Synthesis of (V,Cr)(C,N) nanopowders via carbothermal reduction nitridation method: Influence factors and morphology evolution

(V,Cr)(C,N) nanopowders, in the form of globular-like with an average grain diameter of about 60 nm, were successfully synthesized via carbothermal reduction nitridation (CRN) of the precursor of ammonium vanadate (NH₄VO₃), ammonium dichromate ((NH₄)₂CrO₄) and nano carbon black. The influence factors, including synthesis temperature, isothermal holding time and nitrogen pressure, were studied through X-ray diffraction (XRD), and the morphology evolution was investigated by a scanning electron microscope (SEM). The results show that the phase evolution follows: amorphous → VO₂ → V₃O₅ + V₂O₃ + Cr₂O₃ → V₂O₃ + Cr₂O₃ + (V,Cr)(C,N) → (V,Cr)(C,N) with temperature increases. Long isothermal holding time is beneficial to the dissolution of Cr atom into (V,Cr)(C,N), but it leads to grain growth and nitrogen content reduction of the product. On the contrary, high nitrogen pressure contributes to grain refinement and nitrogen increment of (V,Cr)(C,N). In addition, synthesis temperature has a significant effect on the morphology evolution of (V,Cr)(C,N) nanopowders. The microstructure and chemical composition of the product were also studied.     

In-situ synthesis of chromium carbide (Cr3C2) nanopowders by chemical-reduction route

Chromium carbide nanopowder (Cr₃C₂) has been synthesized from chromium oxide (Cr₂O₃) by chemical-reduction route under autogenic pressure of hydrogen and CO gases at 87 MPa. The reduction of Cr₂O₃ to Cr₃C₂ has taken place at a relatively low temperature (700 °C) in the presence of Mg in an autoclave. The increased pressure of hydrogen and CO gases facilitates the reduction and carburization simultaneously which helps in reducing the reaction temperature. The nanopowder shows faceted morphology as observed in TEM. High resolution transmission electron micrographs reveal that the size of Cr₃C₂ powders varies in the range of 30–40 nm and is coated by 12–13 layers of carbon with average thickness of 3.5–4 nm. Thermal stability of the as-obtained product was investigated by TGA/DTA analysis. Based on the experimental results, possible reaction mechanism for the transformation and formation of nanopowder is discussed in the light of the obtained structure.     

Synthesis of V8C7–Cr3C2 nanocomposite via a novel in-situ precursor method

V₈C₇–Cr₃C₂ nanocomposite has been synthesized by a novel in-situ precursor method, and the raw materials are ammonium vanadate (NH₄VO₃), ammonium dichromate ((NH₄)₂Cr₂O₇) and glucose (C₆H₁₂O₆). The products were characterized by thermogravimetric and differential scanning calorimetry (TG-DSC), X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that V₈C₇–Cr₃C₂ nanocomposite with an average crystallite size of 31.5 nm can be synthesized at 900 °C for 1 h. The powders show good dispersion and are mainly composed of spherical or nearly spherical particles with a mean diameter of about 100 nm. The weight loss ratio of the precursor throughout the reaction process reaches 70 wt.%, and it changes rapidly before 400 °C (about 35 wt.%). Four endothermic peaks and three exothermic peaks occur during the reaction. The surface of the specimen is mainly composed of V, Cr, C and O four elements. The synthesis temperature of V₈C₇–Cr₃C₂ nanocomposite by the method (900 °C) is 500 °C lower than that of the conventional method (1400 °C).     

Synthesis of chromium carbide (Cr3C2) nanopowders by the carbonization of the precursor

The solution-derived precursor method was used to synthesize chromium carbide (Cr₃C₂) nanopowders, ammonium dichromate ((NH₄)₂Cr₂O₇) and nanometer carbon black were used as raw materials. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the single phase Cr₃C₂ can be synthesized under the conditions of 21 wt.% C, 1100 °C and 30 min, and the average crystallite size is 27.2 nm. The powders show good dispersion and are mainly composed of spherical or near- spherical particles with a mean diameter of ~ 30 nm. The surface of the specimen mainly consists of Cr, C and O three species elements. The XPS spectrum of Cr2p consists of two peaks with the binding energies of 577.5 eV and 575.3 eV, which are assigned to the Cr2p_3/2 species of Cr₂O₃ and Cr₃C_2 − x (0 ≤ x ≤ 0.5), respectively. The XPS spectrum of O1s energy region for chromium carbide contains three peaks (Oa, Oh and Od), which are considered to be due to O⁻, OH⁻ and Cr₂O₃, respectively.     

Investigating mechanochemical behavior of Cr2O3–B2O3–Mg–C quaternary system

Fundamental aspects of reaction behavior and formation path in the Cr₂O₃–B₂O₃–Mg–C quaternary system have been studied to synthesize chromium boride–chromium carbide nanocomposite. In order to find the influence of simultaneous presence of magnesium and carbon on final products, various powder mixtures were chosen according to following reaction: B₂O₃ + Cr₂O₃ + (9 − x) Mg + x C. The value of x varied from 0 to 4. In the absence of carbon (x = 0), CrB₂ was synthesize through mechanically induced self-propagating reaction (MSR). In the presence of 8 mol Mg and 1 mol C (x = 1), the dominant boride phase was CrB while no chromium carbide was detected. By increasing C content (x = 2), the magnesiothermic reduction occurred in MSR mode; whereas, the synthesis of Cr₃C₂ initiated after combustion reaction and completed gradually during milling for 6 h. Further increase in C amount (x = 3) resulted in formation of Mg₃(BO₃)₂ as unwanted phases as well as CrB and Cr₃C₂. In the presence of 6 mol Mg and 4 mol (x = 4), no mechanical reaction was observed even after 8 h of milling. Optimum value of x for the formation of CrB–Cr₃C₂ nanocomposite was 2. Based on the morphological evolutions, it is evident that the mechanosynthesized powder is made up of nanometric particles.     

Chromium carbide–CNT nanocomposites with enhanced mechanical properties

Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr₃C₂ coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr₃C₂ reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH_0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation.     

Reaction chemistry in the Mg–B2O3–MoO3 system reactive mixtures

Fundamental aspects of reaction path in the MoO₃ + B₂O₃ + Mg system to synthesize molybdenum boride have been investigated. The phase transformation and structural evaluation were studied by means of differential thermal analysis (DTA) techniques, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thermodynamic evaluations indicated that the reaction was highly exothermic and should be a mechanically induced self-sustaining reaction (MSR). According to DTA results, for unmilled sample, the reaction's sequence includes the following reactions: MoO₃ reduction → B₂O₃ reduction → molybdenum boride formation. For 3 h-milled sample, the temperature of exothermic reaction decreased significantly and all the reactions occurred, simultaneously. Based on XRD results, the mechanochemical products including MgO and mix of molybdenum boride phases were achieved after 4 h of high energy ball milling. SEM and TEM observations confirmed that the range of particle size was within 100 nm.     

Oxidation behavior of nano-scaled and micron-scaled TiC powders under air

The oxidation behavior of several batches of TiC nanopowders and micropowders under air has been studied. Using TGA, the oxidation of nanopowders is completed faster and at a lower temperature compared to micropowders. This is related to the higher specific surface area of the nanopowders. Using an amount of powder of about 41.6 mg through DTA, three different exothermic peaks were observed for both kinds of powders. Correlated to the in-situ temperature XRD diffractograms, these peaks were attributed to the following oxidation reactions: i) TiC → TiC_xO_1 − x up to TiO, ii) → Ti₃O₅, and iii) → TiO₂. The same successive oxide phases for nanopowders and micropowders were observed, meaning that the oxidation mechanism is similar in both cases but with a shift of temperature. In this article, the role of the oxygen trapped in the powder and the effect of powder quantity on the oxidation kinetics and on the occurrence of one or three oxidation peaks are discussed.     

Microstructures and properties of Ni based composite coatings prepared by plasma spray welding with mixed powders

The Ni based composite coatings have been obtained by using the plasma spray welding process and mixed powders (NiCrBSi + NiCr-Cr₃C₂ + WC). Their microstructures and properties were studied. The results showed that the coatings consist mainly of γ-Ni, WC, Cr₂₃C₆, Cr₇C₃, Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases, and the Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases mainly segregated between the carbide grains. The carbide contents in the coatings increased with increasing the mass fractions of NiCr-Cr₃C₂ and WC powders in the mixed powders, which results in enhancing the coating hardness. The abrasive wear resistance of the coatings depends on their hardness. The higher the coating hardness, the stronger the wear resistance is. When the mixed powder (15wt%WC + 30 wt% NiCr-Cr₃C₂ + 55wt%NiCrBSi) was used, the composite coating has higher hardness and more excellent wear resistance, and the coating hardness and weight loss after wear tests are 991 HV and 8.6 mg, respectively.     

In situ synthesis of V8C7 nanopowders from a new precursor

Vanadium carbide (V₈C₇) nanopowders were prepared through simultaneous thermal decomposition and carbothermal reduction of the precursor VOC₂O₄/sucrose in vacuum. The products were characterized by X-ray diffractometer (XRD) and transmission electron microscopy (TEM) techniques. The thermolysis process of the precursor has been investigated by themogravimetric analysis and differential thermal analysis (TG-DSC). The results showed that the pure V₈C₇ powders can be obtained at 950 °C for 30 min and the particle size is in the range from 30 to 50 nm with good dispersion. The effects of experimental parameters and reaction mechanism have been explored. The lower synthesis temperature and shorter reaction time were attributed to take the quadrivalence vanadium as a reactant, as well as carbon source and vanadium source intimate contact in the precursor. This facile and quickly synthetic strategy may open a new route to the preparation of other carbide nanomaterials.     

Synthesis,morphology, microstructure and magnetic properties of hematite submicron particles

We report on hydrothermal synthesis, plate-like morphology, microstructure and magnetic properties of hematite (α-Fe₂O₃) plate-like particles. The sample is obtained immediately after the hydrothermal process without using any template and without further heat treatment. The so-obtained sample is characterized by X-ray powder diffraction (XRPD), energy-dispersive X-ray spectroscopy (EDX), field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and superconducting quantum interference device (SQUID) magnetometer. XRPD confirms the formation of a single-phase hematite sample whereas EDX reveals that iron and oxygen are the only components of the sample. SEM, FE-SEM, TEM and HRTEM show that the sample is composed of plate-like particles. The width of the particles is ～500 nm whereas thickness is ～100 nm (aspect ratio 5:1). The HRTEM images exhibit well defined lattice fringes of α-Fe₂O₃ particles that confirm their high crystallinity. Moreover, the HRTEM analysis indicates the plate-like particles preferring crystal growth along [0 1 2] direction. Magnetic measurements display significant hysteretic behavior at room temperature with coercivity H_C = 1140 Oe, remanent magnetization M_r = 0.125 emu/g and saturation magnetization M_S = 2.15 emu/g as well as the Morin transition at T_M ～ 250 K. The magnetic properties are discussed with respect to morphology and microstructure of the particles. The results and comparison with urchin-like, rods, spherical, hexagonal, star-like, dendrites, platelets, irregular, nanoplatelets, nanocolumns and nanospheres hematites reveal that the plate-like particles possess good magnetic properties. One may conjecture that the shape anisotropy plays an important role in the magnetic properties of the sample.     

Growth and spectroscopic characteristics of Cr3+:CsAl(MoO4)2 crystal

This paper reports the growth and spectroscopic characteristics of Cr³⁺:CsAl(MoO₄)₂ crystal. A Cr³⁺:CsAl(MoO₄)₂ crystal with dimensions of 42 mm × 37 mm × 10 mm has been successfully grown from a flux of Cs₂Mo₃O₁₀ by the TSSG method. The absorption and emission spectra were investigated. The absorption cross sections σ_a are 5.05 × 10^?20 cm^?2 at 481 nm for the ⁴A₂ → ⁴T₁ transition and 3.06 × 10^?20 cm^?2 at 670 nm for the ⁴A₂ → ⁴T₂ transition of Cr³⁺ ions, respectively. The emission cross section σ_e of ⁴T₂ → ⁴A₂ transition is 4.27 × 10^?20 cm² at 818 nm and fluorescence lifetime is 21 μs. Based on the absorption and emission spectra, the crystal field strength Dq, the Racah parameters B and C, the effective phonon energy ?ω and the Huang–Rhys factor S were calculated. The investigated results show that Cr³⁺:CsAl(MoO₄)₂ crystal may be regarded as a potential tunable laser crystal material.     

Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide

For the production of hard, high temperature and abrasion resistant parts, like water-jet nozzles or pressing tools for forming glass lenses, binderless cemented carbide is used. In this work, the consolidation of tungsten carbide with additions of VC and Cr₃C₂ grain growth inhibitors is studied. Tungsten carbide powder dry or wet milled was consolidated by dry pressing, debindering and gas pressurized sintering and, alternatively, by spark plasma sintering. The effect of adding VC and Cr₃C₂ to binderless tungsten carbide on the grain growth was studied with contents being 0; 0.1; 0.3; 0.5; 0.7 and 1.0 wt.%. Samples with an ultrafine microstructure free of abnormal grain growth, a hardness of 25.5 GPa and a fracture toughness of 7.2 MPa·m^1/2 were archived by conventional sintering. Both carbides reduce grain growth, but with Cr₃C₂ a finer microstructure can be achieved at lower amounts. Compared to the same amount of Cr₃C₂, the addition of VC results in smaller grains but lower hardness and fracture toughness.     

Improvement in adhesion of diamond film on Cu substrate with an inlay structured interlayer

Diamond films were fabricated by using a two-step process on copper substrates in this study. The first step involved electroplating a chromium (Cr)–diamond composite interlayer on copper substrate and in the second step continuous diamond film was deposited on top using the hot-filament chemical vapor deposition (HFCVD) method. The interfacial characteristics was investigated by indentation tests and the thin film surface morphology, phase structure and residual stress analyzed by scanning electron microscopy (SEM), X ray diffraction (XRD) and Raman spectroscopy. The results show that the diamond particles are deeply imbedded in the chromium layer and the amorphous Cr in the composite interlayer was carburized to Cr₃C₂ during the CVD process. Low residual stress was detected in the diamond film and good adhesive strength between film and substrate was obtained due to the diamond particles anchored deep in the Cr₃C₂ matrix. Concentric cracks but no delaminated areas and radial cracks were observed on the periphery of the indentation at indentation load of 441 N.     

Oxidation of Fe–10Cr in O2 and in O2 + H2O environment at 600 °C: A microstructural investigation

Oxidation of Fe–10Cr in dry and wet O₂ was studied at 600 °C for up to 168 h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O₂ gives a Cr-rich protective (Fe_1−xCr_x)₂O₃ scale. The same protective oxide initially forms in O₂ + H₂O environment, but after an incubation period scale breakdown is triggered by CrO₂(OH)₂ evaporation that depletes the substrate in Cr and converts (Fe_1−xCr_x)₂O₃ to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.     

Epitaxial Ti2AlN(0 0 0 1) thin film deposition by dual-target reactive magnetron sputtering

Ultrahigh-vacuum dual-target reactive magnetron sputtering, in a mixed Ar/N₂ discharge was used to deposit epitaxial single-crystal MAX phase Ti₂AlN(0 0 0 1) thin films, without seed layers, onto Al₂O₃(0 0 0 1) substrates kept at 1050 °C. By varying the N₂ partial pressure a narrow process window was identified for the growth of single-crystal Ti₂AlN. The film microstructure was characterized by a combination of X-ray diffraction, spherical aberration (C_s) corrected transmission electron microscopy (TEM), high-resolution image simulation and high-resolution scanning TEM. Nitrogen-depleted deposition conditions resulted in the concurrent formation of N-free Ti–Al intermetallics at the film/substrate interface and a steady-state growth of Ti₂AlN together with N-free intermetallic phases. At higher N₂ partial pressures the growth assumes a columnar epitaxial nature. 1 Å resolution of the lattice enabling location of all elements in the Ti₂AlN unit cell is demonstrated.     

Preparation and electrochemical properties of LiNi1/3Co1/3Mn1/3O2–PPy composites cathode materials for lithium-ion battery

Layered LiNi_1/3Co_1/3Mn_1/3O₂ was synthesized by co-precipitation method, and a series of polypyrrole–LiNi_1/3Co_1/3Mn_1/3O₂ composites were then prepared by polymerizing pyrrole monomers on the surface of LiNi_1/3Co_1/3Mn_1/3O₂. The bare sample and composites were subjected to analysis and characterization by the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The electrochemical properties of the composites were investigated with galvanostatic charge–discharge test and AC impedance measurements, which show that the formed coats of polypyrrole (PPy) significantly decrease the charge-transfer resistance of LiNi_1/3Co_1/3Mn_1/3O₂. And the composite containing 2.0 wt% PPy exhibits a good electrochemical performance, its specific discharge capacity is 182 mAh g^?1 at 0.1C rate and voltage limits of 2.8–4.6 V, while the capacity of the bare sample is only 134 mAh g^?1.     

Fine platelet-like grained WC–Co cemented carbides: Preparation,characterization, properties and application in PVD coating substrates

Strengthening of the adhesion strength and its stability of PVD coated cemented carbides has always been the focus of attention. Commercial ultrafine WC–12Co composite powder by spray conversion processing was used as the raw material. WC–12Co–0.05La₂O₃, WC–12Co–0.9Cr₃C₂–0.05La₂O₃, WC–12Co–0.5Cr₃C₂–0.4VC–0.05La₂O₃ and WC–12Co–0.9Cr₃C₂–0.4VC–0.05La₂O₃ alloys were prepared by a conventional long-time ball-milling. The results show that fine platelet-like grained WC–12Co–0.9Cr₃C₂–0.4VC–0.05La₂O₃ alloy is characterized with a homogenous microstructure, the best property combination of high strength, high hardness and high toughness and the highest WC (0001) texture coefficient. By using fine platelet-like grained WC–Co cemented carbides as the substrates, larger than 100 N adhesion strength expressed by critical load L_C2 for AlCrN, AlTiN and (AlCrSiWN + AlCrN) PVD coatings is achieved. The related strengthening mechanisms are discussed.     

VC and Cr3C2 doped WCoB-TiC ceramic composites prepared by hot-pressing

The grain growth inhibitors (GGIs) VC and Cr₃C₂ doped WCoB-TiC ceramic composites were fabricated by hot-pressing. The microstructure, hardness, transverse rupture strength (TRS), fracture toughness (K_IC) and wear-resistance of WCoB-TiC ceramic composites were investigated. The results reveal that the grains can obviously become refined and the densification temperature of WCoB-TiC ceramic composites will be increased due to the VC and Cr₃C₂. The typical microstructure of WCoB-TiC ceramic composites mainly consist of bright W₂CoB₂ grains, gray TiC particles, dark TiB₂ and pores. WCoB-TiC ceramic composites doped with 0.3 wt% VC and 0.3 wt% Cr₃C₂ hot-pressing at 1420 °C show the optimum mechanical properties (hardness, TRS and K_IC are 92.6 HRA, 1976 MPa and 14.8 MPa m^1/2, respectively) and the best dry sliding wear-resistance.