Subsequent Volatilization Research Articles

Controlling Selenization Equilibrium Enables High-Quality Kesterite Absorbers for Efficient Solar Cells

Kesterite Cu2ZnSn(S, Se)4 is considered one of the most competitive photovoltaic materials due to its earth-abundant and nontoxic constituent elements, environmental friendliness, and high stability. However, the preparation of high-quality Kesterite absorbers for photovoltaics is still challenging for the uncontrollability and complexity of selenization reactions between metal element precursors and selenium. In this study, we propose a solid-liquid/solid-gas (solid precursor and liquid/vapor Se) synergistic reaction strategy to precisely control the selenization process. By pre-depositing excess liquid selenium, we provide the high chemical potential of selenium to facilitate the direct and rapid formation of the Kesterite phase. The further optimization of selenium condensation and subsequent volatilization enables the efficient removal of organic compounds and thus improves charge transport in the absorber film. As a result, we achieve high-performance Kesterite solar cells with total-area efficiency of 13.6% (certified at 13.44%) and 1.09 cm2-area efficiency of 12.0% (certified at 12.1%).

Seasonal trends and potential sources of aliphatic amines in the aerosols and gas phase at a forested site on the northern foot of Mt. Fuji, Japan

The present study measured the concentrations of aliphatic amines (methylamine (MA), ethylamine (EA), dimethylamine (DMA), diethylamine (DEA), trimethylamine (TMA), and triethylamine (TEA)) in both the aerosols and gas phase at a forested site in Japan, and discussed their seasonal trends and potential sources. These measured amines were more preferentially partitioned into the aerosols than into the gas phase, and mainly distributed into the fine-mode range in the aerosols, although the DMA and TMA were also abundant in the gas phase and the TMA in the aerosols was mainly distributed into the coarse-mode range. The most abundant amine both in the aerosol and gas phase was the DMA, followed by the MA. The MA, EA, DMA, and DEA in the gas phase and aerosols showed higher concentrations in the summer, although the coarse-mode EA concentration did not show the seasonality. The gaseous TMA also showed higher concentrations in the summer, but the coarse-mode TMA increased in the autumn and no seasonality was found in the fine-mode TMA concentrations. The TEA in the gas phase showed higher concentrations in the autumn, but that in the aerosols did not show the seasonality. The decomposition of organic materials in the soil and subsequent volatilization would be an important source of the gaseous MA, DMA, DEA, and TMA. Various biogenic activities can be also expected to be potential sources of the gaseous MA and DMA. The fine-mode MA, EA, DMA, DEA, TMA, and the coarse-mode DMA would be derived from their gaseous form through direct dissolution. Some portions of the fine-mode MA, DMA, and the coarse-mode DMA can be expected to be derived from the primary sources such as biogenic particles like plant debris. The transport of the anthropogenic aerosols can be also expected to contribute to the fine-mode MA. Only the TEA in the gas phase was suggested to be derived from the different sources in the forest from the other five gaseous amines and not contribute to the formation of the fine-mode TEA. It can be considered that the coarse-mode MA, EA, DEA, TMA, TEA, and the fine-mode TEA were affected by the sources independent from the local emission of the gaseous amines, such as the primary sources or the transport from distant sources, although some portions of the coarse-mode MA, EA and DEA might be derived from the dissolution of their gaseous form. Biogenic and pedospheric processes in the forest would be important sources for the amines, especially the gaseous and fine-mode amines, in a forest atmosphere.

Insights into internal oxidation of SiC/BN/SiC composites

AbstractThe article presents new observations of the physical manifestations of internal oxidation and volatilization in SiC/BN/SiC composites. The observations are made on both unbroken and broken minicomposite specimens before and after 12 h exposures at 1000°C in dry air with 10 ppm water vapor. The observations are enabled by a sample preparation method involving ion‐mill sectioning and polishing. Complementary analyses of volatilization and closure of resulting gaps are also presented. The observations show that BN is generally consumed in two stages: (i) through reaction with oxygen along the interfaces with both the fiber and the matrix, producing two concentric annular pockets of borosilicate glass and an intervening annulus of progressively thinning BN; and (ii) subsequent volatilization, through the reaction of boria with trace amounts of water vapor in the environment to form borohydroxide gases. The spatial extent to which these processes proceed is governed by a competition between the outward diffusion of reaction gases through both matrix cracks and interface gaps produced by boria volatilization, and the formation of oxides on the newly exposed surfaces of fibers, matrix, and coating.

Inhibition of methanogenesis leads to accumulation of methylated arsenic species and enhances arsenic volatilization from rice paddy soil

Flooded soils are important environments for the biomethylation and subsequent volatilization of arsenic (As), a contaminant of global concern. Conversion of inorganic to methylated oxyarsenic species is thought to be the rate-limiting step in the production and emission of volatile (methyl)arsines. While methanogens and sulfate-reducing bacteria (SRB) have been identified as important regulators of methylated oxyarsenic concentrations in anaerobic soils, the effects of these microbial groups on biovolatilization remain unclear. Here, microcosm and batch incubation experiments with an Arkansas, USA, rice paddy soil were performed in conjunction with metabolic inhibition to test the effects of methanogenic activity on As speciation and biovolatilization. Inhibition of methanogenesis with 2-bromoethanesulfonate (BES) led to the accumulation of methylated oxyarsenic species, primarily dimethylarsinic acid (DMAs(V)), and a four-fold increase in As biovolatilization compared to a control soil. Our results support a conceptual model that methanogenic activity suppresses biovolatilization by enhancing As demethylation rates. This work refines understanding of biogeochemical processes regulating As biovolatilization in anaerobic soil environments, and extends recent insights into links between methanogenesis and As metabolism to soils from the mid-South United States rice production region.

An environmentally-friendly process for recovering, solidifying As and preparing bronze material from tin-bearing middling

Tin-bearing middling, a hazardous solid waste produced during beneficiation of tin deposits and tin-bearing tailings, presents a serious environmental pollution risk due to high levels of As. This paper aims to explore an environmentally friendly technology to reduce toxic substances and prepare a high value-added bronze material from tin-bearing middling. The co-process for tin-bearing middling, anthracite, and copper is first proposed at 1073–1123 K. Variables like temperature, amount of added anthracite, and holding time were optimized. Those results showed that 95.56% Sn and 92.79% As were recovered; levels of Fe in the residue reached 73.52 wt%. During roasting, SnO2 in the tin-bearing middling was initially transformed into SnO(g), then into an Fe–Sn spinel (Fe3-xSnxO4) and Ca2SnO4, and finally a Cu–Sn alloy. Generation of the Fe–Sn spinel came primarily from substitution of Fe2+ by Sn2+, but small amounts of Sn4+ replaced Fe3+ in Fe3O4. Meanwhile, FeAsS and FeAsO4 were initially transformed into As4(g) and then solidified as a Cu–As alloy. Interestingly, the residual As existed as Fe–As alloy, but As leaching levels did not exceed 0.02 mg/L. After initial treatment, the Cu alloy can be prepared for bronze. Meanwhile, subsequent volatilization and solidification of As resulted in a significant reduction of arsenic-bearing waste that minimizes environmental risk.

ArsM-mediated arsenite volatilization is limited by efflux catalyzed by As efflux transporters

Arsenic (As) methylation is regarded as an efficient strategy for As contamination remediation by As volatilization. However, most microorganisms display low As volatilization efficiency, which is possibly linked to As efflux transporters competing for cytoplasmic As(III) as a substrate. Here, we developed two types of As biosensors in Escherichia coli to compare the As efflux rate of three efflux transporters and to further investigate the correlation between As efflux rates and As volatilization. The engineered As-sensitive E. coli AW3110 expressing arsBRP, acr3RP or arsBEC displayed a higher As resistance compared to the control. The fluorescence intensity was in a linear correlation in the range of 0–2.0 μmol/L of As(III). The intracellular As(III) concentration was negatively related to As efflux activity of As efflux transporter, which was consistent with the As resistance assays. Moreover, arsM derived from R. palustris CGA009 was subsequently introduced to construct an E. coli AW3110 co-expressing arsB/acr3 and arsM, which exhibited higher As(III) resistance, lower fluorescence intensity and intracellular As concentration compared to the engineered E. coli AW3110 expressing only arsB/acr3. The As volatilization efficiency was negatively related to As efflux activity of efflux transporters, the recombinants without arsB/acr3 displayed the highest rate of As volatilization. This study provided new insights into parameters affecting As volatilization with As efflux being the main limiting factor for As methylation and subsequent volatilization in many microorganisms.

Production of Secondary Organic Aerosol During Aging of Biomass Burning Smoke From Fresh Fuels and Its Relationship to VOC Precursors

AbstractAfter smoke from burning biomass is emitted into the atmosphere, chemical and physical processes change the composition and amount of organic aerosol present in the aged, diluted plume. During the fourth Fire Lab at Missoula Experiment, we performed smog‐chamber experiments to investigate formation of secondary organic aerosol (SOA) and multiphase oxidation of primary organic aerosol (POA). We simulated atmospheric aging of diluted smoke from a variety of biomass fuels while measuring particle composition using high‐resolution aerosol mass spectrometry. We quantified SOA formation using a tracer ion for low‐volatility POA as a reference standard (akin to a naturally occurring internal standard). These smoke aging experiments revealed variable organic aerosol (OA) enhancements, even for smoke from similar fuels and aging mechanisms. This variable OA enhancement correlated well with measured differences in the amounts of emitted volatile organic compounds (VOCs) that could subsequently be oxidized to form SOA. For some aging experiments, we were able to predict the SOA production to within a factor of 2 using a fuel‐specific VOC emission inventory that was scaled by burn‐specific toluene measurements. For fires of coniferous fuels that were dominated by needle burning, volatile biogenic compounds were the dominant precursor class. For wiregrass fires, furans were the dominant SOA precursors. We used a POA tracer ion to calculate the amount of mass lost due to gas‐phase oxidation and subsequent volatilization of semivolatile POA. Less than 5% of the POA mass was lost via multiphase oxidation‐driven evaporation during up to 2 hr of equivalent atmospheric oxidation.

Multi-channel-contained few-layered MoSe2 nanosheet/N-doped carbon hybrid nanofibers prepared using diethylenetriamine as anodes for high-performance sodium-ion batteries

A facile new strategy for the synthesis of multi-channel-contained N-doped carbon nanofibers composed of few-layered MoSe2 nanosheets (denoted as MC-NCNF/MoSe2) was introduced and the composite was demonstrated as an anode material for sodium-ion batteries. This was the first time that diethylenetriamine was introduced as a pore generator in the electrospinning process and played a key role in generating multi-channels in the structure by phase-separation from the molybdenum salt and subsequent volatilization without any additional process. Polyvinylpyrrolidone was used as a carbon precursor and played the role of a N-doping source for the carbon matrix. MC-NCNF/MoSe2 achieved a high reversible discharge capacity of 386 mA h g−1 at a current density of 0.5 A g−1 after the 300th cycle and superior rate capability of 285 mA h g-1 at 10.0 A g−1. The multi-channeled structure of MC-NCNF/MoSe2 facilitated effective Na+ and electron diffusion during repeated discharge/charge processes and accommodated the huge volume expansion of the MoSe2 nanosheets induced by electrochemical reaction of the Na+ ion.

Timing and Appearance of Postmortem Root Banding in Nonhuman Mammals.

A study was undertaken using nonhuman mammal specimens to better understand environmental influences on postmortem hair root band (PMRB) formation and to see whether PMRBs would occur in nonhuman mammal hairs in a similar fashion to human hairs. Carcasses from surrounding roadways were the primary source of specimens for this study, augmented by donated deceased domestic pets. Sections of pelt from each specimen were placed in controlled environmental conditions while the remainder of the carcass was left in a secure outdoor setting. Hair samples were collected daily from outdoor and control specimens and examined for evidence of PMRBs. Several environmental factors were also recorded on a daily basis. Results demonstrate PMRBs can occur in nonhuman mammal hairs, and they have microscopic characteristics similar to human PMRBs. Factors found to correlate with PMRB formation include postmortem interval, temperature, pH, and the formation and subsequent volatilization of ammonia from the surrounding tissue.

Microbial mercury methylation in Antarctic sea ice.

Atmospheric deposition of mercury onto sea ice and circumpolar sea water provides mercury for microbial methylation, and contributes to the bioaccumulation of the potent neurotoxin methylmercury in the marine food web. Little is known about the abiotic and biotic controls on microbial mercury methylation in polar marine systems. However, mercury methylation is known to occur alongside photochemical and microbial mercury reduction and subsequent volatilization. Here, we combine mercury speciation measurements of total and methylated mercury with metagenomic analysis of whole-community microbial DNA from Antarctic snow, brine, sea ice and sea water to elucidate potential microbially mediated mercury methylation and volatilization pathways in polar marine environments. Our results identify the marine microaerophilic bacterium Nitrospina as a potential mercury methylator within sea ice. Anaerobic bacteria known to methylate mercury were notably absent from sea-ice metagenomes. We propose that Antarctic sea ice can harbour a microbial source of methylmercury in the Southern Ocean.

The natural chlorine cycle – Formation of the carcinogenic and greenhouse gas compound chloroform in drinking water reservoirs

Chlorine cycle in natural ecosystems involves formation of low and high molecular weight organic compounds of living organisms, soil organic matter and atmospherically deposited chloride. Chloroform (CHCl3) and adsorbable organohalogens (AOX) are part of the chlorine cycle. We attempted to characterize the dynamical changes in the levels of total organic carbon (TOC), AOX, chlorine and CHCl3 in a drinking water reservoir and in its tributaries, mainly at its spring, and attempt to relate the presence of AOX and CHCl3 with meteorological, chemical or biological factors.Water temperature and pH influence the formation and accumulation of CHCl3 and affect the conditions for biological processes, which are demonstrated by the correlation between CHCl3 and ΣAOX/Cl− ratio, and also by CHCl3/ΣAOX, CHCl3/AOXLMW, CHCl3/ΣTOC, CHCl3/TOCLMW and CHCl3/Cl− ratios in different microecosystems (e.g. old spruce forest, stagnant acidic water, humid and warm conditions with high biological activity). These processes start with the biotransformation of AOX from TOC, continue via degradation of AOX to smaller molecules and further chlorination, and finish with the formation of small chlorinated molecules, and their subsequent volatilization and mineralization. The determined concentrations of chloroform result from a dynamic equilibrium between its formation and degradation in the water; in the Hamry water reservoir, this results in a total amount of 0.1–0.7 kg chloroform and 5.2–15.4 t chloride. The formation of chloroform is affected by Cl− concentration, by concentrations and ratios of biogenic substrates (TOC and AOX), and by the ratios of the substrates and the product (feedback control by chloroform itself).

RECOVERY OF METALS FROM ZINC-CARBON AND ALKALINE SPENT BATTERIES BY USING AUTOMOTIVE SHREDDER RESIDUES

ABSTRACT The main aim of this work is the thermal recovery of manganese and zinc from a mixture of zinc-carbon and alkaline spent batteries containing 40.9% of Mn and 30.1% of Zn after a pre-liminary physical treatment followed by the removal of mercury. Separation of the metals is car-ried out on the basis of their different phase change temperatures, in fact the boiling point of mercury and zinc are 357 °C and 906 °C, respectively, and the melting point of Mn 3 O 4 , the main Mn-bearing phase in the mixture, is 1564 °C. After wet comminution and sieving to remove the anodic collectors and most of the chlorides contained in the mixture, chemical and X-ray powder diffraction (XRPD) analyses were per-formed. The mixture was initially heated in an air flow at temperatures ranging from 300 °C to 400 °C to eliminate mercury, then, the flow of air was turned into an inert carbon dioxide at-mosphere. The volatilization of metallic zinc starts at a temperature 850 °C. At higher temperature the reduction of zinc oxide and subsequent volatilization of metallic zinc is carried out by the carbon present in the original mixture or in the automotive shredder residue (car fluff) added from the outside. By optimization of temperature, stoichiometric ratio and residence time, a product suit-able for production of new batteries after refining was obtained. The treatment residue consist-ed of manganese and iron oxides that could be used to produce manganese-iron alloys.

Microbial- and thiosulfate-mediated dissolution of mercury sulfide minerals and transformation to gaseous mercury.

Mercury (Hg) is a toxic heavy metal that poses significant environmental and human health risks. Soils and sediments, where Hg can exist as the Hg sulfide mineral metacinnabar (β-HgS), represent major Hg reservoirs in aquatic environments. Metacinnabar has historically been considered a sink for Hg in all but severely acidic environments, and thus disregarded as a potential source of Hg back to aqueous or gaseous pools. Here, we conducted a combination of field and laboratory incubations to identify the potential for metacinnabar as a source of dissolved Hg within near neutral pH environments and the underpinning (a)biotic mechanisms at play. We show that the abundant and widespread sulfur-oxidizing bacteria of the genus Thiobacillus extensively colonized metacinnabar chips incubated within aerobic, near neutral pH creek sediments. Laboratory incubations of axenic Thiobacillus thioparus cultures led to the release of metacinnabar-hosted Hg(II) and subsequent volatilization to Hg(0). This dissolution and volatilization was greatly enhanced in the presence of thiosulfate, which served a dual role by enhancing HgS dissolution through Hg complexation and providing an additional metabolic substrate for Thiobacillus. These findings reveal a new coupled abiotic-biotic pathway for the transformation of metacinnabar-bound Hg(II) to Hg(0), while expanding the sulfide substrates available for neutrophilic chemosynthetic bacteria to Hg-laden sulfides. They also point to mineral-hosted Hg as an underappreciated source of gaseous elemental Hg to the environment.

Identification and characterization of arsenite methyltransferase from an archaeon, Methanosarcina acetivorans C2A.

Arsenic is a ubiquitous toxic contaminant in the environment. The methylation of arsenic can affect its toxicity and is primarily mediated by biological processes. Few studies have focused on the mechanism of arsenic methylation in archaea although archaea are widespread in the environment. Here, an arsenite [As(III)] methyltransferase (ArsM) was identified and characterized from an archaeon Methanosarcina acetivorans C2A. Heterologous expression of MaarsM was shown to confer As(III) resistance to an arsenic-sensitive strain of E. coli through arsenic methylation and subsequent volatilization. Purified MaArsM protein was further identified the function in catalyzing the formation of various methylated products from As(III) in vitro. Methylation of As(III) by MaArsM is highly dependent on the characteristics of the thiol cofactors used, with some of them (coenzyme M, homocysteine, and dithiothreitol) more efficient than GSH. Site-directed mutagenesis demonstrated that three conserved cysteine (Cys) residues (Cys62, Cys150, and Cys200) in MaArsM were necessary for As(III) methylation, of which only Cys150 and Cys200 were required for the methylation of monomethylarsenic. These results present a molecular pathway for arsenic methylation in archaea and provide some insight into the role of archaea in As biogeochemistry.

Silicon Carbide Oxidation in Steam up to 2 MPa

Growth and microstructure of a protective or nonprotective SiO2 scale and the subsequent volatilization of scale formed on high‐purity chemical vapor deposited (CVD) SiC and nuclear‐grade SiC/SiC composites have been studied during high‐temperature 100% steam exposure. The environmental parameters of interest were temperature from 1200°C to 1700°C, pressure of 0.1 to 2 MPa and flow velocities of 0.23 to 145 cm/s. Scale microstructure was characterized via electron microscopy and X‐ray diffractometry. The Arrhenius dependence of the parabolic oxidation and linear volatilization rate constants were determined. The linear volatilization rate exhibited a strong dependence on steam partial pressure with a weaker dependence on flow velocity. At high steam pressures, the oxide scale developed substantial porosity, which significantly accelerated material recession. The dominant oxide phase for the conditions studied was cristobalite. The oxidation behavior of SiC/SiC composite was strongly dependent on the state of the surface, specifically whether steam could find easy entry into the material via surface‐exposed interface layers. For the case where these as‐machined interfaces were surface coated with matrix CVD SiC, composite recession was found to be essentially that of high‐purity CVD SiC.

Study of XeF2 fluorination potential against Rh2O3, RuO2, ZrO2, and U3O8 for use in reactive gas recycle of used nuclear fuel

Triuranium octoxide (U3O8) and stable surrogate oxides of selected fission product species representative of a Used Nuclear Fuel (UNF) matrix typical of Light Water Reactor (LWR) service were fluorinated using an alternate, solid-phase fluorinating agent, XeF2. This fluorination reaction formed volatile and non-volatile compounds and demonstrated the possibility of chemical and thermal separations of the UNF matrix based on this approach. A series of experiments was conducted at the milligram quantity scale using a Shimadzu DTG-60 TG/DTA for testing of all non-radioactive samples and a Netzsch STA 409 TGA for testing of all radioactive samples. The fluorination and subsequent volatilization potentials were analyzed by mixing excess fluorinating agent with an oxide form of the UNF material and then heating to elevated temperatures for analysis. Thermogravimetric and differential thermal analysis allowed reaction pathways to be analyzed and suggested windows both thermally and chemically for separations of these various components. The differences in thermophysical properties of these products can be utilized as a starting point to effectively separate, isolate, and collect product streams with different compositions for further processing. The study of these chemistries could be incorporated into advanced separations methods to provide another possible solution for the long-term sustainability of nuclear energy applications.

Natural wetland emissions of methylated trace elements

Natural wetlands are well known for their significant methane emissions. However, trace element emissions via biomethylation and subsequent volatilization from pristine wetlands are virtually unstudied, even though wetlands constitute large reservoirs for trace elements. Here we show that the average volatile fluxes of selenium (<0.12 μg m(-2) day(-1)), sulphur (<37 μg m(-2) day(-1)) and arsenic (<0.54 μg m(-2) day(-1)) from a pristine peatland are considerable and consistent over two summers. We compare these fluxes with the total concentrations in the peat and show that selenium is up to 40 times more efficiently volatilized than arsenic, and over 100 times more efficiently volatilized than sulphur. We further show that the volatilization of selenium and arsenic increases with temperature, implying that emissions of these health-relevant trace elements will increase with global warming. We suggest that biomethylation and volatilization in wetlands play a crucial role in the mobilization and global biogeochemical cycling of trace elements.

Mercury detoxification using genetic engineered Nicotiana tabacum

Phytoremediation is a low cost alternative solution to soil contamination compared with traditional removal and/or disposal techniques. One of the phytoremediation technologies is the phytovolatilization, whereby the contaminant is not primarily accumulated in above-ground tissues, but is instead transformed by the plant into the atmosphere. The detoxification of highly toxic organmercurial compounds and subsequent volatilization of elemental mercury is a unique example for the successful phytoremediation based on genetic engineering approach. For mercury removal techniques we constructed a dicistronic construct containing the bacterial mercury detoxification genes merA and merB under the control of the Arabidopsis Actin2 promoter and terminator. The resulted construct was introduced to Agrobacterium competent cells using heat shock transformation method. In the mean time, we germinated tobacco (Nicotiana tabacum var. Gold leaf) seeds on MS media and infected leaf discs of 6-8 weeks tobacco seedlings with Agrobacterium containing Ti plasmid harboring merA/merB dicistronic construct. The results showed that about 90% of tobacco seedlings were carrying the mer genes. Tobacco seeds were collected from wild type and transgenic lines and tested for mercury resistance. The results showed that transgenic plants are resistant to both Phenyl Mercuric Acetate (PMA) and HgCl2. The root length and dry weight of wild and transgenic seedlings growing on both media amended with mercury compounds and media without mercury (control) were scored. The results showed that the root lengths and dry weight of the transgenic lines are significantly higher by 60 and 17-folds, respectively, compared to wild type. The results showed clear evidence that the transgenic plants are resistant to both organic and inorganic mercury compounds and can be used to clean up mercury contaminated sites.

Hydride generation – in-atomizer collection of Pb in quartz tube atomizers for atomic absorption spectrometry – a 212Pb radiotracer study

Lead preconcentration on the quartz surface after plumbane generation was extensively studied in the separate quartz trap or in the compact trap-and-atomizer device by means of a laboratory prepared 212Pb radioactive indicator. Plumbane generation efficiency of 95% was found. A large surface area is required for sufficient analyte trapping as found by autoradiography. The lead trapping efficiency in the separate trap is around 85% at 300 °C whereas 100% trapping is reached in the trap-and-atomizer device at the same temperature due to its greater inner surface. Volatilization of the trapped lead species at 800 °C is efficient for both the separate quartz trap and the trap-and-atomizer device. The non-volatilized lead fraction was below 5% in the trap-and-atomizer device at 800 °C. The non-volatilized lead fraction increases substantially in the presence of Bi. This was proved to be the mechanism of the serious Bi interference. Although 1–10 ng of analyte is typically preconcentrated in ultratrace analysis the quartz trap is capable of efficient trapping and subsequent volatilization of up to 200 ng Pb.

Degradation of oxide fibers by thermal overload and environmental effects

High strength (2–3GPa) of polycrystalline oxide fibers used as reinforcements of ceramic bodies is achieved as a result of submicron grain size. The fine-grained microstructure, however, is not stable at high temperatures and hence grain coarsening must be considered. Coarsening of alumina in commercial 3M Nextel fibers occurs above 1200°C going along with significant strength reduction. Analysis of isothermal grain growth reveals relative little time dependency which is attributed to limited grain boundary mobility. Activation energy of alumina grain growth is 660kJ/mol. Moderate grain boundary mobility of Nextel 650 and 720 fibers is attributed to second phase blocking while in case of Nextel 610 alumina fibers sub-nanometer silica grain boundary films are assumed to affect the grain boundary mobility. Pronounced grain growth occurs in Nextel 610 fibers as soon as stabilizing silica films dissipate either by outwards diffusion or by hydroxylation and subsequent volatilization in the presence of hot water vapour. Mullite grains are less prone against coarsening than alumina as evidenced in activation energy (900kJ/mol) and very low grain boundary mobility at temperatures below 1600°C.