Phase Thermal Stability Research Articles

Low thermal conductivity without oxygen vacancies in equimolar YO 1.5 + TaO 2.5- and YbO 1.5 + TaO 2.5-stabilized tetragonal zirconia ceramics

A narrow range of composition exists along both the ZrO 2–YTaO 4 and ZrO 2–YbTaO 4 quasi-binaries over which the tetragonal zirconia phase can be retained on cooling. Unlike other stabilized zirconia materials which have low thermal conductivity as a result of phonon scattering by oxygen vacancies, these compositions do not contain oxygen vacancies and yet an equimolar YO 1.5 + TaO 2.5 composition has been reported to also exhibit low thermal conductivity [1]. We find that zirconia compositions along the quasi-binaries have low and temperature-independent thermal conductivities, and that the thermal conductivities and their temperature dependence are consistent with a defect scattering model that takes into account a minimum phonon mean free path due to the inter-atomic spacing. Furthermore, the conductivities of the Yb and Y trivalent-doped compositions scale in a predictable manner with atomic site disorder effects on the cation sub-lattice associated with the lighter Y 3+ ions and the heavier Yb 3+ and Ta 5+ ions. The lowest thermal conductivity measured was ∼1.4 W mK –1 at 900 °C. The low thermal conductivity and phase stability makes these systems promising candidates for low conductivity applications, such as thermal barrier coatings.

Microstructure and mechanical properties of ZA62 based magnesium alloys with calcium addition

As-cast microstructure and mechanical properties of Mg-6Zn-2Al-0.3Mn (ZA62) alloys with calcium addition were investigated. The as-cast microstructure of the base alloy ZA62 consists of the a-Mg matrix and eutectic phase Mg 51Zn 20. The Mg 51Zn 20 eutectic was gradually replaced by MgZn phase and Mg 32(Al,Zn) 49 phase when calcium is added into the base alloy. Further addition of calcium leads to the increase of grain boundary phases and formation of a new quaternary Mg-Zn-Al-Ca eutectic compound. In comparison with the base alloy, the increase of calcium addition to the base alloy results in the reduction of both strength and ductility at ambient temperature, but increase at elevated temperatures due to the thermal stability of Ca-containing phases. At elevated temperatures, the creep resistance of ZA62 based alloys containing calcium is significantly higher than that of AZ91 which is the most commonly used magnesium alloy.

The Effect of the Addition of Carbon Nanotubes in the Hydrothermal Synthesis and in the Thermal Phase Stability of Nanozirconia

The yttria partially stabilized zirconia is a very attractive material for orthopaedic applications. It exhibits excellent biocompatibility, high fracture toughness, high strength and low wear rates. But case studies show that delayed failure can occur in vivo due to crack propagation. Carbon nanotubes could avoid the slow crack propagation and enhance the toughness of the ceramic material used for prostheses fabrication. In this work, X-ray diffraction has been used to study the influence of the addition of MWCNT on the hydrothermal synthesis of tetragonal zirconia nanoparticles and on the phase stability of the CNT-nanozirconia nanocomposite with the temperature. First, the influence of the processing variables on the hydrothermal synthesis has been studied. The theoretical mathematical models that relate the percentage of tetragonal zirconia nanocrystals and the relative crystallinity with the processing variables in the range of analyzed values have been obtained. The values that give the maximum percentage and crystallinity of tetragonal phase in the studied range have been established. No significant differences were observed in the crystalline phases obtained when adding MWCNT during the synthesis. Nanozirconia partially coated MWCNT synthesized under the optimized parameters were added to commercially available nanozirconia particles and their influence in the phase stability of the zirconia with the temperature was studied by XRD. It was concluded that the addition of the carbon nanotubes delays both the monoclinic phase decomposition and the grain growth.

Thermal stability of gas phase magnesium nanoparticles

In this work we present a unique transmission electron microscopy study of the thermal stability of gas phase synthesized Mg nanoparticles, which have attracted strong interest as high capacity hydrogen storage materials. Indeed, Mg nanoparticles with a MgO shell (∼3 nm thick) annealed at 300 °C show evaporation, void formation, and void growth in the Mg core both in vacuum and under a high pressure gas environment. This is mainly due to the outward diffusion and evaporation of Mg with the simultaneously inward diffusion of vacancies leading to void growth (Kirkendall effect). The rate of Mg evaporation and void formation depends on the annealing conditions. In vacuum, and at T=300 °C, the complete evaporation of the Mg core takes place (within a few hours) for sizes ∼15–20 nm. Void formation and growth has been observed for particles with sizes ∼20–50 nm, while stable Mg nanoparticles were observed for sizes >50 nm. Furthermore, even at relative low temperature annealing (as low as 60 °C), void formation and growth occurs in 15–20 nm sized Mg nanoparticles, indicating that voiding will be even more dominant for nanoparticles smaller than 10 nm. Our findings confirm that Mg evaporation and void formation in nanoparticles with sizes less than 50 nm present formidable barriers for their applicability in hydrogen storage, but also could inspire future research directions to overcome these obstacles.

Structure and electrochemical properties of novel mixed Li(Fe1−xMx)SO4F (M = Co, Ni, Mn) phases fabricated by low temperature ionothermal synthesis

In the current scenario, Li-ion batteries are no longer limited to portable electronic devices, but are rapidly gaining momentum to enter the large-scale hybrid automotive market owing to their adequate energy density coupled with their low cost and safety. LiFePO4 is the front-runner candidate in this sector mainly due to its economic cost and operational safety. Recently, our group has discovered a novel 3.6 V metal fluorosulfate (LiFeSO4F) electrode system, which combines sulfate polyanions with fluorine chemistry to deliver excellent conductivity and electrochemical capacity. In the current study, we extend our effort to investigate the structure and electrochemical properties of 3d-transition metal (M = Co, Ni, Mn) substituted fluorosulfates. Toward this goal, we have adopted ionothermal synthesis to fabricate three families of solid-solution systems, namely Li(Fe1−xCox)SO4F, Li(Fe1−xNix)SO4F and Li(Fe1−xMnx)SO4F at temperatures as low as 300 °C. The structure, thermal stability and electrochemical properties of these mixed sulfate phases along with the end members (LiCoSO4F, LiNiSO4F and LiMnSO4F) have been examined using a suite of characterization techniques. Overall, a 3.6 V Fe2+/3+ redox reaction is observed with no signature of Co2+/3+, Ni2+/3+ or Mn2+/3+ reaction. These metal fluorosulfate systems, delivering near theoretical capacity, stand as an alternative new class of electrodes for varied commercial applications.

Synthesis of lanthanum oxide doped photocatalytic nano titanium oxide through aqueous sol–gel method for titania multifunctional ultrafiltration membrane

An all aqueous sol gel route has been demonstrated for the synthesis of titania multifunctional ultrafiltration membrane on porous alumina substrate. Doping with lanthana was used to increase the thermal stability of anatase phase. This resulted in a nanoporous membrane with considerable percentage of anatase phase even after consolidation at 800°C. The highly homogeneous membranes with a thickness of ~4–5 μm could be observed in SEM. The presence of anatase phase ensured higher photoactivity for the doped membrane compared to the undoped one. The performance of the membrane was tested based on molecular weight cut off and a very high (>90%) filtration efficiency was observed on Bovine Serum Albumin of molecular weight 66 kD. Rejection of 11 nm sized particles points to the uniform nanoporous nature of these highly efficient membranes.

Preparation of nano-TiO2 photocatalysts and their decomposition activity in phenol-contaminated water

TiO2 was prepared by the hydrolyzation method in (NH4)2SO4-modified TiCl4 solution, and TiO2 photocatalysts were obtained by accelerating the precipitation of TiO2 powder in a high-temperature water bath. The photocatalysts were characterized by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Raman spectrum and UV-Vis (Ultraviolet-Visible) spectrometry techniques, and the photocatalytic activity in phenol-contaminated water was investigated. The results showed that photocatalysts calcined at 400 °C had a specific surface area of 138.2 m2/g and an average particle size of 9 nm, and a significant increase in thermal stability of anatase phase. At the calcination temperature of 700 °C, the crystal form of TiO2 started to change into rutile (anatase: 97%, rutile: 3%). The activity of TiO2 photocatalysts prepared with (NH4)2SO4-modified TiCl4 solution was markedly stronger than that without (NH4)2SO4-modified TiCl4 solution. Maximal photocatalytic activity was observed at the mole ratio of Ti:(NH4)2SO4=1:2, the water-bath temperature of 90 °C and the calcination temperature of 700 °C.

Oxygen permeation and stability of La 0.6Sr 0.4Ti xFe 1− xO 3− δ ( x = 0.2 and 0.3) membrane

An oxygen transport ceramic membrane was prepared with La 0.6Sr 0.4Ti x Fe 1− x O 3− δ ( x = 0.2 and 0.3) oxides which was synthesized by polymerized complex method. In order to investigate the effects of Ti substitutions into lanthanum–strontium ferrites, material property, thermal expansion coefficient, oxygen permeation, electrical conductivity and phase stability were examined according to Ti contents. The thermal expansion coefficients of LSTF6428 and LSTF6437 were 25.76 × 10 −6 K −1 and 21.32 × 10 −6 K −1 in the temperature range of 973−1273 K, respectively. The oxygen permeation fluxes through a disk type dense membrane of LSTF6428 and LSTF6437 with 1.6 mm thickness increased exponentially with increasing temperature and were 0.17 mL min −1cm –2 and 0.13 mL min −1cm −2 with P h = 0.21 atm (O 2 rich side) and P l = 10 −5 atm (lean side) at 1223 K, respectively. In the result of phase stability test with various conditions such as He, H 2/N 2 and CO 2 atmosphere, La 0.6Sr 0.4Ti x Fe 1− x O 3− δ was confirmed to be more stable than La 0.6Sr 0.4Ti 0.2Fe 0.8O 3− δ through XRD and SEM analysis.

Environmentally benign sol–gel derived nanocrystalline rod shaped calcium doped cerium phosphate yellow-green pigment

Nanocrystalline rod shaped calcium doped cerium phosphate yellow-green pigment particles having an average length of ~100 nm and aspect ratio 10 even after calcination at 600 °C have been realized through an aqueous sol–gel process. The morphology, particle size and identification of the phase are determined by using different analytical tools such as transmission electron microscopy (TEM), photon correlation spectroscopy (PCS), Fourier-transform IR (FTIR) and X-ray diffraction (XRD). Brunauer–Emmett–Teller (BET) nitrogen adsorption analysis showed the pigment particles are mesoporous texture having specific surface area 42 m2 g−1 and average pore size 153 A. Thermogravimetric (TG) analysis is used to explain the thermal phase stability of the pigment. UV–Visible spectroscopy and colorimetric analysis are also done. The typical yellow-green color has been obtained even after heating to as low as 600 °C, which is 300 °C lesser than reported. Systematic study on synthesis and effect of temperature on color are presented.

Induction and Enhancement of Ferroelectric Smectic C* Phase in Multi-Component Room Temperature Mixtures

Six multi-component room-temperature ferroelectric liquid crystal (FLC) mixtures have been formulated and investigated using optical polarizing microscopy, dielectric spectroscopy as well as electro-optic methods. In all the mixtures smectic C* phase is either induced or enhanced. Effect of various dopants on the occurrence and thermal stability of different phases has been discussed. Spontaneous polarizations, measured as a function of temperature by reversal current method, are found to vary between 13–62 nC/cm2 at room temperature in all the mixtures. Different types of dielectric absorption behavior were observed in planar geometry of the FLC cells. Critical frequencies of Goldstone and soft modes have been determined by frequency dependent dielectric study. Rigidity of the core structure, nature of chirality and extent of fluorination of the constituent molecules are found to have pronounced effect on the collective mode relaxation behavior of the mixtures.

Thermal Phase Stability and Catalytic Properties of Nanostructured TiO2-MgO Sol–Gel Mixed Oxides

Several compositions in the system TiO2-MgO were prepared via sol-gel method. The structural evolution of the samples was followed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermal analysis (DTA/TG). In order to study the crystalline phase stability over time, the amorphous fresh gels were subjected to successive thermal treatments from 100 to 1100 degrees C, with a total annealing time of 442 h. Below 350 degrees C, magnesium acetates and oxalates phases were detected; at higher temperatures the binary compounds MgTi2O5, MgTiO3 and Mg2TiO4 appeared over a wide range of compositions. In the titania rich end, brookite, anatase and rutile were detected, these phases easily reacted to form the binary oxides; in contrast, on the rich-end magnesia, periclase showed an overall high stability. The successive thermal treatments over an extended period of time allowed a better control of nanosized crystallite growth; for anatase was 30 nm, below 850 degrees C, and for rutile was 45 nm, between 600 degrees C and 1000 degrees C. Magnesia showed a constant value of 22 nm on the overall studied temperatures. The best catalytic performance for 2-propanol decomposition was achieved by 90Ti/10Mg, annealed at 400 degrees C; this indicates that when magnesium is added to titania, the catalytic activity improves due to the introduction of structural defects and charge deficiencies. For 2-butanol decomposition, the formation of cis and trans isomers indicates the presence of high acidity sites.

Synthesis of Fe3O4-based catalysts for the high-temperature water gas shift reaction

The water gas shift reaction is an essential process to adjust the CO/H2 ratio in the industrial production of hydrogen. FeCr catalysts have been widely used in this reaction at high temperature but have environmental and safety concerns related to chromium content. In this work, the replacement of chromium by molybdenum in magnetite-based catalysts is studied. The materials were prepared by oxidation–precipitation and wet impregnation and they were characterized using X-ray powder diffraction, X-ray fluorescence, transmission electron microscopy, and temperature programmed reduction. Specific surface areas of samples were also measured. The results obtained indicate that molybdenum increases thermal stability of the magnetite active phase and prevents metallic iron formation during the reaction. The oxidation–precipitation method allows obtaining the material directly in the active phase and molybdenum is incorporated into magnetite lattice.

Thermal stability of the omega phase in Zr-containing TiAl alloys

The thermal stability of omega phase in TiAl alloys has been investigated using scanning and transmission electron microscopy and it has been found that Zr has a strong effect in stabilising the omega phase. In Ti–44Al–4Nb–4Zr–0.2Si–1B the omega solvus was determined as just over 900 °C. Increasing additions of Zr to Ti–44Al–4Nb–2Mo resulted in an increase in the volume fraction of omega phase at 700 °C and the omega particle distribution was changed from localised to uniformly distributed omega. In the alloy with 2 at% Zr, omega particles were found uniformly throughout the B2 matrix at 700 °C.

Effect of ion irradiation on the stability of amorphous Ge2Sb2Te5 thin films

The archival life of phase-change memories (PCM) is determined by the thermal stability of amorphous phase in a crystalline matrix. In this paper, we report the effect of ion beam irradiation on the crystallization kinetics of amorphous Ge2Sb2Te5 alloy (GST). The transition rate of amorphous GST films was measured by in situ time resolved reflectivity (TRR). The amorphous to crystal transformation time decreases considerably in irradiated amorphous GST samples when ion fluence increases. The stability of amorphous Ge2Sb2Te5 thin films subjected to ion irradiation is discussed in terms of the free energy variation of the amorphous state because of damage accumulation.

Experimental study of the thermal conductivity of metal oxides co-doped yttria stabilized zirconia

Doping of yttria stabilized zirconia (YSZ) with metal oxides has been reported to have the potential of increasing the thermal insulation capability and phase stability of YSZ at higher application temperatures. In order to select the dopant oxides, it is important to understand the effects of the dopants, in terms of the ionic radius, mass, and valence, on various thermal and mechanical properties of the doped YSZ. In this study, a series of metal oxides, tantalum pentoxide (Ta 2O 5), niobium pentoxide (Nb 2O 5), scandium oxide (Sc 2O 3), ytterbium oxide (Yb 2O 3) and cerium oxide (CeO 2) were incorporated to 7YSZ (7 wt% yttria) by mechanical alloying and sintering; the thermal conductivity changes of the doped 7YSZ were investigated. The oxide dopants that exist as defect clusters within the zirconia lattice structure played an important role in reducing the thermal conductivity of the ternary oxide system. Yb 2O 3 co-doped 7YSZ exhibited the most significant effect on reducing the thermal conductivity while CeO 2 co-doped 7YSZ showed marginal effect. The measured results were also found to be consistent with that obtained from a semi-empirical thermal conductivity model established based on the assumption of defect clusters acting as the phonon scattering centers.

Synthesis and Characterization of Novel All-Organic Liquid Crystalline Radicals

We have synthesized a new series of all-organic liquid crystalline (LC) radical compounds (2), which are the structural isomers of the previously reported series (1) containing a chiral nitroxide unit in the mesogen core. Only difference between the two isomers is the orientation in the ester functionality (‒OCO‒ vs ‒COO‒). The new LC compounds 2 with C4 to C8 alkoxy terminal chains showed the distinct odd-even effects of alkoxy terminal chains with respect to the thermal stability of nematic phases, while such effects were not observed for the isomers 1.

Synthesis and Phase Stability of Silicate-Substituted α-Tricalcium Phosphate

Silicate-substituted calcium phosphates have been shown to result in enhanced biological performance compared to the corresponding, silicate-free, calcium phosphates. We have produced a range of silicate-substituted alpha-TCP compositions using two different synthesis methods and two different substitution mechanisms. Single phase compositions were only observed for a silicate substitution of 1.3 wt% by both solid state synthesis and aqueous precipitation synthesis, although the latter was the result of a design composition with a higher silicate substitution (3 wt%). The silicate substitution resulted in small changes in the unit cell parameters of the alpha-TCP. More importantly, this small level of silicate substitution had a strong effect on the thermal stability of the alpha-TCP phase, with the silicate substitution stabilising the alpha-polymorph to lower temperatures. This has an immediate advantage in that the quenching conditions are not as critical for the production of silicate-substituted alpha-TCP compositions compared to silicate-free alpha- TCP.

Influence of Ti, Mn, Fe, and Ni addition upon thermal stability and decomposition temperature of the MgH2 phase of alloys synthesized by reactive mechanical alloying

Influence of addition of some transition metals (TMs), mainly Ti, Mn, Fe and Ni, to magnesium upon thermal stability of the hydride phase MgH2 synthesized due to reactive mechanical alloying (RMA) in hydrogen atmosphere at pressure 1.2 MPa was studied employing the thermodesorption spectroscopy (TDS) method. The TDS spectra registered differ from each other by their shapes and fine-structure peculiarities. This fact allows to conclude about a different influence of the TMs under consideration upon the feature of hydrogen distribution on location sites in the hydride phase and, consequently, upon its decomposition temperature. We have made an attempt to elucidate the origin of the mentioned influence of the TMs upon the effects derived. A correlation between the degree of dispersion of TM alloying the MgH2 hydride and its decomposition temperature was observed. It has been established that, addition of 10 wt% Ti reveals the maximum influence on decreasing decomposition temperature of the hydride phase. The X-ray photoelectron O 1s core-level spectrum of the specimen contained the addition of 10 wt% Ti shows decreasing a quantity of oxygen-contained structures, catalyst poisons, adsorbed on its surface. © 2007 Elsevier B.V. All rights reserved.

Dry sliding wear behavior of magnesium alloys

Dry sliding tests were performed on as-cast magnesium alloys Mg 97Zn 1Y 2 and AZ91 using a pin-on-disc configuration. Coefficients of friction and wear rates were measured within a load range of 20–380 and 20–240 N at a sliding velocity of 0.785 m/s. X-ray differactometer, scanning electron microscopy, tensile testing machine were used to characterize the microstructures and mechanical properties of Mg 97Zn 1Y 2 alloy and AZ91 alloy. Worn surface morphologies of Mg 97Zn 1Y 2 and AZ91 were examined using scanning electron microscopy. Five wear mechanisms, namely abrasion, oxidation, delamination and thermal softening and melting, have been observed. Mg 97Zn 1Y 2 exhibited good wear resistance compared with AZ91 for applied loads in excess of 80 N, which have been explained in terms of thermal stability of intermetallic phase and elevated temperature mechanical properties of the two materials tested, using surface temperature analysis.

Thermal stabilization of tin- and cobalt-doped manganese dioxide

We study the thermal stability, local structure, and electrical properties of the α-MnO2 phase doped with Sn and Co. It is found that doping prevents the transformation from α-MnO2 to α-Mn2O3 that occurred in the temperature range of 500–600 °C. Samples have been synthesized in an acidic medium using the reduction of potassium permanganate by fumaric acid. X-ray diffraction patterns (XRD) of pure and doped α-MnO2 prepared at 450 °C do not show new peaks related to dopant species. Thermogravimetric analysis (TGA) of the Sn and Co doped MnO2 reveals that transformation from MnO2 to α-Mn2O3 starts above 700 °C. The increase in the thermal stability is attributed to the presence of Sn or Co ions incorporated inside the large 2 × 2 tunnels as revealed by Fourier transform infrared (FTIR) spectra measurements. An increase in the electrical conductivity with the presence of Sn or Co ions is observed. Electrochemical features of the doped MnO2 samples in alkaline cells are reported and compared with that of the pristine α-MnO2 phase.