Synthesis of C3N4 crystals under high pressure and high temperature

C₃N₄ crystals with the size of several micrometers have been synthesized from C₃N₄H₄ in the presence of nickel-based alloy or cobalt as catalyst under high pressure of 7 GPa and temperature of about 1400°C for 10 min. Scanning electron microscopy, energy-disperse X-ray analysis, and X-ray diffraction were used to examine the grown crystals. The general rule on selecting the starting materials for synthesis of carbon nitride crystals at high pressures and high temperatures is suggested.     

Atomic scale epitaxial growth of BaTiO3 thin films by laser molecular beam epitaxy

By using laser molecular beam epitaxy (L-MBE), atomic scale epitaxid growth of BaTiO₃(BTO) thin films on SrTiO₃ (STO) substrates is achieved. Measurements of reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy reveal that the BTO films arec-axis oriented single crystals with smooth surface. The multi-layer ferroelectric/superconducting heterostructures are also prepared and the ferroelectric properties of BTO films are studied. The results show that by using L-MBE technique, high quality BTO films and improved device performance can be obtained. Project supported by the National Natural Science Foundation of China, the National Department of Finance, and the National Center for R and D on Superconductivity of China.     

Formation of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy under high pressure

The formation of nanocrystalline Fe_73.5 Cu₁Nb₃Si_13.5 B₉ alloy by annealing an amorphous Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy at a temperature of 823 K under pressures in the range of 1–5 GPa is investigated by using X-ray diffraction, electron diffraction, and transmission electron microscopy. The high pressure experiments are carried out in belt-type pressure apparatus. Experimental results show that the initial crystalline phase in these annealed alloys is a-Fe solid solution (named a-Fe phase below), and high pressure has a great influence on the crystallization process of the a-Fe phase. The grain size of the a-Fe phase decreases with the increase of pressure (P). The volume fraction of the a-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (Pδ2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy is discussed Project supported by the National Natural Science Foundation of China (Grant No. 19674070) and the Natural Science Foundation of Hebei Province.     

Synthesis and characterization of C3N4 crystal (II)

Single C₃N₄ crystals with 1–3 μ in length and 300 nm in cross area was obtained on nickel substrate. The results rule out the uncertainty of the experimental lattice parameters caused by C-Si-N phase when the growth was on silicon. The X-ray diffraction and transmission electron microscopy with selective-area electron diffraction give the lattice constantsa = 0.624 nm andc = 0.236 nm for β-C₃N₄, and α=0. 638 nm andc = 0.464 8 nm for α-C₃N₄, which are respectively 2.5% and 1.3% lower than those of the latest first-principle calculations. An N:C ratio of 1.30–1.40 was determined by energy dispersive X-ray. Based on the experimental lattice constants, the bulk modulus of the obtained β-C₃N₄ are in the region of 425–445 GPa.     

Synchrotron radiation photoemission spectrum study on K3C60 film

K₃C₆₀, single crystal film was prepared on the cleaved (111) surface of C₆₀, single crystal. Synchrotron radiation angle-resolved photoemission spectra were measured at normal emission with sample temperature at × 150K. Up to four subpeaks of LUMO-derived band were observed. These sub-peaks exhibit distinct energy dispersions which resemble in general the theoretical ones calculated for K₃C₆₀ at low temperature with the so-called one-dimensional disordered structure. But there is large deviation of experimental sub-band intervals from the theoretical values. This result is meaningful for the studies of the physical properties of alkali-doped C₆₀ solids, e.g. the mechanism for superconductivity.     

Differences in microstructure of Pd77.5Au6Si16.5 alloy solidified under microgravity and gravity conditions

The microstructure of Pd_77.5Au₆Si_16.5alloy solidified both on board a Chinese Retrievable Satellite and on the earth is studied. Postmortem analyses of microstructure presented that the same types of phases, primary phase (Pd₃Si) and eutectics (Pd₃Si + Pd solid solution) were formed in both cases. But the phase morphologies were quite different. It was dendritic for the primary phase and lamellar for the eutectics under normal gravity condition. However, under microgravity condition the primary phase was granular and the eutectic was peculiar network. Detailed analysis showed that the differences in morphologies of the microstructure were due to the existence of gravity-induced buoyancy convection on the earth which increased the mass transport abilities and decreased the thickness of the solute boundary in front of the solid-liquid interface during solidification under normal gravity condition.     

Diamond nucleation on surface of C60 thin layers

Diamond nucleation on the surface of C₆₀ thin layers and intermediate layer of Si substrates are studied by scanning electron microscopy (SEM). The cross-section SEM images of diamond films show that diamond grains really nucleate on the surface of C₆₀ thin layers. The SEM images of diamond nucleating sites show the nucleating aggregation of diamond on C₆₀ surfaces. The preferential oriented diamond films are observed. The plasma pretreatment of C₆₀ sublimating layers is a key factor for diamond nucleation.     

Synthesis and characterization of C3N4 hard films

C₃N₄ films have been synthesized on both Si and Ft substrates by microwave plasma chemical vapor deposition (MPCVD) method. X-ray spectra were calculated for single phase α-C₃N₄ and β-C₃N₄ respectively. The experimental X-ray spectra of films deposited on both Si and Pt substrates showed all the strong peaks of α-C₃N₄ and β-C₃N₄ so the films are mixtures of α-C₃N₄ and β-C₃N₄. The N/C atomic ratio is in the range of 1.0–2.0. X-ray photoelectron spectroscopy (XPS) analysis indicated that the binding energy of Is and N ls are 286.2 eV and 399.5 eV respectively, corresponding to polarized C-N bond. Fourier transform infrared absorption (FT-IR) and Raman spectra support the existence of C-N covalent bond in the films. Nano-indentation hardness tests showed that the bulk modulus of a film deposited on Pt is up to 349 GPa     

Liquid structure of undercooled Cu70Ni30 alloy

Experiments of X-ray diffraction for liquid Cu₇₀Ni₃₀ alloy above and below its liquidus (l 230°C) have been carried out. By the analysis of experimental results, it is discovered that difference between structures of liquid and undercooled Cu₇₀Ni₃₀ alloy is their cluster sizes. The correlation radius of cluster is 1.125 nm and the atom number of cluster is 403 at l 250—l 400°C, and they are 1.3 nm and 704 respectively at the undercooled liquid state (1 200°C). The structure of liquid Cu₇₀Ni₃₀ alloy is fcc short order and its solid structure, fcc, is kept from liquid fcc short order.     

Gorenstein Injective and Projective Complexes with Respect to a Semidualizing Module

Let R be a commutative (possibly non-Noetherian) ring (in order to make things less technical) and C a semidualizing R-module. In this article, we introduce and investigate the notion of G _C -injective (G _C -projective) complexes. This extends Enochs and García Rozas's notion of Gorenstein injective (Gorenstein projective) complexes. We then show that a complex X is G _C -injective (G _C -projective) if and only if X ^m is a G _C -injective (G _C -projective) module for each m ∈ ?.     

Crystal structure of zirconia by Rietveld refinement

The crystal structures and phase transformation of zirconia ceramics have been investigated by means of X-ray powder diffraction and Rietveld powder diffraction profile fitting technique. A structural transition from monoclinic to tetragonal occurs when Y₂O₃ and₂ are doped into zirconia. The space group of the tetragonal structure is P4₂/nmc,Z=2. The lattice parameters are α=0.362 6(5)nm,c=0.522 6(3)nm for CeO₂ doped zirconia and α=0.360 2(8)nm,=0.517 9(1)nm for Y₂O₃ doped zirconia, respectively. In each unit cell, there are two kinds of equivalent positions, i.e. 2b and 4d, which are occupied by Zr⁴⁺, M(M=Y³⁺, Ce⁴⁺) cations and O^2- anions, respectively. The crystallographic correlation among the cubic, tetragonal and monoclinic structures of ZrO₂ is discussed.     

Magnetovolume and chemical bonding effects of Ni atom in γ’-(Fe1- x Ni x )4N compounds

By X-ray diffraction and high pressure Mossbauer spectroscopy, the structure and the hyperfine parameters of Ni substituted γ’-Fe₄N were investigated. The results of X-ray diffraction indicate that single phase γ’-(Fe₁-_xNi_x)₄N compounds can be prepared in the composition range of 0⩽x⩽0.6, and with the increase of Ni content the lattice parameter is fit for the relationshipa ₀(x)=3.790 5-0.021 57x-0.031 67x ². By high pressure Mossbauer spectra, effects of magnetovolume and chemical bonding of Ni atom on hyperfine magnetic field and isomer shift of iron were distinguished for the first time, and their composition dependences for different lattice sites were studied simultaneously. It is found that the magnetovolume and chemical bonding have different influences on the properties of γ’-(Fe₁-_xNi_x)₄N, and the latter one plays a key role in the property changes of γ’-(Fe₁-_xNi_x)₄N. Project supported by the National Natural Science Foundation of China, the Natural Science Fund of Gansu Province and the Postdoctoral Fund of China.     

When Some Complement of a Submodule Is a Summand

A module M is said to satisfy the C ₁₁ condition if every submodule of M has a (i.e., at least one) complement which is a direct summand. It is known that the C ₁ condition implies the C ₁₁ condition and that the class of C ₁₁-modules is closed under direct sums but not under direct summands. We show that if M = M ₁ ⊕ M ₂, where M has C ₁₁ and M ₁ is a fully invariant submodule of M, then both M ₁ and M ₂ are C ₁₁-modules. Moreover, the C ₁₁ condition is shown to be closed under formation of the ring of column finite matrices of size Γ, the ring of m-by-m upper triangular matrices and right essential overrings. For a module M, we also show that all essential extensions of M satisfying C ₁₁ are essential extensions of C ₁₁-modules constructed from M and certain subsets of idempotent elements of the ring of endomorphisms of the injective hull of M. Finally, we prove that if M is a C ₁₁-module, then so is its rational hull. Examples are provided to illustrate and delimit the theory.     

Laser molecular-beam epitaxy and second-order optical nonlinearity of BaTiO3/SrTiO3 superlattices

A series of c-axis oriented BaTiO₃/SrTiO₃ superlattices with the atomic-scale precision were epitaxially grown on single-crystal SrTiO₃(l00) substrates using laser molecular-beam epitaxy (LMBE). A periodic modulation of the intensity of reflection high-energy electron diffraction (RHEED) in BaTiO₃ and SrTiO₃ layers was observed and attributed to the lattice-misfit-induced periodic variation of the terrace density in film surface. The relationship between the second-order nonlinear optical susceptibilities and the superlattice structure was systematically studied. The experimental and theoretical fitting results indicate that the second-order nonlinear optical susceptibilities of BaTiO₃/SrTiO₃ superlattices were greatly enhanced with the maximum value being more than one order of magnitude larger than that of bulk BaTiO₃ crystal. The mechanism of the enhancement of the second-order optical nonlinearity was discussed by taking into account the stress-induced lattice distortion and polarization enhancement.     

Experimental study of scaling properties and roughing mechanisms in C60-Ag thin films

Surface morphologies and microstructures of C₆₀/Ag composite films were studied by atomic force microscope (AFM) and transmission electron microscope (TEM). The surface roughness depended on the substrate temperature, and the transition of surface morphology of rough→smooth→rough was observed when the substrate temperature increased from −50 to 120°C. Although the rms values are similar, the scaling properties of the thermal roughing and the kinetic roughing surfaces are quite different. The relations between the scaling properties, microstructures and roughing mechanisms are discussed based on the AFM and TEM results. Project supported by the National Natural Science Foundation of China (Grant No. 59529204).     

Strip Bundles and the Highest Weight Crystals Over Quantum Infinite Rank Affine Algebras

We give new realizations of the highest weight crystals B(λ) over the quantum infinite rank affine algebras U _q (A _∞), U _q (B _∞), U _q (C _∞), and U _q (D _∞) using strip bundles, which are related with Nakajima monomials.     

Bicarbonate effect on electron flow in a cyanobacteriumSynechocystis PCC 6803

In this communication, evidence is presented from the kinetics of Q_A ^– decay (where Q_A is the first plastoquinone electron acceptor of photosystem II) and oxygen evolution for the requirement of bicarbonate in the electron transport in a cyanobacteriumSynechocystis (Pasteur Culture Collection 6803). A large slowing down of Q_A ^– oxidation, measured from the variable chlorophylla fluorescence after saturating actinic flashes, was observed in the thylakoids ofSynechocystis 6803 depleted of bicarbonate in the presence of 25 mM formate. Qualitatively similar results were obtained with DCMU-treated thylakoids. This shows that bicarbonate depletion inhibits electron transport on the acceptor side of photosystem II between Q_A and the plastoquinone (PQ) pool in cyanobacteria. Addition of 2.5 mM HCO₃ ^– fully reversed the inhibition of electron flow caused by bicarbonate depletion. Two exponential phases of Q_A ^– decay, a fast one and a slow one, were observed with halftimes of approx. 400 s (fast) and 26 ms (slow) at pH 6.5. At pH 7.5, these phases were approx. 330 s (fast) and 21 ms (slow), respectively. The amplitude, but not the halftime, of the fast component decreased by about 70% (pH 6.5) or 50% (pH 7.5); this was accompanied by a concomittant increase in the slow phase. Twenty mM bicarbonate stimulated, by a factor of 4, the Hill reaction in bicarbonate-depletedSynechocystis cells. This effect is independent of CO₂ fixation as it was observed even in the presence of an inhibitor DBMIB.Abbreviations Chl Chlorophyll - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMQ 2,5-dimethyl-p-benzoquinone - S₁₃ 5-Cl, 3-t-butyl, 2-Cl, 4-NO₂-salicylanilide - PCC Pasteur culture collection     

Influence of SiO2 in SiCp on the microstructure and impact strength of Al/SiCp composites fabricated by pressureless infiltration

The effect of SiO₂ in SiC_p and the following processing parameters on the microstructure and impact strength of Al/SiC_p composites fabricated by pressureless infiltration was investigated: Mg content in the aluminum alloy, SiC particle size, and holding time. Preforms of SiC_p in the form of rectangular bars (10 × 1 × 1 cm) were infiltrated at 1150°C in an argon→nitrogen atmosphere for 45 and 60 min by utilizing two aluminum alloys (Al-6 Mg-11 Si and Al-9 Mg-11 Si, wt.%). The results obtained show that the presence of SiO₂ in SiC affects the microstructure and impact strength of the composites significantly. When Al₄C₃ is formed, the impact strength decreases. However, a high proportion of SiC to SiO₂ limits the formation of the unwanted Al₄C₃ phase in the composites. Also, a higher content of Mg in the Al alloy lowers the residual porosity and, consequently, increases the composite strength. The impact strength grows with decrease in SiC particle size and increases considerably when the residual porosity is less than 1%. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 3, pp. 401–418, May–June, 2006.     

Phase transition in layered perovskite-like manganate Ca3Mn2O7 under high pressure

In situ high pressure energy dispersive X-ray diffraction measurements on the layered perovskite-like manganate Ca₃Mn₂O₇ powder under pressures were performed by using the diamond anvil cell with synchrotron radiation. The results show that the structure of layered perovskite-like manganate Ca₃Mn₂O₇ is unstable under pressure due to the easy compression of NaCl-type blocks. The structure of Ca₃Mn₂O₇ underwent two phase transitions under pressures in the range of 0–35 GPa. One was at about 1.3 GPa with the crystal structure changing from tetragonal to orthorhombic. The other was at about 9.5 GPa with the crystal structure changing from orthorhombic back to another tetragonal.     

Interface interaction and inter-osmosis effect of Fe x (SiO2)1-x nanocomposite materials on magnetic properties

Fe _x (SiO₂)_1-x nanocomposites prepared by using mechanical alloying method were reported. The microstructure character and magnetic properties of Fe _x (SiO₂)_1-x nanocomposite samples with different Fe content and different ball milling time were studied by using X-ray diffraction (XRD), transmission electron microscopy (TEM), Mössbauer spectroscopy, and Faraday magnetic balance in a wide temperature range. The results indicate that the rnicrostructure and magnetic properties are closely related to ball milling time and Fe content. When Fe content is less than 20 wt%, the sample after 80-h ball milling has very complex microstructure. Small α-Fe grains and Fe cluster are implanted in SiO₂ matrix. And there are not only isolated α-Fe granular and Fe cluster, but also nanometer scaled sandwich network-like structure. Fe _x (SiO₂)_1-x nanocomposite samples display a rich variety of physical and chemical properties as a result of their unique nanostructure, strong interface interaction and inter-osmosis effect in Fe-SiO₂ boundaries, and the grain size effect.