Water Vapor–Mediated Volatilization of High-Temperature Materials

Meschter, P. J.; Opila, Elizabeth J.; Jacobson, Nathan

doi:10.1146/annurev-matsci-071312-121636

Cited by 120 publications

(85 citation statements)

References 107 publications

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“…Introducing trace amounts of water into metal oxide based CVD constitutes a possibility to increase the volatilization of the metal oxide at a constant growth temperature, which is an important parameter for optimizing the growth conditions. Further, we speculate that use of water as a transport agent may not be limited to the oxides of W and Mo; oxides of V, Mn, Cr, Ni, Zr, and Sr are also candidates for water-assisted CVD growth [55].…”

Section: Resultsmentioning

confidence: 99%

The important role of water in growth of monolayer transition metal dichalcogenides

et al. 2017

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The important role of water in growth of monolayer transition metal dichalcogenides Interest in transition metal dichalcogenides (TMDs) has been renewed by the discovery of emergent properties when reduced to single, two-dimensional (2D) layers. The transition to direct band gap [1,2], emerging charge density waves [3,4], high mobility [5][6][7], and valley polarization [8][9][10] are some of the many exciting properties that have been reported in the TMD literature recently. A major bottleneck to this research is the lack of reproducible and large scale synthetic methods for high quality, consistent monolayer TMD samples. The dominant growth method is the vaporization and subsequent chalcogenization of solid metal oxides in the presence of gaseous chalcogen precursors. This process is commonly referred to as chemical vapor deposition (CVD) or powder vaporization [11][12][13][14]. Due to its simplicity, CVD is extensively used by the TMD community to produce high quality, micron-sized single crystals [11][12][13][15][16][17][18][19]. Understanding the vaporization chemistry of solid transition metal precursors and vapor transport of volatilized precursors, particularly with respect to the influence of water vapor, is critical. Humidity, i.e. water content of the reaction environment, is an important parameter in the gas phase synthesis of inorganic materials, and while it is typically thought of as a contaminant, water is also an effective transport agent [20][21][22][23]. In this communication, we describe the synthesis of luminescent monolayer TMD islands by introducing water vapor as a simple means of controlling the volatilization and transport of the metal oxide precursor. Our experiments demonstrate a direct correlation between gas phase water content and the morphology of the resulting films. In particular, explicit control of the in situ water vapor concentration allows us to switch between two modes of growth: one in an effectively dry environment, in which the transition metal oxide source is converted directly to TMD material through a solid state reaction with the chalcogen source, and another in which the transition metal oxide undergoes vapor transport followed by reaction with the chalcogen source. We show that a small amount of water enhances the volatilization, and hence vapor transport, of the oxides of tungsten and molybdenum at the elevated temperatures (500-800 °C) used in the conversion or growth of their TMD counterparts. We attribute this effect to the enhanced vaporization of WO 3 and MoO 3 in the presence of water, first demonstrated in the 1930s and Our results show a direct correlation between gas phase water content and the morphology of TMD films. In particular, we show that the presence of water enhances volatilization, and therefore the vapor transport of tungsten and molybdenum oxide. Surprisingly, we find that water not only plays an important role in volatilization but is also compatible with TMD growth. In fact, carefully controlled humidity can consistently produce high qual...

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Section: Resultsmentioning

confidence: 99%

The important role of water in growth of monolayer transition metal dichalcogenides

et al. 2017

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“…It is noteworthy, that those cracks are filled easily by the borosilicate glass layer resulting in a self-healing effect. Since the exposure of borosilica to high temperature is known to promote boron loss [28,29] EPMA measurements were conducted to determine the boron content in the coating on a Mo sample (without Ti) after pack cementation with a Si/B ratio of 35/1 and conditioning at 1400 °C for 25 h. This is a longer conditioning treatment than that used for the Mo-Si-B-Ti alloys. The results are given in Table 3 and clearly reveal that there is retention of boron at a level close to that in the initial pack content.…”

Section: Microstructure and Layer Arrangement After Conditioningmentioning

confidence: 99%

Enhanced Oxidation Resistance of Mo–Si–B–Ti Alloys by Pack Cementation

et al. 2017

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Abstract.As new high temperature structural materials, Mo-Si-B alloys satisfy several requirements such as oxidation and creep resistance. Recently, novel Ti-rich Mo-Si-B alloys have shown an increased creep resistance compared to Ti-free alloys. However, due to the formation of a duplex SiO 2 -TiO 2 oxide layer, which allows for fast ingress of oxygen, the oxidation resistance is poor. To improve the oxidation resistance a borosilicate based coating was applied to a Mo-12.5Si-8.5B-27.5Ti (in at.%) alloy. After co-deposition of Si and B by pack cementation at 1000 °C in Ar, a conditioning anneal at 1400 °C is used to develop an outer borosilicate layer followed by an inner MoSi 2 and Mo 5 Si 3 layer. During both isothermal and cyclic oxidation after an initial mass loss during the first hours of exposure, a steady state is reached for times up to 1000 h at temperatures ranging from 800 to 1200 °C demonstrating a significantly enhanced oxidation resistance.

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“…Water vapor increased the loss, presumably by forming more volatile hydroxides, e.g., MoO2(OH)2, by reacting with MoO3 vapor [28,29]. Ni(Mo,W)O4 scales are occasionally observed for conventional superalloys [2,24].…”

Section: Effects Of High Mo Content On Oxidationmentioning

confidence: 99%

Cyclic Oxidation of High Mo, Reduced Density Superalloys

Smialek

Garg

Gabb

et al. 2015

Metals

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Cyclic oxidation was characterized as part of a statistically designed, 12-alloy compositional study of 2nd generation single crystal superalloys as part of a broader study to co-optimize density, creep strength, and cyclic oxidation. The primary modification was a replacement of 5 wt. % W by 7% or 12% Mo for density reductions of 2%-7%. Compositions at two levels of Mo, Cr, Co, and Re were produced, along with a midpoint composition. Initially, polycrystalline vacuum induction samples were screened in 1100 °C cyclic furnace tests using 1 h cycles for 200 h. The behavior was primarily delimited by Cr content, producing final weight changes of −40 mg/cm 2 to −10 mg/cm 2 for 0% Cr alloys and −2 mg/cm 2 to +1 mg/cm 2 for 5% Cr alloys. Accordingly, a multiple linear regression fit yielded an equation showing a strong positive Cr effect and lesser negative effects of Co and Mo. The results for 5% Cr alloys compare well to −1 mg/cm 2 , and +0.5 mg/cm 2 for Rene′ N4 and Rene′ N5 (or Rene′ N6), respectively. Scale phases commonly identified were Al2O3, NiAl2O4, NiTa2O6, and NiO, with (Ni,Co)MoO4 found only on the least resistant alloys having 0% Cr and 12% Mo. Scale microstructures were complex and reflected variations in the regional spallation history. Large faceted NiO grains and fine NiTa2O6 particles distributed along NiAl2O4 grain boundaries were typical distinctive features. NiMoO4 formation, decomposition, and volatility occurred for a few high Mo compositions. A creep, density, phase stability, and oxidation balanced 5% Cr, 10% Co, 7% Mo, and 3% Re alloy was selected to be taken forward for more extensive evaluations in single crystal form. OPEN ACCESSMetals 2015, 5 2166

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Water Vapor–Mediated Volatilization of High-Temperature Materials

Cited by 120 publications

References 107 publications

The important role of water in growth of monolayer transition metal dichalcogenides

The important role of water in growth of monolayer transition metal dichalcogenides

Enhanced Oxidation Resistance of Mo–Si–B–Ti Alloys by Pack Cementation

Cyclic Oxidation of High Mo, Reduced Density Superalloys

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