Speckle interferometry for analysing anisotropic thermal expansion—application to specimens and components

This paper describes a non-destructive optical technique, digital speckle pattern interferometry (DSPI), that has been developed particularly for strain analysis and has proved well suited for thermal deformation measurement. Fibre-reinforced composites with both metal and polymer matrices have been analysed by DSPI to determine their thermal expansion behaviour as a function of direction and temperature. Complete series of measurements can be performed quickly and without any restriction on the specimen shape. Engineering components including composite structures have been the subject of investigation. Besides quantitative results, real-time observation provides basic information for materials understanding.     

Characterization of oxygen passivated iron nanoparticles and thermal evolution to γ-Fe2O3

Nanocrystalline iron powders have been prepared by the inert gas evaporation method. After preparation the material has been passivated by pure oxygen and air exposure. In the present paper we describe new characterization studies of this sample by Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), X-ray Absorption Spectroscopy (XAS), Electron Energy Loss Spectroscopy (EELS) and Mössbauer Spectroscopy (MS), giving a complete chemical and structural characterization of the nanocomposite material in order to correlate its microstructure with its singular magnetic behavior.This nanocomposite was later heated following different thermal treatments. It was found that the sample heated successively in high vacuum (10^–7 torr) at 383 K for 1 h and under a residual oxygen pressure of 4 × 10^–4 torr at 573 K for 3 h, results in a powder formed by nanoparticles of -Fe₂O₃ as stated from XRD, XAS and MS. This material is stable during several years and behaves almost totally like superparamagnetic at room temperature.     

Ultralow density carbon aerogels with low thermal conductivity up to 2000 °C

Ultralow density (0.052 g cm^?3) carbon aerogels (CAs) were prepared for ultrahigh temperature thermal insulation, and their thermal conductivities were determined by laser flash method. The CAs have a total thermal conductivity as low as 0.601 W m^?1 K^?1, which is only one third of the value for closed-pore carbon foam (CF) with a density of 0.054 g cm^?3, at 2000 °C under 0.15 MPa argon. The solid, gaseous, and radiative conductivities of the CA are all much lower than those of the CF, because of the special nanoporous and pearl-necklace nanoparticle structures of the CA. The ultralow density CA clearly demonstrates its great potentials as thermal insulations for extreme applications.     

Hydrothermal synthesis of β-Ni(OH)2 platelets and their thermal conversion to NiO

The β-Ni(OH)₂ platelets and nanorods with length ranging from 2 to 6 μm have been successfully synthesized via a hydrothermal method, and no templates or additives were employed. The products were characterized by X-ray powder diffraction and scanning electron microscopy techniques. The dosage of sodium hydroxide and temperature were found to play important roles in controlling the size and morphology of β-Ni(OH)₂. The shape of β-Ni(OH)₂ platelets was successfully sustained during the thermal transformation to NiO. Optical property of the NiO platelets was investigated by means of UV–Vis spectrum, exhibiting small red shift compared to the bulk counterpart. Such β-Ni(OH)₂ and NiO microstructures may find potential applications in micro/nano-devices.     

Coupled thermal–electrical analysis for carbon fiber/epoxy composites exposed to simulated lightning current

A coupled thermal–electrical analysis of carbon fiber reinforced polymer composites (CFRP) exposed to simulated lightning current was conducted in order to elucidate the damage behavior caused by a lightning strike with the numerical results being compared to experimental results. Based on the experimental results and a preliminary analysis, the specific mechanism of electrical conduction through the thickness direction of CFRP following thermal decomposition was revealed to be a key parameter for accurate numerical simulation. In particular, assuming the electrical conductivity in the thickness direction to be linear with respect to temperature in the range from the epoxy decomposition temperature to carbon sublimation temperature produced reasonable numerical results. The delamination area and damage depth were estimated from numerical results and thermal decomposition behavior of CFRP with the estimated damage area agreeing qualitatively with the experimental results. Numerical results suggest that Joule heat generation significantly influences lightning strike damage.     

The thermal decomposition mechanism of iron(III) hydroxide carbonate to α-Fe2O3

The thermal decomposition mechanism of iron(III) hydroxide carbonate prepared through hydrolysis and oxidation of FeCO₃ is studied under both static and dynamic conditions. It is established that the precursor is decomposed to -Fe₂O₃ forming an intermediate phase, -FeOOH. Up to 523 K the structural transformations proceed with limited cation diffusion which leads to the pseudomorphic formation of Fe₂O₃ with large specific area.     

Measurement of linear thermal expansion coefficient of alkali-activated aluminosilicate composites up to 1000 °C

The effect of high temperatures up to 1000 °C on the length changes of two alkali-activated aluminosilicate composites, one of them with quartz sand aggregates, the second with electrical porcelain, is analyzed in the paper. The thermal strain vs. temperature functions of both materials are found to increase monotonically in the whole temperature range studied so that the thermal expansion mismatch (the gel undergoes thermal shrinkage, the aggregates expand with increasing temperature) results in positive values of the apparent linear thermal expansion coefficient. The composite material with electrical porcelain aggregates exhibits a more desired thermomechanical behavior which is a consequence of the better high-temperature thermal stability of electrical porcelain as compared to quartz. In a comparison with Portland-cement based composites, the linear thermal expansion coefficient of both studied aluminosilicates is substantially lower in the whole temperature range of 20–1000 °C.     

Relation of the thermal emf to temperature in telescopes type tera -50/900–1800

Conclusions As the result of the above investigation we obtained a mean calibration curve and an analytical equation which represents with sufficient accuracy the relation between the thermal emf and temperature for the types of telescopes we investigated. This makes it possible to recommend a new, more efficient technique of calibrating grade 2 telescopes at four temperatures in the range of 900–1800°C.     

Hollow-grained “Voronoi foam” ceramics with high strength and thermal superinsulation up to 1400 °C

Nanostructures tend to be unstable at high temperatures due to large capillary energies, and therefore nanotechnology has not yet found many high-temperature applications at 1000 °C and above. By taking advantage of the high-temperature stability of refractory ceramics, here we develop a new approach of making hollow nano-grained materials to achieve thermal superinsulation across a wide temperature range, where the gaseous voids are mostly isolated within individual grain, with size comparable to the mean free path of air molecules to lower the thermal conduction by Knudsen effect. We have proved this general concept with hollow-grained La₂Zr₂O₇ ceramic, and demonstrated exceptionally low thermal conductivity (0.016 W/(m⋅K)), the lowest ever reported for hard materials at or above room temperature. The centimeter-scale samples also have ultrahigh compressive strength (251 MPa), tensile strength in bending up to 100 MPa, and excellent thermal stability up to 1400 °C in air, due to monodispersity of pores that delays coarsening.     

Stress–strain time-dependent behavior of A356.0 aluminum alloy subjected to cyclic thermal and mechanical loadings

This article presents the cyclic behavior of the A356.0 aluminum alloy under low-cycle fatigue (or isothermal) and thermo-mechanical fatigue loadings. Since the thermo-mechanical fatigue (TMF) test is time consuming and has high costs in comparison to low-cycle fatigue (LCF) tests, the purpose of this research is to use LCF test results to predict the TMF behavior of the material. A time-independent model, considering the combined nonlinear isotropic/kinematic hardening law, was used to predict the TMF behavior of the material. Material constants of this model were calibrated based on room-temperature and high-temperature low-cycle fatigue tests. The nonlinear isotropic/kinematic hardening law could accurately estimate the stress–strain hysteresis loop for the LCF condition; however, for the out-of-phase TMF, the condition could not predict properly the stress value due to the strain rate effect. Therefore, a two-layer visco-plastic model and also the Johnson–Cook law were applied to improve the estimation of the stress–strain hysteresis loop. Related finite element results based on the two-layer visco-plastic model demonstrated a good agreement with experimental TMF data of the A356.0 alloy.     

Structure of thermal stresses arising in a viscoelastic half-space with thermal “memory”

We develop the structure of thermal stresses arising in a viscoelastic half-space owing to the thermal impact of a heat flux at the boundary.Notation z coordinate normal to the surface of the half-space - t time - =T–T_o temperature of the half-space - (t) relaxation function of the heat flux - (t) relaxation function of the internal energy - c_v specific heat at constant volume - q_o heat flux acting on the boundary of the half-space - _xx, _yy, _zz normal stresses - density of the material - _t coefficient of linear thermal expansion - u=[(0)/c_v]^1/2 heat-propagation velocity - _t coefficient of thermal conductivity - _r relaxation time of the heat flux - relaxation time of the stresses Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 5, pp. 894–897, November, 1979.     

A guarded hot-plate apparatus for thermal conductivity measurements over the temperature range −75 to 200‡C

A guarded hot-plate apparatus for small circular samples has been developed for the temperature range from –75 to 200C. To avoid edge losses, the apparatus is immersed in a liquid whose temperature is a few degrees lower than the mean temperature of the samples. A detailed evaluation procedure with several correction calculations leads to a remaining uncertainty of measurement of 0.5% for measurements on glass samples. This has been confirmed by experiment. Measurements on glass and on insulation material showed that the developed apparatus and the evaluation procedure applied can be used in a relatively wide range of thermal conductivity values (factor 50).     

A new route to graphene starting from heavily ozonized fullerenes: Part 1—thermal reduction under inert atmosphere

It is shown that heavily ozonized C₆₀ or C₇₀ fullerenes (known also as “fullerene ozopolymers”) are suitable substrates for the preparation of graphene or nanographene in place of graphite oxide (GO) by thermal reduction in inert atmosphere. TGA-FTIR study shows that the release profile of CO₂ and CO from fullerene ozopolymers in the temperature range between 25°C and 900°C is comparable to that shown by GO. Furthermore, the FT-IR spectral evolution of fullerene ozopolymers from room temperature to 630°C under inert atmosphere is once again strikingly comparable to that observed on GO under the same conditions.     

A combined synchrotron powder diffraction and vibrational study of the thermal treatment of palygorskite–indigo to produce Maya blue

The heating process (30–200 °C) of a palygorskite-indigo mixture has been monitored in situ and simultaneously by synchrotron powder diffraction and Raman spectroscopy. During this process, the dye and the clay interact to form Maya blue (MB), a pigment highly resistant to degradation. It is shown that the formation of a very stable pigment occurs in the 70–130 °C interval; i.e., when palygorskite starts to loose zeolitic water, and is accompanied by a reduction of the crystallographic a parameter, as well as by alterations in the C=C and C=O bonds of indigo. Mid- and near-infrared spectroscopic work and microporosity measurements, employed to study the rehydration process after the complex formation, provide evidence for the inhibition of the rehydration of MB as compared with palygorskite. These results are consistent with the blocking of the palygorskite tunnel entrance by indigo molecules with a possible partial penetration inside the tunnels. The surface silanols of palygorskite are not perturbed by indigo, suggesting that MB is not a surface complex.     

A new approach to the design of a low Si-added Al–Si casting alloy for optimising thermal conductivity and fluidity

The effect of Ni on the thermal conductivity and fluidity of a low Si-added Al–Si casting alloy was investigated. The room temperature thermal conductivity of an Al–2 wt%Si alloy instantly dropped when adding 0.5 wt%Ni, whereas further additions of Ni up to 3 wt% had little influence on the thermal conductivity, which was in the range of 180–185 W/mK. The thermal conductivity was also estimated for the alloys cast at various cooling rates by measuring the electrical conductivity using the Wiedemann–Franz law and increased nearly linearly with an increase in the cooling rates. At such a low level of 2 wt%Si, the castability of the Al–Si alloys, which was evaluated by a fluidity spiral test, was enhanced by the Ni additions, exhibiting the maximum fluidity length at 0.5 wt%Ni.     

Microstructure evolution in a Ni–Mo–Cr superalloy subjected to simulated heat-affected zone thermal cycle with high peak temperature

A typical Ni–Mo–Cr superalloy with basic composition of Ni–17Mo–7Cr (wt.%) was fabricated and the relationship between the microstructure and mechanical properties while it underwent simulated heat-affected zone thermal cycle (HAZ) treatment was investigated. The results show that the Ni–Mo–Cr alloy was mainly made up of Ni based solid solution and MoC carbides. The critical peak temperature that a unique lamellar-like structure occurred in the alloy was found to be 1300 °C, and they were firstly determined to be Ni matrix and carbides (MoC and chromium carbides) generated through local melting. Due to the formation of unique structure, the alloy exposed to HAZ thermal cycle with a peak temperature of 1300 °C could still maintain excellent high-temperature mechanical performance. The work carried out here will provide valuable guidelines in designing and applying the Ni–Mo–Cr series superalloys.     

Response functions of the Andersson–Braun and extended range rem counters for neutron energies from thermal to 10 GeV

This work is devoted to the calculation of responses as functions of neutron energy for a paired set of Andersson–Braun rem counters, which is commercially available. Different Monte Carlo codes such as MCNP, LAHET, HADRON and MCNPX were applied in the calculations. The study extended to frontal, lateral and isotropic neutron incidence. For an estimation of the contribution of charged high-energy particles to the reading, the responses to protons and pions were also determined. The results obtained give good bases for the practical use of the new instrument in high-energy neutron fields.