Introduction

Ceramics have attracted attention from the viewpoint of functional materials with excellent thermal stabilities and chemical durabilities. Considering the Sustainable Development Goals (SDGs), the fabrication of functional materials is not only important scientifically but also environmentally. Because transparent ceramics are used for various optical applications, the available fabrication methods and materials have been investigated worldwide. The synthesis of conventional ceramics require energy, and an energy-less fabrication process is an important aspect of SDGs. One preparation method for obtaining transparent materials is spark plasma sintering (SPS)1,2,3,4,5,6,7,8,9,10. In contrast to conventional methods for ceramic synthesis, the SPS process enables sintering at a lower temperature and a shorter time by utilising electrical energy and the high energy of discharge plasma1. Although the size of the obtained solid is limited, sintering at a high pressure and low temperature is attractive for the preparation of novel functional materials, such as phosphors7,8,9,10.

It has been reported that the properties of solid materials obtained by SPS are different from those of conventional materials or materials obtained by conventional sintering. Because the SPS technique has been developed for fabrication of materials, functionality seems to be the top priority for researchers. Detailed studies of physical and structural properties are important not only for the analysis of materials but also for the improvement of the SPS technique. Nevertheless, microscopic structural analysis by spectroscopy is considered secondary to the examination of functionality.

SiO2 glass has been used as a fundamental optical material with high durability and chemical stability in optical fibres and substrates. Because of the high temperature involved in the fabrication, several attempts have been made to fabricate SiO2 glass at low temperatures; these include liquid-phase methods (such as sol–gel) and SPS. SiO2 glass prepared by SPS was first reported in the 1990s. The luminescence of SiO2 glass containing activators prepared by sintering has been also reported7,8,9,10. Because SiO2 glasses prepared by different methods exhibit different properties for different purposes and applications, it is very important to investigate the relationship between the structure and the physical properties of the prepared SiO2 glass. However, a detailed study of the structure of SPS-SiO2 glass is lacking. In this study, we performed a structural analysis of SiO2 glass prepared by the SPS method and compared its characteristics with those of conventional SiO2 glass. In addition, space-selective microscopic analysis was used to determine the spatial heterogeneity of SPS-SiO2 glass.

Results and discussion

Optical and physical properties

The obtained SPS-SiO2 glasses were transparent to the naked eye. In the previous report, SiO2 glasses were prepared in the temperature range of 727–1427 °C at 100 MPa pressure, but transparent bulk material was obtained with the sintering temperature above 1250 °C6. Since transparent SPS-SiO2 glass is the target of the study, four preparation conditions were used: 6 MPa, 1300 °C; 6 MPa, 1400 °C; 70 MPa, 1300 °C; and 70 MPa, 1400 °C. The physical properties of the prepared glasses are listed in Table 1. All the SPS-SiO2 glasses exhibit higher densities than that of conventional SiO2 glass. In addition, the dense SPS-SiO2 glasses possess a higher G0, E0, and K0 than those of conventional SiO2 glass. Despite the same preparation conditions, the densities of the SPS-SiO2 glasses were changed depending on the weight of starting chemicals. The lighter the starting material, the heavier the density. Notably, the dependence of the preparation conditions (temperature and pressure) on the elastic properties is obscure. Therefore, it is expected that not only the temperature, pressure, and heating program but also the volume of the starting material affects the nature of the obtained samples. Considering the mechanism of the SPS method, the distance between the graphite punches might affect the sintering efficiency and the resulting properties14,15,16.

Table 1 Physical properties of the SPS-SiO2 glasses compared with those of conventional glass.
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Figure 1 shows the optical absorption spectra of the glasses along with that of conventional SiO2 glass. Small absorption bands are observed below 350 nm, which originate from the generation of defects, such as oxygen deficiency centres and dangling oxygen bonds11,12,13. It is notable that absorbance of the samples prepared at 6 MPa is higher than that of ones prepared at 70 MPa under the same preparation temperature. We also found that the increasing absorbance, i.e. defect formation, is suppressed by applying lower temperature under the same preparation pressure.

Figure 1
figure 1

Optical absorption spectra of SPS-SiO2 glasses prepared at different conditions along with that of SiO2 glass.

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Positron annihilation spectroscopy (PAS) analysis

For structural analysis of the SPS-SiO2 glasses, we performed PAS, which is used to quantify the cavity size in materials17,18,19,20. Figure 2 shows the positron decay curves of the SPS-SiO2 and conventional SiO2 glasses; there is no significant difference in the curves. For insulators, the decay constant of ortho-positronium (the third component in fitting the decay curve) is used for size calculation. The cavity radius of the SPS-SiO2 glass was calculated to be 0.245 (± 0.002) nm, which is almost identical to that of standard SiO2 glass (0.247 nm) (Supplementary Table 1). This result is in accordance with the values published in the literature for densified silica glasses20. As reported previously19, PAS tends to detect larger cavities (rather than smaller ones) in SiO2 glass. Hence, the small changes observed in the density cannot be explained using the cavity sizes obtained by PAS as it is based on preferential annihilation of positrons in cavities. A microscopic analysis is expected to provide additional information to explain the densification of SPS-SiO2 glasses.

Figure 2
figure 2

Positron decay curves of SPS-SiO2 glasses prepared at different conditions along with that of conventional SiO2 glass.

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Microscope observation

We performed Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) imaging of the surface and the interior to analyse the morphologies of the SPS-SiO2 glasses.

Figure 3 shows the SEM images of SPS-SiO2 glasses prepared at different conditions. Inset shows the surface of the samples by mechanically polishing. Inside the samples prepared by ion-milling, there are no grain boundaries originating from the SiO2 powder, resulting in a uniform morphology. The fully sintered images at 1300 and 1400 °C are very similar to those in the previous paper6. It is natural that the obtained SPS-SiO2 glasses without grain boundary exhibit the transparency over a wide wavelength range.

Figure 3
figure 3

SEM observation of SPS-SiO2 glasses. SEM images of interiors of the SPS-SiO2 glasses prepared at 6 MPa, 1300 °C (a) and 70 MPa, 1400 °C (b), respectively. The insets show the surface of the samples. The surface and the interior of the samples were polished mechanically and by ion-milling, respectively.

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Figure 4 shows the TEM images of SPS-SiO2 glasses prepared at different conditions. Both the surface and the interior of the samples are homogenous with no precipitation of crystallites. Hence, all the SPS-SiO2 glasses are expected to be amorphous, which was confirmed by conventional XRD (discussed later). Because there is no clear evidence of densification in the TEM results, other analytical approaches are required.

Figure 4
figure 4

TEM observation of SPS-SiO2 glasses. TEM images of the (a, c) surfaces and (b, d) insides of the SPS-SiO2 glasses prepared at 6 MPa, 1300 °C and 70 MPa, 1400 °C, respectively.

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High energy XRD (HEXRD) analysis

Based on the relationship between the first sharp diffraction peak (FSDP) of the X-ray total structure factor S(Q) and the density of SiO2 glasses (as reported in our recent study)16, we focused on the FSDP profile of the SPS-SiO2 glasses. The FSDP height at Q = 1.55 Å−1 strongly correlates with the structural disordering of glasses21,22,23,24,25. Figure 5a shows the S(Q) of the SPS-SiO2 glasses prepared at 70 MPa, 1400 °C along with that of SiO2 glass. The X-rays were irradiated at the centre of the samples. The spectral shapes of both the samples are similar, and no sharp diffraction peak attributable to crystalline SiO2 is observed. Similar shapes in the low- to high-Q regions confirm that no cluster-like structure is formed in the SPS-SiO2 glass. Figure 5b shows the enlarged S(Q) values of these glasses in the low-Q regions. Notably, there is a slight difference in the height of the FSDP. However, the relationships between the FSDP height and the SPS conditions or the density of SPS-SiO2 glasses are not completely understood. Although we can conclude that the small difference in the FSDP height originates from the denser packing of the SiO4 network by the SPS method, the detailed structure is not clear from these results.

Figure 5
figure 5

Comparison of S(Q) of SPS-SiO2 glass and SiO2 glass. (a) S(Q) of SPS-SiO2 glass prepared at 70 MPa, 1400 °C along with that of SiO2 glass. (b) Enlarged S(Q) of SPS-SiO2 glasses prepared at different conditions along with that of SiO2 glass.

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IR and Raman spectroscopy

We used IR and Raman spectroscopy techniques to study the spatial heterogeneity of the SPS-SiO2 glass. Figure 6a shows the micro-IR spectra of the SPS-SiO2 glasses obtained from the centres of the SPS-SiO2 and conventional SiO2 glasses. Although the spectral shapes are similar, there is a slight shift in the band at approximately 2260 cm−1. This vibration mode is an overtone of the Si–O–Si vibration mode, which is occasionally discussed from the viewpoint of fictive temperature (Tf) of SiO2 glasses15,16,26,27,28,29,30,31,32,33. Notably, SPS-SiO2 glass prepared at 6 MPa, 1400 °C, which exhibits the highest density among all the samples, exhibits the largest shift in the Si–O–Si peak. Figure 6b shows the enlarged microscopic IR spectra obtained from different positions of the SPS-SiO2 glass prepared at 6 MPa, 1400 °C. The spectra obtained from the edges of the SPS-SiO2 glass are different from that obtained from the centre of the sample; the Si–O–Si peak shifts to lower wavenumbers when moving from the left edge to the right. A shift to a lower wavenumber can be assigned to a higher Tf, i.e., not annealed. However, the peak shift in the present data is too large to explain this phenomenon from the viewpoint of merely the Tf of SiO2 glass16,29. In addition, although the SPS-SiO2 glasses were obtained by cooling without temperature control, the quenching rate was slower than the water-quenching rate of SiO2 glass with a high Tf. Therefore, it is expected that this shift originates from the reaction at the interface between the SiO2 powder and the surrounding graphite die.

Figure 6
figure 6

Spatial heterogeneity by micro-IR spectroscopy. (a) Micro-IR spectra of SPS-SiO2 and SiO2 glasses obtained from the centre. (b) Enlarged micro-IR spectra obtained from different positions of the SPS-SiO2 glass prepared at 6 MPa, 1400 °C. The inset shows the photograph of the SPS-SiO2 glass; the black dots at the edges indicate the measurement points.

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In Raman spectroscopy, we focused on the boson peak and the vibrational modes at 490 cm−1 (D1) and at 600 cm−1 (D2)34,35,36,37,38,39. Therefore, we discuss Raman spectra measured with HH (parallel nicol) polarisation (Supplementary Fig. 1(a)). Although the origins of both these vibrational modes are not completely understood, the boson peak has been correlated with the free volume of glasses40. On the other hand, D1 and D2 are assigned to the vibration of four- and three-membered rings of SiO4 tetrahedral units, respectively39. In contrast to conventional SiO2 glass, SPS-SiO2 glass exhibits fluorescence upon laser irradiation, suggesting the formation of defects in the matrix (Supplementary Fig. 1(b)). The expected defect generation is consistent with the results of optical absorption shown in Fig. 1. Considering the results of micro-IR spectroscopy, the Raman spectra were also obtained from three different points (shown in the inset of the photograph) on the samples. The broad baseline due to fluorescence was removed by applying an extended multiplicative signal correction (EMSC) algorithm, adopting the bulk SiO2 glass spectrum without fluorescence as a reference. 41 Figure 7a shows the micro-Raman spectra of the SPS-SiO2 (prepared at 6 MPa, 1400 °C) and conventional SiO2 glasses; these spectra were recorded with HH polarisation. Although the spectral shapes are roughly similar, there is a slight difference between the spectra of SPS-SiO2 and SiO2 glasses. Figures 7b–d show the enlarged Raman spectra highlighting the boson, D1, and D2 peaks, respectively. The height of the boson peak of SPS-SiO2 glass is comparable to that of the SiO2 glass, and a remarkable peak shift is not observed. On the contrary, the intensities of the D1 peak at 490 cm−1 (Fig. 7c) and the three-membered-ring (D2) peak at 600 cm−1 (Fig. 7d) of SPS-SiO2 glass increase. It is suggested that D2 structures exist at the vicinity of SiO2 surface39. Considering the result of PAS, in which a large cavity is not diminished by SPS, it is suggested that small silica units are formed at the edges of the sample (near the interface with the mould). Because the decrease in the height of FSDP in HEXRD (Fig. 5) also suggests a less ordered glass network, it can be concluded that a defect-like structure is generated in the SPS-SiO2 glass. Notably, the height of the boson peak is comparable, although the shift in the Si–O–Si vibrational peak is the largest at the left edge of the sample (interface with the mould). The correlation between the Boson peak of silica glass and the average of distribution of Si–O–Si bonding angle is well established42,43. Since there is no visible correlation between the boson peak in the Raman spectrum and the Si–O–Si vibrational peak in the IR spectrum, it is expected that Raman spectroscopy is less sensitive to small density changes than IR spectroscopy.

Figure 7
figure 7

Micro-Raman spectra of SPS-SiO2 glass recorded with HH polarisation. (a) Micro-Raman spectra of SPS-SiO2 glass prepared at 6 MPa, 1400 °C along with that of SiO2 glass. The inset shows the photograph of the SPS-SiO2 glass marked with the measurement positions. Enlarged Raman spectra showing the (b) boson peak, (c) D1 peak, and (d) D2 peak.

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The results of the present study suggest that the interface with the mould affects the properties of ceramics obtained by pressure-assisted fabrication under reduced atmosphere. We assume that the highest density of SPS-SiO2 glass prepared at 6 MPa and 1400 °C is mainly due to reduction reactions, rather than conventional densification by applying pressure above several GPa. In addition, it is expected that higher pressure and lower temperature are effective to prevent reduction reaction. Even in the aerodynamic levitation method44, where samples are not in contact with any container, the interface between the materials and surrounding atmosphere is considered important. We believe that the influence of the interface on the structure and physical properties should be considered not only in SPS but also other manufacturing methods. Thus, ceramics with core–shell-like structures can be spontaneously prepared by selecting the fabrication method. In addition, we emphasise that different probes are required for different scales of constituents in a solid-state matrix. For example, because the relationship between elastic (macroscopic) properties and spectroscopic (microscopic) approaches is not straightforward, a combination of analytical methods is necessary for complete characterisation45,46,47. A deeper understanding of the structure of the materials is required for more precise control of the properties.

Conclusion

Transparent SPS-SiO2 glasses without grain boundary or precipitation of crystallites were successfully prepared. The results of HEXRD, micro-IR spectroscopy, and micro-Raman spectroscopy suggest the formation of irregular silica species in the SPS-SiO2 glass near the interface with the mould. Since the highest density of SPS-SiO2 glass was attained prepared at lower pressure and higher temperature, it is expected that that the densification of SiO2 glass is mainly due to reduction reactions, rather than conventional densification by applying GPa pressure. Although sintering can produce novel solid-state matrices, spatial analysis and macroscopic properties are important for understanding the nature of the sample.

Methods

Preparation

SiO2 glasses were prepared by SPS of silica powder with a diameter of approximately 25 μm (99.999% purity; High Purity Chemicals). The SiO2 powder was loaded into a graphite die with an inner diameter of 10 mm and sealed with two graphite punches. The sintering temperature was controlled according to the following sequence: the temperature was increased to 600 °C from approximately 25 °C in approximately 5 min, maintained at 600 °C for approximately another 5 min, increased again to 1300 or 1400 °C at a constant rate of 10 °C/min and then maintained for 15 min. The entire sintering process was carried out in vacuum, and a pressure of 6 or 70 MPa was applied between both ends of the graphite punch. After sintering, the sample was cooled without controlling the temperature. Four different preparation conditions were used in this study: 6 MPa, 1300 °C; 6 MPa, 1400 °C; 70 MPa, 1300 °C; and 70 MPa, 1400 °C. The obtained SiO2 glass was surface polished and characterised using different methods.

Characterisation

A high-energy X-ray diffraction (XRD) experiment was performed using a two-axis diffractometer dedicated to the study of disordered materials at the BL04B2 beamline of the SPring-8 synchrotron radiation facility (Hyogo Japan)48. The energy of the incident X-rays is 61.43 keV. The raw data were corrected for polarisation, absorption, and background, and the contribution of Compton scattering was subtracted using a standard data analysis software.

The morphology of the samples was measured using a scanning electron microscope (SEM), where SEM images were taken using a JSM-6510(JEOL). An HF-2000 (Hitachi) microscope was used to obtain transmission electron microscopy (TEM) images. Transmittance measurements were performed at 25 °C using a spectrophotometer (UH-4150; Hitachi, Ltd.). The ultrasonic velocities of the longitudinal (VL) and transverse (VT) waves were measured at room temperature using the ultrasonic pulse-echo method (DPR300, JSR Ultrasonics). The frequencies of the longitudinal and transverse transducers are 10 and 5 MHz, respectively. The Young's modulus (E0), instantaneous shear modulus (G0), bulk modulus (K0), and Poisson’s ratio (ν) were calculated according to previously reported methods47. The errors in E0, G0, and K0 were less than ± 0.1 GPa.

The micro-Raman spectroscopy was performed in backscattering geometry using a single-frequency diode-pumped solid-state laser operating at 532 nm (Oxxius LCX-532S-300) and a custom-built microscope with ultra-narrowband notch filters (OptiGrate). The incident laser was attenuated to 7 mW and focused using a 50 × objective lens. The scattered light, collected by the same lens, was analysed using a single monochromator (Jovin-Yvon, HR320, 1200 grooves/mm) equipped with a charge-coupled device camera (Andor, DU420). The measurement system for Raman spectra is shown in a previous study49. We employed a multivariate analysis software Unscrambler 11 (Camo Analytics), which provides EMSC, to remove fluorescence-like broad baselines for the SPS samples.

Positron annihilation lifetimes were measured using a PSA TypeL-II system (Toyo Seiko Co., Ltd.) with an anti-coincidence system50. The 22Na source, with a diameter of 15 mm, was encapsulated in a Kapton film. The accumulated count for each sample was 107.

Infrared spectra (in the mid-infrared (IR) region from 600 to 8000 cm−1) were measured using the IR beamline BL43IR at the SPring-8 synchrotron facility (Hyogo, Japan). A Fourier transfer infrared (FTIR) microspectrophotometer (BRUKER model HYPERION IR microscope with a VERTEX70 FTIR spectrometer) was used with IR synchrotron radiation. The microscope has a motorised xy-stage, which is used to specify the measurement position. The magnification of the objective mirrors is 36 × . The spatial resolution is approximately 20 µm at 2000–3000 cm−1. The wavenumber resolution is 1 cm−1, and the number of accumulations is 4000 times. The sample was set on a stainless steel mesh with a honeycomb array with a hole diameter of 2 mm. All the measurements were performed at room temperature. The infrared optical path was purged with dry air to remove water and CO2 molecules (FT-IR purge gas generator; Parker Co., Ltd.).