MMC Structure Research Articles

Analytical Efficiency Evaluation of Modular Multilevel Converter for HVDC System

In this paper, the structure of MMC based HVDC system is discussed. Then, an analytical method based on average current and root mean square (RMS) of the current passing through the sub module is presented to calculate the conversion losses in the switching devices of MMC. The method is applied considering the full bridge sub module topology. Then, considering a case with certain parameters, a MATLAB program is developed and the losses and efficiency curves for each switching device and sub module are obtained respectively. Also, those factors which affect the losses and efficiency of the sub module are discussed.

Current Stability in Multi-Mesa-Channel AlGaN/GaN HEMTs

We develop and characterize multi-mesa-channel (MMC) AlGaN/GaN high-electron mobility transistors (HEMTs), in which a periodic trench structure is fabricated only under the gate electrode. A surrounding-field effect in the MMC structure results in a shallower threshold voltage and a smaller subthreshold slope than those of the standard planar-type HEMT. In addition, the MMC HEMT shows a low knee voltage and a weak dependence of on-resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> ) on the gate-drain distance. Following identical off-state bias stress, the MMC HEMT exhibits low current collapse. The relative decrease in access resistance of the MMC device compared with the planar device can reduce the effects of access resistance on the drain current. It is likely that a high impedance of each nanochannel also contributes to the current stability of the MMC HEMT.

Application of Diffraction and X-Ray Microanalysis in Study of MMC Structure

Quality of metal-matrix composite castings, similarly as quality of conventional metal castings is defined by the collection of data concerning chemical constitution, properties (e.g. strength) and structure of final products. During study aimed at determining these parameters, the priority is to maintain the highest possible repeatability and automation of measurement. In case of chemical constitution or mechanical properties analysis it is considerably easy to achieve thanks to application of appropriate research procedure. Analysis of structure of studied materials is however a much bigger problem. To improve it, different tools of digital image analysis are being used, but because of only slight differences (e.g. in color) between different elements of structure, conducting such analysis without performing additional study may result in obtaining false results. In such case, identification of an object is very important: unequivocal conclusion whether a given object is a reinforcement or maybe an impurity or an inclusion foreign matter, or is it a structure of improper, unintentional chemical constitution. It is particularly important concerning composites for which on the images obtained using both optical and scanning microscopy individual phases (reinforcement, matrix and impurities) are difficult to distinguish. In such case identification of particular phases is aimed at avoiding mistakes, for example while analyzing the quantity or uniformity of spacing of reinforcement phase in given space. X-ray diffraction may be a solution for this problem. An example of identification (quantitative and qualitative description) of reinforcement phase in suspension composites is presented further in the paper. The authors present also a chemical constitution of selected elements of structure of metal-matrix composites from ex-situ group.

Effects of the optical microcavity on the performance of ITO-free polymer solar cells with WO3/Ag/WO3 transparent electrode

We fabricate ITO-free bulk-heterojunction polymer solar cells (PSCs) with the structure glass/WO3/Ag/WO3/P3HT:PCBM/LiF/Al. According to the J–V characterization, the device efficiency is improved by 27.4% for the ITO-free device with a 70nm-thick active layer but reduced by 19.9% for the device with a 150nm-thick active layer, when compared with that of the PSC device with an ITO transparent electrode. Further investigation based on IPCE spectra test and transfer matrix simulations reveals that it is the optical resonance effect of the metal-mirror-microcavity (MMC) constructed between the semitransparent Ag electrode and top Al electrode that leads to the efficiency improvement for the device with 70nm-thick active layer. Meanwhile, the efficiency reduction of the device with a 150nm-thick active layer is caused by the off-resonance suppression of the MMC structure. It demonstrates that the MMC structure is an important element to be considered when the WO3/Ag/WO3 multilayer is used to substitute the ITO electrode as a transparent electrode of the PSC devices.

Reduction of current collapse in the multi‐mesa‐channel AlGaN/GaN HEMT

AbstractTransport properties and off‐stress‐induced current collapse characteristics of the multi‐mesa‐channel (MMC) AlGaN/GaN HEMTs were investigated. The surrounding field effect in the MMC structure led to a shallower threshold voltage and a smaller subthreshold slope compared to those of the conventional planar HEMT. After applying off‐state bias stress, the planar HEMT showed remarkable current collapse, including significant increase in on‐resistance increasing the drain stress bias. On the other hand, for the MMC HEMT, we observed less change in on‐resistance (RON) even after the application of high drain bias stress. This shows that the MMC structure has resistance to the off‐stress‐induced current collapse. In the case of a mesa‐top width (Wtop) of less than 100 nm, the channel resistance of the MMC HEMT is estimated to be about one order of magnitude higher than the increase in access resistance induced by the off‐state stress. It is thus likely that the operation of the MMC HEMT is rather insensitive to change in access resistance. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba 2BiIrO 6 and Ba 3BiIr 2O 9

The Ba–Bi–Ir–O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba 2BiIrO 6; and a 6H-type hexagonal perovskite Ba 3BiIr 2O 9. Ba 2BiIrO 6 undergoes a series of symmetry-lowering phase transitions on cooling Fm 3 ¯ m → R 3 ¯ c → 12 / m ( C 2 / m ) → I 1 ¯ ( P 1 ¯ ) , all of which are second order except the rhombohedral→monoclinic one, which is first order. The monoclinic phase is only observed in a 2-phase rhombohedral+monoclinic regime. The transition and 2-phase region lie very close to 300 K, making the room-temperature X-ray diffraction patterns extremely complex and potentially explaining why Ba 2BiIrO 6 had not previously been identified and reported. A solid solution Ba 2Bi 1+ x Ir 1− x O 6, analogous to Ba 2Bi 1+ x Ru 1− x O 6, 0≤ x≤2/3, was not observed. The 6H-type phase Ba 3BiIr 2O 9 undergoes a clean second-order phase transition P6 3/ mmc→ C2/ c at 750 K, unlike 6H-type Ba 3LaIr 2O 9, the P6 3/ mmc structure of which is highly strained below ∼750 K but fails to distort coherently to the monoclinic phase.

Potassium under Pressure: A Pseudobinary Ionic Compound

Experimentally, we have found that among the "complicated" phases of potassium at intermediate pressures is one which has the same space group as the double hexagonal-close-packed structure, although its atomic coordination is completely different. Calculations on this P6(3)/mmc (hP4) structure as a function of pressure show three isostructural transitions and three distinctive types of chemical bonding: free electron, ionic, and metallic. Interestingly, relationships between localized metallic structures and ionic compounds are found.

Structural, mechanical, and electronic properties of [formula omitted], TaB, [formula omitted], and IrB: First-principle calculations

First-principle calculations were performed to investigate the structural, elastic, and electronic properties of TaB 2 , TaB, IrB 2 , and IrB. The calculated equilibrium structural parameters, shear modulus, and Young's modulus of TaB 2 are well consistent with the available experimental data, and TaB 2 with P 6 / mmm space group has stronger directional bonding between ions than WB 2 , OsB 2 , IrN 2 , and PtN 2 . For TaB 2 , the hexagonal P 6 / mmm structure is more stable than the orthorhombic Pmmn one, while for IrB 2 the orthorhombic Pmmn structure is the most stable one. The high shear modulus of P 6 / mmm phase TaB 2 is mainly due to the strong covalent π -bonding of B-hexagon in the (0001) plane. Such a B-hexagon network can strongly resist against an applied [ 11 2 ¯ 0 ] (0001) shear deformation. Correlation between the hardness and the elastic constants of TaB 2 was discussed. The band structure shows that P 6 / mmm phase TaB 2 and Pmmn phase IrB 2 are both metallic. The calculations show that both TaB and IrB are elastically stable with the hexagonal P 6 3 / mmc structure.

Mg substitution effect on the hydrogenation behaviour, thermodynamic and structural properties of the La 2Ni 7–H(D) 2 system

The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg) 2Ni 7 alloys. Substitution of La in La 2Ni 7 by Mg to form La 1.5Mg 0.5Ni 7 preserves the initial Ce 2Ni 7 type of the hexagonal P6 3/ mmc structure and leads to contraction of the unit cell. The system La 1.5Mg 0.5Ni 7–H 2 (D 2) was studied using in situ synchrotron X-ray and neutron powder diffraction in H 2/D 2 gas and pressure–composition–temperature measurements. La replacement by Mg was found to proceed in an ordered way, only within the Laves-type parts of the hybrid crystal structure, yielding formation of LaMgNi 4 slabs with statistic and equal occupation of one site by La and Mg atoms. Mg alters structural features of the hydrogenation process. Instead of a strong unilateral anisotropic expansion which takes place on hydrogenation of La 2Ni 7, the unit cell of La 1.5Mg 0.5Ni 7D 9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δ a/ a=7.37%) and along [001] (Δ c/ c=9.67%). In contrast with La 2Ni 7D 6.5 where only LaNi 2 layers absorb hydrogen atoms, in La 1.5Mg 0.5Ni 7D 9.1 both LaNi 5 and LaMgNi 4 layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg) 2Ni 2, (La,Mg)Ni 3, Ni 4, tetragonal pyramidal La 2Ni 3 and trigonal bipyramidal (La,Mg) 3Ni 2 interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD 6 and a 16-vertex polyhedron LaD 16 around La. The interatomic distances are in the following ranges: La–D (2.28–2.71), Mg–D (2.02–2.08), Ni–D (1.48–1.86 Å). All D–D distances exceed 1.9 Å. Thermodynamic PCT studies yielded the following values for the Δ H and Δ S of hydrogenation/decomposition; Δ H H=−15.7±0.9 kJ (mol H) −1 and Δ S H=−46.0±3.7 J (K mol H) −1 for H 2 absorption, and Δ H H=16.8±0.4 kJ (mol H) −1 and Δ S H=48.1±1.5 J (K mol H) −1 for H 2 desorption.

Hyperfine interactions on iron in R2−xFe14+2xSi3 (R=Ce, Nd, Gd, Dy, Ho, Er, Lu, Y) compounds studied by Mössbauer spectroscopy

Abstract Compounds R 2− x Fe 14 + 2 x Si 3 with (R = Ce, Nd, Gd, Dy, Ho, Er, Lu, Y) were prepared by the modified Czochralski method and investigated by means of the 57 Fe 14.4 keV Mossbauer spectroscopy and powder X-ray diffraction. The magnetic moment was additionally measured for samples being single crystals. Iron experiencing low magnetic hyperfine field and electric quadrupole interaction was found on (4e) sites in the P 6 3 / mmc structure. Similar iron was found in the R 3 ¯ m structure as well. The non-stoichiometry parameter was determined as being close to 2 x = 0.5 for all compounds investigated. Compound with R = Ho crystallizes in both structures. Our results show that silicon avoids (4f) and (6g) sites for compounds with the P 6 3 / mmc structure (R = Dy, Ho, Er, Lu, Y), whereas it avoids (6c) and (9d) sites for those with R 3 ¯ m structure (R = Nd, Gd, Ho).

Structure and thermal stability of mesostructured zirconium oxophosphates

Highly ordered mesoporous zirconium oxophosphates (designated as ZOP) with hexagonal P6 3/ mmc and cubic Fm 3 ¯ m symmetries were firstly prepared by using gemini cationic surfactants as templates. It has been found that the thermal stability was elevated with the structure curvature order: cubic Ia 3 ¯ d < 2 D-hexagonal p 6 mm < cubic Fm 3 ¯ m , hexagonal P6 3/ mmc and cubic Pm 3 ¯ n . The ZOP mesoporous materials with cubic Fm 3 ¯ m and hexagonal P6 3/ mmc structures were stable up to 800 °C, which provides a new insight into the structural factors governing self-assembly of thermally stable mesoporous materials and would open up new possibilities of porous materials for advanced applications.

Influence of nitrogen stoichiometry on properties of low-compressibility advanced nitrides

Abstract The properties of two classes of recently synthesized advanced nitrides are investigated with the aim of considering how nitrogen stoichiometry affects the cohesive properties of this material. One is the class of cubic nitrides with an I43d structure of which Zr- and Hf-nitrides and the other a hexagonal P6 3 /mmc structure and of which MoN and CoN have also recently been synthesized. All materials are considered to have low compressibility and thus speculated to have a high material hardness. Both categories of materials have an underlying nitrogen sublattice structure. Using ab initio techniques the sublattice structure is considered as being a way through which properties of non-stoichiometric forms of these materials can be examined. Consequences of N stoichiometry on the crystal structure and elastic properties of these materials are suggested.

Structural and Dynamical Aspects of Structural Phase Transitions on Incommensurate A2BX4 compounds

At high temperature, the A 2 BX 4 crystals exhibit P6 3 /mmc structure. With temperature decreasing, a rich diagram of structural phases and phase transitions are observed, including different incommensurate phases. Ordering of the BX 4 tetrahedra orientation in the unit cell drives the transitions, and they can be well described by an Ising model with competing interactions. Raman and infrared spectroscopy, differential scanning calorimetry (DSC) and thermogravimetry (TGA) are used to characterize the phase transitions in Cs 2 HgBr 4 and Rb 2 WO 4 crystals.

Synthesis and search for superconductivity in LiBC

Following the recent theoretical prediction of superconductivity in hole doped LiBC by Rosner et al. [Phys. Rev. Lett. 88 (2002) 127001], we have attempted to synthesize Li deficient Li x BC ( x=1, 0.8, 0.6 and 0.4) and looked for superconductivity in this system. Our synthesis procedure, following the recipe for MgB 2, involves reaction of elemental components in a Ta crucible at 900 °C under 50 bar of argon pressure. X-ray diffraction measurements indicate the formation of P6 3/ mmc structure up to x=0.6. However, no diamagnetic signal or zero resistance, corresponding to the superconducting transition, were observed in the temperature range of 4–300 K. This is possibly related to the presence of disorder in the B–C stacking; evidence for which is suggested from a study of the vibrational modes of Li x BC through infrared spectroscopy.

Crystal structure of α-momorcharin in 80% acetonitrile–water mixture

Crystals of α-momorcharin (MMC) were cross-linked and soaked in an 80% acetonitrile–water mixture and X-ray data were collected to 2.2 Å resolution. MMC is a ribosome-inactivating protein with a sugar chain on Asn-227. In previous studies, the whole conformation of the sugar chain could not be obtained in the aqueous system. Here the structure of MMC in a low water system is shown to be similar to the native one, but the sugar chain on Asn-227 is defined by the electron density map. Several oxygen atoms of the oligosaccharide formed intramolecular hydrogen bonds to the protein moiety. The conformation of the residues in the active center is similar to that in the aqueous system. Our results show conformational alteration of the tetrasaccharide of MMC in organic media. They indicate that the oligosaccharides are more rigid in organic solvents. X-ray determination in organic media may be used to solve some structures of oligosaccharides linked to glycoproteins.

Hexagonal Ba-ferrite: a good model for the crystal structure of a new high-pressure phase CaAl 4Si 2O 11?

A new calcium aluminosilicate phase of composition CaAl 4Si 2O 11 has been encountered amongst the transformation products of CaAl 2Si 2O 8 (anorthite composition) at 14 GPa (Gautron et al., 1996). X-ray diffraction (XRD) confirms that its crystal structure is essentially the same as that of a new complex CaAl-silicate (abbreviated CAS phase) first reported by Irifune et al. (1994). The crystal structure of the CAS phase has been investigated by transmission electron microscopy (TEM). It has a hexagonal unit cell with lattice parameters a = 5.4Å and c = 12.7Å, and its space group is either P6 3mc , P 62c or P6 3 mmc . It is proposed that this CAS phase has a six-layer, close-packed structure so that Z = 2 and density is 3.94 g cm −3, reasonable for a phase stable at transition-zone pressures. The most plausible model for the structure of this phase arises from published refinements of hexagonal Ba-ferrites. This postulated P6 3 mmc structure consists of octahedral layers, 3 4 occupied, separated by 12-coordinate Ca atoms, and by Al and Si in face-shared octahedra and in complex trigonal bipyramidal polyhedra, i.e. some Si would be five-fold coordinated. Observed TEM and XRD data are compared with calculated reflection intensities for this CAS model.

Hybrid plasma CVD of diamond-like carbon (DLC) at low temperatures

Diamond-like carbon coatings have been deposited onto various substrates at 100–150°C using a hybrid plasma assisted chemical vapour deposition technique activated by radio frequency at 13.56 MHz. The coatings have been characterized using a number of techniques including scanning electron microscopy, Raman spectroscopy, thermoanalysis and pin-on-disc wear testing. Results show the films to be diamond like, with the addition of nitrogen (prior to deposition) promoting the formation of crystallites. In addition the condition and type of substrate have been found to have a strong influence on the structural characteristics of the deposited diamond-like films. SEM analysis of diamond-like carbon coatings deposited onto metal matrix composite materials such as Si-Al MMC is reported. The hybrid CVD technology enabled films to be deposited evenly onto the porous MMC structure. Commercially manufactured drills, coated with DLC and titanium nitride (TiN), have been compared to examine their cutting wear resistance characteristics.

Infrared activity in elemental crystals.

In a previous paper, Zallen [Phys. Rev. 173, 824 (1968)] reported a group-theoretical analysis of the competition between unit-cell complexity and crystal symmetry in determining the presence or absence of infrared-active phonons in an elemental crystal. Here we correct an error in that paper's treatment of certain hexagonal space groups. Our results modify the minimum-complexity condition for infrared activity: For 228 of the 230 space groups, a necessary and sufficient condition for the existence of symmetry-allowed infrared-active modes in an elemental crystal is the presence of three or more atoms in the primitive unit cell. The two exceptional space groups are P6/mmm (${\mathit{D}}_{6\mathit{h}}^{1}$) and P${6}_{3}$/mmc (${\mathit{D}}_{6\mathit{h}}^{4}$); for each of these symmetries, there exists one structure with four atoms per cell and no infrared modes. The P${6}_{3}$/mmc structure includes, as special cases, Lonsdaleite (or ``wurtzite silicon'') as well as a c-axis-aligned hcp arrangement of diatomic molecules which is relevant to models of solid molecular hydrogen at high pressure.

Microstructure of α-Al base matrix and SiC particulate composites

The microstructures of a high temperature monolithic AlFeVSi alloy and a metal matrix and SiC particulate composite (MMC) based on this alloy matrix were characterized by conventional and high resolution transmission electron microscopy, microdiffraction and X-ray spectroscopy techniques. Silicide particles of average composition Al 3(Fe,V) 3Si were present in both of these materials. These particles were unstable under the electron beam at voltages above 200 kV, and exhibited radiation-induced disordering. SiC particulates present in the MMC structure were predominantly of the hexagonal 6H polytype, but the rhombohedral 87R-VII polytype was also observed. A very thin reaction layer was present between the matrix and SiC particles. No segregation of alloying elements such as iron, vanadium, or silicon at thee matrix-SiC interface was observed. The second phase present at the α-AlSiC interface was probably a disordered silicide.

The Influence of Interfacial Modification on Tensile Strength of Gr/Al Composites

AbstractVarying the composition of the matrix alloy was a useful technique for tailoring the interfacial structure of MMC. In this paper, the influence of different interfacial states on the tensile strength of Gr/Al composites were investigated. Three aluminum alloys, pure Al, Al-Ti alloy (Ti oversaturated) and LY12 (an Al-Cu-Mg alloy) were chosen as the matrix materials. The results showed that the composites with Al-Ti matrix had excellent tensile properties, even after heat-treatment at 600°C, while with LY12 matrix exhibited poor properties compared with pure Al matrix. Microanalysis revealed that the interfacial structures of the composites with the above matrices were different. In addition, an initial study of mechanism of these matrix effects on the properties of the composites was also carried out.