Thermal Expansion Curves Research Articles

Crack Healing, Reopening and Thermal Expansion Behavior of Al<sub>2</sub>TiO<sub>5</sub> Ceramics at High Temperature

The low thermal expansion (α25-1100oC = 0.05 ~ 1.6 × 10–6/K) of Al2TiO5 ceramics are apparently due to a combination of grain boundary micro cracking caused by the large thermal expansion anisotropy of the crystal axes of the Al2TiO5 phase. During the reheating run, the individual crystallites expanded at low temperature; thus, the solid volume of the specimen expanded into the micro cracks, where as the macroscopic dimensions remained almost unchanged. As a result, the material expanded very little. The micro cracks closed at higher temperatures. This result is closely related to relatively steeper thermal expansion curves.

Effect of changing rate of residual stress on thermal expansion behavior of magnesium borate whisker-reinforced aluminum composite

Magnesium borate whisker-reinforced aluminum matrix composite is a new aluminum matrix composite with high properties and low cost. In the study, the residual stress changing behaviors on heating and cooling of the composite were investigated through the coefficient of thermal expansion (CTE) curves. On heating, the tensile stress relaxation and compressive stress generation in the matrix of the composite results in the CTE of composite is smaller than that predicted by mixture rule, but the compressive stress relaxation in the matrix of composite causes the CTE of composite is larger than that predicted by mixture rule. The tensile and compressive stress changing rates with temperature are linear on heating in the matrix of the composite at lower and higher temperature ranges. The CTE of the composite on cooling is smaller than that on heating.

Thermal Expansion Characteristics Associated with Spin Fluctuations under Applied Magnetic Field and High Pressure for Lu(Co0.9Ga0.1)2Laves-Phase Compound

The thermal expansion measurements under several kinds of conditions have been carried out for Lu(Co 0.90 Ga 0.10 ) 2 Laves-phase compound. With decreasing temperature, the significant increase of the thermal expansion due to the spontaneous volume magnetostriction ω m ( T ) appears at low temperatures. Under magnetic fields, ω m ( T ) spreads over a wide range of temperature due to the induced ferromagnetic moment above T C . On the other hand, ω m ( T ) becomes smaller by applying hydrostatic pressure p , in addition to the significant decrease of T C . At p = 1.1 GPa, ferromagnetic state disappears and the thermal expansion curve becomes close to that of paramagnetic Lu(Co 0.91 Ga 0.09 ) 2 .

Decay of natural stones caused by fire damage

Abstract Almost every representative ancient building suffered from a fire during its history. Therefore, several limestones, sandstones, a gypsum, granites, tuffs, an orthogneiss and two marbles have been tested to analyse the effect of fire. Thermal expansion measurements up to 1000 °C reveal that every rock shows a specific expansion behaviour. Variations are caused by the single crystal thermal expansion properties of rock-forming minerals and by different damage processes. In silicate rocks, intragranular fracturing is the predominant damage phenomenon. Carbonate rocks show, at low temperatures, a behaviour mainly controlled by the anisotropic expansion of calcite. At higher temperatures, mineral reactions, such as decarbonatization, are directly evidenced by sudden jumps in thermal expansion curves. If water is present, a second stage of deterioration follows fire damage: the huge volume increase due to portlandite formation from decarbonized CaO causes severe scaling at the outermost surface of limestone when exposed to the environment. Small amounts of silicates in carbonate rocks may improve the stability of those rocks due to dicalciumsilicate formation. At high temperatures, an increase in the expansion coefficient may be explained by partial melting for some rock types. Phase changes (e.g. quartz) are monitored by a sudden increase in the expansion coefficient. Investigations on gypsum reveal that dehydration reactions reduce fire temperatures in the vicinity of gypsum rocks significantly. In general, all experiments show that samples are severely damaged after being subjected to fire. Real fire tests show that the penetration depth of heat and the associated damage types vary as a function of lithology. While for granites, cracks in feldspars predominate, the firing of limestone causes a scaling of the outermost layer. The investigations may lead to an improved assessment of natural building stones that have been damaged by fire. Implications can also be drawn for the recent use of facade panels made of natural building stones in case of a future fire.

Microstructure and Crystal Phase Development of Y2O3-Stabilized ZrO2 Polycrystal Fabricated by the Solid Phase Mixing and Sintering Method

The microstructure and crystal phase in 2.8 mol% yttria-stabilized zirconia polycrystals were investigated. Starting powders were prepared by wet dispersing and mixing using an agitator mill or by the liquid phase precipitation method. The composites powders were sintered at 1275-1600°C and then analyzed. Solid phase method (SPM) powders showed higher sinterability than those of the liquid phase precipitation method (LPM) powders, and high density samples consisting of fine grains could be obtained at lower sintering temperatures than from LPM powders. Unlike the LPM sample, the cubic and tetragonal phase was formed in the SPM sample at low sintering temperature, because there was the region of high and low Y3+ ion concentration in the material. As the sintering temperature increased, the cubic phase decreased and tetragonal phase increased by the progress of the diffusion of Y3+ ions. And then, Y3+ ions segregated at grain boundaries, and tetragonal grains containing cubic phase at grain boundary regions were formed. The thermal expansion curve for the SPM sample samples sintered at 1275°C exhibited a monoclinic to tetragonal transformation during the heating stage; however, the expansion was linear during the cooling stage, as was the case for both SPM samples sintered at higher temperatures and the LPM samples. The thermal expansion behavior of the SPM sample suggested that the monoclinic phase was essentially not present within the sample, although the transformed monoclinic phase in a surface layer by grinding stresses was exist.

(Pr0.7Ca0.3)0.9MnO3−δ–SDC cathode for IT-SOFC

(Pr0.7Ca0.3)0.9MnO3−δ was synthesized by an EDTA–citrate complexing method. The powder was characterized with X-ray diffraction and transmission electron microscope. The thermal expansion curve and electrical conductivity of (Pr0.7Ca0.3)0.9MnO3−δ was measured. Electrochemical properties of (Pr0.7Ca0.3)0.9MnO3−δ cathode with and without SDC impregnation were studied by ac impedance analyzer. The results suggest that (Pr0.7Ca0.3)0.9MnO3−δ impregnated with SDC is a suitable cathode for IT-SOFCs based on YSZ electrolyte.

Impact of domain anisotropy on CTE of isotropic microcrystalline material

An analytical, phenomenology-based model is proposed for thermal expansion of isotropic single-phase polycrystalline materials. Standard rules of mixing fail to predict correctly the coefficient of thermal expansion (CTE) of such materials. The model introduced in this paper explains and accurately reproduces the U-shaped thermal expansion curve of low-CTE ceramics (cordierite and β-eucryptite) quantitatively. The integrity function of a material is introduced as a measure of microcracking as a function of temperature. This function can be useful for analysis of different bulk properties of isotropic polycrystalline materials that depend on domain microcracking.

Microstructures and electrical properties of calcium substituted LaFeO 3 as SOFC cathode

Mixed ionic and electronic conductors of La 1− x Ca x FeO 3− δ (LCF, x =0.0–0.5) have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometry, and four-probe electrical conductivity measurements. Ca substitution on La site reduces the LaFeO 3 sinterability and the cell volume of this orthorhombic crystal. Dense samples for property studies can be sintered at 1320 °C. Nevertheless, the sintering temperature is near the decomposition temperature of LCF for those solid solutions of x ≥ 0.3. The LCF decomposition is evident when a La-poor secondary phase, not detected in XRD, was revealed in SEM micrographs of 1270 °C thermally etched samples of x ≥ 0.3. Dilatometric studies demonstrate linear increments in thermal expansion with increasing temperature in samples of x ≤ 0.2, while show strange bendings in thermal expansion curves of x = 0.4 and 0.5. The bending in thermal expansion indicates influences of the secondary phase. The TEC value of compositions of x ≤ 0.2 is between 10.8 and 11.7 × 10 −6 °C −1. The LCF electrical conductivity increases with the Ca content and its temperature dependence can be described by the small polaron hopping mechanism. The composition around x = 0.15 promises to be a superior cathode for SOFC since it thermally matches with 8 mol% YSZ and 10 mol% Dy, Er substituted LAMOX electrolytes and possesses electrical conductivity near 90 S cm −1 at 800 °C.

Crack healing, reopening and thermal expansion behavior of Al 2TiO 5 ceramics at high temperature

Thermal expansion behavior and its correlation with microstructure of ZAT (ZrTiO 4–Al 2TiO 5) composites were investigated. Thermal expansion coefficient increased from 0.05 [mm/(mm°C) × 10 6] at room temperature to 1.6 [mm/(mm°C) × 10 6] at 1100 °C. The low thermal expansion of Al 2TiO 5 ceramics is apparently due to a combination of grain boundary micro-cracking caused by the large thermal expansion anisotropy of the crystal axes of the Al 2TiO 5 phase. During the reheating run, the individual crystallites expanded at low temperature; thus, the solid volume of the specimen expanded into the micro-cracks, whereas the macroscopic dimensions remained almost unchanged. As a result, the material expanded very little up to 1000 °C. The micro-cracks closed at higher temperatures. This result is closely related to relatively steeper thermal expansion curves.

Study on La and Y co-doped ceria-based electrolyte materials

Co-doped ceria electrolytes of Ce 0.8La 0.2− x Y x O 1.9 ( x = 0.02, 0.06, 0.10, 0.14, 0.20) fine powders were prepared with the sol–gel method. The results of X-ray diffraction and Raman spectroscopy showed that all powders crystallite calcined at 800 °C were single phase with cubic fluorite structure, the average crystallite sizes calculated by the Scherrer formula were between 27 and 34 nm, which was in good agreement with the results of TEM and particle size distribution measurements. The thermal expansion curves of C e 0.8 L a 0.2 − x Y x O 1.9 were measured and the thermal expansion coefficients (TEC) between 100 and 800 °C were calculated. For the samples of Ce 0.8La 0.2− x Y x O 1.9, in the temperature range of 700–850 °C, when x = 0.06, 0.10 and 0.14, much higher ionic conductivity was observed than those of the singly doped ceria with same dopant concentration (20% trivalent rare earth) and when x = 0.06, maximal conductivity is obtained. It suggested that co-doping with appropriate ratio of lanthanum and yttrium can further improve the electrical performance of ceria-based electrolytes, and these co-doped samples may be the better electrolytes for intermediate-temperature solid oxide fuel cells.

Thermal expansion behavior and chemical compatibility of Ba xSr 1− xCo 1− yFe yO 3− δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba x Sr 1− x Co 1− y Fe y O 3− δ (BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co 4+ and Fe 4+ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10 − 6 K − 1 between 25 °C and 850 °C. Investigations showed further that Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ was completely converted to phases like SrCoO 3− δ , BaCeO 3, BaZrO 3, etc.

Intermartensitic transformations in Ni2Mn1−xCoxGa Heusler alloys

Ni 2 MnGa that possesses a Heusler L21 structure undergoes a martensitic transformation from the parent cubic (austenitic) phase to a low temperature complex tetragonal structure at TM=202K and has a Curie temperature of TC=376K. Some research groups have observed an intermartensitic transformation at a temperature TI<TM, in Ni2MnGa single crystals. In this work detailed studies on the influence of substitution in the Mn subsystem by magnetic Co on the intermartensitic transformation properties of Ni2Mn1−xCoxGa compounds have been done by magnetization (5–400K) and thermal expansion (80–300K) measurements. The samples Ni2Mn0.92Co.08Ga, Ni2Mn0.91Co.09Ga, and Ni2Mn0.90Co.10Ga were studied. The intermartensitic transformation is observed in all of these alloys. As the Co concentration increases, the transformation becomes more pronounced and the temperature range for which the alloys stay in the intermartensitic state decreases as Co concentration increases. Similarities were observed between the magnetization and thermal expansion curves. The results are discussed in terms of the internal stress produced as a result of the Co substitution.

Measurement of thermal expansion by double extensometers between 290 K and 5 K

We developed an apparatus for the measurement of thermal expansion using double extensometers between 290K and 5K. The sample was set vertically in a variable temperature cryostat and its expansion or contraction was measured using the extensometers directly attached to the sample. The temperatures of the samples were controlled with He gas from the cryostat and a heater wounded around the sample chamber. In order to ensure the validity of this system, we measured the thermal expansion of Ag plates with different thicknesses. As a result, the measured thermal expansion curves were confirmed to show good agreement with each other for Ag plates with thickness of 3mm, 1mm and 0.2mm. Good agreement was also ensured for the curves measured during both the cooling and warming processes. These results suggest that our system will be applicable for superconducting composites with a cross-section as small as 1mm2.

Thermal and mechanical properties of Cu–Au intermetallic alloys

The thermal and mechanical properties of Cu, Au pure metals and their ordered intermetallic alloys of Cu3Au(L12), CuAu(L10) and CuAu3(L12) are studied by using the molecular dynamics simulation. The effects of temperature and concentration on the physical properties of CuxAu1−x are analysed. Sutton–Chen (SC) and quantum Sutton–Chen (Q-SC) many-body potentials are used. The simulation results such as cohesive energy, density, elastic constants, bulk modulus, heat capacity, thermal expansion, melting points and phonon dispersion curves are in good agreement with the available experimental data at the various temperatures. Q-SC potential parameter results are usually closer to experimental values than the ones predicted from SC potential parameters.

Preparation and Characteristics of a Glass Sealant for IT-SOFC

Intermediate temperature plate type solid oxide fuel cells (IT- pSOFC) are operated at 500∼850°C, and the working gases between the two sides of electrolyte, fuel in anode chamber and air in cathode chamber, must be separated, so that a hermetic seal between cell components is essential. In this paper, a glass system based on SiO 2 -Bi 2 O 3 -Al 2 O 3 -Na 2 O-PbO 2 -ZnO was prepared and the contents were optimized to determine the best material named T24. The softening point and viscosity at high temperature calculated based on thermal expansion curve are 840 K and 10 5 Pa.s (at 1198 K), respectively. The optimum sealing temperature range is between 1138 and 1173 K. The particle size distribution curves of powders ball-milled for different time were measured by laser light scatter method. After sealing, T24 glass sealant adhered with YSZ electrolyte well and there were no perforative pores in the sealant. The open circuit voltage of single cell sealed with this sealant was near theoretical value.

Comparative Specular X-ray Reflectivity, Positron Annihilation Lifetime Spectroscopy, and Incoherent Neutron Scattering Measurements of the Dynamics in Thin Polycarbonate Films

There are many technological applications using thin polymer films that would be crucially influenced by confinement-induced changes in the transport properties of the film. In the present, we utilize specular X-ray reflectivity (SXR, to measure film thickness h), beam positron annihilation lifetime spectroscopy (PALS, to measure the nanometer-sized domains of unoccupied volume v), and incoherent neutron scattering (INS, to measure the mean-square atomic displacements 〈u2〉) to quantify the influence of film thickness in thin polycarbonate (PC) films. The thermal expansion coefficients of both h and v, as well as the amplitudes of 〈u2〉, indicate that thin film confinement affects reduced molecular mobility in PC. This reduced mobility is not necessarily reflected in the apparent glass transition temperature (Tg) derived from the same techniques. Specifically, SXR and PALS indicate a weak suppression of apparent Tg (the kink in the thermal expansion curve) when the film thickness becomes less than 200 A, wh...

The effect of oxidation and reduction on thermal expansion of magnetite from 298 to 1173 K at different vacuum conditions

Three thermal expansion curves of natural and synthetic magnetite (Fe 3O 4) have been measured under different vacuum conditions (10 −4 and 10 −6 mbar). The different behavior in thermal expansion depends on vacuum level, which controls the partial oxidation of the sample. If the vacuum is poor, that is in mildly oxidizing conditions, the thermal expansion curve presents a discontinuity at 875 K and the samples oxidizes. In non-oxidizing conditions the discontinuity is not present and the thermal expansion coefficient is markedly smaller. The experimental curves indicate that virtually all thermal expansion data on magnetite in the literature were measured on oxidized or partially oxidized samples.

Mechanical and physical properties of Bi-2223 and Nb3Sn superconducting materials between 300 K and 7 K

Within the framework of IEC/TC90-WG5 and VAMAS/TWA16, superconducting (SC) materials are investigated with respect to their mechanical properties between 300 K and 7 K. Besides the mechanical tests, physical and electrical properties are also determined for high Tc SC-tapes. The mechanical tests comprised the characterization of tensile properties at ambient temperature as well as at 7 K of Nb3Sn-reacted strands, Bi2223 tapes, pure silver tapes, silver bars, silver alloy tapes and bare filaments extracted from Bi-2223 tapes. All these investigations are carried out using a variable temperature helium gas flow cryostat equipped with a servo hydraulic tensile machine (MTS, model 810). For the load measurements specially developed, highly sensitive cryogenic proof in situ working load cells are used. For the strain determination of the wires, a high resolution ultra-light double extensometer system with a specially developed low noise signal conditioner is used. The engineering parameters such as yield strength and elastic modulus are evaluated using the obtained data with newly developed software. For the tiny and brittle filaments load versus displacement data are obtained. A determined master line (Young's modulus versus machine compliance) established by thin 0.125 mm Ø wires of different pure metals is used for the Young's modulus estimation of filaments. For the 4 K electrical voltage–current measurements under magnetic fields of up to 13 T, an existing test facility is used for the high Tc tapes. No dependency between applied strain up to 0.3% and the critical current under magnetic field could be observed for the selected specific Bi-2223 tapes. In addition, thermal expansion curves of Bi-2223 tapes along with pure silver and silver alloy (AgMg) are determined between 290 K and 7 K using in situ working extensometers. The coefficient of thermal expansion is evaluated by the determined thermal expansion versus temperature curve.

Simultaneous measurements of heat flux and thermal expansion on hyperfine multistage transitions of single crystalline BaTiO 3

The heat flux and the thermal expansion of BaTiO 3 were simultaneously measured with a temperature resolution of milli-Kelvin to observe the multistage transitions. Heat flux was detected by the thermomodule and thermal expansion was detected by observing the change of capacitance due to the change of the sample length. At least six thermal anomalies were observed between 402 and 405 K in BaTiO 3 simultaneously in heat flux and thermal expansion in the cooling process. The correspondence of the anomalies observed in the two physical properties was excellent. The thermal expansion curves along 〈0 0 1〉 direction obtained at the cooling rates less than 20 μK s −1 consisted of several steps with different number and magnitude depending on the cooling rate. The hysteresis behaviors for the phase transition of BaTiO 3 are discussed in terms of domains.

Magneto-volume effect in the ground and field-induced states of Ce 2Fe 17

X-ray diffraction measurements on Ce 2Fe 17 have been performed in the temperature range from 8 to 300 K at the magnetic fields up to 5 T . The thermal expansion curves along the principal axes for antiferromagnet and ferromagnet Ce 2Fe 17 are presented. A distinct lattice expansion was observed simultaneously with the metamagnetic transition. This suggests that the hybridization between Fe 3d- and Ce 4f-electrons in Ce 2Fe 17 is suppressed by the application of magnetic field.