. A metallic transition at around 133 GPa and an isostructural transition in pressure range of 75-133 GPa are predicted.

High performance cobalt-free perovskite cathode for intermediate temperature solid oxide fuel cells

Abstract: Bi doping of SrFeO3−δ results in the formation of a structure with high symmetry and extraordinary electrochemical performance for Bi0.5Sr0.5FeO3-δ, which is capable of competing effectively with the current Co-based cathode benchmark with additional advantages of lower thermal expansion and cost.

Tuning the thermal expansion properties of optical reference cavities with fused silica mirrors

Abstract: We investigate the thermal expansion of low thermal noise Fabry–Perot cavities made of low thermal expansion (LTE) glass spacers and fused silica (FS) mirrors. The different thermal expansion of mirror and spacer deforms the mirror. This deformation strongly contributes to the cavity’s effective coefficient of thermal expansion (CTE), decreasing the zero crossing temperature by about 20 K compared to an all-LTE glass cavity. Finite element simulations and CTE measurements show that LTE rings optically contacted to the back surface of the FS mirrors allow to tune the zero crossing temperature over a range of 30 K.

Damage and Changes in Mechanical Properties of a Gabbro Thermally Loaded up to 1,000°C

Abstract: Thermal loading of rocks at high temperatures induces changes in their mechanical properties. In this study, a hard gabbro was tested in the laboratory. Specimens were slowly heated to a maximum temperature of 1,000°C. Subsequent to the thermal loading, specimens were subjected to uniaxial compression. A drastic decrease of both unconfined compressive strength and elastic moduli was observed. The thermal damage of the rock was also highlighted by measuring elastic wave velocities and by monitoring acoustic emissions during testing. The micromechanisms of rock degradation were investigated by analysis of thin sections after each stage of thermal loading. It was found that there is a critical temperature above which drastic changes in mechanical properties occur. Indeed, below a temperature of 600°C, microcracks start developing due to a difference in the thermal expansion coefficients of the crystals. At higher temperatures (above 600°C), oxidation of Fe2+ and Mg2+, as well as bursting of fluid inclusions, are the principal causes of damage. Such mechanical degradation may have dramatic consequences for many geoengineering structures.

Material characterization and residual stresses simulation during the manufacturing process of epoxy matrix composites

Abstract: A thermal, rheological and mechanical material characterization of an aeronautic epoxy resin from commercial prepreg is reported in this article. The kinetic of the crosslinking reaction of the resin is characterized and modeled. The specific heat, the glass transition temperature, the thermal expansion coefficients, the chemical shrinkage coefficients and the thermo-mechanical properties have been investigated as a function of temperature and degree of cure. Dynamic mechanical measurements are used to determine the gel point. Finally, the residual stresses developed during the curing process are calculated using a finite element simulation, taking into account the material properties evolutions according to proposed models. The results highlight the importance of the characterization accuracy and the associated models.

Mechanism of heat-modification inside a glass after irradiation with high-repetition rate femtosecond laser pulses

Abstract: Accumulation of thermal energies by highly repeated irradiation of femtosecond laser pulses inside a glass induces the heat-modification whose volume is much larger than that of the photoexcited region. It has been proposed that the heat-modification occurs in the region in which the temperature had overcome a threshold temperature during exposure of laser pulses. In order to understand the mechanism of the heat-modification, we investigated the temperature distribution during laser exposure and the threshold temperature by analyzing the volume of the modification based on a thermal diffusion model. We found that the threshold temperature becomes lower with increasing laser exposure time. The dependence of the threshold temperature on the laser exposure time was explained by the deformation mechanism based on the temperature-dependent viscosity and viscoelastic behavior of a glass under a stress loading by thermal expansion. The deformation mechanism also could simulate a tear-drop shape of a heat-modification by simultaneous double-beams’ irradiation and the distribution of birefringence in a heat-modification. The mechanism proposed in this study means that the temperature-dependence of the viscosity of a glass should be essential for predicting and controlling the heat-modification.

Unusual deviations from bulk behavior in ultrathin films of poly(tert-butylstyrene): can dead layers induce a reduction of T(g)?

Abstract: The temperature and thickness dependence of the thermal expansivity of ultrathin thin films of poly(tert-butylstyrene) capped between aluminum layers revealed an unusual and intriguing confinement scenario. Below 50 nm, both the glass transition temperature and the thermal expansion coefficients decreased. Such a mixed behavior implies an enhancement of the molecular mobility, without the presence of any free surface, but dead layers. On the basis of a careful analysis of averaged quantities measured by capacitive dilatometry, we deduced a profile of thermal expansivity that explains our experimental data and previous observations in polymer nanocomposites. The effect of density-conformation coupling in proximity of a nonattractive interface allows the coexistence of an immobilized fraction in direct contact with the metal and an excess of thermal expansivity, arising from the long-range effects of packing frustration penetrating inside the bulk-like core of the film.

Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene NEMS resonators

Abstract: We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 K to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300K and 30K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With lowering of temperature, we find that the positively dispersing electromechanical modes evolve to negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature compensated sensors. PACS numbers: 85.85.+j, 81.05.ue, 65.40.De, 65.60.+a Graphene NEMS resonator 2

Zero thermal expansion in a flexible, stable framework : tetramethylammonium copper(I) zinc(II) cyanide.

Abstract: Tetramethylammonium copper(I) zinc(II) cyanide, which consists of N(CH(3))(4)(+) ions trapped within a cristobalite-like metal cyanide framework, has been studied by variable-temperature powder and single-crystal X-ray diffraction. Its coefficient of thermal expansion is approximately zero over the temperature range 200-400 K and comparable with the best commercial zero thermal expansion materials. The atomic displacement parameters, apparent bond lengths, and structure of a low-temperature, low-symmetry phase reveal that the low-energy vibrational modes responsible for this behavior maintain approximately rigid Zn coordination tetrahedra but involve significant distortion of their Cu counterparts.

Thermal Expansion, Ferroelectric and Magnetic Properties in (1 − x)PbTiO3−xBi(Ni1/2Ti1/2)O3

Abstract: A zero thermal expansion and multiferroic compound 0.8PbTiO3−0.2Bi(Ni1/2Ti1/2)O3 was developed by a chemical modification route. The structure studies showed that the tetragonality of (1 − x)PbTiO3−xBi(Ni1/2Ti1/2)O3 was gradually weakened to cubic by introducing the dopant Bi(Ni1/2Ti)1/2O3, and the thermal expansion coefficient changed from −8.81 × 10−6/°C to 8.46 × 10−6/°C in 0.1 ≤ x ≤ 0.3 around a wide temperature range (from RT to about 500 °C). Weak ferromagnetic behavior was observed in the solid solutions, and the superexchange interaction was incorporated to explain its nonmonotonous evolution. Meanwhile, the good piezoelectricity and ferroelectricity were well retained. Further investigations demonstrated that the (1 − x)PbTiO3−xBi(Ni1/2Ti1/2)O3 ceramics possessed good mechanical properties, such as high density and excellent fracture toughness. The improved behaviors make the (1 − x)PbTiO3−xBi(Ni1/2Ti1/2)O3 promising piezoceramics with high thermal stability and mechanical performance. The presen...

The β-δ-Phase Transition and Thermal Expansion of Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine

Abstract: Phase behavior of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is investigated by X-ray powder diffraction (XRD). The XRD patterns at elevated temperature show that there is a co-existing temperature range of β- and δ-phase during the phase transition process. Additionally, mechanical forces can catalyze the conversion from δ- back to β-phase. Based on the diffraction patterns of β- and δ-phase at different temperatures, we calculate the coefficients of thermal expansion by Rietveld refinement. For β-HMX, the linear coefficients of thermal expansion of a-axis and b-axis are about 1.37×10−5 and 1.25×10−4 °C−1. A slight decrease in c-axis with temperature is also observed, and the value is about −0.63×10−5 °C−1. The volume coefficient of thermal expansion is about 1.60×10−4 °C−1, with a 2.2% change from 30 to 170 °C. For δ-HMX, the linear coefficients of thermal expansion of a-axis and c-axis are found to be 5.39×10−5 and 2.38×10−5 °C−1, respectively. The volume coefficient of thermal expansion is about 1.33×10−4 °C−1, with a 2.6% change from 30 to 230 °C. The results indicate that β-HMX has a similar volume coefficient of thermal expansion compared with δ-HMX, and there is about 10.5% expansion from β-HMX at 30 °C to δ-HMX at 230 °C, of which about 7% may be attributed to the reconstructive transition.