Successive Thermal Cycles Research Articles

Dielectric relaxation behavior of poly(acrylonitrile- co-methacrylonitrile) microcapsules dispersed in a silicone matrix

The dielectric relaxation behavior of poly(acrylonitrile- co-methacrylonitrile) dispersed in a cured polydimethyl siloxane (PDMS) matrix as microcapsules was investigated over multiple thermal cycles and at varying concentrations. The copolymer microcapsules contained an isopentane core. In the PDMS matrix this copolymer displayed a pronounced relaxation signal at temperatures above the glass transition of the copolymers due to Maxwell–Wagner–Sillars (MWS) relaxation. The mechanism of MWS relaxation interpreted by the Havriliak–Negami and Kohlrausch–Williams–Watts relaxation functions was found to be very similar to previous studies of neat polyacrylonitrile and its copolymer. The activation energy of the relaxation decreased over successive thermal cycling coincident with a decreasing strength of the relaxation. These observations were attributed to the decreasing concentration of nitrile groups due to intramolecular cyclizations.

High-temperature ferroelectric domain stability in epitaxial PbZr0.2Ti0.8O3 thin films

Using high-resolution atomic force microscopy, we have shown extremely high stability of linear ferroelectric domains in epitaxial PbZr0.2Ti0.8O3 thin films heated up to 735°C, a significant advantage for technological applications. An elevated transition temperature ∼785°C is observed even in relatively thick (91nm) films, despite relaxation of in-plane film-substrate lattice-mismatch-induced strain. We also demonstrate the negligible role of the film surface in determining the written domain-wall configuration, both by direct comparison of the surface roughness with domain-wall position at successive thermal cycles, and by measurements of domain-wall dynamics before and after heating.

Linewidth dependence of grain structure and stress in damascene Cu lines

Damascene Cu interconnects show significant differences in both their microstructural and stress behavior as compared to those of Al interconnects patterned using the etching process. Thermal stresses build up during the successive thermal cycles due to the differences in the coefficients of thermal expansion of the component materials. Other than these thermal stresses, growth stresses originating from grain growth develop in damascene Cu interconnects as well. In this study, the linewidth dependence of the stress in damascene Cu was examined experimentally, as well as by numerical simulation. The stresses of damascene Cu with widths ranging from 0.13to2μm were measured using x-ray diffraction, and the measured hydrostatic stress was found to increase with increasing linewidth, in contrast to the typical behavior of Al interconnects. Microstructure analysis using transmission electron microscopy revealed that the grain sizes increased with increasing line dimensions. The increase in stress in the interconnect with increasing dimensions is attributed to the larger grain size, which induces higher growth stress in addition to the thermal stress. The contribution of the growth and thermal stresses of the damascene lines were quantified based on the grain size data utilizing finite element analysis. In this way, the linewidth dependence of the hydrostatic stress of damascene Cu was clearly explained. Finally, the effect of growth stress on the stress-related reliability is discussed.

Production of macroporous ceramics from fibres dispersed mortars (FDM)

We examined the production of macroporous monolithic ceramic from mortars prepared using aluminate cements, one or two different types of alumina powders, superfluidificant, water and polyurethane fibres. Some relevant parameters for the preparation process of the mortar are: (i) the weight ratio between cement powder and water, (ii) the weight ratio between superfluidificant and cement powder and (iii) that between superfluidificant and water. Thirty-six percent is the upper limit for the volumetric content of the fibres that can be introduced into the mortar without leading to a material that breaks up by handling after the successive thermal cycle. We also optimized the thermal cycle that may be used in order to avoid the spontaneous break-up of the material during the sintering treatment. Materials containing fine and coarse particles of alumina powders exhibit extended ruptures probably dependent on the thermal stresses that grow on cooling after the sintering process, whereas in materials containing only fine alumina powders the presence of ruptures is more limited so that the fired materials have a sufficient strength and can be handled without a particular care. The ageing time at the maximum temperature plays an important role on the microstructure of the sintered material since it has been seen that after 2 h at 1250 °C the residual porosity seems to be completely closed and the large grain size shows the presence of structures which can be due to the presence of aluminates.

On the high‐temperature phase transitions of NH4HSO4

AbstractModulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA) and X‐ray diffraction (XRD) were used to study the phase behaviour of NH4HSO4 (AHS) above room temperature. For powder AHS measured during successive thermal cycles, the MDSC curves show an endothermic peak at 147 °C on heating scans and an exothermic peak at about 115 °C on subsequent cooling runs. However, after the second thermal cycle, a second endothermic peak starts to appear at about 135 °C on heating runs whose intensity increases on subsequent cycles. The thermogravimetric measurements on powder AHS show weight loss above 135 °C. X‐ray powder diffraction data for AHS heated to 140 °C during 24 hours, show the diffraction pattern for (NH4)2S2O7. We discuss the endothermic effect at 135 °C in terms of a partial dehydration setting in at reaction sites distributed on the surface of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Li addition on the properties of Bi-based superconductors

The effect of Li addition on the formation and properties of Pb containing Bi-based superconductors has been studied. Three successive thermal sintering cycles have been applied to the oxides adjusted for the nominal composition, Bi/sub 1.6/Pb/sub 0.4/Sr/sub 2/CaCu/sub 2/O/sub 8+d/. Eight different samples having Li/sub 2/CO/sub 3/ from 0.0 up to 3.0 weight% have been synthesized. X-ray diffraction patterns, resistivity-temperature behaviors and differential thermal analysis of the samples have been examined. Li/sub 2/CO/sub 3/ addition at 650 /spl deg/C decreases the fraction of (Bi,Pb)-2212 and increases that of Ca/sub 2/PbO/sub 4/. The normal state resistances of the samples increase, the onset T/sub c/ values decrease and the lattice parameters almost remain same with Li/sub 2/CO/sub 3/ addition. Li/sub 2/CO/sub 3/ addition up to 3.0 wt% decreases the melting temperature of (Bi,Pb)-2212 phase by about 50/spl deg/C inducing a liquid phase at about 750 /spl deg/C. Our observations suggest that the high T/sub c/ phase, (Bi,Pb)-2223 may be formed at about 750-800 /spl deg/C using the precursors, (Bi,Pb)-2212 and Ca/sub 2/PbO/sub 4/ doped with Li/sub 2/CO/sub 3/.

Effect of thermal cycling on R-phase stability in a NiTi shape memory alloy

The stability of R-phase during thermal cycling has been studied for the near equiatomic, 40% cold worked, NiTi wire samples using electrical resistivity measurements. Thermal cycles are given to the specimens heat treated in the temperature region of 340–620 °C. In the samples having only M↔A transformations after heat treatment, successive thermal cycles promote and result in the stabilisation of intermediate R-phase. During the initial thermal cycles, M s is found to decrease facilitating the development of intermediate R-phase as indicated by the emergence of peak in the cooling part of the transformation profile. Once the R-phase is completely developed, the transformation parameters such as R s, R f, M s, M f and peak resistivity of the transformation profile remains unaffected during thermal cycling. There exists a critical number of thermal cycles for the complete development of R-phase and this critical number is a function of heat treatment temperature.

Chemical, electrical, and structural properties of Ni/Au contacts on chemical vapor cleaned p-type GaN

Chemical vapor cleaned, Mg-doped, p-type GaN(0001) surfaces and Ni/Au contacts deposited on these surfaces have been studied using several characterization techniques. Stoichiometric surfaces without detectable carbon and an 87% reduction in the surface oxygen to 2±1 at. % were achieved. The binding energies of the Ga 3d and N 1s core level photoelectron peaks were reduced by 0.5±0.1 eV following the chemical vapor clean. The band bending at the clean surface was measured to be 0.8±0.1 eV. As-deposited Ni/Au contacts on chemical vapor cleaned surfaces exhibited significantly less rectification in the low voltage region (<2 V) compared to identical contact structures on conventional HCl treated surfaces. The specific contact resistance of these contacts deposited on chemical vapor cleaned surfaces and subsequently annealed at 450 °C for 30 seconds was 3±2 Ω cm2. Improved ohmic behavior and a specific contact resistance of 4±2 Ω cm2 was obtained for contacts deposited on HCl treated surfaces and annealed using the same schedule. The formation of Au:Ga and Au:Ni solid solutions was observed for contacts on HCl treated surfaces following the 450 °C anneal. There were significantly less interfacial reactions for annealed contacts on chemical vapor cleaned surfaces. The values of specific contact resistance, sheet resistance, and transfer length of the annealed contacts deposited on both chemical vapor cleaned and HCl treated surfaces and measured from room temperature to 140 °C did not change during three successive thermal cycles within this range.

Thermopower behavior in the Gd5(Si0.1Ge0.9)4 magnetocaloric compound from 4 to 300 K

Recent studies on the orthorhombic Gd5(Si0.1Ge0.9)4 compound show, upon heating, a ferromagnetic to antiferromagnetic-like (AFM*) transition at TS=78 K, coupled with a first-order structural martensitic transformation keeping the orthorhombic symmetry but producing a large increase in the interlayer Si(Ge) distances leading to covalent bond-pair breaking. A second-order AFM*→(paramagnetic)PM transition occurs at TN=125 K. We report thermopower (S) measurements for the Gd5(SixGe1−x)4 series, performed on an x=0.1 sample, from 4 to 300 K, with increasing and decreasing temperatures through successive thermal cycling. Resistivity measurements show a systematic increase in the residual resistivity and a dramatic change in the ρ(T) behavior upon thermal cycling. In spite of this, the thermopower data show a common intrinsic behavior both in the ferromagnetic phase (T<TS=78 K) and above ∼230 K, i.e., independent of the number of thermal cycles, increasing or decreasing temperature, and of the particular residual resistivity. Also no drastic differences are seen in S(T) from TS to TN, upon thermal cycling. The structural transition is marked (upon heating) by a sharp increase in the S magnitude, reaching a deep negative minimum of −23 μV K−1 at ∼95 K. Then S(T) rises with increasing slope as T approaches TN, where dS/dT exhibits a singularity. S(T) hysteresis sets in at TS, reaches a maximum around 100 K, does not disapear at TN but persists in the PM phase up to ∼230 K. The later feature correlates well with a similar effect observed in ρ(T). The striking differences between S(T) and ρ(T) behavior under thermal cycling are analyzed.

Thermotropic behaviour of covalent fullerene adducts displaying 4-cyano-4′-oxybiphenyl mesogens

The synthesis of the covalent fullerene derivatives 2, 3 and 4 with an increasing number (2, 4 and 6) of 4-cyano-4′-oxybiphenyl mesogenic groups is described. The thermotropic behaviour of these compounds was investigated and compared with the liquid crystalline properties of the non-fullerene containing precursors 6 and 9, and the D2h-symmetric tetrakis(methano)fullerene 1 bearing eight long alkyl chains instead of mesogenic groups. Tetrakis(methano)fullerene 1 without and methanofullerene 2 with two mesogenic groups only showed an amorphous phase with glass transition temperatures Tg = 27 and 50 °C, respectively. Samples of hexakis(methano)fullerene 3 with four mesogenic groups obtained by slow crystallisation gave a nematic mesophase on the first heating (Cr 85 N 157 I) that disappeared after isotropisation. No mesogenic behaviour could be recovered for 3 in successive thermal cycles. Unlike 3, the fullerene dendrimer 4 with six 4-cyano-4′-oxybiphenyl groups showed mesogenic properties independent of the thermal history: low intensity first order phase transitions were recorded in the DSC and confirmed by optical microscopy analysis. Owing to the lack of a characteristic texture, the unambiguous identification of the mesophase was not possible. The effect of the introduction of mesogens onto C60 on the stability of the mesophases and on the preservation of liquid crystalline properties is discussed with respect to the number of mesogenic moieties per fullerene sphere.

Electric Response of ZrO<sub>2</sub> - 3 mol% Y<sub>2</sub>O<sub>3</sub> nanocrystals

The evolution of microstructure and electrical behaviour of nanocrystalline ZrO 2 - 3mol% Y 2 O 3 samples with respect to thermal cycles have been investigated. The sample, that initially consisted of 77.8%wt of tetragonal and 22.3%wt monoclinic phases, eliminates the tetragonal phase by means of a low temperature t → m phase transformation in which the content of monoclinic phase reaches 96%wt. This transformation affects significantly the electrical properties of the sample, increasing the grain boundary resistivity in successive thermal cycles, whereas the bulk contribution remains unchanged. The growth of the grain boundary resistivity is believed to be due to the presence of microcracks that accommodate the larger size of monoclinic grains.

Low-temperature phonon thermal conductivity of the quasi-onedimensional single crystals (Ta1-xNbxSe4)2I

We report on thermal conductivity measurements of the quasi-onedimensional single crystals (Ta1−xNbxSe4)2I (x = 0, 0.008, 0.01) between 50 mK and 6 K. The thermal conductivity of a (TaSe4)2I sample obeys a cubic variation below 0.8 K, in good agreement with the boundary scattering regime. For two Nb doped samples, the thermal conductivity between 0.1 K and 1 K obeys a quasiquadratic regime, which is ascribed to extended “linear or planar defects,” specific to this one-dimensional structure, and introduced either by doping or by successive thermal cycling, while it obeys a cubic variation below 0.1 K, as in the case of pure (TaSe4)2I. In the vicinity of 1 K and 2 K for pure and 1 at. % Nb doped samples, thermal conductivity shows a very sharp peak, which exceeds the low-temperature cubic boundary Casimir regime, or the quasi quadratic regime, respectively. The presence of the peak in these systems is explained by a phonon Poiseuille flow, originating from their strong anharmonicity, amplified by the lattice anisotropy.

Binary caesium–lanthanum oxide supported on MCM-41: A new stable heterogeneous basic catalyst

Heterogeneous mesoporous stable basic catalysts have been prepared by wet or solid-state impregnation of MCM-41 with caesium acetate and lanthanum nitrate followed by thermal decomposition. 133 Cs MAS NMR data of CsLa/MCM-41 show an increase in the Cs–O bond length in the CsLa mixed oxides compared with that in Cs oxides supported on MCM-41. A small difference in chemical shift between the hydrated and dehydrated materials is observed, indicating a weak interaction of water with Cs + cations. CO 2 temperature-programmed desorption (TPD) suggests that these materials possess small oxide clusters of mild basicity. The thermal stability of the materials, which is independent of the framework aluminium content, is reflected by an unaffected CO 2 desorption and BET surface area on successive thermal cycles. Alkali metal–lanthanum binary oxides of the Na, K and Rb-type appear to be thermally less stable, in the sense of a significantly smaller CO 2 desorption indicative of particle clustering. 23 Na MAS NMR shows several resonances for the NaLaO x materials. The presence of the single Rb-oxide component in the RbLaO x materials is confirmed by 87 Rb MAS NMR. The mild base strength of the CsLaO x /MCM-41 is demonstrated by its ability to remove a proton from enolates having a pK a ⩽10.2. The presence of lanthanum and the mesoporous framework govern the activity and product selectivity in the liquid-phase Michael addition of ethyl cyanoacetate to ethyl acrylate. The Knoevenagel addition of enolates to benzaldehyde in aqueous media is also catalysed by the CsLa-oxide/MCM-41 system. The catalyst is re-usable after regeneration at high temperature without loss of activity.

Quantification of ANP mRNA in primary cultures of adult rat atrial myocytes by image processing: in situ hybridization to multiple parallel samples using single-stranded cDNA probes.

In primary cultures of adult rat atrial myocytes, we quantified the accumulation of atrial natriuretic peptide (ANP) mRNA in parallel with ANP secretion. ANP mRNA was quantified by image analysis of myocytes hybridized in situ with single-stranded cDNA probes generated by two successive thermal cycling procedures. In situ analysis permitted measurement of many small experimental samples in tandem while avoiding the possibility of differential extraction and processing of mRNA from sample to sample. The single-step application of 32P-labeled probes allowed processing of many parallel samples and generated intense punctate autoradiographic signals that were readily countable by image processing. Biotin-labeled probes, in conjunction with gold-labeled anti-biotin antibodies and silver intensification, gave an apparently equivalent specific signal but presented more difficulty in uniform processing of many samples and was harder to quantify by our image processing system. Measurement of ANP mRNA during atrial myocyte culture showed that ANP mRNA accumulated from undetectable levels after 1 day of culture to maximal levels by Day 8. In contrast, secretion of ANP (which is stored in atrial granules) slowly decreased, but was not abolished, during the first 5 days of culture. Subsequently, ANP secretion increased, with the increase trailing ANP mRNA accumulation by at least 24 hr.

Anomalous phase behavior of (NH4) HSO4

We have studied the phase behavior at normal pressure of NH4HSO4 (AHS) over the 100–420 K temperature region using ac calorimetry on single crystal samples. From the changes in the ac-calorimetric curves recorded on successive thermal cycles of the samples, it is inferred that metastable phases can be induced in the previously reported region of its intermediate ferroelectric phase (154 K <T < 220 K) at 1 atm. An additional thermal anomaly was found to occur at 220 K that seems to indicate the coexistence of the low and intermediate phase in the 154 K <T < 220 K temperature range. No thermal anomalies were observed in the paraelectric region 270 K <T < 420 K, where it has been claimed that some proton transport occurs in AHS.

A New Oxide Ion Conductor Family: Bi 7(P 1- yV y)O 13

A new phase, Bi 7PO 13, obtained at room temperature in the Bi 2O 3-BiPO 4 system, has been identified as a superstructure of the δ Bi 2O 3 fcc form. Its conductivity level has been improved by a substitution of V for P, in solid solution Bi 7(P 1- y V y )O 13 with 0 ≤ y ≤ 0.5. Above 400°C the conduction mechanism is due to the mobility of the O 2- ions. The electrolyte P O 2 response time has been investigated vs time for successive thermal cycles.

Identification of IRO structures in photodarkening: A high pressure study of As 2S 3

The pressure dependence of reversible photodarkening was determined for glassy As 2S 3 in the range from 1 to 60 kBar. Annealed samples were pressurized in a diamond anvil cell for successive light soaking and thermal annealing cycles at constant pressure. Similar to PD at atmospheric pressure, the absorption edge, determined from in-situ transmission measurements, exhibits a parallel shift to lower energy under illumination with band gap light. Hydrostatic pressure causes the magnitude of the reversible edge shift to increase in the low pressure region and then to decrease rapidly at higher pressures, leading to a vanishing of the effect at 60 kBar. The results are interpreted in terms of two IRO structural states: Application of low pressures further promotes a structural configuration responsible for PD at atmospheric pressure whereas at higher pressures, a second structural configuration that exhibits no PD becomes increasingly dominant.

ChemInform Abstract: FAILURE PREDICTION OF THE THERMAL FATIGUE OF SILICON NITRIDE

AbstractAm Beispiel von Si3N4, das für Schaufeln in Hochtemperaturturbinen Verwendung findet, wird gezeigt, daß Daten für langsames Rißwachstum zur Vorhersage des thermischen Ermüdungsverhaltens von Strukturkeramiken geeignet sind.

Failure Prediction of the Thermal Fatigue of Silicon Nitride

Slow crack growth data were used to predict the thermal fatigue life of silicon nitride parts subjected to the cyclic thermal environment of a turbine engine. For this purpose, a computer program was devised which numerically calculates the extent of slow crack growth for each successive thermal cycle until catastrophic failure occurs. Reasonable agreement was found between predicted and experimental fatigue life. Discrepancies were attributed to uncertainties in the values for the property data required for the fatigue calculations, as well as to statistical aspects of brittle fracture. Results of failure predictions for materials having flaw depths smaller than those encountered in the present specimens suggest that silicon nitride can be made more resistant to thermal fatigue by minimizing the amount of viscous grain‐boundary phase, which is responsible for slow crack growth.

Electrical Conduction in Amorphous and Semicrystalline Ni–Cr–SiO Films

A resistance maximum with respect to temperature has been observed in 40 Ni-10 Cr-50 SiO v/0 films of thickness 500–2200-Å vacuum coevaporated onto amorphous SiO2 substrates maintained at room temperature. The maximum occurs at a temperature TM between 275° and 325°C. On successive thermal cycling to 500°C the resistivity of the film decreases and the resistivity peak shifts to somewhat higher temperatures. The phenomena are attributed to short-range ordering processes occurring in the temperature range 200°–350°C due to the absence of significant oxidation. When the films of resistivity 200–1000Ω per square were deposited onto substrates maintained at 300°C, the resistivity maximum disappears and the films showed a near zero temperature coefficient of resistivity and excellent cyclic stability from 25° to 350°C. All films were apparently amorphous as determined by transmission electron microscopy and electron diffraction. Depositions and annealing of films at temperatures up to 600°C led to crystalline films and to the general rule that films of up to a thousand ohms per square deposited or annealed at a temperature TM exceeding 300°C are cyclically stable at least up to TM and exhibit near zero temperature coefficient of resistance.