Nowotny Phase Research Articles

First-principles investigation of physical properties of transition metal-based ternary TM5Si3C (TM = Nb, Mo, Ta, W) silicides of nowotny phase

In this work, the structural, electronic, mechanical, elastic anisotropy, thermodynamic, and optical properties of TM5Si3C silicides have been examined using first-principles calculations. All studied silicides fulfill the structural stability criteria. Computed mechanical properties reveal the ductile nature of Mo5Si3C, Nb5Si3C, and W5Si3C silicides, while Ta5Si3C silicide is brittle in character. Elastic anisotropy plots, including 2D and 3D, show highly anisotropic behaviour. The ascertained electronic and dielectric properties substantiate the conductive nature of all these silicides. All silicides in the visible region have an average reflectivity of 46 %, making them ideal for coatings to reduce solar heating. Debye temperature, melting point, thermal conductivity, and Gr u¨ neisen constant has also been investigated in current work. Thermodynamic properties have been computed at different pressure and temperature values including bulk modulus (B), volume (V), thermal expansion coefficient (α), Debye temperature (θD), and lattice heat capacity (Cv). The probed results of TM5Si3C ternary silicides demonstrate their viability for applications at elevated temperatures and as well as solar reflectors. Our findings may provide a gateway for exploring new ternary silicides and can act as an efficient, cost-effective guide for experimentalists.

Microstructure and properties of molybdenum composite coatings by plasma spraying of Mo–SiC and MoO3–Al–SiC composite powders

To improve the sliding wear resistance of plasma sprayed Mo coating, Mo–SiC and MoO3–Al–SiC composite powders were used to prepare Mo composite coatings by plasma spraying. The results show that the introduction of SiC improved the mechanical properties and sliding wear resistance of plasma sprayed Mo coating. The reactivity of Mo and SiC was low during plasma spraying Mo–SiC composite powder. The thermite reaction for MoO3 and Al was not conducive to the densification of the composite coating prepared by MoO3–Al–SiC composite powder, but improved the reactivity of in-situ generated Mo and SiC, and the Nowotny phase Mo4.8Si3C0.6 was observed in the coating. The coating prepared by Mo–SiC composite powder displayed better mechanical properties and sliding wear resistance than that of the coating prepared by plasma spraying MoO3–Al–SiC composite powder. The coating obtained by Mo–SiC composite powder revealed a combination of adhesive wear, abrasive wear and oxidation wear.

Electronic, elastic and thermal properties of hexagonal TM5Si3N investigated by first-principles calculations

During the current study, structural, electrical, elastic, and thermal properties of TM5Si3N (TM = V, Nb, and Ta) Nowotny phase were researched using first-principles calculations. The calculated formation enthalpies and phonon dispersion spectrums of these TM5Si3N Nowotny phases indicate that they are dynamically and thermodynamically stable. Because of the strong hybridization between TM-d state and Si-p and N-p states, TM5Si3N can form TM-Si and TM-N bonds. The obtained elastic anisotropy indexes, two-dimensional (2D) planar projection, and three-dimensional (3D) surface constructions demonstrate that the elastic modulus of TM5Si3N is anisotropic, and the order of elastic anisotropy is Ta5Si3N > Nb5Si3N > V5Si3N. In addition, the minimum thermal conductivity of TM5Si3N was calculated using the Clarke's and Cahill's models, which was found to be anisotropic and in the following order of Ta5Si3N > Nb5Si3N > V5Si3N.

First-principles investigation of equilibrium phase, mechanical and thermodynamic properties of the Nowotny TM5Si3C ternary phase

Although the TM-Si-C ternary compounds are potential high-temperature ceramics, the equilibrium phase, mechanical and thermodynamic properties of Nowotny phase (NP) are still unclear. Here, the structural stability, mechanical and thermodynamic properties of the TM5Si3C Nowotny phase (NP) are studied by the first-principles calcualtions. Four TM5Si3C ternary phases (TM = Mo, Nb, W and Ta) are considered. It is found that the equilibrium structure of the Nowotny phase (NP) is a TM5Si3C. In particular, the Mo5Si3C, Nb5Si3C, W5Si3C and Ta5Si3C are firstly predicted. Nb5Si3C has better thermodynamic stability in comparison to the other TM5Si3C. Furthermore, the calculated elastic properties show that four TM5Si3C not only exhibit similar strength in comparison to the corresponding TM5Si3 and TMSi2, but also show better ductility compared to the corresponding TM-Si binary silicides. The nature of mechanical properties of TM5Si3C is revealed by the electronic structure and difference charge density. In addition, it is further found that four TM5Si3C have high melting points and excellent thermodynamic stability. Naturally, the better thermodynamic properties of TM5Si3C is related to the vibration of C atom in a system.

Highly ordered carbon penetration into the Mn5Ge3Cx lattice: A superstructure in Mn5Ge3C0.5 inferred from a Mn55 NMR study

Local magnetic properties of the ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}{\mathrm{C}}_{x}$(001) epitaxial films grown on Ge(111) with a nominal carbon concentration $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.85$ have been studied by means of $^{55}\mathrm{Mn}$ nuclear magnetic resonance (NMR). The NMR spectra were recorded from the demagnetized films and from the films fully magnetized along the $c$ direction as well as in the hexagonal $c$ plane. The data unambiguously show a synchronized carbon penetration of the crystal lattice ($D{8}_{8}$ structure, Nowotny phase, space group $P{6}_{3}/mcm$), occupying every second $2(b)$ void located within the chain of $\mathrm{Mn}[6(g)]$ octahedra and setting the limit for the carbon uptake into the ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$ lattice at $x=0.5$. Moreover, the NMR data indicate that the chains of carbon-filled voids are correlated also in plane. Eventually, for the end concentration of $x=0.5$, a highly ordered superstructure is proposed, responsible for the observed two magnetically inequivalent positions within the $\mathrm{Mn}[4(d)]$ sublattice.

The effect of the impurities on the magnetic, electronic and optical properties of Mn5Ge3

Earlier, we experimentally showed a significant effect of oxygen on the magnetic and structural properties of Mn5Ge3 due to the formation of a Nowotny phase of Mn5Ge3Ox. Here, in continuation of this study, we present a theoretical study of the magnetic and electronic properties of Mn5Ge3 and Mn5Ge3Dx (D = B, C, O). It was found that hexagonal Mn5Ge3 is a ferromagnetic metal with two nonequivalent manganese atoms in the structure. Our ab initio calculations also predict the existence of a spin-crossover in Mn5Ge3 under pressure. Impurities reduce saturation magnetization and electrical and thermal conductivity; however, the magnetic susceptibility and Curie temperature increase. Microscopic mechanisms of the effect of the impurities on the magnetic and electronic properties Mn5Ge3 are discussed.

Dielectric Properties and Electromagnetic Wave Absorbing Performance of Single-Source-Precursor Synthesized Mo4.8Si3C0.6/SiC/Cfree Nanocomposites with an In Situ Formed Nowotny Phase.

For the first time, dielectric properties and electromagnetic wave (EMW) absorbing performance of single-source-precursor derived Mo4.8Si3C0.6/SiC/Cfree ceramic nanocomposites with a highly electrically conductive intermetallic Nowotny phase (NP, i.e., Mo4.8Si3C0.6) are reported. High-temperature phase evolution of the nanocomposites reveals that free carbon (Cfree) plays a crucial role in the in situ formation of the NP, indicating that the microstructure of the nanocomposites can be tailored via molecular design of the single-source precursors. Compared with SiC/Cfree and MoSi2/SiC/Cfree nanocomposites obtained under the same conditions, the Mo4.8Si3C0.6/SiC/Cfree nanocomposites exhibit significantly enhanced EMW absorbing performance. A minimum reflection loss (RL) of -59 dB was achieved at 8 GHz for the thickness of 2.46 mm, proving the superiority of the Mo4.8Si3C0.6/SiC/Cfree nanocomposite as an outstanding EMW absorbing material. On the basis of our previous discovery that the Mo4.8Si3C0.6 embedded in a SiC-based matrix with high specific surface area exhibits excellent electrocatalytic properties suitable for the electrochemical hydrogen evolution reaction, the present results prove that Mo4.8Si3C0.6/SiC/Cfree nanocomposites have to be considered as novel multifunctional materials with tailorable microstructure and excellent performance in two different fields including electrochemical water splitting and EMW absorption.

Enhanced hydrogen evolution reaction catalyzed by carbon‐rich Mo4.8Si3C0.6/C/SiC nanocomposites via a PDC approach

AbstractIn this study, mesoporous carbon‐rich Mo4.8Si3C0.6/C/SiC ceramic nanocomposites were successfully prepared via a single‐source precursor route, starting from allylhydridopolycarbosilane (AHPCS, SMP‐10), bis(acetylacetonato) dioxomolybdenum (VI) [MoO2(acac)2], and divinylbenzene (DVB). Besides, polystyrene (PS) was used as a pore former. The obtained carbon‐rich single‐source precursor/PS mixtures were pyrolyzed at 1100°C, and then annealed at 1350°C‐1600°C to fabricate a series of carbon‐rich Mo4.8Si3C0.6/C/SiC ceramics comprised of high carbon content above 50 wt%. In comparison to the carbon‐poor materials, the carbon‐rich samples retain the higher specific surface area up to 214.6‐304 m2/g at higher annealing temperatures (1350°C‐1600°C) due to the enhancement of carbothermal reaction. The carbon‐rich samples synthesized at 1500°C, denoted as SM/Mo/PS/DVB 2‐1‐4‐2 1500 exhibit enhanced electrocatalytic performance with ultra‐low overpotentials of 119 mV vs reversible hydrogen electrode at a current density of 10 mA cm−2 in acidic media, which is superior to that of the Mo4.8Si3C0.6/C/SiC ceramic (138 mV) with lower carbon content reported in our previous study. Therefore, our porous materials comprised of high carbon content and Nowotny phase (Mo4.8Si3C0.6, NP) are considered as promising catalysts for the hydrogen evolution reaction (HER).

Mesostructure - thermoelectric properties relationships in VxMn1−xSi1.74 (x = 0, 0.04) higher manganese silicides prepared by magnesiothermy

The synthesis of pure pristine and vanadium-doped MnSiγ (γ = 1.74) powder by a relatively fast, ‘low temperature’ and high yield magnesioreduction process is described. The powder obtained by this innovative route is composed of well crystallized grains with sizes ranging from 20 to 500 nm and free from any MnSi precipitates. Mesostructured densified pellets with average grain sizes as small as 550 nm are obtained by spark plasma sintering (SPS). Detailed structural and microstructural characterization of the samples were realized at every stage of the process, highlighting a high concentration of defects such as orientational or spacing anomalies of the Nowotny phase, γ variations within a single grain and dislocations. Accordingly a significant decrease of the lattice thermal conductivity is evidenced in comparison to conventionally synthesized (arc-melting/SPS) samples having similar density and (V/)Mn/Si stoichiometry. The thermoelectric properties of these materials are discussed in regard of their complex microstructure.

Nowotny phase Mo3+2xSi3C0.6 dispersed in a porous SiC/C matrix: A novel catalyst for hydrogen evolution reaction

AbstractThe ternary Nowotny phase (NP), with a composition Mo3+2xSi3C0.6 (x = 0.9‐0.764), is found to be catalytically active in the field of electrochemical water splitting. The NP embedded in a porous SiC/C nanocomposite matrix is synthesized via a single‐source‐precursor approach which involves the reaction of allylhydridopolycarbosilane with MoO2(acac)2. Thermal treatment of the single‐source‐precursor up to 1400°C in a protective atmosphere results in the in situ formation of nanocrystalline Mo3+2xSi3C0.6 immobilized in a thermally and corrosion‐stable SiC/C matrix. The weight fractions of the observed crystalline phases Mo3+2xSi3C0.6 and SiC amount to ca. 28 (26) and 72 (74) wt%, respectively, when prepared at 1400°C (1350°C). The porosity of the formed nanocomposite is adjusted by the addition of polystyrene (PS) as a pore former to the single‐source‐precursor resulting in a specific surface area up to 206 m2/g. The electrocatalytic activity of the Mo3+2xSi3C0.6/C/SiC nanocomposite with respect to the hydrogen evolution reaction (HER) is characterized by low over potentials of 22 and 138 mV vs reversible hydrogen electrode (RHE) for applying 1 and 10 mA cm−2 of current density, respectively. The analyzed electrocatalytic performance exceeds that of most Mo‐based electrocatalysts and shows high stability (over 90%) during 35 hours.

Compositional dependence of epitaxial Tin+1SiCn MAX-phase thin films grown from a Ti3SiC2 compound target

The authors investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 °C as well as the influence of the sputtering power at 850 °C for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in the angular distribution between C, Si, and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at. % in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by x-ray photoelectron spectroscopy shows C–Ti and Si–C bonding in the Ti3SiC2 films and Si–Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 °C and with the growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 °C. Four-point probe resistivity measurements give values in the range ∼120 to ∼450 μΩ cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.

Experimental evidences of Nowotny phase incongruent melting

The Nowotny phase (NPh) Mo4.8Si3C0.6 was synthesized from preliminary obtained Mo5Si3 powder mixed with 1.7 wt% of carbon black with the help of reaction sintering in a solid state at 1750–1850 °C. Sintered and quenched samples were investigated with a help of the X-ray phase analysis, the scanning electron microscopy and X-ray microanalysis. Significant amount of Mo5Si3 in a mixture with NPh and some Mo2C was found after quenching of the samples melted in levitation conditions. Quenching of almost 100% NPh sintered sample melted in a narrow temperature interval above solidus temperature also results in a drastic increase of Mo5Si3 volume fraction. The analogous sample after being slowly cooled near the crystallization interval demonstrates rather high volume fraction of NPh. We suppose that here described results may be caused by the incongruent melting of NPh.

Melting behavior of the Nowotny phase Mo4.8Si3C0.6

Mixtures of the Mo5Si3 silicide with 1.1–2.4 wt % carbon were melted after reaction sintering and the formation of the Nowotny phase (Mo4.8Si3C0.6), and the effect of various heat treatments on the phase composition of the samples was studied. The crystallization of the melted samples leads to a reduction of the volume fraction of the Nowotny phase relative to that after reaction sintering, but subsequent annealing below the liquid phase formation temperature allows the volume fraction of Mo4.8Si3C0.6 to be again raised. We discuss possible causes of the observed phase transformations.

Solid state synthesis of Mn5Ge3 in Ge/Ag/Mn trilayers: Structural and magnetic studies

The thin-film solid-state reaction between elemental Ge and Mn across chemically inert Ag layers with thicknesses of (0, 0.3, 1 and 2.2µm) in Ge/Ag/Mn trilayers was studied for the first time. The initial samples were annealed at temperatures between 50 and 500°C at 50°C intervals for 1h. The initiation temperature of the reaction for Ge/Mn (without a Ag barrier layer) was ~ 120°C and increased slightly up to ~ 250°C when the Ag barrier layer thickness increased up to 2.2µm. In spite of the Ag layer, only the ferromagnetic Mn5Ge3 compound and the Nowotny phase were observed in the initial stage of the reaction after annealing at 500°C. The cross-sectional studies show that during Mn5Ge3 formation the Ge is the sole diffusing species. The magnetic and cross-sectional transmission electron microscopy (TEM) studies show an almost complete transfer of Ge atoms from the Ge film, via a 2.2µm Ag barrier layer, into the Mn layer. We attribute the driving force of the long-range transfer to the long-range chemical interactions between reacting Mn and Ge atoms.

Synthesis of the Nowotny phase Mo4.8Si3C0.6 from Mo5Si3 + carbon mixtures

A simple, efficient method for the synthesis of the Nowotny phase Mo4.8Si3C0.6 from mixtures of the Mo5Si3 silicide and carbon in the form of carbon black or diamond powder has been proposed. The synthesis of the Nowotny phase via reaction sintering has been shown to be more complete in samples prepared at reduced compaction pressures, which has been tentatively attributed to the considerable role of carbon transport through the gas phase. In samples quenched from the liquid state in a levitation melting system, the highest volume fraction of Mo4.8Si3C0.6 has been obtained using starting mixtures of Mo5Si3 and diamond powder.

Incommensurate structure of a new Nowotny phase in Fe-Ge system

We studied post-annealed Fe-Ge alloy by X-ray diffraction and transmission electron microscopy. A new Nowotny chimney ladder phase has been discovered in such Fe-Ge system. The crystal structure and the composition have been calculated based on selected area electron diffraction. The formation of the incommensurate structure is attributed to the shrinkage of the Ge sublattice to meet the 14 electrons rule.

Synthesis of Ferromagnetic Germanides in 40Ge/60Mn Films: Magnetic and Structural Properties

Solid-state reactions between Ge and Mn films are systematically examined using X-ray diffraction, photoelectron spectroscopy and magnetic measurements. The films have a nominal atomic ratio Ge:Mn = 40:60 and are investigated at temperatures from 50 to 500 °С. It is established that after annealing at ~120 °С, the ferromagnetic Mn5Ge3 phase is the first phase to form at the 40Ge/60Mn interface. Increasing the annealing temperature to 500 °С leads to the formation of the ferromagnetic phase with a Curie temperature TC ~ 360 K and magnetization MS ~ 140-200 emu/cc at room temperature. Analysis of X-ray diffraction patterns and the photoelectron spectra suggests that the increased Curie temperature and magnetization are related to the migration of C and O atoms into the Mn5Ge3 lattice and the formation of the Nowotny phase Mn5Ge3СxOy. The initiation temperature (~120 °С) of the Mn5Ge3 phase is the same both for solid-state reactions in Ge/Mn films, as well as for phase separation in GexMn1-x diluted semiconductors. We conclude that the synthesis of the Mn5Ge3 phase is the moving force for the spinodal decomposition of the GexMn1-x diluted semiconductors.

Formation of ferromagnetic germanides by solid-state reactions in 20Ge/80Mn films

Solid state reactions between Ge and Mn films are systematically examined using X-ray diffraction, photoelectron spectroscopy, and magnetic and electrical measurements. The films have a nominal atomic ratio Ge:Mn=20:80 and are investigated at temperatures from 50 to 500ºC. It is established that after annealing at ~120ºC, the ferromagnetic Mn5Ge3 phase is the first phase to form at the 20Ge/80Mn interface. As the annealing temperature increases to 300ºC, the weak magnetic Mn5Ge2+Mn3Ge phases simultaneously begin to grow and they become dominant at 400ºC. Increasing the annealing temperature to 500ºC leads to the formation of the ferromagnetic phase with a Curie temperature TC~350–360K and magnetization 14–25kA/m at room temperature. The X-ray diffraction study of the samples shows the reflections from the Mn5Ge3 phase, and the photoelectron spectra contain the oxygen and carbon peaks. The homogeneous distribution of oxygen and carbon over the sample thickness suggests that the increased Curie temperature and magnetization are related to the migration of C and O atoms into the Mn5Ge3 lattice and the formation of the Nowotny phase Mn5Ge3СxOy.The initiation temperature (~120°С) is the same in the Mn5Ge3 phase with the solid-state reactions in the Ge/Mn films as well as in the phase separation in the GexMn1−x diluted semiconductors. Thus, we conclude that the synthesis of the Mn5Ge3 phase is the moving force for the spinodal decomposition of the GexMn1−x diluted semiconductors.

The interaction of carbon with Mo5Si3 and W5Si3 silicides. Nowotny phase synthesis

High-temperatures reactions of Mo5Si3 and W5Si3 silicides powders with the carbon soot were studied experimentally on sintered and melt-quenched samples. It was shown that Nowotny phase Mo4.8Si3C0.6 may be synthesized after annealing at 1600–2000 °C of initial mixture of Mo5Si3 with the carbon soot compacted at room temperature, simultaneously the small quantity of Mo2C phase was synthesized also. We have not found the Nowotny phase for W (like W4.8Si3C0.6) after reactions of W5Si3 or W5Si3 + WSi2 eutectic powders mixtures with carbon neither for the solid state reactions, nor for the melt-quenched samples. Only in some experiments with quenching of melted on the graphite substrate W5Si3 and WSi2 silicides eutectic there were found some evidences of the phase like W4.8Si3C0.6 possible presence along with W5Si3 and WSi2 silicides and WC carbide. In the melt-quenched Mo5Si3 + C sample Nowotny phase for Mo was distinguished from the Mo5Si3 phase in the back-scattered electron image, Mo2C carbides were revealed also.

Thermoelectric Properties of Higher Manganese Silicides Prepared by Mechanical Alloying and Hot Pressing

Higher manganese silicides (HMS), MnSi1.75–δ, were synthesized by mechanical alloying and consolidated by hot pressing. The optimum condition of mechanical alloying was ball milling at 400 rpm for 6 h, and sound sintered compacts could be obtained by hot pressing at temperature higher than 1073 K. The phase fraction of HMS showed no significant difference with compositional (δ) variation, but the MnSi1.75 specimen had the lowest fraction of MnSi of approximately 3%. The lattice constants of HMS with compositional variation were similar to values reported in the literature. All specimens showed Nowotny phase with tetragonal structure, and exhibited i-type conduction at measuring temperatures between 323 K and 823 K. HMS behaved as degenerate semiconductors in that the absolute values of the Seebeck coefficient increased and the electrical conductivity slightly decreased with increasing temperature. MnSi1.73 showed the highest figure of merit of 0.28 at 823 K.