Glasses and Glass–Ceramics from Li2O-KF-TiO2-SiO2 System Doped with SiC

Abstract
Unfamiliar invert host silicate glass from the system SiO₂-TiO₂-KF-Li₂O was prepared together with SiC doped samples by traditional melt-quenching technique. Collective characterization of the prepared glasses through FTIR, optical, thermal and microhardness properties were measured to justify the efect of silicon carbide (SiC) on the resultant data. Silicon carbide is selected because its known high mechanical and thermal properties and extended applications of SiC- containing candidates in several domains. Structural FTIR characterization of the prepared glasses reveals familiar silicate network in spite of their invert percent together with the suggested sharing of Si-Ti or Ti-Ti or fuoride units. Optical spectra show only distinct UV absorption with additional small peaks at 380–420 nm in high percent of SiC. Such UV spectra are assumed to originate from unavoidable traces of ferric ions contaminated within the raw materials used for the preparation of glasses. The known high thermal and mechanical properties of silicon carbide are identifed to be refected on the measured thermal expansion and Vickers microhardness data. Samples from the parent glasses were thermally heated to produce their corresponding glass-ceramic derivatives. X-ray difraction analysis indicate the formation of the peculiar orlovite crystalline phase as a major one due to the presence of all the constituents within the chemical composition of the invert glass. Also, some minor crystalline phases of lithium silicate or lithium titanium silicate are identifed. It is assumed that the presence of self-nucleation Li₂O and high negatively charged fuoride anions beside the conditional oxide of TiO₂ facilitate the ease of nucleation and crystallization of the formed crystalline phase. SEM results confrm the x-ray data showing diferent crystalline features with the addition of SiC.

Keywords Glass · Glass-ceramics · Orlovite · Silicon carbide

1 Introduction

The scientifc era of glass-ceramic began with the invention of Stookey from Corning Glass Work of the crystallized glass (Pyroceram) ^[1]. Subsequently in 1972, all crystalline derivatives (e.g., slag-ceram, photosital, etc.) were classifed and nominated as glass-ceramic materials. Stookey opened a whole feld of science—the nucleation and crystallization of glass that produced all kinds of new crystalline products with so many diferent useful and exciting properties ^[2].

Glass-ceramics are accepted to be marvelous materials with ceramic luster formed through the controlled nucleation and crystallization of parent glasses. The best route lies in efcient nucleation from the component of the glass or by the introduction of specifc nucleating agent that allows the development of fne, randomly oriented grains or seeds and generally without voids, microcracks, or other porosity ^[3].

It is recognized that most of the commercially fabricated glass-ceramics are of silicate candidates. This can be related to the known excellent chemical stability of the commercial silicate glasses and the long-term experience of melting of such silicate glasses from the ancient times for thousands of years. Silicate glasses containing Li₂O are identifed to be easily nucleated and crystallized due to the presence of self-nucleator Li₂O as evidenced in recent book in glassceramics ^[4]. Studies on invert silicate glasses ^[5] containing SiO₂ as glass-forming oxide with lower percent than the rest of modifer oxides or conditional oxides such as TiO₂ have shown interesting crystallization behavior. Titania as essential component in such glasses acts as initiator for phase separation and can also share in the formed crystalline phases derived from thermal heat-treatment ^[6].

It has been established the silicon carbide (SiC) has been recommended to be involved in glasses or ceramics as a dopant or as a composite for its excellent overall mechanical and physical properties ^[7]. These specifc properties include low density, good thermal-mechanical behavior, high temperature oxidation and resistance to radiation. As a result, the Si-C based materials have been recommended to be used in some advanced felds involving aerospace, aviation, nuclear power ^[8]. Silicon carbide is a highly covalent material possessing 88% covalent and 12% as ionic bond which is refected on their properties including high modulus, high melting point and good wear and corrosion resistance.

In the present research, a host base glass of the detailed composition SiO₂ 44.17, TiO₂ 29.38, KF 21.37, and Li₂O 5.08 (mol%) was prepared together with samples containing the same basic composition beside added dopant of SiC (1.5,2 or 3 gm /100 gm glass). This composition was chosen to verify the stoichiometric formula of orlovite phase 
(KLi₂TiSi₄O₁₀(OF) after heat-treatment regime. The derived parent glasses were characterized primarily by FTIR analysis, optical spectral and thermal expansion measurements. The study also includes the conversion of samples from the parent glasses to their corresponding glass-ceramic derivatives through controlled heat-treatment. The derived glass-ceramics were examined by FTIR, X-ray difraction and scanning electron microscopic (SEM) measurements to identify the detailed structural units and the crystalline phases formed by controlled heat-treatment beside their surface morphological textures.

2 Experimental Details

2.1 Preparation of the Parent Glasses

The glasses were prepared through traditional melting-quenching technique. The laboratory chemicals used include purifed sand (SiO₂), titanium dioxide (TiO₂, Fluka, Germany), lithium carbonate (Li₂CO₃ TechnoFarmchem. India), and potassium fuoride (KF, Fluka, Germany), Silicon Carbide (SiC, Electroscmeltz Werk Kempten, Germany). Chemicals with (>99% purity) were used to prepare samples. The melting of glasses was carried out in platinum crucibles in an electric SiC furnace (Vecstar, UK) at 1400±10 °C and the melting time was extended to 90 min.

The crucibles were rotated at intervals to promote complete mixing and homogeneity. The melts were then poured into preheated stainless-steel molds at about 400 °C. The prepared glassy samples were immediately transferred to an annealing mufe regulated at 400 °C. The mufe was then shut down after 1 h and left to cool to room temperature with rate 0.5 °C/min.

2.2 Optical Absorption Measurements

UV-visible absorption spectra were derived out from polished samples (2 mm±0.1 mm) using a recording spectrophotometer (type Jasco, V-570, Japan) covering the range from 200 to 1100 nm.

2.3 Diferential Thermal Analysis

Diferential thermal analysis for the glass samples was performed (DTA-Shimadzu DTG60 micro-diferential thermoanalyser, Japan) with heating rate 5 °C/min. up to 1000ºC.

2.4 Preparation of the Corresponding Glass–Ceramics

Samples from the parent glasses were thermally heat-treated through specifc two steps regime to convert them to their corresponding glass-ceramic derivatives. The selected temperatures for thermal heat-treatment, were derived from collective DTA and thermal expansion measurements. The glasses were primarily heated to the nominated temperature selected (480 °C) with a low rate (5 °C/min.) and kept at this temperature for 4 h and then the temperature was raised to (700 °C). The furnace was kept at this temperature for 2 h. The furnace was then switched of and left to cool to room temperature with the heated samples inside.

2.5 Structural FTIR Spectral Measurements

Fourier-transfer infrared absorption measurements of the glasses were carried out at room temperature within the wavenumber range 400 – 4000 cm⁻¹ by a computerized spectrometer (FTIR 4600 Jasco corp., Japan) using the KBr disc technique. The IR spectra were also obtained for the prepared glass-ceramic derivatives.

2.6 Thermal Expansion Measurements

The thermal expansion of the parent glasses was obtained using a computerized dilatometer (type NETSCH-402 PC, Germany) with a heating rate of 5 °C/min. The thermal measurements were done within the range from room temperature up to the dilatometric softening temperature of each glass.

2.7 X‑ray Difraction Analysis

The identifcation of the various crystalline phases formed through controlled heat- treatment was conducted using a (Bruker AXS difractometer CD8- Advance) with Cu-Ka radiation operating at 40 kV and 10 mA. The difraction patterns were recorded for 2θ values between 4° and 70° and the scanning rate was 10 °C/min.

2.8 Measurements of Microstructures using 

Scanning Electron Microscope (SEM)The surfaces of the heat-treated samples were examined by SEM type (JEOL- JSM- T2O Japan) after coating with surface layers of gold.

2.9 Vickers Microhardness Measurements

The microhardness values of the glasses and glass-ceramic derivatives were obtained by a Vickers microhardness tester (type Shimadzu, Japan) with a load of 100 g for 15 s. The measurements were done for polished samples and each sample was tested in diferent locations for 5 times and the net average value was obtained.

3 Results and Discussion

3.1 Structural FTIR Spectra of the Parent Glasses and their Glass–Ceramics Derivatives

Figure  1 reveals the FTIR spectral curves of both the base and SiC-doped glasses. The IR spectrum of the base undoped glass shows extended absorption from 400 to about 1700 cm⁻¹ revealing small far IR peaks at 440, 520, 595 and 687 cm⁻¹. This is followed by distinct absorption with two major peaks at 1018 and 1387 cm−1 and with an attached peak at 1033 cm⁻¹ on the descending lobe. The rest of the IR spectrum reveals small peaks at about 2420, 2840 and 2940 cm⁻¹ and a fnal broad near IR band extending from 3000 to 3750 cm⁻¹ with a broad peak centered at 3452 cm⁻¹.

Fig. 1 FTIR absorption spectra of the base and SiC doped glasses

The IR spectra of the SiC-doped glasses reveal similar spectra to the undoped but show marked splitting of the main absorption from 400 to 1640 cm⁻¹ but still the IR absorption at 1350–1390 cm⁻¹ is the highest intensity.

The understanding and assignments of the IR spectral results are based on the following basis ^[9–12]:

[a] The resultant IR absorption bands are recognized to be originating from the vibrations of the structural groups or units within the network structures of studied glasses or assumed to be fngerprints of these groups. The structural groups are dependent on all the constituent oxides or fuorides which produce the expected types of groups 
arrangements within the network.

[b] The base glass composition consists of familiar glass forming oxide (SiO₂ with 44.17%) and conditional oxide (TiO₂ 29.38%), alkali oxide (Li₂O 5.08%) and alkali fuoride (KF 21.37%).

[c] The expected main structural groups are SiO₄ with the sharing of titanium ions as Si-O-Ti or TiO6 groups or sharing as TiO₄ groups with the presence of alkali oxide (Li₂O) ^{[4, 5]}. Also, there is the possibility of the presence of fuoride ions as revealed before ^[5].

[d] The assignments of the resultant IR spectra are introduced in accordance with previous published studies ^{[9, 10]}:

(i) The far-IR peaks at 400–450 cm⁻¹ are related to vibrations of alkali cations (Li⁺, K⁺) in their characteristic sites.
(ii) The vibrational peaks within the range 460–620 cm⁻¹ are correlated with bending vibrations of Si-O-Si and O-Si–O bending mode.
(iii) The absorption of the IR peaks within the range 700–800 cm⁻¹ are attributed to vibrations due to Si-O-Si symmetric stretching of bridging oxygens between SiO₄ tetrahedra.
(iv) The absorption peaks at 900–1180 cm⁻¹ are related to antisymmetric stretching of Si-O-Si bonding.
(v) The IR peaks at 1300–1390 cm⁻¹ are related to carbonate groups.
(vi) The absorption peak at 1640 cm⁻¹ and extend to the near IR broad band with a peak at about 3450 cm⁻¹ are related to vibrations of water, OH, SiOH.
(vii) The TiO₄ groups possess vibrations within the range 400- 850 cm⁻¹, and therefore the collected IR spectra are considered as composite one involving mixed Si-O-Si and Ti-O vibrations as reached before ^[5].

Figure 2 reveals the FTIR of the glass-ceramic derivatives and they are similar to that of their parent glasses.

Fig. 2 FTIR spectra of base and doped SiC glass-ceramics

3.2 Thermal Expansion Measurements Data

Figure 3 illustrates the thermal expansion curves of the prepared glasses. The thermal curves reveal nearly similar behavior in the rate of increase with the progressive increase of temperature.

Fig. 3 Thermal Expansion data of the base and SiC doped glasses

The increase in thermal expansion steadily increases until reaching the transition region after which the increase is sharp reaching the dilatometric softening temperature after which the dilatometer is stopped causing a rapid drop in the property.

The present thermal curves indicate that the increase of dopant SiC percent added increases the dilatometric softening temperatures as follows:

undoped (464°C) → 1.5% SiC (516°C) → 2% SiC (519°C)→3.0% SiC (530°C)

The resultant thermal expansion data are explained as follows ^[13–16]:

(i) The thermal expansion of glass is not a simple property because primarily the lack of periodicity within all the structural network of the glass. In crystalline solids the efect of heat is extended in similar structural units and comparable bond lengths throughout the entire structural units in the solids. In perfect crystalline solids the thermal expansion is the direct result of an increase on bond length units with increasing temperature.
(ii) Rawson ^[14] described normal expansion in solids (including glasses) as being the response of the increasing amplitude of the atomic vibrations of the 
constituent ions. The vibrations are anharmonic and as a result with increasing the amplitude of the vibrations, the interionic distance increases.
(iii) The identifed continuous increase in length at low temperatures can be related to increase in the interionic distance with increase in temperature and the change in the rate at higher temperature is assumed to be due to accumulated energy. The increase in the dilatometric softening temperature with the increase of SiC percent is correlated with the high covalent bonding of this silicon carbide material and its known high melting temperature.

3.3 Optical Absorption Spectral Results

Figure 4 illustrates the UV-visible spectra of both the parent undoped and SiC doped glasses.

Fig. 4 Uv–Visible optical absorption spectra of base and doped SiC glasses

All the optical spectral curves reveal only distinct UV absorption which is extended to about 322 nm. The optical spectra of the glasses containing dopant of SiC show additional small peak at 347.

Extended glass scientists ^[17–19] have identifed the appearance of distinct UV absorption in the optical spectra of undoped silicate, borate, and phosphate glasses. They assumed that the origin of such UV spectra within the range 200–310 nm to originate from unavoidable trace ferric ions present as impurities contaminated within the raw materials. Many authors ^{[16, 17, 20]} have recognized that such UV spectra can be identifed even the traces are present within the ppm level and recommended the need for ultrapure chemicals to produce high quality optical glasses for specifc lenses and lasers.

The appearance of extra small peak at 347 nm can be related to the introduction of extra ferric ions in the resultant SiC-doped glasses which can be assumed to be identifed by many authors ^{[21, 22]}. It is assumed that SiC added as carborundum contains extra ferric ions as impurities which make it easier to identify the additional peak at 347 nm.

3.4 Vickers Microhardness Data

Table 1 depicts the indentation microhardness values of the prepared glasses. It is obvious that the data indicate that the addition of SiC progressively increases the M.H.V. upon the introduction of 1, 2 or 3 wt % with more SiC contents.

Table 1 Chemical composition of the studied samples and their microhardness values

The experimental microhardness data of the studied glasses can be explained as follows:

(a) In indentation process, the glass is suggested to perform both compression and shear ^[23]. The collective two routes are believed to cause stresses which are responsible in the frst for the identifed deformations. The net result is one or more of the following behaviors: (1) viscous or plastic fow due to shear efect. (2) Elastic deformation caused by load compression and shear.
(b) It is to be referred that the microhardness data bears high resemblance to the measured softening temperatures derived by thermal expansion measurements. The same behavior has been identifed for diferent glass systems ^{[24, 25]}. These results refer to the assumption that microhardness process is mainly correlated with viscous behavior especially in glasses containing modifer ions.
(c) The increase of the microhardness data upon the frst increase of SiC can be related to the known high cohesive bonding strength of the silicon carbide leading to higher mechanical properties.

3.5 X‑ray Difraction Analysis of the Glasses and Glass–Ceramic Derivatives Containing Varying SiC

Figure 5 illustrates the x-ray difraction of the base glass without SiC and glasses doped with highest SiC (3 wt%) percent.

Fig. 5 DTA of Glass samples containing diferent concentrations of SiC

The x-ray curves of the glasses reveal the familiar broad hump which is indicative to the non-crystalline or amorphous non periodic texture. The curves also show 2 lines at the left side of the hump at θ which is indicative of traces of low quartz. All the prepared glasses either parent or doped or parent are clear and didn’t show any crystalline parts and SEM measurements confrm the amorphous nature of the glasses.

The x-ray difraction of the glass-ceramic derivatives are shown in Fig. 6. The crystalline phases identifed in the undoped and SiC-doped glass-ceramics are mainly the orlovite phase (KLi₂TiSi₄O₁₀(OF) and sharing of secondary lithium silicate phase and lithium titanium silicate phase. The ease of crystallization of this specifc orlovite phase can be realized by the following reasons:

(a) The presence of all the constituting crystalline components within the composition of the base glass SiO₂-Li₂O- KF-TiO₂
(b) The ease of the crystallization process in the studied system is assumed to be due to the presence of selfnucleating oxide (Li₂O) and fuoride (F⁻) as the highest negatively charged anions together with the collective presence of TiO₂ which is accepted to be an conditional oxide which can be easily nucleated and crystallized. The combinations of all the previous (Li⁺, F⁻ and TiO₂) constituents facilitate the process of nucleation and crystallization to produce the identifed orovite crystalline phase. Also, the separation of the traces of lithium silicate and lithium titanium silicate phases are expected due to the presence of nucleator of Li⁺ and TiO₂. These assumptions have been cited in reference books in glass-ceramics by several authors e.g. ^{[2, 3]}.

Fig. 6 XRD patterns of base and SiC doped glass-ceramics

3.6 Scanning Electron Microscope of Glass–Ceramic

The SEM figure of the undoped glass-ceramic shows tiny particles separated or connected in parallel lines in glassy groundmass base glass-ceramic microstructure. Whereas the sample which contains 3 wt.% SiC shows similar microstructure to the base glass-ceramic but with scattered pores with different sizes, such pores show interlocked rods in sub-micro-scale tiny particles in between. These results confirm the identification of different crystalline phases with the addition of SiC in x-ray measurements (Fig. 7).

Fig. 7 SEM of base and SiC doped glass-ceramics

4 Conclusion

The authors have prepared a selected unfamiliar invert host glass from the composition SiO₂ (44.17), TiO₂ (29.38), KF (21.37), Li₂O (5.08) (mol%) to conduct its behavior and responses upon the addition of silicon carbide as dopant. Detailed investigations of the optical, FTIR, thermal expansion and Vickers microhardness properties of the 
glasses were carried out. Furthermore, samples of the parent glasses were thermally heat-treated to convert them to their corresponding glass-ceramics. The structural evaluation, type of crystalline phases formed together with their morphological features after crystallization were derived by FTIR, X-ray difraction, and SEM measurements.

The collected FTIR spectra of the glasses or glassceramics reveal familiar IR curves of silicate network with minor variations depending on the partner oxides (TiO₂) or fluorides (KF) and the SiC dopant. Optical spectra show distinct UV absorption beside small peaks at 380 – 420 nm at high SiC-doped samples and all are correlated with ferric ions present as trace impurities in the raw materials. The measured thermal expansion and microhardness data justify and confirm that SiC possess high thermal and mechanical properties which are reflected on the properties even its presence in the doping level (1–3 wt %). Also, there is obvious correlation between softening temperature and microhardness data referring to the viscous flow is the dominant factor for forming indentation. The identification of the diffraction patterns of orlovite crystalline phases as a main component can be related to the accumulation of such chemical components (SiO₂, TiO₂, KF, Li₂O) within the composition of the host glass which takes advantage of selfnucleated Li⁺ ions and both of highly negative fluoride anions and conditional oxide of TiO₂ which all initiate and support the ease of nucleation and voluminous crystallization. SEM measurements indicate the amorphous nature of the glasses with clear matrix.

References: Omitted