Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Radiation-Induced Changes in Quartz, A Mineral Analog of Nuclear Power Plant Concrete Aggregates Chinthaka M. Silva,*,† Thomas M. Rosseel,† and Marie C. Kirkegaard‡,§ †
Materials Science and Technology Division and ‡Nuclear Security and Isotope Technology Division, Oak Ridge National Laboratory, One Bethel Road, Oak Ridge, Tennessee 37831, United States § Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee 37996, United States ABSTRACT: Quartz single-crystal samples consisting of αquartz crystal structure were neutron irradiated to fluences of 5 × 1018, 4 × 1019, and 2 × 1020 n/cm2 (E > 0.1 MeV) at two temperatures (52 and 95 °C). The changes in the α-quartz phase as a function of these two conditions (temperature and fluence) were studied using X-ray powder diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM), and the results acquired using these complementary techniques are presented in a single place for the first time. XRD studies showed that the lattice parameters of α-quartz increased with increasing neutron flux. The lattice growth was larger for the samples that were neutron irradiated at 52 °C than at 95 °C. Moreover, an amorphous content was determined in the quartz samples neutron irradiated at 4 × 1019 n/cm2, with the greater amount being in the 52 °C irradiated sample. Complete amorphization of quartz was observed at a fluence of 2 × 1020 n/cm2 (E > 0.1 MeV) using XRD and confirmed by TEM characterization and Raman spectroscopic studies. The cause for α-quartz lattice expansion and sample amorphization was also explored using XRD and Raman spectroscopic studies.
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INTRODUCTION With the increasing demand for electrical power, the extended operation of currently existing US nuclear power plants (NPPs) beyond 60 years is under consideration. Among the materialsrelated aging issues being addressed, evaluation of the behavior of concrete structures is of high importance because concrete is used in many functions and parts of NPPs, including radiation shielding, spent-fuel pool girders, ice condenser floor systems, and reactor cavity walls in prestressed concrete conditions. As NPP concrete structures age, concrete can degrade due to factors such as hydrogen stress cracking, (microbiological) corrosion, sustained temperature cycles and fatigue, high humidity, retention or spills of water or borated water, carbonation, chemical reactions such as alkali−silica reaction, machine vibrations, and irradiation.1−3 These age-related degradations of structural materials have been increasingly reported.4 Although degradation of concrete structures due to environmental causes has been reported, deterioration caused by direct radiation has not been observed in decommissioned NPPs that have operated for less than 40 years. While periodic inspections and controls are in place to mitigate degradation issues, an in-depth understanding of the behavior of NPP concrete structures under various operating conditions, such as irradiation, is necessary to inform programs that manage aging structures. Specifically, there are knowledge gaps especially with respect to the reference levels for the onset of changes in © XXXX American Chemical Society
mechanical properties and possible effects on structural integrity, as well as constitutive models to predict radiationinduced damage.5−7 These studies also discuss several potential areas requiring additional research. Development of a research plan to investigate aging and degradation of concrete structures in NPPs has been initiated in the United States to provide consistent knowledge on irradiation effects on concrete. NPP concrete structures consist of Portland cement (CaO, SiO2, Al2O3, Fe2O3, and SO3) and ∼70 vol.% site-specific aggregates, which are composed of minerals (e.g., silicates, carbonates including calcite and dolomite [CaCO3 and CaMg(CO3)2], and various oxides), and rocks such as granite, gabbro, basalt, and serpentinite.8 Due to the multiconstituent and multiphase aggregates and their different behaviors under irradiation conditions, reactor cavity concrete structures exhibit different radiation-induced defects. For example, differences in crystallinity of the constituent phase can result in different levels of radiation-induced defect-cluster formation in the phase crystals.9 The internal structure defects such as porosity and interstitials with different chemical and physical properties of hardened cement paste (HCP) will also exhibit varied responses to radiation.9 Moreover, the effects of irradiation on concrete in NPPs are dose and temperature dependent and Received: January 11, 2018
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DOI: 10.1021/acs.inorgchem.8b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Williamson−Hall relationship, βτ cos θ = (Kλ/τ)+ 4ε sin θ, where βτ, ε, K, λ, and τ are the peak broadening due to sample particles, microstrain, shape factor, wavelength, and crystallite size, respectively. Microstrain was determined using the slope (4ε) of the Williamson− Hall plot method. Crystallite size can also be calculated using the Scherrer equation, τ = Kλ/βτ cos θ, using the broadening of each peak. The crystallite sizes reported here were averaged using the first eight Bragg peaks of α-quartz. Amorphous phase content was determined using Rietveld quantitative amorphous content analysis (RQACA).18 Raman Spectroscopy. Raman spectra of pre- and postirradiated quartz samples were acquired using an inVia confocal Raman microscope (Renishaw Inc.). An Ar ion laser with the excitation wavelength of 785 nm, 2 s exposure, 100 accumulations, and 10% intensity (40 μW; power density 892 Wcm−2) was used in collecting the spectra at a resolution of ∼0.04 cm−1 (1200 I/mm grating). All spectra were collected from below the energy cutoff of ∼30−1250 cm−1. Peak fitting was carried out using OriginPro, version b9.2.214. Background subtraction was carried out using a second derivative baseline and the line interpolation method. A first derivative with the Savitzky−Golay second-order polynomial smoothing method was used for the peak fitting. Transmission Electron Microscopy. Powdered quartz samples in an ethanol or isopropyl alcohol solution were immersed and mixed well in a sonication bath. A few drops of the solution containing the sample microparticles were added onto Lacey carbon 200 mesh copper grids (Ted Pella, Inc., Redding, CA). These samples, prepared using the solution-drop method, were characterized using a 2100F JEOL field emission electron microscope, which was operated at a maximum voltage of 200 kV. TEM micrographs in conventional bright field (BF) mode were used to study sample particles, and selected area electron diffraction (SAED) patterns were collected and used to verify the crystalline character of the selected particles.
may also be dependent on neutron flux.5,10,11 Specifically, changes in the mechanical properties of concrete have been observed for neutron fluence values >1 × 1019 n/cm2.10,12,13 The American Concrete Institute recommends a conservative limit of 1 × 1017 n/cm2 fluence for preventing radiationinduced degradation.14 Research performed at Oak Ridge National Laboratory (ORNL), sponsored by the US Department of Energy (DOE) Light Water Reactor Sustainability (LWRS) Program, has focused on the evaluation of long-term irradiation and temperature effects on concrete aggregates. Specifically, a select group of mineral analogs of aggregates found in reactor cavity concrete was irradiated at the ORNL High Flux Isotope Reactor (HFIR). The purpose was to evaluate the evolution of aggregate and mineral amorphization at or above neutron fluences expected in US NPP concrete. The mineral analogues, including calcite (CaCO3), dolomite (CaMg(CO3)2), and quartz (SiO2) samples, were neutron irradiated at three different fluence levels (5 × 1018, 4 × 1019, and 2 × 1020 n/ cm2; E > 0.1 MeV) and at two temperatures (52 and 95 °C). Only the research conducted on quartz samples is however discussed in this current manuscript. The pre- and postirradiated samples were characterized using a variety of techniques including X-ray powder diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). Results obtained for the quartz samples are presented here.
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EXPERIMENTAL DETAILS
Quartz Samples and Neutron Irradiation. Single-crystal quartz (X- and Y-cut) boules purchased from MTI Corporation (Richmond, CA) were cut into samples using a Buehler IsoMet slow-speed saw. An MTI diamond wire saw was used to prepare additional quartz samples for unirradiated baseline measurements. The samples were cut so that the X and Y directions were along the C-axis of the parent crystal. The maximum dimensions of specimens were determined by the capsule geometries and the estimated maximum swelling (∼18%) at complete amorphization.10 Each sample was engraved or marked with a unique identification to ensure proper tracking. Using two positions centered on the peak fluence position within the hydraulic tube of the HFIR, eight quartz samples were irradiated simultaneously in two capsules (sealed and perforated) in three separate irradiations to obtain doses of 0.5, 4.0, and 20 × 1019 n/cm2 (E > 0.1 MeV) at two temperatures (52 and 95 °C). The lowertemperature (perforated) capsule mimics the normal operating temperature (65 °C) of the concrete biological shield, while the higher-temperature (sealed) capsule mimics the maximum design temperature (95 °C) of the biological shield. X-ray Powder Diffraction. XRD patterns of the samples were acquired using a D2 Phaser benchtop X-ray diffractometer (Bruker Inc., Billerica, MA) equipped with Cu Kα radiation (30 kV and 10 mA). Quartz single-crystal samples were powdered using a mortar and pestle. The microsized fine powder was admixed with Si SRM640d or LaB6 SRM660b National Institute of Standards and Technology standards. The LaB6 standard was used where any peak overlapping of the sample due to peak shifts and/or peak broadening at peak trails was present. The powder mixture was then placed on a zerobackground SiO2 single-crystal wafer. A fine and flat slurry of the powder sample was then spread on the zero-background plate using an alcohol solution such as isopropyl or ethanol. If the sample was neutron irradiated, it was covered using Kapton tape to prevent any loss of powder and any potential contamination of personnel and instrument. The sample plate was then placed on a XRD sample holder, which was then placed in the D2 Phaser. XRD patterns were fitted to a quartz-low (α-SiO2) phase of P 32 2 1 space group (ICSD #16331) using GSAS15 or GSAS-II16 profile-fitting software. VESTA software was used to visualize unit cell models.17 Microstrain and crystallite sizes of the samples can be determined using the
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RESULTS Quartz samples studied were low- or α-quartz with a trigonal (space group P3221) crystal system. This trigonal crystal system is distorted from the simple rhombohedral coordinate system, which generally consists of three equal axes and interaxial equal angles, forming a hexagonal-type stacked lattice with a = b ≠ c and α = β = 90° and γ = 120°. Si and O reside at 3a and 6c symmetry sites in its unit cell, respectively. The experimental XRD pattern and its profile fit of the nonirradiated quartz sample are shown in Figure 1. The XRD results show that the material is mostly single-phase α-quartz, with some minor impurity peaks, especially at ∼24° and ∼25.8° 2θ, which could not be identified because the peak intensities are low, and the number of peaks is not sufficient for a proper phase identification. The Rietveld-refined α-SiO2 lattice parameters of the sample are a = b = 4.9138 ± 0.0001 and c = 5.4057 ± 0.0001 Å with a cell volume of ∼113 Å3. These refined lattice parameters are very close to that reported by Le Page et al. and Cohen et al.19,20 In order to obtain in-depth information on the effects of irradiation on the α-quartz lattice, XRD patterns were fitted and the lattice parameters were refined. This was performed on multiple samples of quartz to verify any trend in the lattice parameter variation. As shown in Figure 2, it was observed that both a and c lattice parameters were larger in the postirradiated samples and increased as a function of fluence. Moreover, the increase in lattice parameter a is greater than that of c, which agrees with previous studies.21−23 Additionally, as can be seen in Table 1, there is a greater difference in the increase in lattice parameters for higher neutron fluences (i.e., difference of 0.001 and 0.0002 vs 0.0469 and 0.0155 Å for a and c at 5 × 1018 and 4 × 1019, respectively). At similar neutron fluence values, larger lattice parameters were B
DOI: 10.1021/acs.inorgchem.8b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
the samples as the dose increases. The nonirradiated sample has a c/a ratio of 1.1001(1), and the value decreased to 1.0989(1) at 95 °C and 1.0987(1) at 52 °C, where absolute errors are denoted in parentheses, for the 5 × 1018 n/cm2 irradiated samples. The c/a ratio for β-quartz is 1.0925 (ICSD #26430) or 1.0916 (ICSD #89284), and these values are very close to the value (1.0906(4)) of the sample irradiated at 4 × 1019 n/cm2, 95 °C. The c/a ratio is lowest (1.0835(5)) in the case of the sample irradiated at 4 × 1019 n/cm2, 52 °C, indicating the existence or initiation of structure disorder in the quartz, as the α-to-β quartz phase transition is dominated by structural disorder.24 Relative to the increase in the lattice parameters, the cell volume also increases as the neutron flux on the samples increased. The percent volume increases with respect to the unirradiated sample are 0.35 and 0.39% (Table 1) for the two samples irradiated at 52 and 95 °C to a fluence of ∼5 × 1018 n/ cm2, respectively. The largest unit cell volume is in the sample irradiated to 4 × 1019 n/cm2, at 52 °C. The increase in the unit cell volume compared with that of the unirradiated sample is 5%. A comparison of the experimental XRD patterns of α-SiO2 samples as a function of irradiation conditions is depicted in Figure 3. Narrow and intense peaks in XRD patterns clearly indicate the high crystallinity of the control sample and the samples irradiated to ∼5 × 1018 n/cm2. Broadening and a decrease in intensity of the peaks in the XRD pattern is observed for the 4 × 1019 n/cm2 irradiated samples. In the case of the sample irradiated (4 × 1019 n/cm2) at low temperature (52 °C), some broad features were also observed adjacent to the Bragg peak positions of α-quartz. For samples irradiated to ∼2 × 1020 n/cm2 at 52 °C, no significant peaks were identified in their XRD patterns, except for some broad humps near the Bragg angles of the α-SiO2, indicating extensive or complete amorphization of the samples. The sample irradiated to ∼2 × 1020 n/cm2 at 95 °C also showed no significant peak intensities for a full profile fit, but the peak associated with the (101) Miller indices at ∼26.3° showed narrower peak characteristics compared with that of the ∼2 × 1020 n/cm2/52 °C sample, suggesting less amorphous character of the sample irradiated at ∼2 × 1020 n/cm2 at 95 °C. The peak positions of these broad features closely match that observed for the sample irradiated to 4 × 1019 n/cm2 at low temperature (52 °C), confirming the amorphous content of the latter sample as well. While no amorphous content could be quantified for the ∼5 × 1018 samples, the RQACA technique could be used to quantitatively determine the amorphous content of samples irradiated at 4 × 1019 n/cm2. A 30 wt % amorphous content was determined for the sample irradiated to 4 × 1019 n/cm2 at 95 °C, while the
Figure 1. Fitted XRD pattern of the quartz unirradiated control sample. The experimental XRD and fitted patterns are indicated in red and green, respectively. The residual between experimental and fitted patterns is shown in pink. Black and red tick marks indicate Bragg peak positions of α-SiO2 and Si SRM640d internal standard, respectively. The inset is a magnified area showing SiO2 (100), SiO2 (101), and Si (111) peaks. Arrows indicate the first two impurity peaks in the pattern.
Figure 2. Variation of lattice parameters of quartz samples as a function of irradiation fluence and irradiation temperature. Lattice parameters of multiple samples prepared from each irradiation condition are plotted.
observed for the low-temperature irradiated sample. A decreased c/a lattice parameter ratio can also be observed for
Table 1. Refined Lattice Parameters of Quartz Samples as a Function of Irradiation Conditionsa Irradiation conditions Sample Quartz Q9 Q11 Q5 Q8 Qx Q3, Q4
Fluence (1020 n/cm2, E > 0.1 MeV) 0.0504 0.0492 0.403 0.394 1.97 1.97
Lattice parameters (Å) Temp. (°C) 95 52 95 52 95 52
a 4.9138 4.9213 4.9223 4.9739 5.0208
c (1) (1) (2) (10) (13)
5.4057 5.4081 5.4083 5.4243 5.4398
c/a lattice parameter ratio (1) (1) (2) (10) (13)
1.1001 1.0989 1.0987 1.0906 1.0835
Cell volume (Å3) 113.036 113.434 113.484 116.216 118.757
(1) (2) (3) (1) (2)
Vol. % increase 0.35 0.39 2.81 5.06
a Note: Errors in refined lattice parameters are given inside parentheses. Samples irradiated at ∼2 × 1020 n/cm2 did not show enough peak intensities to refine lattice parameters.
C
DOI: 10.1021/acs.inorgchem.8b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Comparison of XRD patterns of quartz samples (top). Bragg peaks of the samples in a range of ∼20 to ∼34° 2θ are shown in the bottom plots. Peak intensities were normalized to the highest (101) peak of SiO2 acquired for the unirradiated sample for comparison. The asterisk marks show the internal standard peak positions. Broad features observed in low angles are also highlighted for the 4 × 1019 n/cm2/52 °C and 2 × 1020 nm/cm2 samples.
Table 2. Amorphous Content, Microstrain, And Crystallite Size Values of the Samples Acquired Using XRD Analysis Irradiation conditions
Microstrain (%)
Crystallite size (nm)
Sample no. Sample Quartz Q9 Q11 Q5 Q8
Fluence (1020 n/cm2) 0.0504 0.0492 0.403 0.394
Temp. (°C) 95 52 95 52
Sample no.
Amor. Content (RQACA)
1
2
Avg.
1
2
Avg.
30 50
0.030 0.050 0.096 0.332 0.026
0.040 0.232 0.095 0.280 0.251
0.030 0.141 0.096 0.306 0.139
141 167 94 35 103
247 48 108 41 47
194 108 101 38 75
× 1019 n/cm2, higher microstrain and smaller crystallite size were obtained for the sample irradiated at 95 °C in both samples used, indicating greater peak broadening in the 95 °C sample. Crystallite size and lattice strain, however, did not exhibit a clear trend with temperature, as compared with that of the lattice parameters and amorphization, possibly due to variations in the strain uniformity. These observations are discussed in greater detail later in the text. The crystal class of the trigonal unit cell of α-quartz is 3 2 (D3 symmetry), giving Raman scattering tensors of A1, E(X), and E(Y). Several phonon modes are present for this structure due to the presence of nine atoms per unit cell. Quartz should have 13 Raman-active modes: 4 nondegenerate A1 modes (3 6c and 1 3a) and 9 doubly degenerate E modes {6 6c (x,y,z) and 3 3a (x,0,1)}, with one being an acoustic vibration and eight being optical vibrations, which are the fundamental vibrations for α-
amorphous content was 50 wt % for the sample irradiated to 4 × 1019 n/cm2 at 52 °C. The greater amorphous content of the 52 °C sample is also evident from the broad features observed in the XRD pattern. Microstrain and crystallite sizes of the samples were determined using XRD analysis of two patterns of two powdered samples from each irradiated quartz sample. Table 2 shows the results from the analyses. In the first set of ∼5 × 1018 n/cm2 irradiated samples, higher microstrain and smaller crystallite size were obtained in the 52 °C sample. However, this order was reversed in the second set of samples, with a greater increase in the microstrain compared with the first sample. The average microstrain value was higher in the 95 °C sample, with only a minor increase in the crystallite size. Therefore, it can be concluded that the greater peak broadening occurred in the 95 °C sample. For the samples irradiated at ∼4 D
DOI: 10.1021/acs.inorgchem.8b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 3. Raman Bands Observed for Quartz Samples Neutron Irradiated at 5 × 1018 and 4 × 1019 n/cm2a ν (cm−1) Fluence (n/cm2), Temperature (°C) Mode of symmetry
Control sample
E(t+l) A1 E(t+l) A1 E(l) E(l) E(T) A1 E(l) E(t+l) E(t) E(l) E(t) A1 E(l+t) E(l)I E(l)II
130 207 267 357 395 405 452 466 511 697 795 810 1083 1161
4.92 × 1018, 52
5.04 × 1018, 95
3.94 × 1019, 52
4.03 × 1019, 95
128 206 265 356 394 404
128 203 265 355 394 (small) 402
Broad shifted Broad
465 508 696 Broad 808
465 508 696 Broad 808
Broad Broad Broad Broad Broad Broad Broad Broad Broad
126 201 263 356 Broad Broad
1083 1161
1084 1161
Broad Broad
463 504 Broad Broad 808 1082 Broad
Basic bands in the 30−1250 cm−1 range are assigned based on the Raman active bands of the quartz control sample. Polarization assignments cited in Kingma and Scott.25,39 LO and TO represent longitudinal and transverse optical modes of E vibrations due to electric forces, respectively. The peak at 452 in the control sample was broader compared with the other peaks.
a
Figure 4. Raman spectra of quartz as a function of irradiation fluence and temperature. (a) Full spectra (40−1250 cm−1) of all samples. Highresolution plots of samples (control and irradiated up to 4 × 1019 n/cm2) for (b) 45−425, (c) 425−525, and (d) 525−1250 cm−1 Raman shifts.
unit length) range of 30−1160 cm−1. The most intense band for unirradiated and 0.5 and 4 × 1019 n/cm2 irradiated samples lies at ∼465 cm−1, which is due to A1 mode optic vibrations of quartz. This band is a highly polarized A1 mode that is due to
quartz.25−27 The main Raman-active phonon frequencies observed in the control sample and their modes are listed in Table 3. Figure 4 displays Raman spectra of the unirradiated and neutron-irradiated quartz samples over a frequency (per E
DOI: 10.1021/acs.inorgchem.8b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. Low-frequency Raman spectra of quartz irradiated at 4 × 1019 and 2 × 1020 n/cm2 at both 52 and 95 °C temperatures. The insets show spectra in the 45−300 cm−1 range to capture the Boson peak features. This feature was not observed in 4 × 1019 n/cm2, 95 °C, irradiated sample.
919 cm−1 is also present. These broad features represent amorphization of the samples, as was observed by XRD patterns due to a loss of long-range order of the lattice. As depicted in Figure 4b−d, a slight blue shift (i.e., peaks shift to lower wavenumber) can be observed in samples as the neutron dose is increased (up to 4 × 1019 n/cm2). This indicates a radiation-induced bond length increase of the samples, which will be discussed later. Because the quartz samples irradiated at 4 × 1019 and 2 × 1020 n/cm2 consist only of broad, asymmetric peaksexcept for the sample irradiated at 4 × 1019 at 95 °C, used here for comparisonthey were fitted to understand the consisting components under each broad feature. In the case of the 4 × 1019 at 95 °C sample, no features were observed that correspond to the Boson peak in the ∼30−250 cm−1 spectral region (Figure 5). For the other three samples, several distinct bands could be determined in the 30−745 cm−1 range. Specifically, three distinct broad peak regions could be observed. In the first region (30−250 cm−1), two distinct bands were determined for the 4 × 1019 n/cm2, 52 °C, and 2 × 1020, 95 °C, samples, while the 2 × 1020 n/cm2, 52 °C, sample shows a single band with extended peak tails, and this band corresponds to the Boson peak.29 For the 4 × 1019 n/cm2, 52 °C, irradiated sample, a comparatively intense peak was
the αzz polarizability tensor of α-quartz. A similar observation can be made for the A1 species at 207 cm−1. This band is uniquely broad (c.f. peaks at 130 and 357 cm−1) and is attributed to the anharmonic coupling of an A1 phonon with a two-phonon (acoustic) mode.28 The other two bands observed at 357 and 1083 cm−1 are also from the A1 mode of symmetry. A few of the less intense bands can also be observed at low and high frequencies (e.g., the bands at 267, 357, 405, 512, 810, and 1083 cm−1). The bands at 130 and 267 cm−1 represent the E vibrations in both longitudinal optical (LO) and transverse optical (TO) modes, while low (405 cm−1) and high (512 cm−1) are the tail ends of the A1 mode at 466 cm−1 split into LO modes. Another LO split mode was observed at 810 cm−1 within the scanned range of the Raman shift. As the neutron fluence increased to 4 × 1019 n/cm2, broadened features with decreased intensities appeared. These features are more significant in the low-temperature 4 × 1019 n/ cm2 irradiated sample in which only the 463 cm−1 band shows a comparatively sharp feature. The samples irradiated at 2 × 1020 n/cm2 show only broad features that present at different Raman shift values. Moreover, these samples exhibit a broad peak in the low-frequency (peak maximum at
