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.     

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.     

Study on oxidation mechanism and kinetics of MoO2 to MoO3 in air atmosphere

The oxidation mechanism and kinetics of MoO₂ to MoO₃ in air atmosphere from 750 K to 902 K have been investigated in the present work. These results show that temperature has significant effects on the oxidation process. It is found that the produced MoO₃ has a tendency to form a big platelet-shaped particle and the surface appears to be smooth at the high reaction temperature (902 K); while at the low reaction temperature (750 K), the micrographs of final products MoO₃ become rough and irregular. The intermediate product Mo₄O₁₁ will be formed only when the temperature is above 810 K. It is found that the oxidation reaction was controlled by the interface chemical reaction at the reaction interface (from MoO₂ to Mo₄O₁₁) and diffusion (from Mo₄O₁₁ to MoO₃), respectively, by using the dual-interface reaction model in the temperature range of 810 K to 902 K. While in the temperature range of 750 K to 779 K, the oxidation reaction (one-step reaction, from MoO₂ to MoO₃ directly) was controlled by the diffusion model.     

A study on mechanochemical behavior of MoO3–Mg–C to synthesize molybdenum carbide

The influence of Mg value in the MoO₃–Mg–C mixture on the molybdenum carbide formation and the mechanism of reactions during mechanochemical process were investigated. In keeping with this aim, magnesium and carbon contents of the mixture were changed according to the following reaction: 2MoO₃ + (6 − x) Mg + (1 + x) C = (6 − x) MgO + Mo₂C + x CO. The value of x varied from 0 to 6. Differential thermal analysis (DTA) results for sample with stoichiometric ratio (x = 0) revealed that in the early stage, carbon reduced the MoO₃ to MoO₂ and subsequently highly exothermic magnesiothermic MoO₂ reduction occurred after magnesium melting. Also, it was indicated that the exothermic reaction temperature shifted to before magnesium melting in the 11 h-milled sample (x = 0) and all the exothermic reactions happened, simultaneously. According to the experimental findings, molybdenum carbide (Mo₂C) was synthesized in the mixture powder with stoichiometric ratio (x = 0) after 12 h milling process and the type of reactions was mechanically induced self-sustaining reaction (MSR). However, at lower Mg content in the MoO₃–Mg–C mixture (0 < x ≤ 2), the magnesiothermic reduction occurred in MSR mode and activated the carbothermal reaction. Further decrease in Mg value (2 < x ≤ 3) resulted in MSR mode magnesiothermic reaction and gradual carbothermal reduction. In samples with lower magnesium contents, partial molybdenum oxide reduction proceeded through a gradual mode magnesiothermic reaction.     

Agglomeration during reduction of MoO3

Molybdenum powder is manufactured in a two step process starting from MoO₃. The first step reduction of MoO₃ to MoO₂ is carried out in rotary calciners. Agglomeration of powder occurs during this reduction stage resulting in several manufacturing issues. The evolution of agglomeration during the reduction of MoO₃ was investigated in the current study. As-received MoO₃ and MoO₃ milled for 0.5 h were used as the starting powders. The powders were reduced at 550 °C, 650 °C and 750 °C in a hydrogen atmosphere. The starting and reduced powders at various temperatures were analyzed using BET surface area, XRD, and SEM techniques. The surface area of the reduced powders was monitored for quantifying the degree of agglomeration. The surface area was found to be minimum for the samples reduced at 650 °C. SEM observations confirmed the agglomeration of powders during reduction process. XRD analysis showed complete reduction of MoO₃ to MoO₂ at 650 °C and 750 °C. The agglomeration of the powders was either due to melting of eutectic formed between MoO₃ and Mo₄O₁₁ or due to partial melting of MoO₃. The reduction of MoO₃ is recommended to be completed at a low temperature to prevent agglomeration of the oxide powders.     

Transient stage oxidation behavior of Mo76Si14B10 alloy at 1150 °C

The transient stage during isothermal oxidation of Mo₇₆Si₁₄B₁₀ has been studied at 1150 °C. The maximum mass loss is found to occur by volatilization of MoO₃ within the first 480 s of exposure, which is considered to be the transient period. Scanning electron microscopic studies have revealed that a glassy borosilicate layer flows to cover the top surface, and thereby it acts as a protective coating. The thickness of the borosilicate layer increases with exposure time following a parabolic relationship. The mechanism of oxidation during transient period has been discussed with the help of an analytical model.     

Thermodynamic analysis and synthesis of porous Mo2C sponge by vapor-phase condensation and in situ carburization of MoO3

Spongy porous MoO₃ deposits were grown by vaporization, vapor-phase transportation and condensation of MoO₃ in Ar flow. It was observed that increased source temperature (≥1200 K) and temperature gradient (≥100 K/cm) favor the formation of spongy deposit owing to high supersaturation of the oxide vapor at ～900 K. Spongy Mo₂C deposits consisting of intermingled platelet crystals with thin walls were synthesized by in situ carburization of the condensed MoO₃ using 0.05–0.1 mol of CH₄ and 1 mol of H₂ at 900 K. Thermodynamic analysis in the Mo–O–C–H system was used as a guide to predict the conditions for the formation of Mo₂C from the MoO₃–CH₄–H₂ reactants at 900 K. X-ray diffraction analysis showed that the carburized deposits consisted of single phase Mo₂C, in agreement with the thermodynamic prediction. The equilibrium analysis was also used to reveal possible reaction pathways to Mo₂C formation from MoO₃–CH₄–H₂ reactants which yielded gaseous products of H₂O, CO₂, CO, C₂H₆ and C₂H₄.     

Effect of ammonium dimolybdate (ADM) on the reduction of molybdenum trioxide

Ammonium dimolybdate ((NH₄)₂Mo₂O₇) or molybdenum trioxide (MoO₃) is used as starting raw materials for manufacturing Mo powders. In the initial step, usually carried out in rotary calciners, (NH₄)₂Mo₂O₇ or MoO₃ is reduced to MoO₂. Agglomeration of powder due to melting of eutectic formed between MoO₃ and Mo₄O₁₁ and due to melting of MoO₃ occurs during this reduction step resulting in several manufacturing issues. The reduction from (NH₄)₂Mo₂O₇ involves an endothermic reaction however, reduction of MoO₃ occurs only by exothermic reaction. It is hypothesized that addition of (NH₄)₂Mo₂O₇ to MoO₃ will decrease agglomeration of powders due to the endothermic reaction involved in the reduction process. The current paper details experiments carried out to verify the hypothesis. MoO₃ containing varying amounts (NH₄)₂Mo₂O₇ were reduced at 550 °C, 650 °C and 750 °C in hydrogen atmosphere. The results show lower agglomeration of powder with addition of (NH₄)₂Mo₂O₇. The thermal analysis results confirm reduction of MoO₃ at lower temperatures with the addition of (NH₄)₂Mo₂O₇.     

Preparation of Mo(Si, Al)2–ZrO2 nanocomposite powders by mechanical alloying

Phase transformations and the final formation of Mo(Si, Al)₂–ZrO₂ nanocomposite during high-energy ball milling of a series of Mo–Si–Al–ZrO₂ powders were investigated. Mechanical alloying led to phase transformations from the initial Mo–Si–Al powders mixture to Mo_ss (2 h) → C40 Mo(Si, Al)₂ (4, 8 h) → Mo_ss (12 h) phases. The phase transformations studied by XRD are discussed considering the alloying and second phase effects. Finally, the Mo_ss matrix reinforced with ZrO₂ particles nanocomposite structure was studied by means of TEM. The Mo_ss matrix phase formed was revealed to be strongly inhomogeneous even after 12 h of mechanical alloying and Mo-, Si- and Al-enriched regions were observed. The ZrO₂ nanostructured phase, evenly distributed in the Mo_ss matrix, had grain size of about 5–20 nm.     

Oxidation behaviour of the Mo–Si–B and Mo–Si–B–Al alloys in the temperature range of 700–1300 °C

The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo₃Si, Mo₅SiB₂ and SiO₂ or α-Al₂O₃ phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO₃ as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO₂ and MoO₃, while those formed at 900 °C or above contain Mo, MoO₂ and SiO₂. In addition, α-Al₂O₃ or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B₂O₃–SiO₂.     

Low driving voltage in organic light-emitting diodes using MoO3 as an interlayer in hole transport layer

Driving voltage of organic light-emitting diodes (OLEDs) was lowered by applying MoO₃ as an interlayer between hole injection layer (HIL) and hole transport layer (HTL). MoO₃ was effective as an interlayer between HIL and HTL due to its valence band of around 5.3 eV which is suitable for hole injection. Hole injection from HIL to HTL was enhanced by MoO₃ interlayer and driving voltage of green fluorescent device could be lowered by 1.3 V at 1000 cd/m² by using thin MoO₃ interlayer.     

Simple solvothermal routes to synthesize nanocrystalline Bi2MoO6 photocatalysts with different morphologies

Nanocrystalline Bi₂MoO₆ photocatalysts were successfully synthesized by conventional solvothermal and microwave–solvothermal routes, respectively. The prepared samples were characterized by X-ray diffraction, BET surface area analysis, UV–vis diffuse reflectance spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The samples have high surface areas in the range of 10–32 m² g⁻¹. Their average crystallite sizes are in the range of 16–35 nm. The absorption edge of the samples is at ∼491 nm, corresponding to a band gap energy of about 2.53 eV. Different morphologies with nanosheets and nanorods were also observed. The photocatalytic activities of Bi₂MoO₆ photocatalysts were evaluated by the decomposition of Rhodamine B under visible-light irradiation (λ > 420 nm). Nanocrystalline Bi₂MoO₆ samples obtained via different conditions exhibited different photocatalytic performances. The effects of the crystallinity, specific surface area and morphology of the samples on the photocatalytic activities are also discussed.     

Preparation of MoO2 sub-micro scale sheets and their optical properties

MoO₂ sub-micro sheets have been synthesized in a large scale on silicon substrate with MoO₃ and C powders as raw materials using a novel chemical vapour deposition method. The lengths and widths of MoO₂ sheets are in the range of several to dozens micrometers, and the thickness of MoO₂ sub-micro sheets is ～200 nm. Transmission electron microscopy and high-resolution electron microscopy show that the MoO₂ sheets are of single crystal with a monoclinic structure. The sheets exhibit fluorescent emissions at 304.4, 343.5 and 350.6 nm when the 220 nm light excitation is applied at room temperature, and the emissions result from some defects and the electron transition between valence band and conduction band. UV–vis spectrum shows the MoO₂ sub-micro sheets have absorption peaks between 200and 300 nm and the emissions should be attributed to the defect states of MoO₂. Furthermore, the band-gap is estimated to be approximately 4.22 eV. The growth mechanism of the two-dimensional MoO₂ sub-micro scale sheets is also discussed.     

The four-step multiple stage transformation in deformed and annealed Ti49Ni51 shape memory alloy

A four-step multiple stage transformation is observed in 20% deformed and 500 °C annealed Ti₄₉Ni₅₁ shape memory alloy. Two extra B2 → B19^′ transformation peaks appear before the previously described B2  → R and R → B19^′ peaks while cooling, and these correspond to one new peak, which appears after the original B19^′ → B2 peak during heating. These two extra peaks are caused by the combined effect of severe cold-working and long-time annealing on Ti₄₉Ni₅₁ alloy, and they come separately from the B2 → B19^′ transformation occurring in regions with low and high dislocation densities, which are originally suppressed by cold-working.     

Effects of excess reactant amounts on the mechanochemically synthesized molybdenum silicides from MoO3, SiO2 and Mg blends

This study reports on the preparation of molybdenum silicide powders from MoO₃-SiO₂-Mg powder blends with a two-step process of mechanochemical synthesis and selective acid leaching. Mechanochemical synthesis was carried out at a short duration of 1 h using a high-energy ball mill. Subsequently, mechanochemically synthesized powders were eliminated from the unwanted Mg-based by-products by HCl leaching. Excess amounts of reactants were utilized with the intention of eliminating Mo phase and their effects were investigated on the yielded products. Phase and microstructural characterizations of the powder products were performed using X-ray diffractometer (XRD), particle size analyzer (PSA) and scanning electron microscope/enery dispersive X-ray spectrometer (SEM/EDX). Quantitative phase analysis (QPA) and crystallite size calculations were also conducted on the mechanochemically synthesized and leached powders using the Bruker AXS TOPAS software. All the leached powders consisted of α-MoSi₂, β-MoSi_2, Mo₅Si₃ and Mo phases. However, in case of using 80 wt.% excess amount of Mg, the occurrence of Mo phase was inhibited and powders containing dominant α-MoSi₂ (~ 73 wt.%), β-MoSi₂ (~ 11 wt.%) and Mo₅Si₃ (~ 16 wt.%) phases were obtained with an average crystallite size of about 70 nm.     

Thermoelectric properties of molybdenum oxides LnMo8O14 (Ln = La,Ce, Pr,Nd and Sm)

The series LnMo₈O₁₄ (Ln = La, Ce, Pr, Nd and Sm) containing bicapped Mo₈ clusters was synthesized via solid state reaction at 1673 K. Oxides of this type were reported to be narrow gap semiconductors. Our Seebeck coefficient measurements show that some of these reduced molybdenum oxides exhibit a thermopower of above ?100 μV/K at room temperature, which is promising for the thermoelectric application. The highest power factor of 71 μW/mK² was obtained for SmMo₈O₁₄ at 1152 K.     

Dynamic self-organization in Cu alloys under ion irradiation

Phase separation in dilute Cu_(1–x)M_x films under irradiation with 1.8 MeV Kr⁺ ions at elevated temperatures has been studied for 0.10 < x < 0.15 and M = Ag, Co, Fe, Mo and Nb. In all systems compositional patterning was observed on a mesoscopic length scale within a fixed temperature interval. At lower temperatures most of the alloys formed nearly homogeneous solid solutions, while at higher temperatures these same alloys underwent macroscopic phase separation. The upper temperature limit for patterning increased with decreasing solute diffusivity in the Cu matrix, reported in the absence of irradiation. The heat of mixing of the alloy, however, also played a role in the patterning behavior. For Cu₉₀Mo₁₀, which has a high positive heat of mixing, no upper or lower temperature limits for patterning were observed. Molecular dynamics computer simulations reported here explain why Cu₉₀Mo₁₀ behaves differently from the other alloys.     

Growth and optical spectroscopy of KPb2Cl5 crystal containing Mn2+

KPb₂Cl₅:Mn²⁺ crystal with sizes of centimeters has been successfully grown by a modified Bridgman method. The electronic spectra of Mn²⁺ in the title compound have been measured at different temperatures and compared, revealing that the red emission observed at 10 and 77 K in the spectral range from 500 to 800 nm, which vanishes at room temperature, is associated with the radiation from Pb²⁺ ions with adjacent defects whereas the orange-red emission in the spectral range from 530 to 680 nm can be undoubtedly attributed to the spin forbidden transition ⁴T_1g (⁴G) → ⁶A_1g(⁶S) of Mn²⁺. The electronic excitation spectra are interpreted in a strong crystal field scheme with the use of the Tanabe–Sugano diagram. Good agreement with the experimental spectroscopic data is obtained with the use of crystal field splitting parameter 10Dq = 7902 cm^?1, Racah parameters B = 618 cm^?1, and C = 3549 cm^?1.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Improvement of the oxidation-proof property and the scale structure of Mo3Si intermetallic alloy through the addition of chromium and aluminum elements

The influences of chromium and aluminum additions to Mo₃Si intermetallic alloy on the microstructure, mechanical and oxidation properties as well as the scale evolution were investigated for alloys with the composition of Mo_75 − yCr_ySi_25 − xAl_x (y = 0, 15, 30, 45, 60, 75 mol%, x = 0, 5, 10 mol%). Cr and Al were added for the aim of not only improving the oxidation property but also changing the compound type from Daltonide to Berthollide ones. The alloys were prepared by arc-melt method and the button ingots were homogenized at 1873 K for 10 h. It was found that the homogenized (Mo,Cr)₃Si alloy consists of continuous solid solution. During oxidation test at 1173 K, the mass of Mo₃Si alloy decreased remarkably due to the evaporation of molybdenum oxides. However, the improvement of the oxidation resistance was confirmed when chromium was added up to at least 15 mol%. In particular, the (Mo,Cr)₃Si alloys with above 30 mol%Cr showed excellent oxidation resistance through the formation of the passive oxide-scale layer. Although the effect is smaller, aluminum addition also contributed to the improvement of oxidation resistance.