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 nanopowders via a microwave heating method

Chromium carbide nanopowders were firstly synthesized via a simple microwave heating technique using nanometer chromic oxide (Cr₂O₃) and nanometer carbon black as raw materials in argon gas atmosphere. The samples were characterized by X-ray diffractometry (XRD), thermogravimetric and differential scanning calorimetry (TG–DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The results show that chromium carbide nanopowders with an average crystallite size of 24 nm can be synthesized at 1000 °C for 1 h. The synthesis temperature required by the method is 400 °C lower than those required by the conventional approaches for preparing chromium carbide. SEM and TEM results show that the powders show good dispersion and are mainly composed of spherical or nearly spherical particles with a mean diameter of about 30 nm. The phase transformation sequence during the heat treatment is: Cr₂O₃ → CrO → Cr₇C₃ → Cr₂C → Cr₃C₂.     

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.     

Semiconductor TiO2–Ga2O3 thin film gas sensors derived from particulate sol–gel route

Nanostructured and mesoporous TiO₂–Ga₂O₃ thin films with various Ti:Ga atomic ratios were prepared by a new straightforward particulate sol–gel route. Titanium isopropoxide and gallium (III) nitrate hydrate were used as precursors, and hydroxypropyl cellulose (HPC) was used as a polymeric fugitive agent (PFA) in order to increase the specific surface area (SSA). XRD and TEM analysis of the powders revealed that the Ga₂O₃ formed from the nitrate precursor retarded anatase-to-rutile transformation, crystallization and crystal growth. The average crystallite size of pure TiO₂ powder annealed at 600–1000 °C were in the range 4–10 nm; the values that could be decreased to 2–6 nm for TiO₂–Ga₂O₃ powders. Furthermore, one of the highest SSA was obtained by introducing Ga₂O₃ into TiO₂, being 305 m² g⁻¹ for TG11 (Ti:Ga = 50:50 atomic ratio) binary oxide annealed at 600 °C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO₂ gases at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption. TG11 sensor showed the highest response towards all CO and NO₂ concentrations operated at 200 °C. The response magnitude of 13.7 and 4.3 with response times of 30 s and 108 s were achieved for TG11 sensor towards 400 ppm CO and 10 ppm NO₂, respectively. Furthermore, calibration curves revealed that TiO₂–Ga₂O₃ sensors follow the power law (S = A[gas]^B) (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and they have excellent capability for the detection of low gas concentrations (25 ppm CO and 0.5 ppm NO₂). The maximum response of TiO₂–Ga₂O₃ sensors towards CO and NO₂ was measured at 450 and 400 °C, respectively. The sensor response decreased with increasing film annealing temperature owing to sintering of the particles. The response magnitude and response time of the sensors obtained in this work is superior to TiO₂-based sensors reported in previous studies.     

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.     

Nanoscaled grain boundaries and pores,microstructure and mechanical properties of translucent Yb:[LuxY(1−x)O3] ceramics

Translucent ceramics of Yb:[Lu_xY_(1?x)O₃] system doped by ZrO₂ was sintered from nanopowder synthesized by laser evaporation. The relative density of the ceramics was 99.97%, residual pores had sizes from 8 nm to 20 nm, Young modulus was 200 GPa at the applied load of 2000 mN, the microhardness was 12.8 GPa. The grains of ceramics had sizes 1–10 μm, but the thickness of grain boundaries was about 1 nm. The transcrystalline type of the crack propagation was detected in the specially broken ceramics. The results indicated high strength of grain bonds and good perfection of grain boundaries in the studied ceramics but an increased content of pores (higher than 10^?3 vol.%) and stoichiometry deviation (Lu:Y:O = 0.21:0.79:3) from the required one (Lu:Y:O = 0.25:0.75:3).     

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.     

Diffusion and solubility of Cr in WC

Although no detailed study on the Cr solubility in WC exists the compilation on the various C–Cr–W phase diagrams [1] suggests this behaviour. In order to prove this and to estimate the diffusivity of Cr in WC we prepared diffusion couples of the type Cr₃C₂–WC by joining and annealing polished fully dense counterparts of the two carbides (temperature range 1550–1750 °C). After thermal treatment the diffusion couples were cut, polished and investigated by metallography. For the measurement of the diffusion profiles the couples were subjected to WDS-EPMA (Cameca SX 100 microprobe). W, Cr, and C concentration profiles were obtained from line scans performed perpendicular to the interface. The analysis of diffusion couples of WC contacted to other carbides used for doping of hardmetals (VC, TaC, NbC, and TiC) did not yield perceptible solubility of the respective metals in WC with respect to the detection limit of EPMA.From the Cr diffusion profiles a diffusion coefficient of Cr in WC of approximately D = 1.70–2.20 × 10^?11cm²/s and an activation energy of E_A = 0.75 eV was estimated. In addition the composition of the ternary phase (W,Cr)₂C in equilibrium with WC and Cr₃C₂ could be measured. For example, in couples annealed at 1750 °C the composition reaches from (W_0.5Cr_0.5)₂C (in equilibrium with WC) to (W_0.2Cr_0.8)₂C (in equilibrium with Cr₃C₂).With the results obtained from the analysis of diffusion couples, the Cr uptake of WC powder as a function of grain size, time and temperature was calculated. Cr saturation in idealised spherical particles of 1 μm occurs only within a few minutes.     

A comparative study of laser cladding high temperature wear-resistant composite coating with the addition of self-lubricating WS2 and WS2/(Ni–P) encapsulation

In order to inhibit its decomposition and improve its compatibility with the metal matrix during laser cladding, WS₂ powder was encapsulated with a layer of micro Ni–P by electroless plating. The microstructure and tribological properties of the NiCr–Cr₃C₂/30%WS₂ and NiCr–Cr₃C₂/30%WS₂(Ni–P) high temperature self-lubricating wear-resistant composite coatings at RT (room temperature), 300 °C and 600 °C were investigated, respectively. It was found that the NiCr–Cr₃C₂/30%WS₂(Ni–P) coating had a microstructure consisting of primary Cr₇C₃ dendrite, γ-(Fe,Ni)/Cr₇C₃ eutectic and solid lubricant particles WS₂ and CrS, Ni–P electroless plating had decreased the decomposition of WS₂ to some extent, the WS₂ solid lubricant particles dispersed in the ductile γ-(Fe,Ni)/Cr₇C₃ matrix. Friction and wear experiments indicated that the tribological properties of the NiCr–Cr₃C₂/30%WS₂(Ni–P) coating was better than that of NiCr–Cr₃C₂/30%WS₂ coating, the NiCr–Cr₃C₂/30%WS₂(Ni–P) coating presents lower friction coefficient at RT and 300 °C and lower wear rate from RT to the elevated temperature of 600 °C.     

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.     

Highly sinter-active (Mg–Cu)–Zn ferrite nanoparticles prepared by flame spray synthesis

Nanoscale ZnFe₂O₄, Mg_0.5Zn_0.5Fe₂O₄ and Mg_0.2Cu_0.2Zn_0.62Fe_1.98O_3.99 powders were prepared for the first time by flame spray synthesis (FSS). Solutions of metal β-diketonates in organic solvents were used as precursor. Crystalline particles of spinel structure with 6–13 nm primary particle size resulted from the flame process. Particle and crystallite size depended on the flow rate of the atomizing gas, the precursor and its molarity. Compacts prepared from Mg–Cu–Zn ferrite nanoparticles revealed an extremely high sinter-activity. A sintered density of 5.05 g cm⁻³ was achieved after firing for 2 h at 900 °C without any sintering additives, while a maximum density of 4.91 g cm⁻³ was obtained with particles from the conventional ceramic route. The permeability of the sintered Mg–Cu–Zn ferrite nanopowder compacts reached μ = 600 at 1 MHz and the saturation magnetisation was 80 emu g⁻¹. The outstanding sintering activity of the flame-made ferrite powders is attributed to their small primary particle size.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

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.     

Optical diffraction of second harmonic generation in SrBi2(Nb0.7V0.3)2O9 in the SrO–Bi2O3–0.7Nb2O5–0.3V2O5–Li2B4O7 glass system

Transparent glasses in the system (100 − 3x)(Li₂O–4B₂O₃)–x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (where x = 10, 30 and 50, in molar ratio) embedded with nanocrystallites of SrBi₂(Nb_0.7V_0.3)₂O₉ exhibited intense second harmonic signals in transmission mode when exposed to IR laser light at λ = 1064 nm. The second harmonic waves were found to undergo optical diffraction. The origin of optical diffraction in these samples was attributed to the self organised structures of fine crystallites of submicrometer size that were inscribed in-situ by the IR laser radiation. Laser Raman studies confirmed these crystallites to be vanadium doped strontium bismuth niobate.     

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.     

Spectral investigations of organic metals β″-(BEDT-TTF)4A[M(C2O4)3]·DMF,where A = NH4+, K+ and M = CrIII,FeIII

Spectral investigations of the new isostructural organic metals of the formula β″-(BEDT-TTF)₄A[M(C₂O₄)₃]·DMF, where A = NH₄⁺, K⁺ and M = Cr^III, Fe^III, were performed. The polarized reflectance spectra of single crystals of these salts were recorded in the range 600–6500 cm⁻¹. Additionally, the optical absorption spectra of the powdered salts dispersed in KBr pellets, in the range 200–40,000 cm⁻¹ were measured. The spectra are characteristic for organic metals and show small anisotropy as typical for quasi-two-dimensional (2D) materials. A Kramers–Krönig analysis of the reflectance data was performed to obtain the optical conductivity spectra. The frequency dependences of the reflectivity were fitted with the Drude or Drude–Lorentz models and the transport parameters were evaluated.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.     

Characterization of mechanically alloyed nanocomposites in TiO2–B2O3–Mg–C quaternary system

The influence of the simultaneous presence of magnesium and graphite on mechanosynthesis of various nanocomposite powders in TiO₂–B₂O₃–Mg–C quaternary system was investigated. A mixture of boron oxide and titanium dioxide powders along with different amounts of magnesium and graphite was milled using a high-energy planetary ball mill to provide necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). In the absence of C (100 wt.% Mg), TiB₂ nanopowder was formed as a result of combustion reaction after 34 min of milling. In the presence of both Mg and C, the mechanochemical reaction was completed after different milling times depending on the weight fraction of the reducing agents in the powder mixture. In the presence of x wt.% Mg–y wt.% C (x = 85 and 90; y = 100 − x), the mechanosynthesized composites contained TiB₂ and TiC as major compounds as well as MgO and Mg₃B₂O₆ as unwanted phases. With further increasing the graphite content to 30 wt.%, no mechanical activation was observed after 90 min of milling. The nanocomposite powders showed a bimodal particle size distribution characterized by the presence of several coarse particles (≈ 250 nm) along with finer particles with a mean size of about 75 nm. Formation mechanism of nanocomposites was explained through the analysis of the relevant sub-reactions.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.