Journal of Nuclear Materials 444 (2014) 170–174

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Journal of Nuclear Materials

journal homepage: www.elsevier .com/locate / jnucmat

Development of a multi-layer diffusion couple to study fission producttransport in b-SiC

0022-3115/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jnucmat.2013.09.040

⇑ Corresponding author. Tel.: +1 734 763 4476 (O); fax: +1 734 763 4540.E-mail addresses: shyamd@umich.edu (S. Dwaraknath), gsw@umich.edu (G.S.

Was).

S. Dwaraknath ⇑, G.S. WasDepartment of Nuclear Engineering and Radiological Sciences, University of Michigan, 2355 Bonisteel Blvd, Ann Arbor, MI 48109-2104, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 September 2013Accepted 20 September 2013Available online 29 September 2013

A multi-layer diffusion couple was designed to study fission product diffusion behavior while avoidingthe pitfalls of direct ion implantation. Thin films of highly anisotropic pyrolytic carbon (PyC) were depos-ited onto CVD b-SiC substrates. The PyC films were subsequently implanted with 400 keV silver, cesium,strontium, europium, or iodine at 22 �C to a dose of 1016 cm�2, such that the implanted species residedwholly within the PyC layer. The samples were then coated with 50 nm of SiC via plasma enhancedCVD (PECVD) to retain the implanted species during post-deposition annealing treatments. The designallows for high spatial resolution tracking of the implanted specie using Rutherford backscatteringspectrometry. Annealing at 1100 �C for 10 h resulted in retention of 100% of implanted cesium, strontium,europium and iodine, and 70% of silver. This diffusion couple design provides the opportunity to deter-mine diffusion coefficients of FPs in PyC and SiC and the role of the PyC/SiC interface in FP transport.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Containment of fission products (FP) within the TRISO fuel par-ticle is critical to the success of the Very High Temperature Reactor(VHTR). In these particles, the SiC layer provides both structuralsupport and serves as the primary diffusion barrier for FPs attemperatures up to 1300 �C during normal operation and 1600 �Cduring accident conditions. While the behavior of several fissionproducts is of interest, the fission products silver, palladium, andeuropium appear to be able to penetrate the SiC layer most easily[1–7], with significant release in both in-pile irradiation and out-of-pile post-irradiation annealing. While less certain, release mea-surements also suggest that cesium and strontium could diffusethrough SiC. No data exists for iodine, which has a half-life tooshort to identify in post-irradiation examination. These measure-ments also exhibit large batch-to-batch variability suggesting thatmicrostructure plays an important role in FP transport across theSiC.

Experiments to isolate FP transport through SiC have focused ondirect ion implantation into SiC for the determination of the diffu-sion coefficient of FPs at temperatures well over the melting tem-perature of their metallic forms [8–11]. Diffusion coefficients canbe extracted by analyzing the broadening of the Gaussian shapedconcentration profile created by implantation [11]. Silver is the fis-sion product most heavily studied by this method for which the re-sults are rather inconsistent, indicating that silver is both immobile

up to 1500 �C in SiC [9] and that it diffuses in a traditional Fickianmanner as low as 1300 �C [11]. The measured diffusion coefficientsusing direct ion implantation are also orders of magnitude lowerthen those required to account for high silver release from TRISOfuel under normal operating conditions [12,13]. Our own direction implantation experiments have exhibited non-Fickian diffu-sion. Fig. 1 shows the silver concentration profiles as a functionof depth for polycrystalline b-SiC implanted with 400 keV silverto a fluence of 1016 cm�2 and after annealing at 1600 �C for 10and 60 h. Note the significant decrease in the integrated concentra-tions with no indications of broadening in the full width half-maxima that would be expected from Fickian diffusion.

One explanation for these inconsistencies is that the largeamount of radiation damage introduced during implantation intothe SiC alters the microstructure. Implantation can also result inFP concentrations well above the solubility limit, resulting in acomplex spatially segregated source term from which diffusioncoefficients are difficult to determine. Both of these factors areshown in Fig. 2, which is a scanning transmission electron micros-copy (STEM) image of as-implanted b-SiC corresponding to theconcentration profiles in Fig. 1. The diffraction pattern correspondsto the implanted region where the SiC has been completely amor-phized and the implanted silver is in the form of large precipitatesof varying size.

Traditional diffusion couples, where the FP of interest is in di-rect contact with SiC either through deposition or mechanical con-tact, have yet to yield meaningful results [9,14,15]. Most FPs ofinterest have melting temperatures well below the temperaturerange of interest: 1100–1600 �C, resulting in a significant FP vaporpressure that quickly removes it from the diffusion couple before it

Fig. 1. Silver concentration profiles from RBS measurements of a SiC sampleimplanted with 400 keV silver followed by annealing at 1600 �C for various times.

S. Dwaraknath, G.S. Was / Journal of Nuclear Materials 444 (2014) 170–174 171

can diffuse into SiC. Efforts to contain the FP within the diffusioncouple have resulted in geometries that are difficult to analyzeand with techniques that do not have the spatial resolution andsensitivity necessary to identify and isolate the possible diffusionmechanisms. Directly coupling SiC with an FP also introduces theFP to SiC at concentrations well above those expected in TRISO par-ticles. This creates the possibility for reactions that hamper theability to measure diffusion coefficients and is limited to FPs thatcan be safely handled in open atmosphere. This method also failsto take into account the effect of the inner pyrolytic carbon–siliconcarbide (IPyC/SiC) interface, which could play a significant role incontrolling FP transport through the TRISO layers.

To avoid these pitfalls, a diffusion couple design should providefor the introduction of FPs into the PyC in realistic quantities andwithout altering the SiC layer. One way to do this is to use a hostlayer. This host layer should be PyC to recreate the local chemistryinvolved with FP transport across the IPyC/SiC interface. A barrier

Silver Clusters

Fig. 2. HAADF image showing SiC implanted with 400 keV silver at roomtemperature. The amorphous diffraction pattern corresponds to the implantedregion.

layer that can be deposited at low temperatures will also be neces-sary to prevent FP escape from the diffusion couple via the front orside surfaces of the PyC host layer. The diffusion couple geometrymust be conducive to high-resolution high sensitivity analyticaltechniques that can be used to resolve the diffusion profiles. Theobjective of this paper is to describe and demonstrate the capabil-ities of such a novel diffusion couple.

2. Diffusion couple fabrication

Diffusion couples are made using thin film techniques such aschemical vapor deposition (CVD) to create the thin films, ionimplantation to introduce various FPs into the host layer and Ruth-erford backscattering spectroscopy (RBS) to profile the FP concen-tration as a function of depth after synthesis and subsequentannealing. The fabrication of the diffusion couple is divided into4 steps, as shown schematically in Fig. 3; substrate preparation,PyC deposition, FP implantation, and SiC cap deposition. Propersubstrate preparation will result in a sharp substrate/host layerinterface to accurately measure the diffusion depth of FPs intothe SiC. The PyC host layer will be deposited via CVD in order tobe chemically similar to TRISO PyC. FP implantation into the PyCwill introduce the FP and provide the capability to investigate awide range of elements. A final SiC diffusion barrier layer will bedeposited using plasma enhanced CVD (PECVD) to prevent prema-ture loss of the FPs during annealing.

2.1. Substrate preparation

Polycrystalline SiC was chosen as the substrate material to be asrepresentative of TRISO SiC as possible. One advantage of ourdiffusion couple design is the ability to tailor the substrate toinvestigate specific mechanisms, e.g. single crystal SiC grown onSi to investigate bulk diffusion. The SiC substrates weresupplied by Rohm and Haas Chemicals LLC in the form of1 cm � 1 cm � 0.6 mm plates made by grinding and cutting a lar-ger plate grown by CVD. X-ray diffraction (XRD) analysis was con-ducted using a Rigaku X-ray diffractometer with Cu Ka radiationfrom 30� to 110� with a step size of 0.01� to identify if any phasesother then b-SiC were present. The XRD spectrum, shown in Fig. 4contains peaks that match only with b-SiC. There is also a broadplateau near the (111) peak that is characteristic of stacking faultsin SiC [16].

The substrates were ground flat using 9 lm diamond solutionand polished using 3 lm, 1 lm, and 0.25 lm diamond solutionsfor 5 min per step prior to a vibratory polish using 0.02 lm colloi-dal silica for 4 h. Surface roughness, measured using a Veeco

a b c d

SiCSubstrate

DepositedPyC

FP Implanted PyC

DepositedSiC

Fig. 3. Schematic illustration of the four steps in preparing the novel diffusioncouple: (a) Preparation of the substrate to create a sharp interface with the PyC. (b)Deposition of a layer of CVD PyC. (c) Implantation of an FP into the PyC. (d)Deposition of a SiC cap to retain the FP within the diffusion couple at hightemperature.

Fig. 4. Cu Ka XRD of the SiC substrates provided by Rohm and Haas Inc. All thepeaks correspond to b-SiC. The inlay shows the broad plateau near the (111) peakthat is characteristic of stacking faults in SiC [16].

172 S. Dwaraknath, G.S. Was / Journal of Nuclear Materials 444 (2014) 170–174

Dimension Icon atomic force microscope (AFM) sampling8000 lm2, was 22 nm Ra after vibratory polishing. While the che-mo-mechanical polishing (CMP) mechanism of the vibratory polishremoves the damage layer introduced by conventional mechanicalpolishing, it also introduces a surface contamination layer [17]. Afinal plasma cleaning, using a 500 W argon plasma at 130 mTorrfor 5 min, was used to remove this layer and improve the surfacefinish by reducing the surface roughness to 6 nm Ra.

2.2. PyC deposition

SRIM [18] simulations were used to determine that 250 nm ofPyC would completely contain the implantation. Fig. 5 shows onesuch simulation of the silver concentration profile and displace-ment damage introduced into to PyC for a 400 keV silver implanta-tion to a fluence of 1016 cm�2. A similar simulation for 400 keVcesium indicated that 300 nm of PyC would be necessary to accom-modate the lighter element. The PyC thickness was standardized to300 nm for all the elements investigated to ensure that similarconcentrations were achieved. PyC depositions were performed

Fig. 5. Implantation range and damage for 400 keV Ag into PyC to a fluence of1016 cm�2. The green lines mark the location of the crossover between the expectedcrystalline and amorphous regions. (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)

in a vertical flow hot-walled graphite reactor at 1300 �C and15 Torr. The furnace was initially evacuated to 50 mTorr using atwo-stage oil-sealed rotary vane pump before flowing 50 sccm ofultra-high purity (UHP) argon controlled by an Alicat Scientificmass flow controller. A throttle valve was used to control pumpingspeed and maintain a pressure of 15 Torr in the furnace. The fur-nace was then heated to 1300 �C over a 1-h period. The tempera-ture was verified using a spot pyrometer measured on thegraphite support on which the samples rest. After allowing thetemperature to settle to within 5 �C of the 1300 �C set point,21.6 sccm of 99.5% purity propylene was introduced via anotherAlicat Scientific mass flow controller and the deposition proceedsfor a total of 35 min. When the deposition was complete, the pro-pylene flow was cut off, and the reactor was cooled down over 4 hto prevent thermally shocking the deposited PyC.

Coating thicknesses were measured using RBS performed at theMichigan Ion Beam Laboratory (MIBL). A 1 mm diameter 2 MeVHe++ beam was placed on the sample at normal incidence to thesample surface with a detector at a backscatter angle of 160�.Fig. 6 shows an RBS spectrum for a 308 nm PyC deposition on aSiC substrate. The green arrow marks the front surface energy forthe PyC, while blue arrows mark the width of the carbon plateaufrom the PyC and the Si edge from the SiC. The PyC thicknesswas calculated by simultaneously fitting the width of the carbonplateau and the shift down in energy of the Si edge due to thePyC, assuming the PyC was at the theoretical density of 2.21 g/cm3. This yielded a thickness of 306 ± 10 nm as measured over 5batches of 10 samples. The sample roughness, as measured over8000 lm2 using previously stated AFM, increased to 11 nm Ra afterPyC deposition.

2.3. Fission product implantation

FP implantation was performed at MIBL using a NationalElectrostatics Ion Implanter. All of the investigated FPs: silver,cesium, strontium, europium and iodine, were implanted at400 keV to a fluence of 1016 cm�2. The implantation energy waschosen to maximize the implanter efficiency, while the fluencewas chosen to be in the order of magnitude of the FP yield of silverper unit IPyC/SiC interface area for a TRISO particle at 20% burn up[19]. The implantation flux was limited to 1.5 � 1011 cm�2 s�1 tomaintain the sample temperature at room temperature as mea-sured by a thermocouple located behind a sample. The resulting

Fig. 6. RBS spectrum of a 320 nm thick PyC layer on a SiC substrate. The greenarrow marks the front surface of the PyC. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. RBS spectrum for a strontium diffusion couple showing the change in thestrontium profile before and after annealing at 1100 �C for 10 h. The dashed greenline marks the front surface energy for strontium. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

S. Dwaraknath, G.S. Was / Journal of Nuclear Materials 444 (2014) 170–174 173

FP concentration profile, was measured using RBS and was foundto be Gaussian in shape and contained within the PyC. Implanta-tion peak depths ranged from 60 nm for europium to 160 nm forstrontium. Peak concentrations ranging from 1.9% to 0.95%. Fullwidth half-maxima (FWHM) ranged from 53 nm to 88 nm. Fig. 5shows the implantation range and damage, as calculated by SRIM[18] using a carbon displacement energy of 24 eV [20] for400 keV silver into PyC to a fluence of 1016 cm�2. The high fluenceinduces a large amount of radiation damage in the PyC. Most of theimplanted silver is within the expected amorphous region, whichoccurs over 1 dpa [21], while a small portion of the PyC near thesubstrate is left undamaged. This makes it possible to determinediffusion coefficients for the FPs in the undamaged PyC.

2.4. SiC cap deposition

It was found early on that FPs easily migrate through the frontsurface and out the sides of the PyC layer at annealing tempera-tures of 1100–1600 �C, well over their respective melting points.Several coatings were tested for their efficacy in retaining fissionproducts within the diffusion couple. Physical vapor depositionsof molybdenum and tungsten delaminated from the PyC at900 �C. Sputter depositions of SiC proved to be ineffective, allowingall of the implanted silver to be released from the diffusion coupleafter annealing at 1100 �C for 10 h. It was postulated that the silverwas diffusing out the edges of the PyC coating. SiC coatings createdby pyrolyzing spin-coated polycarbosilane (PCS), which coveredthe edges of the diffusion couple as well as the front surface,showed promise in retaining both silver and cesium at 1100 �C.The coatings were difficult to reproduce, exhibited a high degreeof non-uniformity and greatly enhanced the roughness of the diffu-sion couple.

PECVD SiC was chosen as the diffusion barrier to further retainFPs within the PyC and thus, allow for diffusion into the SiC sub-strate. This process creates a reproducible, high density, low rough-ness coating. The thickness of this cap was limited to a maximumof 100 nm in order to retain the ability to accurately measure FPconcentration profiles in the PyC via RBS. Initial tests showed thata 50 nm thick was sufficient to retain FPs within the diffusion cou-ple. The PECVD SiC was deposited using a Plasmatherm 790 PECVDtool. The chamber was evacuated using a two-stage turbo-dragpump backed by a semiconductor pump to a pressure of 10�8 Torr

Fig. 7. RBS spectrum of a diffusion couple for strontium prior to annealing. Thefront surface energies for the various elements are marked by arrows with dashedgreen lines. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

before being heated to 250 �C. Then, 1000 sccm of argon wasflowed into the chamber for 2 min to allow the throttle valve tostabilize the chamber pressure at 1 Torr before flowing 50 sccmof methane and 5 sccm of silane. The plasma was powered by a13.56 MHz RF generator, which provides 0.05 W cm�2 for 5 min.The chamber was then re-evacuated to 10�8 Torr and cooled beforeremoving the diffusion couples.

Fig. 7 shows the RBS spectrum for a strontium diffusion coupleafter being coated with the PECVD SiC. Green arrows mark frontsurface energies. The peak closest to the strontium surface energyis from the strontium implantation into the PyC. The PECVD capprimarily shows up as the Si peak at the 1143 keV. There is alsoan oxygen peak and nitrogen peak that are contaminants in thePECVD SiC due to contaminants in the argon. The Si to C ratiowas measured from the ratio of the Si peak to the height of the stepin the carbon edge that marks the transition from PECVD SiC toPyC. The Si/C ratio ranged between 1.0 and 1.2. The sample rough-ness did not change from 22 nm Ra measured post-implantation asa result of the PECVD deposition.

Fig. 9. RBS spectrum for a silver diffusion couple showing the change in the silverprofile before and after annealing at 1100 �C for 10 h. The dashed green line marksthe front surface energy for silver. (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)

174 S. Dwaraknath, G.S. Was / Journal of Nuclear Materials 444 (2014) 170–174

A cross-section sample of the PECVD SiC cap on PyC was pre-pared for transmission electron microscopy (TEM) using the focusion beam (FIB) technique. A diffraction analysis of the SiC cap re-vealed that it was amorphous, as expected of the low temperatureprocess. The density, calculated from the areal density measuredby RBS and the thickness measured in TEM, yielded a value of2.64 g cm�3 or 83% of the theoretical density of SiC.

3. Verification of diffusion couple operation

The functionality of the diffusion couple was assessed using an1100 �C 10 h anneal to test the retention of fission products silver,cesium, strontium, europium, and iodine. An effective diffusioncouple should retain 100% of the FP. The 1100 �C anneal was con-ducted in the same graphite furnace that was used for PyC deposi-tion. A roughing pump was used to evacuate the chamber to50 mTorr. The chamber was heated to 150 �C and allowed to dwelluntil the vacuum returned to 50 mTorr, at which point 50 sccm ofUHP argon was introduced. The throttle valve on the rough pumpwas slowly engaged until it was completely closed and the pres-sure in the chamber was allowed to rise to 860 Torr. At that pointa leak valve was opened on the chamber allowing the pressure todrop to 800 Torr, while argon was still flowing, and establishinga slight over-pressure of UHP argon within the furnace while main-taining a constant flow of argon. The furnace was then heated up to1100 �C over 50 min and then allowed to dwell for 10 h. The finaltemperature was verified by the same spot pyrometer used duringPyC depositions on the same graphite support holding the diffusioncouples. The furnace was cooled down over 2 h to ensure that thethin films did not delaminate.

Fig. 8 shows the RBS spectrum for a strontium diffusion couplebefore and after annealing at 1100 �C for 10 h. Note the redistribu-tion of the strontium from a Gaussian shaped peak after implanta-tion to what is indicative of complete dissolution into the PyC witha constant concentration of 0.2% throughout the PyC. There wasalso significant strontium enrichment at the PyC/SiC cap interfaceto a concentration of 1.4%, with a small amount of the strontiumpenetrating into the SiC cap. Most importantly, 100% of the im-planted strontium was retained within the diffusion couple, indi-cating that the SiC cap is an effective diffusion barrier. Similartests of cesium, europium, and iodine all resulted in 100% retentionof the implanted FP.

A similar test of silver in our diffusion couple demonstrates theability of the silver to penetrate TRISO coatings at high tempera-ture. Fig. 9 shows the RBS spectrum for a silver diffusion couple be-fore and after annealing at 1100 �C for 10 h. The implanted silverredistributed into two peaks, one near the implantation peak andanother towards the front surface of the diffusion couple nearthe PyC/PECVD SiC interface. A small portion of the silver was ableto penetrate through this layer and escape the diffusion coupleleading to a net retention of 71% of the implanted silver. The pres-ence of oxygen and nitrogen in the PECVD SiC likely degraded theretentive capabilities of this layer, existing in structure as SiO2 andSiN, neither of which was as effective a diffusion barrier as SiC.Future modifications to the PECVD device will introduce UHPhydrogen as the carrier gas, allowing for the deposition of pureSiC. This diffusion couple design will permit the determination ofFP diffusion coefficients in PyC and SiC, the role of the PyC/SiCinterface in FP transport and the transport mechanism.

4. Summary

A diffusion couple design for determining the mechanism of dif-fusion of various FPs in SiC has been developed and tested. Thinfilms of PyC were reproducibly deposited on SiC substrates fol-lowed by controlled implantations of silver, cesium, strontium,europium, and iodine such that the implanted FP was confined tothe PyC layer. Because the PyC between the implant distributionand the substrate was undamaged, diffusion coefficients of FPs inPyC are obtainable. A final layer of PECVD SiC was deposited tocontain the FP within the diffusion couple. The FP concentrationprofiles were determined using RBS, demonstrating the relativeease in analyzing the diffusion couple. All of the implanted cesium,europium, iodine, and strontium was retained by the PECVD SiCbarrier within the diffusion couple after annealing at 1100 �C for10 h, demonstrating the versatility of this design in investigatingdifferent FPs. Only silver escaped through the barrier, which islikely due to the presence of nitrogen and oxygen in the cap layer.

Acknowledgements

The authors gratefully acknowledge Ovidiu Toader and FabianNaab for their assistance in conducting RBS measurements andion implantations, and Lumin Wang and Gang Yu for their assis-tance in TEM analysis. The authors also acknowledge the facilitiesprovided by the Michigan Ion Beam Laboratory, the ElectronMicrobeam Analysis Laboratory, and the Lurie NanoFab at Univer-sity of Michigan. SiC substrates were provided by Rohm and HaasChemicals LLC. Support for this research was provided by theDepartment of Energy under award number NEUP-002-10.

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