Thermal behavior of the composite xerogels of zirconium oxyhydroxide and silicic acid

Gels were prepared via sol?Cgel method by addition of zirconium oxychloride solution into sodium metasilicate (SZ) and sodium metasilicate solution into zirconium oxychloride (ZS) at varying final pH. Si/Zr molar ratio equaled 1/1. Synthesized gels were dried with calcium chloride until they reached a constant mass. SEM and nitrogen adsorption analysis have shown that SZ gels have surface area 175?C200?m^2?g^?1, consist of 20?C30?nm grains. ZS samples have surface area about 1?m^2?g^?1, consist of grains smaller than 10?nm. Thermal and X-ray phase analysis have shown that transition of amorphous ZrO₂ to crystalline form shifts from 430 to 850?C870?°C for SZ gels. Unlike zirconia gels phase transitions that proceed in order: ??amorphous (430?°C)??tetragonal (800?°C)??monoclinic (1,000?°C) phases??, the monoclinic phase in ZS gels appears immediately after transition from amorphous to crystalline state; the tetragonal phase in SZ samples is stable until 1,000?°C.     

Photocatalytic activity of ZrO2 nanoparticles prepared by electrical arc discharge method in water

ZrO₂ nanoparticles were synthesized through arc discharge of zirconium electrodes in deionized (DI) water. X-ray diffraction (XRD) analysis of the as prepared nanoparticles indicates formation a mixture of nanocrystalline ZrO₂ monoclinic and tetragonal phase structures. Transmission electron microscopy (TEM) images illustrate spherical ZrO₂ nanoparticles with 7–30 nm diameter range, which were formed during the discharge process with 10 A arc current. The average particle size was found to increase with the increasing arc current. X-ray photoelectron spectroscopy (XPS) analysis confirms formation of ZrO₂ at the surface of the nanoparticles. Surface area of the sample prepared at 10 A arc current, measured by BET analysis, was 44 m²/g. Photodegradation of Rhodamine B (Rh. B) shows that the prepared samples at lower currents have a higher photocatalytic activity due to larger surface area and smaller particle size.     

Zirconia Aerogels: Effect of the Use of Mono- and Dicarboxylic Acids in the Sol-Gel Process on Structural Properties

Zirconia aerogels were prepared from tetra-n-butoxy zirconium(IV) using acid catalysis with different types of mono- and dicarboxylic acids and ensuing supercritical drying of the gels either at high temperature (578 K, HT-aerogels) or by semicontinuous extraction with supercritical CO₂ at low temperature (318 K, LT-aerogels). The resulting HT- and LT-aerogels were calcined in air at 573 and 773 K, respectively, and characterized by nitrogen physisorption, X-ray diffraction, Raman spectroscopy, thermal analysis, and thermal desorption of ammonia. All HT-aerogels were mesoporous and possessed a high specific surface area between ca. 90 and 170 m²·g^-1 after calcination at 773 K. With oxalic acid the pore size distributions were either bimodal (R = 1) or unimodal R = 2), depending on the acid-to-alkoxide ratio, R. In contrast, the use of acetic and adipic acid, respectively, afforded bimodal pore size distributions. Generally, the crystalline part of the raw HT-aerogels prepared with acetic and adipic acid was made up of tetragonal ZrO₂ with traces of monoclinic zirconia, whereas with oxalic acid at higher acid-to-alkoxide ratio (R = 2) the monoclinic phase became dominant. Calcination in air at 773 K had no influence on the phase distribution and crystallite size. With LT-aerogels, the sample prepared with the largest acetic acid-to-alkoxide ratio (R = 2) afforded the highest BET surface area (ca. 90 m²·g^-1). All LT-aerogels were X-ray amorphous after extraction and calcination in air at 573 K. Crystallization resulted in the formation of tetragonal zirconia. The crystallization temperature increased with higher acetic acid-to-alkoxide ratio, whereas the crystallite size decreased.     

Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa

We have studied CaWO₄ under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO₄ beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure–volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of the fergusonite phase and a different transition pressure. These facts suggest that deviatoric stresses influence on the high-pressure structural behavior of CaWO₄ as previously found in related compounds. The reported experiments also provide information on the pressure dependence of interatomic bond distances, shedding light on the transition mechanisms.     

Formation and dielectric properties of anodic oxide films on Zr–Al alloys

Zr–Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm⁻³ ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO₂ with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO₂ phase, are formed on the Zr–Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr–11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.     

Microstructure and optical properties of Ag-doped ZrO2 thin films prepared by sol gel method

Ag doped ZrO₂ thin films were deposited on quartz substrates by sol–gel dip coating technique. The effect of Ag doping on tetragonal to monoclinic phase transformation of ZrO₂ at a lower temperature (500 °C) was investigated by X-ray diffraction. It is found that the Ag doping promotes the phase transformation. The phase transformation can be attributed to the increase in the tetragonal grain size and concentration of oxygen vacancies in the presence of the Ag dopant. Accumulation of the Ag atoms at the film surface and surface morphology changes in the films were observed by AFM as a function of varying Ag concentration. X-ray photoelectron spectroscopy gave Ag 3d and O 1s spectra on Ag doped thin film. The chemical states of Ag have been identified as the monovalent state of Ag⁺ ions in ZrO₂. The Ag doped ZrO₂ thin films demonstrated the tailoring of band gap values. It is also found that the intensity of room temperature photoluminescence spectra is suppressed with Ag doping.     

Über die Koexistenz von tetragonalem und monoklinem CaC2: Strukturelle und spektroskopische Untersuchungen an Erdalkalimetallacetyliden,MC2 (M = Ca,Sr, Ba)

On the Coexistence of Tetragonal and Monoclinic CaC₂: Structural and Spectroscopic Studies on Alkaline Earth Metal Acetylides, MC₂ (M = Ca, Sr, Ba) The alkaline earth acetylides CaC₂, SrC₂ and BaC₂ can be considered to occur in three polymorphic structures each. The monoclinic low-temperature form, the tetragonal form, and the cubic high-temperature form. No deviation from axial symmetry is obtained for the C₂^2– ions in the tetragonal structure determinations, as confirmed by X-ray single-crystal structures and ¹³C MAS NMR studies. The CaC₂ samples prepared by us were always a mixture of monoclinic and tetragonal phase. Their Raman spectra exhibited two distinct C₂ streching vibrations. Problems arising from the coexistence of these two phases for the interpretation of ¹³C MAS NMR spectra are discussed.     

Thermal Plasma Synthesis of Zirconia Powder and Preparation of Premixed Ca-Doped Zirconia

A novel study about the synthesis of zirconia and calcia-stabilized zirconia powders were carried out by DC thermal plasma starting from cheap precursors as the carbonates. Different operational parameters were investigated to explore the effects of the process conditions, such as the plasma torch power and the gas flow rate on the composition and the morphology of the powders. The products phase changes from a metastable tetragonal to monoclinic/tetragonal mixture. Basically a main tetragonal phase was obtained at low torch power (7 kW) while the amount of monoclinic phase linearly rises with the power, up to 66 wt% at 26 kW of plasma power and high gas flow rate. The gas flow rate also affects the shape and the size of the powder, where high values reduce powder aggregation and enhance the spherical shape. The best results were achieved at 22 kW of plasma power and high gas flow rate, with powders of roundness about 79% and a wide particle size distribution. Adding the calcium carbonate to the zirconium carbonate (corresponding to 8 wt% CaO in the final mixture), the plasma treatment mainly produces a tetragonal phase zirconia, that at 1400 °C in furnace changes in a stable cubic phase. These powders could be made suitable for further industrial applications after proper treatments.

     

N,N-dimethyl-selenobenzamide: two solid phases and low-temperature phase change

Two polymorphs (I: mp 49.0–50.0°C; II: mp 80.0–82.0°C) of N,N-dimethyl-selenobenzamide, (CH₃)₂NC(Se)Ph, have been observed. Both I and II can be prepared separately using the same reaction under different conditions. The phase change from phase I to phase II occurs at low temperatures and this has been confirmed by solid state NMR (¹³C), powder X-ray, and single-crystal structural studies. The single-crystal X-ray diffraction study reveals that the lower melting point form (phase I) crystallizes in the triclinic space group P-1 with two conformations in the unit cell, while the higher melting point form (phase II) crystallizes in the monoclinic space group P2₁/c with one conformation in the unit cell. Theoretical calculations on model clusters using the Universal Forcefield (UFF) show that the total energy of phase I (triclinic form) is 5.9 kcal per mol molecule higher than that of phase II (monoclinic form). Also, the immersion energy which is due to non-bonding interactions, namely Van der Waals and Coulombic (electrostatic) terms, has been calculated using UFF. The Van der Waals terms were very similar in the two crystalline forms (triclinic: −43.1 kcal mol⁻¹; monoclinic: −44.8 kcal mol⁻¹), but the Coulombic terms were significantly different (triclinic: −14.0 kcal mol⁻¹; monoclinic: −31.5 kcal mol⁻¹) and favor the monoclinic form. The triclinic form (phase I) is a kinetically favored metastable phase and upon cooling it changes to the monoclinic form (phase II), a thermodynamically stable phase.     

Effect of sol temperature on the structure,morphology, optical and photoluminescence properties of nanocrystalline zirconia thin films

Transparent nanocrystalline zirconia thin films were prepared by sol–gel dip coating technique using Zirconium oxychloride octahydrate as source material on quartz substrates, keeping the sol at room temperature (SET I) and 60 °C (SET II). X-ray diffraction (XRD) pattern shows the formation of mixed phase [tetragonal (T) + monoclinic (M)] in SET I and a pure tetragonal phase in SET II ZrO₂ thin films annealed at 400 °C. Phase transformation from tetragonal to monoclinic was achieved in SET II film annealed at 500 °C. Atomic force microscopy analysis reveals lower rms roughness and skewness in SET II film annealed at 500 °C indicating better optical quality. The transmittance spectra gives a higher average transmittance >85% (UV–VIS region) in SET II films. Optical spectra indicate that the ZrO₂ films contain direct—band transitions. The sub- band in the monoclinic ZrO₂ films introduced interstitial O⁻defect states above the top of the valance band. The energy bandgap increased (5.57–5.74 eV) in SET I films and decreased (5.74–5.62 eV) in SET II films, with annealing temperature. This is associated with the variations in grain sizes. Photoluminescence (PL) spectra give intense band at 384 and 396 nm in SET I and SET II films, respectively. A twofold increase in the PL intensity is observed in SET II film. The “Red” shift of SET I films and “Blue” shift of SET II films with annealing temperature, originates from the change of stress of the film due to lattice distortions.     

Electrochemical characterization of praseodymium centers in Pr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> zirconias using electrocatalysis and photoelectrocatalysis

The voltammetry of nanoparticles and scanning electrochemical microscopy are applied to characterize praseodymium centers in tetragonal and monoclinic zirconias, doped with praseodymium ions (Pr _x Zr_1−x O₂), prepared via sol–gel routes. Doped zirconia nanoparticles were synthesized by a sol–gel liquid-phase route and characterized by different techniques, including X-ray diffraction powder pattern, ultraviolet–visible diffuse reflectance spectroscopy, infrared spectroscopy, and transmission electron microscopy (TEM). Gels annealed at around 400 °C yielded tetragonal Pr _x Zr_1−x O₂ phases. The monoclinic forms of Pr-doped ZrO₂ were obtained by annealing at temperatures higher than 1,100 °C. TEM micrographs proved that the size of the nanoparticles produced was dependent on their crystalline form, around 15 and 60 nm for tetragonal and monoclinic, respectively. The electrochemical study confirmed that a relatively high content of praseodymium cation was in the chemical state (IV), i.e., as Pr⁴⁺, in both zirconia host lattices. The catalytic and photocatalytic effects of Pr⁴⁺ centers located in the monoclinic zirconia lattice on nitrite reduction and oxygen evolution reaction were studied.     

Phase evolution, phase transition, raman spectra, infrared spectra, and microwave dielectric properties of low temperature firing (K(0.5x)Bi(1-0.5x))(Mo(x)V(1-x))O4 ceramics with scheelite related structure

In the present work, the (K(0.5x)Bi(1-0.5x))(Mo(x)V(1-x))O(4) ceramics (0≤x ≤ 1.00) were prepared via the solid state reaction method and sintered at temperatures below 830 °C. At room temperature, the BiVO(4) scheelite monoclinic solid solution was formed in ceramic samples with x < 0.10. When x lies between 0.1-0.19, a BiVO(4) scheelite tetragonal phase was formed. The phase transition from scheelite monoclinic to scheelite tetragonal phase is a continuous, second order ferroelastic transition. High temperature X-ray diffraction results showed that this phase transition can also be induced at high temperatures about 62 °C for x = 0.09 sample, and has a monoclinic phase at room temperature. Two scheelite tetragonal phases, one being a BiVO(4) type and the other phase is a (K,Bi)(1/2)MoO(4) type, coexist in the compositional range 0.19 < x < 0.82. A pure (K,Bi)(1/2)MoO(4) tetragonal type solid solution can be obtained in the range 0.82 ≤ x ≤ 0.85. Between 0.88 ≤ x ≤ 1.0, a (K,Bi)(1/2)MoO(4) monoclinic solid solution region was observed. Excellent microwave dielectric performance with a relative dielectric permittivity around 78 and Qf value above 7800 GHz were achieved in ceramic samples near the ferroelastic phase boundary (at x = 0.09 and 0.10).     

Formation of nanocrystals in the ZrO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O system

Formation of zirconia nanocrystals in the course of thermal treatment of an X-ray amorphous zirconium oxyhydroxide was studied. It was shown that the formation of tetragonal and monoclinic polymorphs of ZrO₂ in the temperature range from 500 to 700°C occurs owing to dehydration and crystallization of amorphous hydroxide. An increase of the temperature up to 800°C and higher activates mass transfer processes and, as a result, activates the nanoparticle growth and increases the fraction of the phase based on monoclinic modification of ZrO₂ due to mass transfer from the nanoparticles with the non-equilibrium tetragonal structure. Herewith, formed ZrO₂ nanocrystals with monoclinic structure have a broad size distribution of crystallites, and the average crystallite size after thermal treatment at 1200°C for 20 min is about 42 nm.     

Röntgenographische Hochtemperaturuntersuchung der Thermischen Zersetzung von Dodekawolframatoborsäure

Investigation on the Thermal Degradation of 12-Tungstoboric Acid by Means of X-ray Heating Photographs By means of X-ray Guinier investigation of 12-tungstoboric acid hexahydrate H₅[BO₄W₁₂O₁₂] · 6H₂O at room temperature a monoclinic lattice was determined, being in disagreement with the literature. The LSQ-refinement of parameters of the monoclinic C-lattice give a = 1.728 nm, b = 1.215 nm, c = 1.216 nm, b? = 135° 34′, Z = 2, d_exp. = 5.44 g cm^?3, d_calcd. = 5.52 g cm ^?3. From X-ray heating patterns (heating rate: 4°C/min, atmosphere: air) the formation of a new monoclinic phase at 185°C was found, being stable till 270°C. From 270–420°C exist a bad crystalline phase and from 420–840°C a monoclinic phase: a = 0.532 nm, b = 0.389 nm, c = 0.522 nm, b? = 91° 09′. Above 840°C a tetragonal phase is formed with a diagram typical for pure WO₃. The relationship between the modifications is discussed.     

Growth and characterization of 2-amino-4-picolinium toluene sulfonate single crystal

2-Amino-4-picolinium toluene sulfonate (2A4PTS), a new organic material, was synthesized and grown as single crystals in room temperature by slow evaporation solution growth technique using water as solvent. The crystal structure of 2A4PTS has been determined using single crystal X-ray diffraction studies. 2A4PTS belongs to monoclinic crystal system. The molecular arrangements in the crystal were studied. The structural perfection of the grown crystals has been analysed by high-resolution X-ray diffraction (HRXRD) rocking curve measurements. Fourier transform infrared (FTIR) spectral studies have been performed to identify the functional groups. The optical transmittance window and the lower cutoff wavelength of the 2A4PTS have been identified by UV–Vis–NIR studies. The nonlinear optical properties have been investigated by Z-scan method. The nonlinear refractive index and linear absorption coefficient of the 2A4PTS are found to be in the order of 10⁻⁸ cm²/W and 10⁻⁴ cm/W, respectively. The laser induced surface damage threshold for the grown crystal was measured using Nd:YAG laser. Thermal analysis carried out on the compound reveals that 2A4PTS is stable up to 133 °C. The microhardness test was carried out and the load dependent hardness was measured.     

Surface analytical study of hydrothermally treated zirconia ceramics

Summary The hydrothermal instability of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) imposes serious problems on their range of applications as ceramic materials. It is well known that a generally uniform distribution of yttrium cations prevents the transformation of the metastable tetragonal into the monoclinic crystallographic modification. The induced crack propagation is widely considered to be a major source of mechanical failure in zirconia ceramics. The segregation was investigated of yttrium and other cations on the surface of hydrothermally treated ceramic samples with angle resolved XPS (X-ray photoelectron spectroscopy). The combination of this method with the high lateral resolution of SAM (scanning Auger microprobe) yields three dimensional distribution profiles of Y³⁺, Al³⁺ and Si⁴⁺. The relative content of the two crystallographic modifications of zirconia at the surface has been determined with XRD (X-ray diffraction). Additional Raman spectroscopic measurements partially confirm these results but suffer from quantification problems. A possible mechanism for the enrichment of yttrium in the surface layer and its promotive function for the phase transformation of zirconia is proposed. Preliminary results from experiments with temperature variable XPS measurements also indicate the formation of either yttrium- or zirconium hydroxide which are both suspected to contribute to crack propagation along the grain boundaries.     

Structural Phase Transitions and Magnetic Superexchange in MIAgIIF3 Perovskites at High Pressure**

Pressure-induced phase transitions of M^IAg^IIF₃ perovskites (M=K, Rb, Cs) have been predicted theoretically for the first time for pressures up to 100 GPa. The sequence of phase transitions for M=K and Rb consists of a transition from orthorhombic to monoclinic and back to orthorhombic, associated with progressive bending of infinite chains of corner-sharing [AgF₆]⁴⁻ octahedra and their mutual approach through secondary Ag⋅⋅⋅F contacts. In stark contrast, only a single phase transition (tetragonal→triclinic) is predicted for CsAgF₃; this is associated with substantial deformation of the Jahn–Teller-distorted first coordination sphere of Ag^II and association of the infinite [AgF₆]⁴⁻ chains into a polymeric sublattice. The phase transitions markedly decrease the coupling strength of intra-chain antiferromagnetic superexchange in MAgF₃ hosts lattices.     

Time‐Resolved Phase Transitions of the Nanocrystalline Cubic to Submicron Monoclinic Phase in Mn2O3‐ZrO2

The nanocrystalline cubic phase of zirconia was found to be thermally stabilized by the addition of 3 to 40 mol % manganese. The nanocrystalline cubic, tetragonal and monoclinic phases of zirconia stabilized with manganese (III)oxide (Mn‐Stabilized Zirconia) were prepared by thermal decomposition of carbonate and hydroxide precursors. Both the crystallization and isothermal phase transitions associated with Mn‐SZ were studied using high temperature x‐ray diffraction and x‐ray diffraction of quenched samples. Cubic Mn‐SZ initially crystallized and progressively transformed to tetragonal, and monoclinic structures above 700°C. The nanocrystalline cubic Mn‐SZ containing 25 mol % Mn was found to have the greatest thermal stability, retaining its cubic form at temperatures as high as 800°C for periods up to 25 hours. Higher than 40 mol %, cubic Mn₂O₃ was found to coexist with cubic Mn‐SZ. The crystallite sizes observed for the cubic, tetragonal and monoclinic Mn‐SZ phases ranged from 50 to 137, 130 to 220, and 195 to 450 Å respectively, indicating, for ZrO₂, that particle size was a primary factor in determining its polymorphs. The classical Avrami equation for nucleation and growth was applied to the observed phase transformations.     

Analysis of silica fume produced by zircon desilication

Novel chemical methods have been developed to allow for the determination of the components of silica fume produced by zircon desilication. Hitherto, no methods have been described for the analysis of this material. The amorphous silica is first removed by treatment with sodium hydroxide. The residue from the hydroxide treatment may then be subjected to a suite of reagents to determine the zircon, the total zirconia, the monoclinic zirconia and the tetragonal zirconia content of the fume. The zircon content of the fume is determined by treatment of the hydroxide residue with concentrated hydrofluoric acid (HF). The total zirconia content of the fume is determined by digestion of the hydroxide residue with fuming sulphuric acid (oleum), while the relative amounts of monoclinic and tetragonal zirconia may be found by treatment of the hydroxide residue with 10%w/v HF, which attacks the less stable tetragonal phase. Both X-ray diffraction and particle size analysis were used to validate the steps in the analytical procedure. An explanation of the presence of tetragonal zirconia in the fume is proposed. A greater understanding of the composition of the fume led to the installation of a separator in the company's production line to remove the zircon. Australian Fused Materials (AFM) now produces a vastly superior grade of fume marketed under the code SF-98.     

Monoclinic phase of the misfit-layered cobalt oxide (Ca0.85OH)1.16CoO2

A monoclinic phase of the misfit-layered cobalt oxide (Ca_0.85OH)_1.16CoO₂ was successfully synthesized and characterized. It was found that this new material is a poly-type phase of the orthorhombic form of (CaOH)_1.14CoO₂, recently discovered by the present authors. Both the compounds consist of two interpenetrating subsystems: CdI₂-type CoO₂ layers and rock-salt-type double-atomic-layer CaOH blocks. However, these two phases exhibit a different stacking structure. By powder X-ray and electron diffraction (ED) studies, it was found that the two subsystems of (Ca_0.85OH)_1.16CoO₂ have c-centered monoclinic Bravais lattices with common a=4.898 Å, c=8.810 Å and β=95.8° lattice parameters, and different b parameters: b₁=2.820 Å and b₂=4.870 Å. Chemical analyses revealed that the monoclinic phase has a cobalt valence of +3.1-3.2. Resistivity of the monoclinic phase is approximately 10¹-10⁵ times lower than that of the orthorhombic phase. This suggests that the monoclinic phase is a hole-doped phase of the insulating orthorhombic phase. Furthermore, large positive Seebeck coefficients (∼100 μV/K) were observed near room temperature.