Stable Monoclinic Phase Research Articles

Metastable Phase Diagram of Nanocrystalline ZrO2−Sc2O3 Solid Solutions

We have investigated the crystal structures and phase transitions of nanocrystalline ZrO2−1 to −13 mol % Sc2O3 by synchrotron X-ray powder diffraction and Raman spectroscopy. ZrO2−Sc2O3 nanopowders were synthesized by using a stoichiometric nitrate−lysine gel−combustion route. Calcination processes at 650 and at 850 °C yielded nanocrystalline materials with average crystallite sizes of (10 ± 1) and (25 ± 2) nm, respectively. Only metastable tetragonal forms and the cubic phase were identified, whereas the stable monoclinic and rhombohedral phases were not detected in the compositional range analyzed in this work. Differently from the results of investigations reported in the literature for ZrO2−Sc2O3 materials with large crystallite sizes, this study demonstrates that, if the crystallite sizes are small enough (in the nanometric range), the metastable t′′-form of the tetragonal phase is retained. We have also determined the t′−t′′ and t′′−cubic compositional boundaries at room temperature and analyzed the...

Analysis of reactions during sintering of CuO-doped 3Y-TZP nano-powder composites

3Y-TZP (yttria-doped tetragonal zirconia) and CuO nano powders were prepared by co-precipitation and copper oxalate complexation–precipitation techniques, respectively. During sintering of powder compacts (8 mol% CuO-doped 3Y-TZP) of this two-phase system several solid-state reactions clearly influence densification behaviour. These reactions were analysed by several techniques like XPS, DSC/TGA and high-temperature XRD. A strong dissolution of CuO in the 3Y-TZP matrix occurs below 600 °C, resulting in significant enrichment of CuO in a 3Y-TZP grain-boundary layer with a thickness of several nanometres. This “transient” liquid phase strongly enhances densification. Around 860 °C a solid-state reaction between CuO and yttria as segregated to the 3Y-TZP grain boundaries occurs, forming Y 2Cu 2O 5. This solid-state reaction induces the formation of the thermodynamic stable monoclinic zirconia phase. The formation of this solid phase also retards densification. Using this knowledge of microstructural development during sintering it was possible to obtain a dense nano–nano composite with a grain size of only 120 nm after sintering at 960 °C.

Overcoming the deleterious effect of hafnium in tungsten–zirconia catalysts: The use of doping and thermal treatments

The effect of hafnium and aluminum addition to Ni/WO 3/ZrO 2 was examined as a function of calcination temperature for the isomerization of a 5 wt% cyclohexane in n-hexane feed. Hafnium, a common and difficult to separate impurity in zirconia, was found to decrease catalytic activity, primarily through the generation of monoclinic and cubic phases that are considered inactive for acid-catalyzed reactions. A hafnium induced monoclinic phase is formed at relatively low calcination temperatures and is fundamentally different from the thermodynamically stable monoclinic phase generated at higher temperatures. The addition of aluminum as well as higher calcination temperatures limited the formation of this monoclinic phase resulting in very active catalysts without the use of more expensive, hafnium-free catalyst precursors. Catalysts with tetragonal zirconia contents less than 40 wt% had initial isomerization activities that scaled linearly with tetragonal zirconia content. Catalysts with a tetragonal zirconia contents greater than 40% had initial n-hexane isomerization activities that were unrelated to zirconia morphology; a limitation that was a result of the hydrogenation function. The lack of correlation between zirconia crystal phases and catalytic activity for high tetragonal zirconia content catalysts implies that the measured isomerization activity may better represent the hydrogen spillover rate of nickel and not necessarily the intrinsic acid-catalyzed rate. Improved control over hydrogenation activity would allow for the preparation of more active catalysts.

Computing the melting point and thermodynamic stability of the orthorhombic and monoclinic crystalline polymorphs of the ionic liquid 1-n-butyl-3-methylimidazolium chloride

The melting point, enthalpy of fusion, and thermodynamic stability of two crystal polymorphs of the ionic liquid 1-n-butyl-3-methylimidazolium chloride are calculated using a thermodynamic integration-based atomistic simulation method. The computed melting point of the orthorhombic phase ranges from 365 to 369 K, depending on the classical force field used. This compares reasonably well with the experimental values, which range from 337 to 339 K. The computed enthalpy of fusion ranges from 19 to 29 kJ/mol, compared to the experimental values of 18.5-21.5 kJ/mol. Only one of the two force fields evaluated in this work yielded a stable monoclinic phase, despite the fact that both give accurate liquid state densities. The computed melting point of the monoclinic polymorph was found to be 373 K, which is somewhat higher than the experimental range of 318-340 K. The computed enthalpy of fusion was 23 kJ/mol, which is also higher than the experimental value of 9.3-14.5 kJ/mol. The simulations predict that the monoclinic form is more stable than the orthorhombic form at low temperature, in agreement with one set of experiments but in conflict with another. The difference in free energy between the two polymorphs is very small, due to the fact that a single trans-gauche conformational difference in an alkyl sidechain distinguishes the two structures. As a result, it is very difficult to construct simple classical force fields that are accurate enough to definitively predict which polymorph is most stable. A liquid phase analysis of the probability distribution of the dihedral angles in the alkyl chain indicates that less than half of the dihedral angles are in the gauche-trans configuration that is adopted in the orthorhombic crystal. The low melting point and glass forming tendency of this ionic liquid is likely due to the energy barrier for conversion of the remaining dihedral angles into the gauche-trans state. The simulation procedure used to perform the melting point calculations is an extension of the so-called pseudo-supercritical path sampling procedure. This study demonstrates that the method can be effectively applied to quite complex systems such as ionic liquids and that the appropriate choice of tethering potentials for a key step in the thermodynamic path can enable first order phase transitions to be avoided.

Thickness-dependent structural and optical properties of sputter deposited ZrO2 films

ZrO2 thin films in thickness of 17nm to 559nm were deposited on Si-(100) and glass substrates using mid-frequency reactive magnetron sputtering method. The microstructure analysis showed that the stable monoclinic phase preferentially grew in the initial growth stage, whereas the relative amount of tetragonal phase increased steadily with the increase of film thickness beyond the critical threshold (about 125nm in the present work). The different evolution of phase component with that in the case of chemical vapor deposition is considered due to the additional impact of energetic particles in the sputtering process. The film thickness also showed significant impacts on residual stress, refractive index and transmittance of the films. The stress magnitude changed remarkably during the early stages of film growth, and then tended to a stable compressive value. The optical transmittance degraded gradually in the initial growth stage, meanwhile the refractive index increased progressively until the bulk value was reached. The fact that both microstructural and optical properties of ZrO2 films are sensitive at thinner thickness indicates that the technical control of the sputtering process in the initial growth stage is crucial for microstructural and functional optimization of ultra-thin films.

CVD deposition and characterization of coloured Al 2O 3/ZrO 2 multilayers

Coloured Al 2O 3/ZrO 2 multilayers have been deposited onto WC–Co based inserts by a CVD process. Through physical as well as optical analysis of such multilayers, colour is believed to originate from interference. The coatings are obtained with good process reproducibility. It was found that the ZrO 2 process used in the multilayer, with ZrCl 4 as the only metal chloride precursor, results in a mixture of tetragonal and monoclinic ZrO 2 phases. However by adding a relatively small amount of AlCl 3 during such a process results in ZrO 2 layers being composed of predominantly tetragonal ZrO 2 phase. Corresponding multilayers seem to have a more fine grained and smoother morphology whereas multilayers containing monoclinic ZrO 2 phase seem to be less perfect with existence of larger grains of ZrO 2 which are believed to scatter light and alter the reflectance of such a multilayer. In addition to this, such multilayers were found to be free of or with greatly reduced amount of thermal cracks, normally present in pure CVD grown Al 2O 3 layers. It is believed that, in the studied Al 2O 3/ZrO 2 multilayers, the observed tetragonal ZrO 2 phase is the result of a size effect, where small enough ZrO 2 crystallites energetically favor the tetragonal phase. However as the ZrO 2 crystallite size distribution is shifted to larger sizes it is believed that a mixture of crystallites with both stable and metastable tetragonal phases as well as a stable monoclinic phase is obtained. The proposed metastable tetragonal ZrO 2 phase may in fact explain the absence of thermal cracks in such multilayers through a transformation toughening mechanism, well known in ZrO 2 based ceramics.

Phase Decomposition and Dielectric Properties of Reactively Sputtered Bismuth Zinc Niobate Pyrochlore Thin Films Deposited from Monoclinic Zirconolite Target

Thin films of Bi2O3-ZnO-Nb2O5 system with cubic pyrochlore structure can be deposited using both cubic and monoclinic (zirconolite) targets. When monoclinic target was used, the as-deposited phase was nonequilibrium cubic monophase. After post-annealing in the 600–800^∘C range, phase decomposition occurred, resulting in more thermodynamically stable cubic pyrochlore and monoclinic zirconolite phases. The dielectric properties, such as dielectric constant and electric field dependent tunability, showed a steep increase along with phase separation. However, dielectric loss had a reverse tendency. The maximum tunability was about 38%, which exceeds that of cubic pyrochlore monophase films deposited from cubic target.

Structural, electronic, and dielectric properties of ultrathin zirconia films on silicon

As high-permittivity dielectrics approach use in metal-oxide-semiconductor field-effect transistor production, an atomic level understanding of their dielectric properties and the capacitance of structures made from them is being rigorously pursued. We and others have shown that crystal structure of ZrO2 films have considerable effects on permittivity as well as band gap. The as-deposited films reported here appear amorphous below a critical thickness (∼5.4nm) and transform to a predominantly tetragonal phase upon annealing. At much higher thickness the stable monoclinic phase will be favored. These phase changes may have a significant effect on channel mobility.

Raman and Brillouin scattering spectroscopy studies of atomic layer-deposited ZrO 2 and HfO 2 thin films

Atomic layer-deposited ZrO 2 (zirconia) and HfO 2 (hafnia) films with various thicknesses, ranging from 112 to 660 nm, have been studied by Raman scattering spectroscopy. Spectral analysis of the excellent quality Raman data obtained by using freestanding edges of the films has unambiguously demonstrated that a metastable tetragonal t-ZrO 2 is coexisting with the stable monoclinic phase in zirconia films. Even though the Raman spectrum signal-to-noise ratio was high, only the monoclinic phase was positively identified from the observed spectral patterns of hafnia films. X-ray diffraction patterns are used to define the structure of metastable phases. Complementary Brillouin light scattering measurements of the freestanding edges are also employed in constraining elastic properties of the 405 nm HfO 2 thin film.

Study of surface morphology and optical properties of Nb2O5 thin films with annealing

Nb2O5 films have been deposited on variety of substrates using the sol-gel dip coating technique. As-deposited films on all substrates are amorphous. Films were annealed under controlled ambience at 300, 400 and 600°C for 5 h. As-deposited films on glass substrate show uniform surface structure. The crystal structure and surface topography are found to depend strongly on the annealing temperature and nature of the substrates. The average grain size of 40 nm is observed in films annealed at 300°C. On annealing at 400°C increasing grain size and resulting fusing of them, enhanced surface roughness. Films deposited on NaCl substrates crystallized into a stable monoclinic phase and those deposited on single crystal Si substrates crystallized into hexagonal phase after annealing at 600°C. The as-deposited films show very high transmittance (>90%) in the visible region. The optical band gap is observed to increase from 4.35 eV when the films are in amorphous state to 4.87 eV on crystallization.

The Two-Component System CCl4 + CBrCl3. Inference of the Lattice Symmetry of Phase II of CBrCl3

The phase diagram of the two-component system between two halomethanes, carbon tetrachloride (CCl4) and bromotrichloromethane (CBrCl3) has been determined by means of X-ray powder diffraction and thermal analysis techniques from 200 K to the liquid state. From the continuous series of mixed crystals, three relations of isomorphism have been evidenced: (i) between the stable low-temperature monoclinic (M) phases, (ii) between the stable orientationally disordered (OD) rhombohedral (R) phases, and (iii) between the stable OD face-centered cubic (FCC) of CBrCl3 and the metastable FCC phase of CCl4. The existence of such a metastable phase gives rise to the appearance, for a large range of composition, of metastable FCC mixed crystals that behave monotropically with respect to the R mixed crystals accordingly with the well-known monotropic character of the R to FCC phase transition of the CCl4. The isomorphism relationships i and ii enable us to validate the previously proposed structure of the monoclinic ph...

Thermal and X-ray measurements on n-hexatriacontane (C36H74)

The polymorphism of n-hexatriacontane (C 36 H 74 ) was investigated by X-ray diffraction and DSC thermal analysis. C 36 H 74 displays at least two stable monoclinic phases and a rotator phase. Starting from a solution-grown sample of very high purity the following phases are observed with increasing temperature: M011 < 345K < M101 < 346K < RIII < 348K < liquid. Depending on the thermal treatment also other forms may be observed. During these transitions, both the crystalline volume and the lamellar thickness increase. The higher order reflections of the layer structure was precisely measured by X-ray diffraction. Using a Fourier synthesis, electron density profiles of the three phases were analysed. The results show that the interlamellar structure is disturbed by increasing molecular motions with elevating temperatures. For some homologous alkanes also p(T) phase diagrams and binary systems have been studied.

Growth of Hafnium Dioxide Thin Films by Liquid‐Injection MOCVD Using Alkylamide and Hydroxylamide Precursors

AbstractThin films of HfO2 have been deposited by liquid‐injection metal–organic (MO)CVD using two nitrogen‐containing precursors [Hf(NMe2)4] and [Hf(ONEt2)4]. The [Hf(NMe2)4] precursor deposits oxide at lower growth temperatures and over a wider temperature range than [Hf(ONEt2)4]. Varying levels of residual carbon and nitrogen were detected in the films grown from each precursor. Analysis by X‐ray diffraction (XRD) indicates that films grown from both precursors exist predominantly in the thermodynamically stable monoclinic phase. Scanning electron microscopy (SEM) shows that the morphology of the HfO2 films from each precursor is markedly different, with films deposited from [Hf(NMe2)4] having a well‐defined columnar crystalline structure, whereas films grown from [Hf(ONEt2)4] are smoother with little evidence of columnar structure. Full crystal structure data for [Hf(NMe2)4] are given, and the dielectric properties of [Al/HfO2/n‐Si(100)] MOS capacitors, fabricated using each precursor, are also reported.

Low temperature martensitic transformation of zirconia with low contents of yttria

Small spherical specimens of the tetragonal phase were obtained for ZrO 2 -1.40∼1.60mol% Y 2 O 3 . These specimens transformed into the stable monoclinic phase, either isothermally or athermally depending on being heated or subzero cooled, respectively. In this paper, the characteristic of the latter transformation was studied. The results include the dependence of M b temperature on the composition, grain size, and specimen size. Other characteristics of the martensitic transformation were also presented.

Isothermal martensitic transformation of zirconia doped with small amount of yttria

The high temperature tetragonal phase of zirconia containing 1.45∼1.60mol% of yttria can be fully retained at room temperature by rapid cooling. The metastable phase transforms into the stable monoclinic phase either isothermally or athermally depending on being heated or subzero cooled. The behavior of the isothermal transformation was studied. Dilatometric measurement resulted in a typical sigmoidal curve after some incubation time. The transformation start time exhibited a C curve in TIT diagram. The C curve shifted toward the short time side as well as toward the high temperature side with decreasing yttria content or increasing grain size. An interruption/resumption experiment of the isothermal heating revealed that the total time expended before the initiation of the transformation was nearly constant regardless of the interruption.

Two New Paracetamol/Dioxane Solvates—a System Exhibiting a Reversible Solid‐State Phase Transformation

This work reports on the crystal structures of two dioxane solvates of paracetamol that are true polymorphs. The high temperature phase is an orthorhombic form, space group Pbca, Z = 8, a = 12.6078(3) Å, b = 12.1129(2) Å, c = 13.4138(3) Å, V = 2048.52(7) Å3, (at 295K) and the low temperature form is monoclinic, space group P21/c, Z = 4, a = 12.325(6) Å, b = 11.965(4) Å, c = 13.384(6) Å, β = 92.01°, V = 1972.6(14)Å3 (at 123K). The structures of these polymorphs are described as is the interrelationship between the two structures. In addition to the structural interrelationship, it is shown that the two forms undergo a reversible phase transformation. Desolvation of either form generates the stable monoclinic phase of paracetamol. © 2003 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:2069–2073, 2003

Effects of initial structure on the deformation behavior of PP hollow fiber in continuous drawing

AbstractThe effects of the microcrystalline structure of undrawn fibers on the rheological behavior in the process of the polypropylene (PP) hollow fiber formation system were studied by a simple model describing the continuous drawing process. The predicted and observed drawing behaviors were explained in view of a simple approach based on the concept of strain‐rate sensitivity and strain‐hardening parameters. Various parametric studies showing the interactive nature of the strain‐rate sensitivity and strain‐hardening parameters on the drawing behavior were also numerically performed. Strain‐rate sensitivity affects mainly the intensity of the neck, and strain hardening has more effects on the position of the inflection point in continuous drawing. Details of the necking mechanism in the drawing process were studied by observing the deformation behavior of the specimens with different initial microcrystalline structures. The distinctive initial structure of undrawn PP hollow fibers can be formed by controlling the quenching condition in melt spinning. It was shown that the water‐quenched PP fiber exhibited an unstable smectic form, whereas the stable monoclinic phase was observed for the fiber prepared without quenching by forced convection. The experimental results of the drawing behavior indicated that the strain‐rate sensitivity of the fiber with a smectic form was larger than that with the monoclinic form. It was also shown that the hollowness was affected by the quenching condition in melt spinning and the deformation behavior in the drawing process. © 2001 John Wiley &amp; Sons, Inc. J Appl Polym Sci 81: 2170–2182, 2001

Formation of niobium-tantalum pentoxide orthorhombic solid solutions under chlorine-bearing atmospheres

This paper studies, by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS), the structural changes suffered by amorphous hydrated Nb oxide and mixtures of amorphous and crystalline Nb and Ta oxides subjected to thermal treatments in air and chlorine atmospheres. The air heating of amorphous Nb2O5·nH2O leads to different crystalline phases depending on the working temperature. The hexagonal phase of Nb2O5 is obtained at 773 K and the monoclinic phase β-Nb2O5 is obtained at 1173 K. The thermal treatment of amorphous Nb oxide in chlorine atmosphere decreases the temperature at which phase γ-Nb2O5 appears and at 1173 K the stable monoclinic phase α-Nb2O5 is obtained. Air calcination for 5 hours at temperatures between 973 and 1273 K of different amorphous Nb2O5·nH2O - Ta2O5·nH2O mixtures does not lead to solid solution of these oxides. Thermal treatment in chlorine atmosphere of amorphous Nb and Ta oxides leads to the formation of Nb2O5 and Ta2O5 orthorhombic solution in one hour at 973 K and in 24 hours at 1223 K, when starting from crystalline oxides. The effect of chlorine is due to the dissolution-recrystallization of the metallic chlorides and the oxygen formed, when the system evolves to chemical equilibrium between solid phases and gaseous chlorine.

Crystal Nucleation and Growth in Large Clusters of SeF6 from Molecular Dynamics Simulations

An account of a computer simulation of the nucleation kinetics and crystal growth during the freezing of a series of SeF6 clusters is presented. Although SeF6 has a stable monoclinic phase at the temperatures studied, the clusters froze initially to the body-centered cubic phase and then transformed to the low-energy structure. The temperature dependence of the nucleation rate obtained in the simulations is in approximate agreement with that predicted by the classical nucleation theory. A theoretical model of cluster crystallization that includes time-dependent nucleation and finite-size effects is proposed, applied, and found to accord well with the molecular dynamics (MD) data. Three order parameter profiles, namely, density, translational order, and molecular orientational order, were calculated for nuclei close to the critical size. The orientational order parameter is a new one, presented here for the first time. The translational order parameter shows a weak temperature dependence, while the orienta...

X-ray diffraction and phenomenological studies of the engineered monoclinic crystal domains in single crystal relaxor ferroelectrics

We present x-ray diffraction data, which demonstrate that after high field poling, solid solutions of lead zinc niobate with 8% lead titanate (PZN–PT) consist of a mixture of a dominant monoclinic phase and a remnant tetragonal phase. To understand the origin of this engineered monoclinic phase we studied a Devonshire phenomenological model that predicts the field induced phase transitions in PZN–PT. Although previous attempts to use this model for these materials have matched the phase diagram and dielectric behavior, we show that the coefficients used in those models cannot be reconciled with recent data on electric field behavior. By modifying the coefficients we are able to explain the phase diagram, dielectric behavior, and high field phase transitions in compositions near the morphotropic phase boundary. Using these coefficients, we find no stable monoclinic phase present in the model. This contradiction is explained by noting that within the engineered domain state the tetragonal regions strain the rhombohedral regions, resulting in a metastable monoclinic phase.