Microcrystalline to nanocrystalline silicon phase transition in hydrogenated silicon-carbon alloy films

Different classes of interesting materials (such as protocrystalline, microcrystalline and nanocrystalline) have been grown under conditions very near to those for the microcrystalline phase. In spite of the importance of these materials, a clear picture regarding their phase transitions is missing. A smooth transition from the microcrystalline to the nanocrystalline silicon phase, distinctly different from an abrupt order-disorder phase transition, has been demonstrated, for the first time, in hydrogenated silicon-carbon alloy films, prepared from a silane-methane gas mixture highly diluted in hydrogen, by varying the rf power in a plasma enhanced chemical vapour deposition system. The study has also provided the signature of medium range order in hydrogenated silicon-carbon alloy films.     

Synthesis of ZrO2 and TiO2 nanocrystalline powders by hydrothermal process

In present work, nanocrystalline zirconia and titania were synthesized by hydrothermal treatment of zirconyl ZrO(NO₃)₂ and titanyl TiO(NO₃)₂ nitrates aqueous solutions and amorphous gels of the corresponding hydroxides. The hydrothermal synthesis was performed in a wide range of temperatures (150–250 °C), concentration of starting solutions (0.25–0.5 M) and duration of the process (from 10 min to up to 24 h). The hydrothermal treatment at high pressure about 2.0–4.0 GPa was also carried out. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and BET. Particle sizes, morphology and properties of the products were investigated. The influence of hydrothermal synthesis conditions on ZrO₂ and TiO₂ phase compositions were established.     

On the orthorhombic phase in ZrO2-based alloys

During TEM observation, a tetragonal (t) to orthorhombic (o) phase transformation often occurs in thin portions of ZrO₂-containing foils. This transformation is stress-induced and in some senses artefactual, in that the reaction product is actually a high-pressure phase, relative to monoclinic (m) ZrO₂, that can form from metastable t-ZrO₂ in the TEM because its density is intermediate between t- and m-ZrO₂. Examples of the formation of o-ZrO₂ in a number of different systems are given.     

Low temperature phase equilibria and ordering in the ZrO2-rich region of the system ZrO2-CaO

From co-precipitated powder samples, the solid state reactions occurring between room temperature and 1500° C in the ZrO₂-CaO system have been studied. At low temperatures, compositions containing < 25 mol% CaO show a complex picture of phase transformation and ordering in the system. From the obtained results the following singular reactions have been established. (i) Tetragonal zirconia solid solution decomposes eutectoidally at 7 mol% CaO and 1048 ± 4° C into monoclinic zirconia solid solution and calcium zirconate (CZ). (ii) Cubic zirconia solid solution undergoes a eutectoidal decomposition at 17.5 mol% Cao and 1080 ±20° C into tetragonal solid solution + calcium zirconate. (iii) The monoclinic ordered phase, CaZr₄O₉ (₁), ), undergoes an order-disorder transformation into cubic zirconia solid solution at 1232 ± 5° C. (iv) Cubic zirconia solid solution undergoes a eutectoidal decomposition into two ordered phases, ₁ + ₂ at 21 mol% CaO and 1200 ± 10°C. (v) Hexagonal ordered phase Ca₆Zr₁₉O₄₄ (₂) decomposes peritectoidally into cubic zirconia solid solution + calcium zirconate at 1360 ± 10° C. The two ordered phases ₁ and ₂ seem to be unstable below 1100° C. By using DTA, X-ray diffraction and SEM techniques, the extent of the tetragonal and cubic zirconia solid solution fields have been established. From the above experimental results a new tentative phase diagram is given for the ZrO₂-rich region of the system, ZrO₂-CaO.     

Temperature-dependent phase stability of nanocrystalline SiO2

Nanocrystalline powder of SiO2, synthesized by gas phase condensation, has been characterized by X-ray and electron diffractometric techniques. The composition of the as-received powder is mostly amorphous and a minor amount of alpha-crystobalite phase. Annealing at higher temperatures, the amorphous phase gradually transforms into small crystallites of alpha-crystobalite. Subsequently, the tiny crystallites form small domains in the structural form of beta-crystobalite.     

Subsolidus phase relationships in the ZrO2-rich part of the ZrO2–Zr3N4 system

Zirconia can be stabilized by incorporation of nitrogen. The phase relationships in the ZrO2-rich part of the system ZrO2–Zr3N4 have been investigated using high-temperature X-ray methods. At temperatures above 1000°C, a tetragonal and a cubic phase with randomly distributed vacancies exists, depending on the amount of incorporated nitrogen. This high-temperature behaviour is similar to that of systems like ZrO2–Y2O3, which is another indication for the important role of anion vacancies in zirconia systems. Below 1000°C, β-type phases with an ordered arrangement of anion vacancies are stable. This revised version was published online in November 2006 with corrections to the Cover Date.     

Zinc vacancy mediated structural phase transition and photoluminescence in nanocrystalline ZnS thin films

Nanocrystalline thin films of zinc sulphide were prepared on glass substrate at various deposition temperatures by thermal evaporation technique. The variation of the structural and optical properties of films deposited at various substrate temperatures was investigated in detailed. X-ray diffraction spectra showed that films deposited at 300 and 400 °C are polycrystalline in nature having cubic and both cubic and wurtzite structure, respectively. However, film deposited at temperature of 200 °C was found to be amorphous in nature. The ultra-violet and visible absorption studies showed that the band gap of films increases with increase in deposition temperatures. Photoluminescence spectra displayed emission near 396 and 444 nm, which arises due to zinc vacancies and sulphur vacancies, respectively and has been correlated to phase transition of the films.     

Synthesis and characterization of nanocrystalline Cr2O3 and ZrO2 ceramic materials

A novel methodology based on a hybrid approach has been evolved for synthesizing nearly monodisperse nanocrystalline oxides. The approach basically involves precipitation of gelatinous hydroxide in liquid phase hydrolysis and subsequent temperature programmed calcination to obtain nanocrystalline oxide. Cr2O3 and ZrO2 have been synthesized through this route. This paper describes synthesis procedures giving details of temperature windows required for this synthesis. In addition, solid state analytical technique like X-Ray Diffraction (XRD) and Photoluminescence (PL) have been used to characterize these materials. Especially PL was used to derive information on confinement effect. Transmission Electron Microscope (TEM), Scanning Probe Microscope (SPM), and Scanning Near Field Optical Microscope (SNOM) were used to derive morphology.     

Solid-state reaction,microstructure and phase relations in the ZrO2-rich region of the ZrO2-Yb2O3 system

Phase relations below 1700°C in the ZrO₂-rich region of the zirconia-ytterbia system have been established using thermal expansion, room-temperature X-ray diffraction, precision lattice parameter measurements and microscopic observations. The solubility limits of ytterbia in both monoclinic and tetragonal zirconia were determined. A eutectoid reaction, tetragonal zirconia solid solution monoclinic + cubic zirconia solid solutions at 400 ± 20°C and 2.4 mol % ytterbia was found. The left-hand boundary of the cubic zirconia solid-solution field was redetermined between room temperature and about 1700°C. Long-range ordering was present at 40 mol % ytterbia and the formation of an ordered phase, Zr₃Yb₄O₁₂, isostructural with M₇O₁₂-type compounds was found. Its thermal stability was established between room temperature and 1630 ± 10°C, in which it decomposes into cubic zirconia solid solution by an order-disorder reaction.     

Effects of ZrO2 addition on phase stability and photocatalytic activity of ZrO2/TiO2 nanoparticles

ZrO₂/TiO₂ nanoparticles with various Zr/Ti ratios (0–0.9) were prepared by a polymer complex solution method (PCSM). The prepared samples were characterized using transmission electron microscopy (TEM), the Brunauer, Emmett & Teller (BET) method, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The ZrO₂/TiO₂ photocatalyst showed a high specific area and small crystal size. The XRD pattern for the Zr/Ti = 0.1 sample indicated that the addition of ZrO₂ stabilized the anatase phase of TiO₂ up to 800 °C. The photocatalytic activity of Zr/Ti = 0.1 sample was higher than that of the TiO₂ sample and commercially available Degussa P25. The high photocatalytic activity can be attributed to stronger adsorption in the visible light region, higher specific area, smaller crystal size and increased surface OH groups.     

Joining of ZrO2-4.5 wt% Y2O3 (Y-TZP) ceramics using nanocrystalline tape cast interlayers

Nanocrystalline Y-TZP tape casts were used as interlayers to join conventional Y-TZP ceramic pellets. The joining experiments were performed by hot pressing at 1000°C to 1300°C under constant pressure of 55 MPa. Two types of joints were obtained with and without a nanocrystalline interlayer. At 1100°C, the successful joints were enabled only with the interlayer; four point bending test results revealed an average joint strength of 206 ± 10 MPa. The joint strength increased with the joining temperature. The specimens joined at 1300°C with an interlayer exhibited a joint strength of 613 ± 40 MPa, which is 96% of the strength of the ceramic pellets. The interlayer at the joint exhibited homogeneous and crack free microstructure and preserved its nanocrystalline nature at all temperatures. The advantage of the nanocrystalline interlayer for joining is pronounced at lower joining temperatures and most probably for pellets with large grain size.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

In situ TEM observation of heterogeneous phase transition of a constrained single-crystalline Ag2Te nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.