Anomalous Thermal Expansion Behavior Research Articles

The phase diagram of ammonium nitrate

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.

Anomalous thermal expansion of cobalt olivine, Co2SiO4, at low temperatures

The temperature variation of the Co2SiO4structural parameters has been investigated by means of both neutron powder and single-crystal diffraction. Measurements were performed in a broad temperature range from 5 to 500 K. There is clear evidence of an anomalous thermal expansion related to the magnetic phase transition at 50 K. Below this temperature an abrupt elongation and contraction were observed along thecandbaxes (space groupPnma), respectively, whereas the lattice parameteraremains constant in the range between 5 and 50 K. The resulting volume of the unit cell is contracted below 50 K. This phenomenon is attributed to magnetostriction effects. A model based on the specific heat capacity data was applied for the quantitative description of the anomalous thermal expansion behaviour. On the other hand, no anomalies can be observed in the temperature dependency of the interatomic distances and angles.

H-BN on Rh(111): Persistence of a commensurate 13-on-12 superstructure up to high temperatures

We present a high-resolution surface X-ray diffraction study of hexagonal boron nitride (h-BN) on the surface of Rh(1 1 1). The previously observed commensurate 13-on-12 superstructure for this system is stable in the temperature range between room temperature and 830 °C. Surface X-ray diffraction measurements up to 830 °C on the superstructure show no sign of a shift towards a different superstructure, demonstrating the high thermal stability and strong bonding between film and substrate. At lower temperatures, an anomalous thermal expansion behaviour of the topmost surface region of rhodium is observed, where the rhodium in-plane lattice constant remains invariant. This can be explained by the (h-BN) single-layer being compressively strained, whereby the strong bonding to the substrate causes the latter to be tensile strained.

High pressure effects on liquid viscosity and glass transition behaviour, polyamorphic phase transitions and structural properties of glasses and liquids

Since the pioneering work of Bridgman it has been known that pressure affects the glass transition of polymers and liquid state viscosities. Usually the Tg and viscosity both increase as a function of pressure as expected from ‘free volume’ theories. However, H2O provided a notable exception in that the viscosity passes through a minimum at low temperature. It was thought that this might be linked to the anomalous thermal expansion behavior. However further research on geologically important aluminosilicate liquids revealed that they could show anomalous viscosity decreases with increasing pressure and this behavior is given a structural interpretation as five-fold coordinated Si4+ and Al3+ species are formed. Also the existence of polyamorphism or density-driven liquid–liquid phase transitions in certain systems can lead to anomalies in the Tg or η vs. P relations. This may be the case for H2O, for example. Current research is focusing on investigating structural changes in liquids and glasses at high pressure as the rich variety of behavior is becoming recognized. Both experimental studies and computer simulations are important as the underlying phenomonology is linked to changes in the glass or liquid structure as a function of densification.

Anomalous thermal expansion behaviour of Ln 2Mo 4O 15 (Ln = Y, Dy, Ho, Tm)

Based on a consideration of the crystal structures of well-known negative thermal expansion (NTE) materials, ZrV 2O 7 and ZrW 2O 8, we have identified a new series of tetramolybdates, Ln 2Mo 4O 15 (Ln = Y, Dy, Ho, Tm), that exhibit an anomalous thermal expansion behaviour. Unlike ZrW 2O 8 and ZrV 2O 7 which are cubic, the tetramolybdates are monoclinic (space group P2 1/ c). Nevertheless, the framework is similar to ZrV 2O 7/ZrW 2O 8 consisting of MoO 4 tetrahedra that are weakly connected to Mo 2O 7 anions, which bind the Ln in a seven-fold anion coordination. An exploratory investigation of these materials using dilatometry, differential scanning calorimetry (DSC) and high-temperature powder X-ray diffraction (XRD) (Dy 2Mo 4O 15) reveals a negative or low thermal expansion in the 30–200 °C temperature range. Since there are no major structural changes in the high-temperature XRD and there is no first-order phase transition in DSC in the 30–500 °C range, we believe the mechanism of anomalous thermal expansion in these molybdates likely involves second-order structural changes of the molybdate oxygens, in a manner similar to the second-order changes found in ZrW 2O 8. Further detailed structural studies are essential to unravel the mechanism of anomalous thermal expansion behaviour of these molybdates.

Inelastic neutron scattering and lattice dynamical calculation of negative thermal expansion inHfW2O8

The compounds ZrW 2 O 8 and HfW 2 O 8 undergo large isotropic negative thermal expansion (NTE) over a wide range of temperatures up to 1443 K and 1050 K, respectively. We have showed previously that large softening of low-energy phonons in ZrW 2 O 8 is responsible for its anomalous thermal expansion behavior. In order to understand the effect of replacing Zr by Hf on NTE behavior we report lattice dynamical calculations and neutron time-of-flight spectroscopic measurements of the phonon density of states for cubic HfW 2 O 8 . The calculated phonon spectrum for cubic HfW 2 O 8 is in fair agreement with the experimental data. The phonon spectra in the Zr and Hf compounds differ at low energies largely due to the mass difference. The calculated negative thermal expansion for HfW 2 O 8 is in good agreement with experimental data from the literature. We further report a calculation of the pressure dependence of the detailed phonon dispersion relation which reveals large softening of several phonon branches on compression associated with the NTE.

Dilatometric approach for the determination of the solid state reaction-onset of the lead based relaxor ferroelectric system

Dilatometer based thermal expansion studies have been carried out on the starting oxide mixtures of some of the important relaxor ferroelectric compositions including lead iron niobate, lead magnesium niobate–lead titanate and lead zinc niobate–barium titanate. An anomalous thermal expansion behavior is observed between 550 and 650 °C corresponding to the onset of the solid state reaction. Further heating of the reaction mixture resulted in a decrease in the dimension of the sample. From the complementary evidence of DTA, X-ray diffraction (XRD) and scanning electron microscopy (SEM), it is shown that the initiation of solid state reaction results in the restructuring of the reacting grains to form the agglomerates of fine particles of reaction intermediates. This phenomenon is shown to be absent in the barium zinc niobate system where the pyrochlore intermediates do not exist. This novel dilatometric approach has been projected as a possible technique to identify optimum calcination temperatures to produce sinter-active powders in the above oxide system, thereby help in reducing the sintering temperatures.

Invar effect in Pu-Ga alloys

Abstract We show that the anomalous thermal expansion behaviour of Pu-Ga alloys can be explained by an Invar effect, which is modelled by assuming the thermal transfer of Pu atoms from a lower-energy higher-volume Δ1, state to a higher-energy lower-volume Δ2 state. The energy separation of these states is found to be 1400 K from a fit to the available neutron diffraction data on the lattice constants of Pu-Ga alloys. The Invar model enables a precise evaluation of the coefficient of thermal expansion and of the Gruneisen constant. The addition of Ga forces a stabilization of the Δ2 state and suppresses the negative thermal expansion.

High-Moment-Low-Moment Description of Magnetovolume Effects in Y(Mn x Al 1− x ) 2 and Y x Sc 1− x Mn 2

Based on the assumption of a high-moment-low-moment instability of the Mn atom, we construct a simple spin model with coupled magnetic and spatial degrees of freedom to describe the Laves phase systems Y(Mn x Al 1 m x ) 2 and Y x Sc 1 m x Mn 2 . Monte Carlo simulations of this model qualitatively reproduce anomalies observed in these materials like a discontinuous giant volume change and anomalous thermal expansion behavior.

Thermal expansion and high-temperature phase transformation of the yttrium silicate Y2SiO5

The linear thermal-expansion coefficients of yttrium silicate Y2SiO5, [Y2(SiO4)O] were measured in the temperature range from 20 to 1400 °C using x-ray diffraction. The anomalous behavior of thermal expansion was observed above Tc = 850 °C and was attributed to the displacive phase transformation. The transformation was reversible and resulted from the local order °C the compositional disorder and local fluctuation in the elastic free energy constrained a secondary transformation related to the polymorphic twin transformation. This created an additional peak in x-ray diffraction patterns at 2 's intensity. The characteristic of phase transformation both on heating and on cooling of the sample was also investigated using the differential thermal analysis method. The thermogravimetric technique did not indicate on a change of weight at Tc.

Stress effects on the anisotropic thermal expansions of “martensitic” A15 compounds

Precision capacitance dilatometry provides a sensitive measure of the thermal strain developed in a sample undergoing a structural distortion with its varying temperature. The A15 structure compounds, V3Si and Nb3Sn, are well known to undergo distortion from their cubic structures at room temperature to tetragonal structures (c/a > 1 for V3Si and c/a < 1 for Nb3Sn) at low temperatures. In the past, highly anomalous thermal expansion behaviour recorded for these materials has been attributed to a strongly anharmonic lattice potential manifesting itself in unusually high, and strongly temperature-dependent, Grüneisen parameters. Further studies on polycrystalline material revealed this anomalous expansion to be highly anisotropic at temperatures for which, according to conventional diffraction data, the materials are cubic. This behaviour was linked to control of sample morphology by a residual stress field resulting from sample preparation. More recent experiments, in which the transformation morphology has been controlled by the application of external stresses to single crystal V3Si and polycrystalline samples of Nb3Sn and Nb3(Sn1-x Sb x ), have confirmed the occurrence of significant anisotropy in the thermal strain in the cubic phase, well above the structural transformation. We link this departure from cubic symmetry with the well-known soft-mode character of these materials and the associated “central peak” scattering which is also observed well above the transformation temperature. We are led to propose that the “central peak” is the precursor to a Bragg reflection for the transformation structure. This coincidence between “central peak” scattering and the reciprocal lattice for the transformed phase in Ti-Ni has been termed a “ghost lattice”.

A micro-Raman investigation of the SCS-6 SiC fiber

The microstructure of the SCS-6 SiC fiber, a fiber widely studied as a reinforcement in metal matrix and ceramic matrix composites, was characterized by spatially resolved Raman spectroscopy. Cross sections of as-received fibers and fibers annealed at 1640 °C were characterized in the backscattering geometry with the 514.5 nm line of an Ar+ laser. Characteristic graphitic peaks were observed in the several regions of the SCS-6 fiber, including the inner SiC region. The SiC transverse optical phonon mode was observed in the outer SiC region as a double peak, suggesting the presence of multiple polytypes in this region. Spectra of the outer SiC layer of this fiber in the zircon-matrix composite also showed a distinct Si peak. Silicon in this fiber is believed to be responsible for the observed anomalous thermal expansion behavior of the fiber between 1300 and 1400 °C.

Observation of Si in SiC composite fibers: A micro-Raman investigation

The chemical composition and microstructure of SiC composite fibers were studied by spatially resolved Raman spectroscopy. The spectra display a strong peak in the outer SiC layer identified as a Si signature and thus provide direct evidence of the presence of excess Si in the fiber composites. The Si crystallite size as well as the stress in the Si were estimated. Excess silicon has been previously suggested as the cause of the unusual creep and the anomalous thermal expansion behavior of these composites at high temperatures.

Anomalous thermal expansion behavior of the YBaCuO superconductor. Indirect evidence of polaron formation

A simple-minded semiclassical method is used to study the thermal expansion data of YBa 2Cu 3O 7−δ ( δ = 0.15) both in the normal and superconducting phases. It is shown that there is an anomaly in the temperature variation of the thermal expansion which begins to show up at a temperature somewhat higher than T c. We argue that this anomaly can be attributed to the phenomenon of polaron formation. This might give an indirect support to the bipolaronic mechanism of high-temperature superconductivity.

High temperature structural stability of chemically vapor-deposited silicon carbide filaments

The structural stability of SCS-6 filament was investigated in the temperature interval 1000–1600 °C by differential scanning calorimetry (DSC) and in situ X-ray diffraction techniques. It was found that this filament is composed mostly of the cubic β polytype of SiC and of graphite, and that these phases are stable in the temperature interval studied. The lattice parameter for β-SiC was determined between room temperature and 1600°C, and the calculated average linear thermal expansion of the lattice in this temperature interval (5.24 × 10−6 °C−1 is in agreement with macroscopic thermal expansion measurements on single filaments. Thermal events observed by DSC occur at 1380 °C on heating and at 1300°C on cooling, and are suggested as being related to the melting and solidification, respectively, of excess silicon. These results are analysed in relation to the anomalous thermal expansion behavior exhibited by this filament.

Retention of Implanted Gases and Enhanced Thermal Expansion of Mica

Helium and xenon gases implanted into muscovite micas increase the reversible thermal expansion perpendicular to the silicate layers by one or two orders of magnitude. The gas‐mica composite behaves like a gas within an elastic membrane, and this can be developed into a thermal actuator or heat sensor. Helium gas is implanted into muscovite mica to simulate the anomalous thermal expansion behavior of water in phlogopite. The implanted Xe gas has been studied by Rutherford backscattering and by counting and measuring gas bubble radii. Helium gas appears to diffuse out of the structure, since the anomalous thermal expansion decreases slowly over a period of several months. All of the implanted Xe is found to remain within the mica structure.

Structural investigation of the silica-titania gel/glass transition

Vitreous silica containing titania has received considerable attention due to its anomalous thermal expansion behaviour, together with high optical quality. The effect and the role of TiO2 in the properties of glass are directly correlated with the structural position of titanium; thus several attempts have been made to elucidate the structural aspects of gels and glasses containing titania. Several SiO2-TiO2 gels containing up to 30 mol% TiO2 prepared using (i) normal drying; (ii) supercritical drying in alcohol; (iii) supercritical drying in CO2 were studied in this work. The aim of this work is to establish the titanium coordination during thermal treatments during the gel/glass system evolution. X-ray diffraction (XRD) and dilatometric analysis have been used to detect the presence of a crystal phase of TiO2 (anatase) in gel/glass samples. The crystal size of anatase was evaluated using the Scherrer relation from XRD patterns. Infrared and diffuse reflectance spectroscopy in the UV-visible region have been used to elucidate the structural aspects and the electronic transitions due to different titanium coordination. These results indicate that drying conditions strongly influence the silicon-oxygen-titanium structure.

Structural Mechanisms of Anomalous Thermal Expansion of Cordierite‐Beryl and Other Framework Silicates

The structural mechanisms controlling the anomalous thermal expansion behavior of the cordierite‐beryl type structure and other framework silicates are discussed. The expansion of a particular silicate framework depends on the temperature response of the framework itself involving T–O–T and O–T–O angular changes, the expansion coefficient(s) of the nontetrahedral bonds, if present, and the interplay between these two factors. For cordierite, beryl, and β‐eucryptite, expanding nontetrahedral bonds play a critical role in driving the anomalous thermal expansion observed. For β‐spodumene and the feldspars, the thermal response of the framework dominates the expansion mechanism. The effects of channel constituents on the thermal expansion of the cordierite‐beryl framework are also discussed.

An Interphasic Region between Nitrocellulose and Nitroguanidine in triple base propellant

AbstractEvidence for the existence of an interphasic region in the nitroglycerin‐nitrocellulose matrix immediately surrounding the crystalline nitroguanidine filler in triple base propellant is presented for the first time. This interphasic region displays anomalous thermal expansion and mechanical relaxation behaviour compared to the bulk nitroglycerin‐nitrocellulose matrix in the propellant. The interphasic region accounts for the highly unusual thermal expansion behaviour of triple base propellants containing long needlelike nitroguanidine crystals. An interphasic region does not occur in similar propellant when the nitroguanidine filler is replaced by crystalline RDX. Thus the interphase region is thought to be a manifestation of a significant bindernitroguanidine interaction. Implications for munition survivability stemming from the existence of a separate polymeric interphase are discussed in terms of the thermal and relaxation behaviour of the interphasic binder‐crystalline filler interface.

Effect of Decomposition on the Densification and Properties of Nasicon Ceramic Electrolytes

Nasicon ceramic electrolytes were processed and characterized using a number of different techniques. The materials were sintered at T = 1100°C to 1375°C. Results of this work demonstrate that Nasicon ceramics with compositions around x =2 (i.e. Na3Zr2Si2POt2), are not monophase, possess poor mechanical strength and an anomalous thermal expansion behavior, and are subject to chemical and phase instability during densification at high temperatures. Wet‐chemical techniques, X‐ray diffraction, and SEM analyses were used to observe the phase and chemical decomposition of these materials. The products of this decomposition process are correlated with the poor physical properties.