Consolidation of Tin Sulfide Chalcogels and Xerogels with and without

Nov 5, 2015 - Tin sulfide (Sn2S3) chalcogels are one of the most effective nonoxide aerogels evaluated to date for iodine gas capture. This is attribu...
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Consolidation of Tin Sulfide Chalcogels and Xerogels with and without Adsorbed Iodine Brian J. Riley,*,† David A. Pierce,† William C. Lepry,† Jared O. Kroll,† Jaehun Chun,† Kota Surya Subrahmanyam,‡ Mercouri G. Kanatzidis,‡ Fares Khamis Alblouwy,§ Aneeruddha Bulbule,§ and Edward M. Sabolsky§ †

Pacific Northwest National Laboratory, Richland, Washington 99352, United States Northwestern University, Evanston, Illinois 60208, United States § West Virginia University, Morgantown, West Virginia 26506, United States ‡

S Supporting Information *

ABSTRACT: Tin sulfide (Sn2S3) chalcogels are one of the most effective nonoxide aerogels evaluated to date for iodine gas capture. This is attributed to the fact that the Sn within the gel network has a strong affinity for chemisorption of iodine to form SnI4. This study demonstrates an approach for consolidating the raw and iodine-sorbed Sn2S3 chalcogels into a chalcogenide glass using GeS2 as a glass-forming additive. Adding GeS2 to iodine-sorbed or iodine-free Sn2S3 chalcogels provides better glass formation than Sn−S or Sn−S−I alone, and the quantity of iodine measured in the bulk glass of the consolidated iodine-sorbed Sn2S3 chalcogel was at ∼45 mass%. Additional experiments were conducted using microwave sintering and hot isostatic pressing with iodine-sorbed Sn2S3 xerogels to evaluate alternative consolidation techniques. sented by Subrahmanyam et al.20 Since Zn−Sn−S chalcogels tend to change from an amorphous aerogel to a crystalline material when heated,12 the Sn−S chalcogels were expected to crystallize as well. Therefore, the Sn−S system was predicted to require a glass-forming additive in order to obtain a glassy product such as Ge or Ge−S, which was previously demonstrated by Ruffolo and Boolchand.31 Ruffolo and Boolchand31 presented the glass transition temperature (Tg) and crystallization temperature (Tc) for the Sn2S3−Ge2S3 binary system (Figure 1). The value of (Tc − Tg) is known to be a rough estimation of glass formability within a particular system where a larger difference allows for slower quench rates to prevent crystallization upon cooling.32 According to Figure 1, the value of Tc − Tg for SnGeS3 was 69 °C but it was 134 °C for Ge2S3. Thus, an additional component such as GeS2 would improve glass formation in the Sn−S system by increasing the value of Tc − Tg. Another option is to synthesize a Sn−Ge−S chalcogel with the variety of Ge−S and Sn−S chalcogel precursors discovered to date that include GeS44−, Ge4S104−, SnS44−, Sn2S64−, and Sn4S104−.33−37 Additionally, glass formation has been demonstrated in the Sb−Sn−S system with iodine38 and similar compositions have been made into chalcogels10 showing promise as a link between the chalcogel chemistry and a known glass-forming system. Work summarizing the consolidation of iodine-sorbed Zn2Sn2S6 chalcogels using Sb2S3 as a glass former was published recently.20 This paper demonstrates an approach for consolidating both iodine-free and iodine-sorbed Sn2S3 chalcogels

1. INTRODUCTION Chalcogels are aerogels that can be made from a variety of chalcogenide building blocks.1−15 These materials have a s el e c t i v e a ffi n i t y f o r v a r i o u s h e a v y m e t a ls a n d gases.3,5,7,10,12,15−17 A summary of the more commonly studied chalcogel chemistries is provided in Table 1.18 Recent work has shown that chalcogels can capture large quantities of iodine at >2 g g−1 of the starting sorbent.5,17,19−21 These sorbents are under development as an alternative to metal−organic frameworks22,23 and the more widely studied silver-exchanged zeolite (AgZ)24,25 for capturing radioiodine (129I and 131I) that is generated during nuclear fission of uranium fuel and released during used fuel reprocessing. However, once the iodine is captured within the chalcogel matrix, the fate of the iodine is uncertain considering that the chalcogel is not a viable waste form by itself due to its high porosity and low density. A feasible option would be to melt this material into a chalcogenide glass since this particular family of glasses can host more iodine than any other type of known glass.26−30 Some difficulties can arise when processing chalcogenide glasses due to the inherent volatility and oxygen sensitivity of the glass constituents, which poses challenges for adapting such a technology for remediation applications. However, chalcogenide glass formation is compositionally flexible so additional chalcogenide compounds can be added as glass formers to the iodine-sorbed chalcogels. This can significantly improve glass formation and aid in adjusting the properties of the mixture by altering the final chemical durability, thermal stability, and mechanical properties. From a glass formation standpoint, two of the most promising chalcogel chemistries are Sn−S and Sn−Se formulations that include Ge or Sb (current work as well as Ruffolo and Boolchand31) or Sb−Sn−S compositions pre© 2015 American Chemical Society

Received: Revised: Accepted: Published: 11259

July 23, 2015 October 19, 2015 October 28, 2015 November 5, 2015 DOI: 10.1021/acs.iecr.5b02697 Ind. Eng. Chem. Res. 2015, 54, 11259−11267

Article

Industrial & Engineering Chemistry Research Table 1. Summary of Different Types of Commonly Studied Chalcogel Chemistries

a

chemistry (family)

methoda

chalcogenide cluster(s)

interlinking metal(s), M

reference(s)

(Cd,Zn,Pb)-(S,Se) Ge-S Pt(Ge,Sn)(S,Se) (Mo,W)-M-S (K,Na)(Sb,As)-Fe-(S,Te) (Sn,Sb)-M-(S,Se) Zn-Sn-S Fe-Sn-S Fe-M-Sn-S Mo-Co-M-S

AN AN CL CL CL CL CL CL CL CL

N/A N/A Ge4(S,Se)104−, Sn4Se104−, Sn2Se64−, Sn(S,Se)44− (Mo,W)S42− AsS33−, SbS33−, SbTe33− Sn2(S,Se)64−, Sn(S,Se)44−, SbSe43− SnS44−, Sn2S64−, Sn4S104− Fe4S4m−, Sn2S64− Fe4S4m−, Sn2S64− MoS42−

N/A N/A Pt2+ Co2+, Ni2+ Fe2+, K+, Na+ Sn2+, Sb3+ Zn2+ Fe4S4m−, Sn2S64− Zn2+, Sn2+, Ni2+, Co2+ Co2+, Pb2+, Cd2+, Pd2+, Cr3+, Bi3+

Mohanan et al.1 Kalebaila et al.2 Bag et al.,3,4 Riley et al.5 Shafaei-Fallah et al.,6 Bag et al.7 Ahmed et al.8,9 Bag and Kanatzidis,10 Riley et al.11 Oh et al.12 Yuhas et al.13 Yuhas et al.14 Polychronopoulou et al.15

AN: aggregation of nanocrystals; CL: chemical linkage of clusters.

ethanol to remove the water. Finally, the gels were rinsed with liquid CO2 and then dried with supercritical CO2 in an autoclave (4762Q, Parr Instrument Company, Moline, IL) to preserve their pore structure as described elsewhere.17 The two chalcogel (CG) products from this process were a granular and a powder form referred to here as CGg and CGp, respectively. 2.2. Sn2S3 Xerogel Synthesis. The Sn2S3 xerogels (XG) were synthesized in a very similar fashion as the chalcogels although, after the ethanol exchanging procedure, the ethanol was not removed using supercritical CO2. Instead, the ethanol was allowed to evaporate in a vacuum oven. The products from this process were granular and powdered forms of Sn2S3 xerogels, or XGg and XGp, respectively. 2.3. Direct Melt Consolidation of Chalcogels without Iodine. A consolidation experiment was conducted with a portion of the as-made CGg. For this experiment, 0.1786 g of granules were added to a fused silica tube (10 × 12 mm) along with 0.1464 g of GeS2, resulting in a molar ratio of Sn:Ge = 1. This tube was added to a secondary tube (22 × 25 mm) that was evacuated and sealed under vacuum. The assembly was loaded into a Deltech furnace (Deltech, Inc., Denver, CO) and heated at 20 °C min−1 to temperatures of 400, 550, 750, and 830 °C where it was removed from the furnace briefly after each temperature for observation, returned to the furnace afterward, and heated to the next temperature. After an observation was made at 830 °C, the sample was quenched in water. The inner ampule was then mounted in resin, crosssectioned, and polished for further observations. 2.4. Iodine Capture Experiments. Both the powdered and granular forms of the CG and XG were evaluated in the iodine capture experiments. (Note that the CGp is discussed in more detail elsewhere.17) The CGg was loaded with iodine by placing 0.6178 g of the as-made material into a preweighed 20 mL glass scintillation vial. This vial was placed into a vacuum desiccator along with an empty vial and ∼10 g of pure iodine (99.9999%, Alfa Aesar) in a separate vial. A dynamic vacuum was pulled on the chamber and, periodically, the vials were removed and weighed so that the sorbent mass uptake could be measured. This process was carried out until no further mass gain was observed at which point the sorbents were saturated with iodine. The CG specimens that underwent maximum iodine capture are denoted as “CGg+I” and “CGp+I” for the granular and powdered forms, respectively. The XG samples were loaded in a separate experiment. Here, 1.7121 g of XGg and 1.3672 g of XGp were loaded into the desiccator with ∼18 g of the same iodine source as mentioned above. Vials were periodically removed in the same fashion as described above until no further mass gain was observed. The

Figure 1. Glass formation tendency in Sn−Ge−S chalcogenide glasses adapted with permission from Ruffolo and Boolchand31 (Copyrighted by the American Physical Society).

and xerogels using GeS2 as a glass-forming additive based off of the Sn−Ge−S glass formation work by Ruffolo and Boolchand.31

2. EXPERIMENTAL SECTION 2.1. Sn2S3 Chalcogel Synthesis. To prepare the Sn2S3 chalcogel, 10 mmol of Na4Sn2S6·14H2O and 20 mmol of Sn(CH3COO)2 were dissolved in separate beakers containing 200 mL of formamide each. The Sn(CH3COO)2 was purchased from Sigma-Aldrich (St. Louis, MO); the Na4Sn2S6·14H2O was made fresh from Na2S·9H2O and SnCl4·5H2O (Sigma-Aldrich) according to the literature,10,16,36 and the formamide was obtained from Sigma-Aldrich. Once dissolved, these solutions were combined, mixed for 10 min, cast into polypropylene vials, and left to undergo gelation for 37 d. Following gelation, the gels were removed from their vials, cut into ∼2−5 mm pieces, and placed in an aging solution of 50/50 ethanol/water (V/V) for 24 h to increase rigidity. Following aging, the gels were rinsed 10 times over the course of 2 d, each time with a fresh 50:50 solution to remove the water-soluble byproducts, followed by 10 rinses with pure 11260

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of 1.1928 g, which included XGp+I and GeS2. Each sample was individually mixed in an agate mortar and pestle and uniaxially pressed into a pellet at ∼24.8 MPa. The can material used for containing the samples was 0.127 mm thick annealed 316 stainless steel (316SS) (Maudlin Products, Kemah, TX) that was cut into discs with a diameter of 38 mm. In order to allow for displacement of the sample, each disc was pressed with a die to form a dimple that would hold a ∼15 mm diameter pellet. Then, the pellets were loaded into a pair of the preformed 316SS discs. For HIP-2, the insides of the discs were coated with a thin layer of carbon to help prevent reaction between the 316SS and the sample. Each assembly was loaded into a copper chuck inside of an electron beam welder where it was evacuated and then welded shut around the circumference. The samples were shipped out to American Isostatic Presses, Inc. in Columbus, OH. The samples were run in an AIP 6-30 molybdenum furnace under Ar gas at different temperatures where HIP-1 was heated at 10 °C min−1 to 800 °C for 30 min (after a hold at 600 °C for 30 min) and HIP-2 was run at 10 °C min−1 to 600 °C for 30 min. Both were run at a pressure set point of ∼207 MPa. 2.8. Characterization. 2.8.1. Electron Microscopy. Scanning electron microscopy (SEM) was performed on polished cross sections with a JSM-7001F field-emission gun microscope (JEOL USA, Inc. Peabody, MA) using a backscattered electron (BSE) detector for atomic number contrast. Energy dispersive spectroscopy (EDS) was performed with an EDAX Si-drift detector (Apollo XL, AMETEK, Berwyn, PA). Acceleration voltages used were 1−20 kV, spot collection times for EDS were 0.5−1 min, and the EDS dot map was collected for 1 h. 2.8.2. X-ray Diffraction. The XRD analysis was performed using two different instruments on powdered specimens that were ground in a mortar and pestle. One instrument was a Philips X’Pert with a radius of 190 mm and variable divergence and antiscatter slits (10 mm irradiated area). The scan range was 5−90° 2θ with 0.03° steps and a 2-s dwell at each step. The second instrument was a Bruker D8 Advance (Bruker AXS Inc., Madison, WI) XRD with Cu Kα emission and a LynxEye position-sensitive detector with a collection window of 3° 2θ. Scan parameters were 5−70° 2θ with a step of 0.009° 2θ and a 1 s dwell at each step. 2.8.3. Specific Surface Area Measurements. The specific surface area of the aged CG was measured using N2(g) adsorption/desorption isotherms collected with a Quantachrome Autosorb-6B (Quantachrome Instruments, Boynton Beach, FL) gas sorption system on degassed samples. Samples were loaded in a glass sample holder and degassed at different temperatures (25−125 °C) while under vacuum so that the temperature dependence on the specific surface area could be evaluated. The degassed samples were analyzed with nitrogen adsorption and desorption at a constant temperature of 77.4 K (−195.75 °C), the temperature of liquid nitrogen. The specific surface areas were determined from the isotherms with the Brunauer−Emmett−Teller (BET) method.39

XG specimens that underwent maximum iodine capture are denoted as “XGg+I” and “XGp+I” for the granular and powdered forms, respectively. 2.5. Direct-Melting with Iodine-Loaded Chalcogels. A consolidation experiment was conducted with CGg+I in an evacuated and sealed fused silica ampule with an approach similar to the experiment without iodine. Here, 0.7238 g of CGg+I (Sn = 1.379 mmol) was placed into an 8 × 12 mm fused silica tube. With a target Sn:Ge molar ratio of 1:1, 0.1885 g of GeS2 was added to the ampule on top of the chalcogel granules without any mixing or crushing. The ampule was evacuated and sealed. The ampule was then heated at 20 °C min−1 sequentially to 400, 500, and 600 °C with 10 min dwells at each temperature. After each dwell, the ampule was taken out for observation. At each temperature, a red vapor was observed above the melt, and the vapor darkened with increasing heat-treatment temperatures [this vapor is believed to be SnI4(g)]. After the final temperature of 600 °C, the ampule was taken out and airquenched to help prevent cracking from rapid thermal shock. The entire consolidated chalcogel was mounted in resin and cross sectioned, and one-half was polished. The chalcogel from the other half of the mounted specimen was extracted from the silica tube and ground to a powder for X-ray diffraction (XRD) analysis. 2.6. Microwave Sintering of Xerogels. The microwave sintering (MS) experiments were conducted using a 6 kW microwave furnace operating at 2.45 GHz under Ar gas (Hadron Technologies; Arvada, CO). A Minolta-Land Cyclops 152 portable pyrometer was used for the temperature measurement reading through the top port of the furnace. The temperature measurement accuracy with the infrared pyrometer was determined to be within 5 °C of the temperature measured with a K-type thermocouple in conventional experiments. An Y2O3-coated SiC suscepter was used within the insulation package to assist in rapid heating of the reactants. Here, XGg and XGg+I were separately mixed with GeS2 in an agate mortar and pestle keeping the Sn:Ge molar ratio at 1:1. The reaction vessels that were used for the experiments were small fused silica vessels that were 5 mm inner diameter and 25 mm tall. All experiments were conducted at 800 °C but with different hold times as shown in Table 2. Following the Table 2. Summary Table for MS Parameters for the Primary Tests That Were Conducteda experiment #

Sn2S3

GeS2

iodine

temp

time

SiC susceptor used?

1 2 3 4

yes yes yes yes

yes yes yes yes

no yes yes yes

800 800 800 800

30 30 0 10

no yes yes yes

a

Additional tests are discussed in the Supporting Information.

experiments, the material left in the silica crucibles was scraped out, ground to a powder in a mortar and pestle, mixed with ethanol, dropped onto a zero background silicon sample holder, and analyzed with XRD. 2.7. Hot Isostatic Pressing of Xerogels. For hot isostatic pressing (HIP), two different samples were made with a targeted Sn:Ge molar ratio of 1:1. The mass of sample 1 (HIP1) was 1.2408 g, which included XGg+I and the GeS2 glassforming additive. Sample 2 (HIP-2) utilized a total sample mass

3. RESULTS AND DISCUSSION 3.1. Analysis of As-Made Chalcogels before Consolidation Experiments. The as-made chalcogel was highly porous with specific surface areas of 364−456 m2 g−1 measured over the range of degas temperatures (Figure 2).16 The decrease in specific surface area as a function of degas temperature is likely due to residual solvent leaving the chalcogel matrix upon heating. The Sn:S ratio measured with 11261

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Figure 2. (a) Specific surface area as a function of degas temperature and (b) an SEM micrograph of as-made and aged Sn−S chalcogel. Reprinted from ref 16. Copyright 2013 American Chemical Society.

EDS was ∼2:3. Inclusions of Na were also observed and attributed to residual impurity from the reactant, Na4Sn2S6· 14H2O. The XRD spectra from the as-made and aged CG are presented in Figure 3a and show a few broad diffraction peaks (humps) corresponding to the amorphous structure in the sample. Additionally, some crystalline peaks were observed, some of which were attributed to SnS and others remained unidentified. 3.2. Analysis of Iodine-Sorbed Chalcogels and Xerogels before Consolidation Experiments. The CGp+I was analyzed with XRD, and the diffraction pattern is shown in Figure 3b. The XRD results showed a crystalline transformation from the chalcogel without iodine following iodine adsorption. These included SnI4 and SnI4(S8)2, both providing evidence of iodine chemisorption involving the Sn in the chalcogel matrix; additionally, S8 was observed showing crystallization of the sulfur network. Although both the SnI4 (cubic space group Pa3)̅ and SnI4(S8)2 (orthorhombic space group Fdd2) phases have tetrahedrally coordinated SnI4 moieties within their structures, both are different from a crystallographic standpoint and the SnI4(S8)2 phase has S8 rings present within the structure (see Figure S1). It is hypothesized that the formation of the SnI4(S8)2 phase is due to the polymerization of the sulfur constituents in the chalcogel matrix as the neighboring Sn atoms are crystallized into SnI4 units. More detailed analysis of the chemistry and phase distribution of the I-sorbed Sn2S3 chalcogels is provided in our earlier work.17 The iodine loadings for CGg+I, CGp+I, XGg+I, and XGp+I were very high at 2.155 g g−1 (68.3% I), 2.049 g g−1 (67.2% I), 1.933 g g−1 (65.9% I), and 1.985 g g−1 (66.5% I) (%’s are by mass), respectively, as seen in Figure 4. The times to reach maximum adsorption for CGg+I, CGp+I, XGg+I, and XGp+I were 25.6, 8.5, 69.5, and 61.7 d, respectively. In both cases, the powders reached saturation earlier than the granules and this is likely due to the pores being more accessible in the powders as compared to the more tortuous pathways to reach inner pores in the granules.

Figure 3. (a) XRD whole pattern fitting results for CGp after aging in air for 12 months (before consolidation experiments) where “Amor.1” and “Amor.2” denote separate amorphous peaks in the spectra. (b) XRD results for CGp+I after maximum iodine adsorption. Note that there are unidentified peaks in each pattern.

Figure 4. Iodine loading time and capacities for granular and powdered forms of Sn2S3 chalcogels and xerogels.

3.3. Chalcogels without Iodine Consolidated with Direct Melting. Figure 5a shows a progression of pictures taken at different temperatures during the consolidation process of the Sn2S3 chalcogel without iodine. On the basis of these results, it appears that the mixture of the Sn2S3 chalcogel and the GeS2 additive melted somewhere in the range of 550−750 °C. The yellow films observed on the walls of the silica tube were very thin films of the mixture that evaporated but condensed upon cooling due to a slightly cooler 11262

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Since the glass reacted with the silica wall, the thermal expansion mismatch between the fused silica and the melt as it cooled into a glass was the cause of the cracking. If this experiment were to be repeated, other types of crucibles could be evaluated such as glassy carbon to avoid rupture of the silica wall. Other than the SiO2 particles, the glass was completely homogeneous according to BSE-SEM and random area EDS analyses. The second half of the specimen after thin sectioning was removed from the fused silica ampule, ground to a powder, and analyzed with XRD. Figure 5g provides the diffraction pattern of the powdered glass. The results show a few broad diffraction peaks corresponding to amorphous structure, which is characteristic of sulfide-based chalcogenide glass.40 A few weak crystalline peaks were attributed to the silica41 from the wall of the ampule as observed with SEM (Figure 5c−f). 3.4. Iodine-Loaded Chalcogels Consolidated with Direct Melting. The diffraction pattern from the as-made Sn−Ge−S−I glass shown in Figure 6 reveals crystalline peaks

Figure 6. XRD whole pattern fitting results for the product from the CGg+I consolidation experiment after a 600 °C final heat-treatment temperature. The SnI4 phases identified are labeled with the International Crystal Structure Database identification number in parentheses.

Figure 5. (a) Progression of CGg consolidation (without iodine) in a fused silica ampule with temperature (as viewed through outer tube). The sample as viewed at 830 °C was following a water quench. (b) Optical and (c−f) SEM micrographs of the heat-treated and polished CGg. Regions denoted with blue circles are regions that are magnified in other micrographs. (g) XRD spectrum of the portion of consolidated glass that was not mounted in resin for SEM observations.

attributed to SnI4, an unidentified peak at 26° 2θ, and an amorphous signature as seen by the broad peaks centered at ∼26 and ∼48° 2θ. Figure 7a shows a picture of CGg+I before the consolidation process, Figure 7b shows the ampule containing the CGg+I granules mixed with the GeS2 glass binder, and Figure 7c shows the polished cross-section of the product following the consolidation experiment. The other half of the cross-sectioned material was removed from the fused silica tube, ground to a powder, and analyzed with XRD. The SEM micrograph in the upper left of Figure 7d (also see Figure S2) was captured at a low magnification and shows the interface of unincorporated SnI4 salt on top of the glass seen as the yellow phase shown in Figure 7c (see Table S2 for EDS results). The presence of SnI4 here was not surprising and is due to the high SnI4 content in the CGg+I that was likely above the glass formation limit for the Sn−Ge−S−I quaternary glass system at this processing temperature of 600 °C. The SEM micrograph in the upper right of Figure 7d shows a magnified view of the melt, and the rest of the micrographs in

temperature at the top of the ampule. The total mass of the film was estimated to be very low based on the small thickness. Figure 5b shows an optical micrograph of a polished crosssection taken from the bottom portion of the heat-treated sample. From the SEM micrographs in Figure 5c−f, it is apparent that the glass reacted with the wall of the silica ampule causing particles of the fused silica inner wall to spall off and enter the melt; these particles can be seen most clearly in Figure 5f. The EDS analysis of the glass composition did not reveal that much Si was incorporated into the melt (∼0.4 mass %) (see Table S1). It is uncommon to see a chalcogenide glass melt attack fused silica, and it is speculated that some surface oxides (e.g., SnO2) present on the chalcogels increased the reactivity between the melt and the silica wall. 11263

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value, and this was attributed to the unincorporated SnI4 crystals on the top of the melt, which were not included in the EDS analysis region. The significance of this is that, in order to fully incorporate all of the SnI4 in the final product, the iodine loading in the sorbent and/or the quantity of glassforming additive (e.g., GeS2) must be controlled to maintain the balance between the solubility limit of SnI4 in the glass system (e.g., Sn−Ge−S) at a given processing temperature and to control the volatility of SnI4, which boils at a temperature of ∼348 °C.42 The targeted molar ratio of Sn:Ge in the final glass was 1:1 but the analyzed ratio was closer to ∼1.5:1. This could be due to pockets of unreacted GeS2 observed in the glass during SEM−EDS analysis (not shown) suggesting that a higher processing temperature could fully melt the GeS2 glass additive in order to obtain full homogenization of this particular mixture. The elemental EDS dot map for the glassy area shown in Figure 7 reveals a homogeneous distribution of Sn, Ge, S, and I but with some unexpected anomalies. Some SiO2 was observed as were C- and Na-rich areas. Some of the Na-rich regions correspond to high-I regions as well, suggesting that residual Na in the chalcogel reacted with the iodine. The only possible source for Na is Na+ from the Na4Sn2S6·14H2O precursor that was not removed from the gel during the solvent exchange steps, suggesting that these chalcogels required a more rigorous rinsing protocol with the 50/50 ethanol/water solution. The potential source of C is from residual CH3COO− anions from the Sn(CH3COO)2 precursor used during gel synthesis, also suggesting that better rinsing was needed. 3.5. Microwave Sintering of Xerogels. For the MS experiments, almost all of the material left inside the crucibles following heat treatment was deposited on the walls of the vessels as shown in Figure 8 along with the corresponding XRD results for each sample. The XRD results reveal a mixture of various phases that were formed in the samples during the consolidation process. One of the primary points of concern is the GeO2 that formed in all of the samples. This is likely due to the presence of oxygen within the microwave or oxygen contamination during specimen preparations considering that Ge was added as pure GeS2 before heating. Unless the oxygen can be removed during heating, it is likely that this process will not work for consolidation. Additionally, several different side products were seen in the samples including various Sn−S compounds (i.e., SnS, SnS2, Sn2S3), GeSnS3, SnO2, and SnI4. Additional experiments were conducted under different conditions where the samples were cross-sectioned and polished so that the microstructure and porosity could be observed, and these are shown in Table S3 and Figure S3. The material did not consolidate in any of these experiments based on the very large porosity observed for all of the samples. It is possible that MS could be an elegant approach for consolidating these materials, but more work is needed to find the optimal processing conditions. 3.6. Hot Isostatic Pressing of Xerogels. Both HIP-1 and HIP-2 samples reacted with the 316SS can material during processing as is shown in Figure 9. The can for HIP-1 was compromised during heating as was evidenced by a thin yellow coating on the inside of the furnace. The sample remaining was very small and had completely alloyed with and embrittled the can material. After getting the HIP-1 sample back, the carbon coating was added to the inside of the can for HIP-2 and a lower processing temperature of 600 °C (versus 800 °C) was used to help prevent this attack; however, it was not successful.

Figure 7. Pictures of (a) raw CGg+I, (b) CGg+I with GeS2 in a fused silica ampule, and (c) the mixture in (b) after the final heat treatment at 600 °C. (d) SEM micrographs at different magnifications and EDS elemental maps of the polished cross-section seen in (c). The callouts of (1) and (2) in (c) are for reference to the images in (d) that show the regions of analysis.

Figure 7d are elemental maps showing the distribution of Sn, Ge, S, I, Na, and Si from that same field of view. The SEM analysis of this specimen showed some heterogeneity, as was expected from the XRD analysis and general physical appearance. The top portion of the consolidated product (yellow phase in Figure 7c) was coated with a mixture of sulfurrich deposits, or a darker layer than the other phases at the top of the SEM-BSE micrograph in Figure 7d, and SnI4 is seen as bright faceted crystals at the top of the SEM micrograph in Figure 7d; see Figure S2 and Table S2 for the corresponding EDS analysis of these different regions. The dark phase on the top of the glass did not show up in the XRD spectrum as a crystalline phase so it is likely a Sn−S−I glass of a slightly different composition than the bulk, according to the composition of this mixture shown in Table S2. The iodine content in the glassy phase was still rather high at ∼45 mass% (Table S2). This is smaller than the predicted 11264

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material was removed from the can and analyzed with XRD. The results from this analysis are presented in Figure 9. The XRD analysis revealed a large fraction of SnI4 (46.8%) and some other species that showed evidence of an alloy between the sample and the can including FeS (32.1%) and Fe2GeS4 (11.1%); additionally, Ge was observed (10.0%) (all values are in mass%). The amorphous fraction was not determined. The large fraction of SnI4 provided evidence that the Sn2S3+I material did not undergo much reaction with the GeS2 during processing. Additional can materials were not evaluated. However, alternative materials might include platinum, aluminum, or even tantalum, which was successfully used for consolidating metal waste forms by Crum et al.21

4. CONCLUSIONS The Sn2S3 chalcogel shows one of the largest iodine capacities of the chalcogel chemistries evaluated to date. From a waste form standpoint, once the iodine is captured in this chalcogel as SnI 4 , which is water-soluble, the material should be consolidated into a final form to reduce porosity and improve the bulk chemical durability. One viable option is to convert iodine-sorbed Sn2S3 chalcogels into a chalcogenide glass using glass-forming additives. The work presented here demonstrates a pathway for consolidating Sn2S3 chalcogels with and without adsorbed iodine using a GeS2 glass-forming additive, and the results show that a relatively homogeneous glass was formed by combining GeS2 with either the iodine-sorbed (CGg+I) or iodine-free (CGg) Sn2S3 chalcogels. However, volatility during heating could lead to heterogeneity in the final product as well as iodine loss, which is expected to be in the form of SnI4. In an industrial process utilizing this approach, the iodine lost during consolidation would need to be captured, posing drawbacks to this approach. During the direct-melting consolidation experiment with the CGg+I, some unincorporated SnI4 was observed suggesting that either the batched composition was above the limit for SnI4 in the ternary Sn−Ge−S glass system or the melting temperature used was not sufficiently high. Nevertheless, the quantity of iodine measured in the bulk glass was sustained up to ∼45 mass%. Additional consolidation experiments with microwave sintering (MS) and hot isostatic pressing (HIP) using Sn2S3 xerogels proved unsuccessful at effectively consolidating the material. For the MS experiment, this was most likely due to oxygen contamination that occurred during preparation of the samples as per the oxide phases that were observed with the XRD analysis. Further experiments could be done to investigate several different parameters including various temperatures, times, and masses for MS. For the HIP experiments, the xerogels reacted with the stainless steel can, even when it was coated with a layer of carbon. Thus, other types of can materials could be evaluated for HIP in the future. More work is currently underway to explore other types of chalcogels, such as Sb4(SnS4)3,20 that have high iodine adsorption potential. For some of these chalcogel chemistries, glass formation experiments are underway and some of that work is published elsewhere.20

Figure 8. (top) Pictures and (bottom) XRD whole pattern fitting results for XGg+I samples consolidated with microwave sintering.

Figure 9. (top) Pictures of an unreacted HIP can and the can and furnace for HIP-1, as well as the can and sample for HIP-2. (bottom) XRD whole pattern fitting results for HIP-1 and HIP-2 samples.

The XRD analysis on the coating on the HIP-1 can showed remnants of Cr2O3 (35.1%), likely causing the green color, and Cr (9.2%) as well as several different compounds providing evidence of the sample reacting with the stainless steel including Fe0.9Ge0.1 (40.7%), FeS (3.0%), Ni0.92Sn0.08 (9.8%), and Ni3Sn2 (2.3%) (all values are in mass%). The amorphous fraction was not determined. The can for HIP-2 was in better condition, but it was still compromised at some point during the heat treatment as pin holes were observed in the can and discoloration was seen on the outer surfaces of the can (see Figure 9). The pelletized

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02697. 11265

DOI: 10.1021/acs.iecr.5b02697 Ind. Eng. Chem. Res. 2015, 54, 11259−11267

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Crystal structure of Sn−I compounds found by X-ray diffraction analysis in consolidated products, scanning electron microscopy and energy dispersive spectroscopy results, additional experimental details, and results of the second batch of the microwave sintering experiments (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (509) 372-4651. Funding

This work was funded by the Department of Energy Office of Nuclear Energy under the Fuel Cycle Research and Development Program. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Pacific Northwest National Laboratory is operated by the U.S. Department of Energy under Contract Number DE-AC0576RL01830. At Northwestern University, this work was supported by the NEUP program of DOE (M.G.K). Authors also thank Benjamin Yuhas of Northwestern University for providing the Na4Sn2S6·14H2O precursor that was used to make the Sn2S3 chalcogel, Andrew Yu from American Isostatic Presses, Inc. for running the hot isostatic press experiments, Xiaohong Li for help with BET, and Mark Bowden for help with XRD analysis on the Philips X’Pert. A portion of the research was performed using capabilities in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL.

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