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Demonstrated viability and utilization of: a) Fluidized powder bed. b) WCl6 CVD process. c) Coated spherical particles with tungsten. The highly corrosive nature of the WCl6 solid reagent limits material of construction. Indications that identifying optimized process variables with require substantial effort and will likely vary with changes in fuel requirements.
WCl6 crystallizes in the trigonal P-3m1 space group. The structure is zero-dimensional and consists of three tungsten(vi) chloride molecules. W6+ is bonded in an octahedral geometry to six Cl1- atoms. All W–Cl bond lengths are 2.31 Å. There are two inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a single-bond geometry to one W6+ atom. In the second Cl1- site, Cl1- is bonded in a single-bond geometry to one W6+ atom.
WCl6 is Copper structured and crystallizes in the trigonal R-3 space group. The structure is zero-dimensional and consists of three tungsten(vi) chloride molecules. W6+ is bonded in an octahedral geometry to six equivalent Cl1- atoms. All W–Cl bond lengths are 2.31 Å. Cl1- is bonded in a single-bond geometry to one W6+ atom.
W-UO2 CERMET fuels are under development to enable Nuclear Thermal Propulsion (NTP) for deep space exploration. Research efforts with an emphasis on fuel fabrication, testing, and identification of potential risks is underway. One primary risk is fuel loss due to CTE mismatch between W and UO2 and the grain boundary structure of W particles resulting in higher thermal stresses. Mechanical failure can result in significant reduction of the UO2 by hot hydrogen. Fuel loss can be mitigated if the UO2 particles are coated with a layer of high density tungsten before the consolidation process. This paper discusses the work to date, results, and advances of a fluidized bed chemical vapor deposition (CVD) system that utilizes the H2-WCl6 reduction process. Keywords: Space, Nuclear, Thermal, Propulsion, Fuel, CERMET, CVD, Tungsten, Uranium
In order to achieve deep space exploration via Nuclear Thermal Propulsion (NTP), Marshall Space Flight Center (MSFC) is developing W-UO2 CERMET fuel elements, with focus on fabrication, testing, and process optimization. A risk of fuel loss is present due to the CTE mismatch between tungsten and UO2 in the W-60vol%UO2 fuel element, leading to high thermal stresses. This fuel loss can be reduced by coating the spherical UO2 particles with tungsten via H2/WCl6 reduction in a fluidized bed CVD system. Since the latest incarnation of the inverted reactor was completed, various minor modifications to the system design were completed, including an inverted frit sublimer. In order to optimize the parameters to achieve the desired tungsten coating thickness, a number of trials using surrogate HfO2 powder were performed. The furnace temperature was varied between 930 C and 1000degC, and the sublimer temperature was varied between 140 C and 200 C. Each trial lasted 73-82 minutes, with one lasting 205 minutes. A total of 13 trials were performed over the course of three months, two of which were re-coatings of previous trials. The powder samples were weighed before and after coating to roughly determine mass gain, and Scanning Electron Microscope (SEM) data was also obtained. Initial mass results indicated that the rate of layer deposition was lower than desired in all of the trials. SEM confirmed that while a uniform coating was obtained, the average coating thickness was 9.1% of the goal. The two re-coating trials did increase the thickness of the tungsten layer, but only to an average 14.3% of the goal. Therefore, the number of CVD runs required to fully coat one batch of material with the current configuration is not feasible for high production rates. Therefore, the system will be modified to operate with a negative pressure environment. This will allow for better gas mixing and more efficient heating of the substrate material, yielding greater tungsten coating per trial.
WFeCl5 crystallizes in the monoclinic C2/c space group. The structure is two-dimensional and consists of two WFeCl5 sheets oriented in the (0, 0, 1) direction. there are two inequivalent W2+ sites. In the first W2+ site, W2+ is bonded to six Cl1- atoms to form WCl6 octahedra that share an edgeedge with one WCl6 octahedra and edges with three FeCl6 octahedra. There are a spread of W–Cl bond distances ranging from 2.38–2.54 Å. In the second W2+ site, W2+ is bonded to six Cl1- atoms to form WCl6 octahedra that share an edgeedge with one WCl6 octahedra and edges with three FeCl6 octahedra. There are a spread of W–Cl bond distances ranging from 2.37–2.54 Å. There are two inequivalent Fe3+ sites. In the first Fe3+ site, Fe3+ is bonded to six Cl1- atoms to form FeCl6 octahedra that share edges with two equivalent FeCl6 octahedra and edges with three WCl6 octahedra. There are a spread of Fe–Cl bond distances ranging from 2.42–2.53 Å. In the second Fe3+ site, Fe3+ is bonded to six Cl1- atoms to form FeCl6 octahedra that share edges with two equivalent FeCl6 octahedra and edges with three WCl6 octahedra. There are a spread of Fe–Cl bond distances ranging from 2.40–2.54 Å. There are five inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a 3-coordinate geometry to one W2+ and two Fe3+ atoms. In the second Cl1- site, Cl1- is bonded in a 2-coordinate geometry to two W2+ atoms. In the third Cl1- site, Cl1- is bonded in a water-like geometry to one W2+ and one Fe3+ atom. In the fourth Cl1- site, Cl1- is bonded in a water-like geometry to one W2+ and one Fe3+ atom. In the fifth Cl1- site, Cl1- is bonded in a 3-coordinate geometry to one W2+ and two Fe3+ atoms.
Cs3W2Cl9 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. there are two inequivalent Cs1+ sites. In the first Cs1+ site, Cs1+ is bonded to twelve Cl1- atoms to form CsCl12 cuboctahedra that share corners with nine CsCl12 cuboctahedra, corners with three equivalent WCl6 octahedra, faces with seven CsCl12 cuboctahedra, and faces with four equivalent WCl6 octahedra. The corner-sharing octahedral tilt angles are 21°. There are a spread of Cs–Cl bond distances ranging from 3.68–3.96 Å. In the second Cs1+ site, Cs1+ is bonded to twelve Cl1- atoms to form CsCl12 cuboctahedra that share corners with twelve CsCl12 cuboctahedra, faces with six equivalent CsCl12 cuboctahedra, and faces with six equivalent WCl6 octahedra. There are six shorter (3.65 Å) and six longer (3.78 Å) Cs–Cl bond lengths. W3+ is bonded to six Cl1- atoms to form WCl6 octahedra that share corners with three equivalent CsCl12 cuboctahedra, faces with seven CsCl12 cuboctahedra, and a faceface with one WCl6 octahedra. There are three shorter (2.42 Å) and three longer (2.50 Å) W–Cl bond lengths. There are two inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a 6-coordinate geometry to four Cs1+ and two equivalent W3+ atoms. In the second Cl1- site, Cl1- is bonded in a distorted single-bond geometry to four Cs1+ and one W3+ atom.
Co(WCl5)2 crystallizes in the monoclinic C2/c space group. The structure is two-dimensional and consists of two Co(WCl5)2 sheets oriented in the (0, 0, 1) direction. there are two inequivalent W4+ sites. In the first W4+ site, W4+ is bonded to six Cl1- atoms to form WCl6 octahedra that share an edgeedge with one WCl6 octahedra and edges with two equivalent CoCl6 octahedra. There are four shorter (2.38 Å) and two longer (2.45 Å) W–Cl bond lengths. In the second W4+ site, W4+ is bonded to six Cl1- atoms to form WCl6 octahedra that share an edgeedge with one WCl6 octahedra and an edgeedge with one CoCl6 octahedra. There are a spread of W–Cl bond distances ranging from 2.31–2.47 Å. Co2+ is bonded to six Cl1- atoms to form CoCl6 octahedra that share edges with three WCl6 octahedra. There are a spread of Co–Cl bond distances ranging from 2.41–2.48 Å. There are five inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a 2-coordinate geometry to two W4+ atoms. In the second Cl1- site, Cl1- is bonded in a water-like geometry to one W4+ and one Co2+ atom. In the third Cl1- site, Cl1- is bonded in a single-bond geometry to one W4+ atom. In the fourth Cl1- site, Cl1- is bonded in a water-like geometry to one W4+ and one Co2+ atom. In the fifth Cl1- site, Cl1- is bonded in an L-shaped geometry to one W4+ and one Co2+ atom.
CsWCl6 crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. Cs1+ is bonded to twelve Cl1- atoms to form CsCl12 cuboctahedra that share corners with four equivalent CsCl12 cuboctahedra, corners with two equivalent WCl6 octahedra, edges with four equivalent CsCl12 cuboctahedra, edges with two equivalent WCl6 octahedra, and faces with two equivalent WCl6 octahedra. The corner-sharing octahedral tilt angles are 46°. There are a spread of Cs–Cl bond distances ranging from 3.74–3.87 Å. W5+ is bonded to six Cl1- atoms to form WCl6 octahedra that share corners with two equivalent CsCl12 cuboctahedra, edges with two equivalent CsCl12 cuboctahedra, and faces with two equivalent CsCl12 cuboctahedra. All W–Cl bond lengths are 2.35 Å. There are three inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a distorted single-bond geometry to two equivalent Cs1+ and one W5+ atom. In the second Cl1- site, Cl1- is bonded in a distorted single-bond geometry to two equivalent Cs1+ and one W5+ atom. In the third Cl1- site, Cl1- is bonded in a distorted single-bond geometry to two equivalent Cs1+ and one W5+ atom.
RbWCl6 crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. Rb1+ is bonded to twelve Cl1- atoms to form RbCl12 cuboctahedra that share corners with four equivalent RbCl12 cuboctahedra, corners with two equivalent WCl6 octahedra, edges with four equivalent RbCl12 cuboctahedra, edges with two equivalent WCl6 octahedra, and faces with two equivalent WCl6 octahedra. The corner-sharing octahedral tilt angles are 46°. There are a spread of Rb–Cl bond distances ranging from 3.60–3.73 Å. W5+ is bonded to six Cl1- atoms to form WCl6 octahedra that share corners with two equivalent RbCl12 cuboctahedra, edges with two equivalent RbCl12 cuboctahedra, and faces with two equivalent RbCl12 cuboctahedra. All W–Cl bond lengths are 2.35 Å. There are three inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a distorted single-bond geometry to two equivalent Rb1+ and one W5+ atom. In the second Cl1- site, Cl1- is bonded in a distorted single-bond geometry to two equivalent Rb1+ and one W5+ atom. In the third Cl1- site, Cl1- is bonded in a distorted single-bond geometry to two equivalent Rb1+ and one W5+ atom.
W3Sn2Cl14 crystallizes in the hexagonal P6_3 space group. The structure is two-dimensional and consists of two W3Sn2Cl14 sheets oriented in the (0, 0, 1) direction. W+3.33+ is bonded to six Cl1- atoms to form WCl6 octahedra that share a cornercorner with one SnCl4 trigonal pyramid and edges with two equivalent WCl6 octahedra. There are a spread of W–Cl bond distances ranging from 2.41–2.50 Å. There are two inequivalent Sn2+ sites. In the first Sn2+ site, Sn2+ is bonded in a 6-coordinate geometry to six Cl1- atoms. There are three shorter (2.72 Å) and three longer (3.13 Å) Sn–Cl bond lengths. In the second Sn2+ site, Sn2+ is bonded to four Cl1- atoms to form distorted SnCl4 trigonal pyramids that share corners with three equivalent WCl6 octahedra. The corner-sharing octahedral tilt angles are 50°. There are one shorter (2.42 Å) and three longer (3.06 Å) Sn–Cl bond lengths. There are six inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a single-bond geometry to one Sn2+ atom. In the second Cl1- site, Cl1- is bonded in a 12-coordinate geometry to three equivalent W+3.33+ atoms. In the third Cl1- site, Cl1- is bonded in a 2-coordinate geometry to two equivalent W+3.33+ atoms. In the fourth Cl1- site, Cl1- is bonded in a 2-coordinate geometry to one W+3.33+ and one Sn2+ atom. In the fifth Cl1- site, Cl1- is bonded in a water-like geometry to one W+3.33+ and one Sn2+ atom. In the sixth Cl1- site, Cl1- is bonded in a distorted single-bond geometry to one W+3.33+ and one Sn2+ atom.
WTl2Cl6 crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. W4+ is bonded to six equivalent Cl1- atoms to form WCl6 octahedra that share faces with eight equivalent TlCl12 cuboctahedra. All W–Cl bond lengths are 2.39 Å. Tl1+ is bonded to twelve equivalent Cl1- atoms to form TlCl12 cuboctahedra that share corners with twelve equivalent TlCl12 cuboctahedra, faces with six equivalent TlCl12 cuboctahedra, and faces with four equivalent WCl6 octahedra. All Tl–Cl bond lengths are 3.56 Å. Cl1- is bonded in a distorted single-bond geometry to one W4+ and four equivalent Tl1+ atoms.
Rb2WCl6 crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Rb1+ is bonded to twelve equivalent Cl1- atoms to form RbCl12 cuboctahedra that share corners with twelve equivalent RbCl12 cuboctahedra, faces with six equivalent RbCl12 cuboctahedra, and faces with four equivalent WCl6 octahedra. All Rb–Cl bond lengths are 3.63 Å. W4+ is bonded to six equivalent Cl1- atoms to form WCl6 octahedra that share faces with eight equivalent RbCl12 cuboctahedra. All W–Cl bond lengths are 2.40 Å. Cl1- is bonded in a distorted single-bond geometry to four equivalent Rb1+ and one W4+ atom.
Cs2WCl6 crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Cs1+ is bonded to twelve equivalent Cl1- atoms to form CsCl12 cuboctahedra that share corners with twelve equivalent CsCl12 cuboctahedra, faces with six equivalent CsCl12 cuboctahedra, and faces with four equivalent WCl6 octahedra. All Cs–Cl bond lengths are 3.75 Å. W4+ is bonded to six equivalent Cl1- atoms to form WCl6 octahedra that share faces with eight equivalent CsCl12 cuboctahedra. All W–Cl bond lengths are 2.40 Å. Cl1- is bonded in a distorted single-bond geometry to four equivalent Cs1+ and one W4+ atom.
K2WCl6 crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. K1+ is bonded to twelve equivalent Cl1- atoms to form KCl12 cuboctahedra that share corners with twelve equivalent KCl12 cuboctahedra, faces with six equivalent KCl12 cuboctahedra, and faces with four equivalent WCl6 octahedra. All K–Cl bond lengths are 3.54 Å. W4+ is bonded to six equivalent Cl1- atoms to form WCl6 octahedra that share faces with eight equivalent KCl12 cuboctahedra. All W–Cl bond lengths are 2.39 Å. Cl1- is bonded in a distorted single-bond geometry to four equivalent K1+ and one W4+ atom.
WTeCl9 crystallizes in the triclinic P-1 space group. The structure is one-dimensional and consists of one WTeCl9 ribbon oriented in the (1, 0, 0) direction. W5+ is bonded to six Cl1- atoms to form WCl6 octahedra that share corners with three equivalent TeCl6 octahedra. The corner-sharing octahedra tilt angles range from 45–57°. There are a spread of W–Cl bond distances ranging from 2.28–2.43 Å. Te4+ is bonded to six Cl1- atoms to form distorted TeCl6 octahedra that share corners with three equivalent WCl6 octahedra. The corner-sharing octahedra tilt angles range from 45–57°. There are a spread of Te–Cl bond distances ranging from 2.33–3.22 Å. There are nine inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a 2-coordinate geometry to one W5+ and one Te4+ atom. In the second Cl1- site, Cl1- is bonded in a distorted bent 120 degrees geometry to one W5+ and one Te4+ atom. In the third Cl1- site, Cl1- is bonded in a single-bond geometry to one W5+ atom. In the fourth Cl1- site, Cl1- is bonded in a single-bond geometry to one W5+ atom. In the fifth Cl1- site, Cl1- is bonded in a single-bond geometry to one Te4+ atom. In the sixth Cl1- site, Cl1- is bonded in a distorted bent 120 degrees geometry to one W5+ and one Te4+ atom. In the seventh Cl1- site, Cl1- is bonded in a single-bond geometry to one Te4+ atom. In the eighth Cl1- site, Cl1- is bonded in a single-bond geometry to one Te4+ atom. In the ninth Cl1- site, Cl1- is bonded in a single-bond geometry to one W5+ atom.
The paper describes a sequential pulsing technique for deposition of refractory alloys and evaluates the technique for the deposition of the tantalum-base alloys Ta-10W (Ta-10 st% W) and T-111 (Ta-8 wt% W-2 wt% Hf). The deposition cycle for Ta-10W was chosen as alternate injections of TaCl5 plus hydrogen and WCl6 plus hydrogen. The cycle for T-111 was chosen as injections of TaCl5 plus hydrogen interspersed with injections of WCl6 plus hydrogen. A temperature range of 900-1300 C was chosen for both alloys. The ability of the pulse process to blanket a uniformly heated section of substrate with a mixture of gases, whose composition varies not with position on the substrate but instead with time of residence in the reactor, allows metal of uniform thickness to be deposited. It is shown that Ta and W can be deposited at high temperature with the formation of a dense columnar grain structure, so that the feasibility of preparing uniformly thick deposits of these elements by a 'pulsing' modification of CVD is demonstrated. A similar attempt to deposit T-111 was unsuccessful due to the difficulty in reducing HfCl4.
K3W2Cl9 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. there are two inequivalent K1+ sites. In the first K1+ site, K1+ is bonded in a 9-coordinate geometry to nine Cl1- atoms. There are three shorter (3.24 Å) and six longer (3.44 Å) K–Cl bond lengths. In the second K1+ site, K1+ is bonded in a 9-coordinate geometry to nine Cl1- atoms. There are a spread of K–Cl bond distances ranging from 3.31–3.63 Å. W3+ is bonded to six Cl1- atoms to form face-sharing WCl6 octahedra. There are three shorter (2.42 Å) and three longer (2.54 Å) W–Cl bond lengths. There are two inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a distorted see-saw-like geometry to three K1+ and one W3+ atom. In the second Cl1- site, Cl1- is bonded in a 5-coordinate geometry to three K1+ and two equivalent W3+ atoms.