Preamble

Effect of Low-Dose Neutron Irradiation on the Microstructure of 6H-SiC Single Crystals

Songbao Zhanga,b, Zhanfeng Yanb, Hao Wangb, Jiting Tianb, Wei Zhoub, Yuchi Cuia, Jian Zhengb,, Zhe Chena,

a School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
b Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China

Abstract

The irradiation response of pure and nitrogen-doped 6H-SiC single crystals was investigated following low-fluence neutron irradiation at 30–60°C. Internal friction, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy were employed to characterize the microstructural changes. Neutron irradiation induced a significant increase in internal friction, which exhibited distinct temperature-dependent behaviors, indicating competition between two opposing contributions from different defect types. The doped nitrogen atoms caused minor lattice shrinkage due to their smaller atomic radius. Neutron irradiation produced obvious volume swelling of 2.4–3.4%, which was slightly larger in nitrogen-doped 6H-SiC. Raman spectra showed significant changes in peak intensity, shift, and broadening, correlating with the formation of irradiation-induced defects. The total disorder, derived from Raman spectra, increased with irradiation dose and reached saturation, with nitrogen-doped samples showing higher disorder than pure ones. These findings highlight the influence of nitrogen doping on the microstructural response of 6H-SiC to neutron irradiation, offering insights into its potential applications in nuclear environments.

Keywords: 6H-SiC, neutron irradiation, swelling, internal friction, XRD, Raman

*Corresponding author. Tel.: +86 816 2491738; fax: +86 816 2493835.
E-mail address: latent89@hotmail.com (Jian Zheng), zhe.chen@sjtu.edu.cn (Zhe Chen)

1. Introduction

Silicon carbide (SiC) is highly attractive for nuclear applications due to its superior high-temperature mechanical properties, high thermal conductivity, low thermal expansion coefficient, low neutron absorption cross-section, and minimal production of long-lived radioisotopes under neutron irradiation [1][2][3]. Significant research has focused on SiC-based materials for their potential use in structural components and accident-tolerant fuel cladding within the nuclear energy sector [4][5]. Additionally, neutron-irradiated SiC, particularly at relatively low fluences, can serve as a temperature monitor in research reactors and for aerospace applications [6][7][8][9][10].

For these applications, high-energy neutrons create atomic displacements, inducing large concentrations of defects that cause significant microstructural changes. Consequently, the material's mechanical, thermal, and electrical properties—which are crucial for performance in extreme conditions—are altered [11]. Extensive studies have been performed to elucidate the mechanisms of irradiation-induced swelling [12][13][14][15][16][17], concluding that swelling occurs primarily due to the accumulation of radiation-induced defects, including point defects, small interstitial clusters, large dislocation loops, and vacancy clusters [18].

In particular, at moderate fluences and low irradiation temperatures, SiC swelling is contributed mostly by point defects [14][15], whose stability can be affected by polytype and impurities [19]. Ion doping, particularly nitrogen doping, has become a routine method to improve the electronic properties of SiC. However, the synergistic effect of nitrogen doping and neutron irradiation-induced defects on the microstructure of 6H-SiC single crystals has not been extensively studied [20][21]. The limited use of techniques such as internal friction (IF), which has proven useful for detecting irradiation-induced microstructural changes, represents a key barrier to studying the irradiation response of SiC [22][23][24].

In this work, we conducted a comparative study of pure and nitrogen-doped 6H-SiC single crystals to reveal microstructural changes after low-fluence neutron irradiation at low temperature. Using internal friction (IF), X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM), we aimed to gain a deeper understanding of irradiation-induced microstructural changes and compare the effects of nitrogen doping on these changes. The findings will provide valuable insights for optimizing SiC-based materials for nuclear applications and enhancing their performance under irradiation.

2. Experimental

The samples consisted of bulk pure 6H-SiC (denoted as 6H-SI, electrical resistivity: ≥1×105 Ω·cm) and nitrogen-doped 6H-SiC (denoted as 6H-N, doping concentration: 1019 ions/cm3, electrical resistivity: 0.015–0.028 Ω·cm). All single crystal wafers were oriented on the (0001) plane with dimensions of 40 mm length, 3 mm width, and 0.30–0.42 mm thickness. They were irradiated under a fast neutron (>0.1 MeV) flux of 0.80 and 0.91×1014 n/cm2·s, reaching fast neutron fluences of approximately 2.31×1020 and 2.44×1020 n/cm2 generated by the China Mianyang Research Reactor (CMRR). A perforated capsule was used to minimize heating effects by allowing maximum water coolant flow during irradiation, as described elsewhere [25]. The irradiation temperature was estimated to be 30–60°C based on neutronics and thermal-hydraulic calculations. The neutron irradiation parameters for all samples are summarized in Table 1 [TABLE:1].

X-ray diffraction (XRD) measurements were performed using a conventional DX-2700BH diffractometer (Haoyuan Instrument Co., China) with a Cu-Kα source operating at 40 kV and 30 mA. Spectra were recorded for the (00012) Bragg reflection plane from 73.5° to 76° with an angular step size of 0.004° and a counting time of 1 s per step.

Raman spectroscopic investigation was conducted on a micro-Raman spectrometer (WITec, alpha300 R). All Raman spectra were excited with a 532 nm laser, with the laser intensity kept around 2 mW to avoid obvious sample heating.

Internal friction measurements were performed using an inverted torsion pendulum (developed by the Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences) operating in free decay mode with a strain amplitude of 80×10-6 and frequencies around 1 Hz. The internal friction coefficient, Q-1, was determined from the free decay signal. Spectra were measured from room temperature to 400°C with a heating rate of 1°C/min.

The microstructure was characterized using a transmission electron microscope (TEM, FEI, Themis Z). TEM samples were prepared using conventional focused ion beam (FIB, Thermo Scientific, Scios 2) lift-out techniques.

3.1 Internal Friction

Fig. 1 shows the temperature-dependent internal friction of virgin and irradiated 6H-SI and 6H-N single crystals. No detectable relaxation peaks were observed in any samples. For the unirradiated samples, internal friction was relatively low and increased slowly with temperature for both 6H-SI and 6H-N, which is typical for single-crystal SiC and similar to sapphire behavior [26]. For the virgin 6H-N sample, an upward shift occurred after the temperature rose to 300–350°C. It has been established that at low interstitial-solute concentrations, the relaxation strength of internal friction at a given temperature depends on the nature, positions, and concentration of interstitial atoms [27]. At the testing frequency of 1 Hz, nitrogen-related Snoek peaks have been observed in various materials, including around 300 K for Fe, 562 K for Nb, and 615 K for Ta [28]. In this work, the increasing internal friction in the virgin 6H-N sample at higher temperatures is mainly attributed to the evolution of doped nitrogen atoms, with the possible nitrogen-related Snoek peak expected to appear at temperatures higher than the tested 673 K.

For both 6H-SI and 6H-N samples, internal friction increased significantly after neutron irradiation. At room temperature, internal friction for both virgin samples was less than 0.5×10-3, but it increased substantially to approximately 2×10-3 for all irradiated samples. A distinct plateau was observed in the internal friction curves between approximately 150°C and 230°C for all irradiated samples. Outside this plateau region, internal friction rose quickly with increasing temperature.

The primary contributor to internal friction in SiC is defect relaxation [29]. The formation of numerous radiation-induced defects leads to the observed increase in internal friction. The nature of radiation-induced defects in SiC is multifaceted, and their recovery behavior at elevated temperatures varies significantly depending on defect type [15][18][4][14], leading to the complex temperature dependence of internal friction. The plateau observed in Fig. 1 suggests the concurrent presence of two competing contributions from different defect types to the internal friction between approximately 150°C and 230°C. This competition warrants further investigation, as it may provide insights into the thermal recovery of specific defects under irradiation.

Fig. 2 shows the relative elastic moduli in virgin and irradiated 6H-SI and 6H-N single crystals. Neutron irradiation caused significant modulus increases for both materials, with higher doses producing higher moduli. This can be attributed to enhanced resistance to deformation caused by the accumulation of radiation-induced defects, whose strain fields impede dislocation motion.

3.2 Irradiation-Induced Swelling

To characterize lattice swelling induced by neutron irradiation, XRD measurements were used to record patterns in the vicinity of the (00012) reflection plane, as shown in Fig. 3. Small peaks accompanying the main peaks correspond to Kα2 spectra. Compared to 6H-SI, the diffraction peaks of 6H-N shift to higher angles under the same irradiation conditions, indicating lattice shrinkage attributed to the smaller atomic radius of N atoms (65 pm) compared to Si (110 pm) and C (70 pm). For both 6H-SI and 6H-N, neutron irradiation caused obvious peak shifts to lower angles. According to Bragg's law (2d sinθ = nλ), this shift indicates expansion of interplanar spacing, signifying lattice swelling induced by neutron irradiation [16]. The extent of lattice swelling increases with irradiation dose.

The XRD data were quantitatively analyzed using Jade software, with results listed in Table 2 [TABLE:2]. Volume swelling was calculated assuming isotropic lattice swelling in 6H-SiC, as demonstrated in previous studies [21]. The results indicate a volume swelling range of 2.4–3.4% following low-dose neutron irradiation at low temperature. These findings are consistent with Kuryachiy et al., who observed lattice volume expansion of 2.5–3% in 3C-SiC following a neutron fluence of approximately 2×1020 n/cm2 (E>0.18 MeV) at about 100°C [30]. Notably, the volume swelling observed in 6H-N samples was higher than in 6H-SI samples under the same irradiation parameters, suggesting that nitrogen doping may enhance swelling behavior. This indicates that neutron-irradiated 6H-N could be more suitable for temperature sensing applications due to its higher swelling response.

3.3 Irradiation-Induced Microstructural Changes

Raman spectroscopy was used to examine the impact of neutron irradiation on the crystalline structure of 6H-SiC. Figure 4 [FIGURE:4] shows Raman spectra for both virgin and irradiated samples. For both 6H-SI and 6H-N, the relative Raman intensity decreased exponentially after irradiation, a direct result of increased optical absorption in the damaged layer where defects accumulate [31]. Theoretical and experimental studies indicate that Raman-active modes in 6H-SiC include 5A1, 5E1, and 6E2 [32]. For the virgin 6H-SI sample, three first-order Si-C vibration peaks were detected in the 200–2000 cm-1 range at 773, 795, and 972 cm-1, corresponding to E2(TO), E2(TO), and A1(LO) modes, respectively [20][33]. In contrast, the virgin 6H-N sample exhibited an additional first-order peak at 505 cm-1, corresponding to the A1(LA) mode, along with two second-order peaks in the 1500–1800 cm-1 range. The asymmetric peak at 505 cm-1 can be divided into two smaller peaks (486 cm-1 and 507 cm-1), attributed to valley-orbit transitions occurring at nitrogen donors located on two inequivalent cubic sites [34].

Significant changes including peak intensity reduction, peak shift, and broadening are observed in the Raman spectra of neutron-irradiated samples compared to virgin ones. These alterations indicate the formation of irradiation-induced defects [35]. As discussed by Chen et al. [20], Raman analysis is highly sensitive to antisite defects and interstitial atoms. An obvious blueshift of the A1(LO) peak is also observed after neutron irradiation due to radiation-induced lattice strain [20][36], consistent with the large lattice swelling discussed in Section 3.2.

To quantitatively assess irradiation-induced damage from Raman spectra, the total disorder (1-Anorm) was calculated based on the definition by Menzel et al., where Anorm corresponds to the total area A under characteristic Raman lines normalized to the value Acryst of the crystalline material (i.e., Anorm = A/Acryst) [36]. Available ion and neutron irradiation data from literature [20][31] are also included in Fig. 5. The calculated total disorder for 6H-N samples follows a similar trend to ion-irradiated SiC at both room temperature and 400°C, with total disorder increasing with irradiation dose and then reaching saturation at nearly similar doses. In contrast, 6H-SI samples exhibit much lower total disorder at equivalent doses. This discrepancy suggests that nitrogen doping significantly impacts the defect structure, as changes in nitrogen concentration can dramatically alter the position and shape of the A1(LO) phonon, leading to higher total disorder [37].

4. Conclusions

This study investigated the microstructural evolution of pure and nitrogen-doped 6H-SiC after low-fluence neutron irradiation at 30–60°C. The results demonstrate that neutron irradiation significantly alters the internal friction, lattice swelling, and mechanical properties of SiC. The internal friction of both pure and nitrogen-doped samples increased markedly post-irradiation, with a characteristic plateau observed between 150°C and 230°C, indicative of competing contributions from different defect types. Additionally, neutron irradiation caused substantial increases in the elastic modulus of both 6H-SI and 6H-N samples, with higher irradiation doses correlating with higher moduli. This suggests that radiation-induced defect accumulation leads to increased material stiffness, consistent with radiation hardening.

XRD measurements revealed that neutron irradiation induced lattice swelling in both pure and nitrogen-doped 6H-SiC, with volume expansions ranging from 2.4% to 3.4%. Notably, swelling was more pronounced in nitrogen-doped SiC, which can be attributed to enhanced defect formation due to nitrogen doping. Raman spectroscopy confirmed that irradiation induces extensive defect formation, as evidenced by changes in peak intensity, shift, and broadening, with nitrogen-doped samples exhibiting higher total disorder compared to pure SiC.

These findings provide valuable insights into the irradiation behavior of SiC, particularly regarding the influence of nitrogen doping on defect formation, lattice swelling, and material properties under neutron irradiation.

CRediT Authorship Contribution Statement

Songbao Zhang: Writing – original draft, Methodology, Investigation. Zhanfeng Yan: Methodology, Investigation. Hao Wang: Methodology, Investigation. Jiting Tian: Investigation. Wei Zhou: Investigation, Funding acquisition. Yuchi Cui: Methodology, Investigation. Jian Zheng: Writing – review & editing, Methodology, Investigation, Conceptualization. Zhe Chen: Supervision, Resources, Conceptualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Nuclear Technology Research and Development Project (Grant No. HJSYF2024(12)). We are grateful to Chuanxin Liu and Zhuoming Xie for their valuable contributions to this work.

Data Availability

Data will be made available on request.

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