Shuai Li

Yinsheng Xu

Xianghua Zhang

Ping Lu
email: lupingwh@whut.edu.cn

Formation and crystal growth of needle-like rutile in glass-ceramics

Keywords: glass-ceramics, needle-like, rutile

Introduction

Glass-ceramic, which is prepared by controlled crystallization from the inorganic glass, 1 has negligible dielectric loss; thus, it has little effect on the transmission of electromagnetic signals and good applicability for 5G communications. It also exhibits remarkable characteristics including high mechanical strength, low CTE, and low density for lightweight design. [START_REF] Song | Crystallization mechanisms and properties of α-cordierite glass-ceramics from K 2 O-MgO-Al 2 O 3 -SiO 2 glasses[END_REF][START_REF] Wange | Microstructure-property relationship in high-strength MgO-Al 2 O 3 -SiO 2 -TiO 2 glass-ceramics[END_REF][START_REF] Dittmer | Self-organized nanocrystallinity in MgO-Al 2 O 3 -SiO 2 glasses with ZrO 2 as nucleating agent[END_REF] Meanwhile, compared with traditional materials, glass-ceramics are more suitable for wireless charging technology. Thus, glassceramic is the best option for back and front cover materials of mobile devices.

Moreover, with the continuous improvement of mobile phone functions, the usage frequency of mobile phones has gradually increased, which has increased the probability of mobile phones falling, resulting in higher requirements for mobile phone packaging materials.

In recent years, Corning company launched the first glass-ceramic used in J o u r n a l P r e -p r o o f iPhone 12 back and front cover materials, setting off a frenzy of research in the related field. Among the most studied glass-ceramics, the MgO-Al 2 O 3 -SiO 2 (MAS) system is known to be suitable for the production of high-strength glass-ceramics.

Peter et al. obtained MAS glass ceramics with hardness of 9.5 GPa by adjusting the treatment procedure. [START_REF] Wange | Microstructure-property relationship in J o u r n a l P r e -p r o o f high-strength MgO-Al 2 O 3 -SiO 2 -TiO 2 glass-ceramics[END_REF] Han et al. obtained MAS glass ceramics with hardness of 7.94 GPa and compressive strength of 509 MPa by adjusting the nucleating agent. [START_REF] Han | Crystallization, structure and properties of MgO-Al 2 O 3 -SiO 2 highly crystalline transparent glass-ceramics nucleated by multiple nucleating agents[END_REF] However, the main crystal phases of the current MAS glass-ceramic system are bulk crystals with relatively small length and diameter. Meanwhile, fiber reinforcement is a commonly used methods to improve mechanical properties. The mechanical properties of glass can be improved by using needle-like crystals. [START_REF] Höland | Formation and crystal growth of needle-like fluoroapatite in functional glass-ceramics[END_REF]8 Höland et al., found that the precipitation of needle-like fluorapatite Ca 5 (PO 4 ) 3 F crystals in glass-ceramics improved the durability of glass-ceramics. [START_REF] Höland | Formation and crystal growth of needle-like fluoroapatite in functional glass-ceramics[END_REF] Bansal et al. prepared unidirectional chemical-vapor-deposited silicon-carbide-fiber-reinforced glass ceramics by hot pressing, resulting in a flexural strength of 825 MPa. [START_REF] Bansal | Mechanical behavior of silicon carbide fiber-reinforced strontium aluminosilicate glass-ceramic composites[END_REF] Three polymorphs of TiO 2 , rutile, anatase and brookite have been found in nature, among which rutile crystals have a needle-like morphology. Yin et al. found that well-crystallized needle-like rutile nano-crystals possessed higher photocatalytic activities than anatase crystals under visible light irradiation with wavelength of >400 and/or >510 nm. [START_REF] Yin | Synthesis of visible-light-active nanosize rutile titania photocatalyst by low temperature dissolutionreprecipitation process[END_REF] However, the research on needle-like rutile crystals primarily focuses on photocatalysis, and its precipitation as the crystal phase has little influence on glass-ceramics.

J o u r n a l P r e -p r o o f

In this work, the needle-like rutile TiO 2 crystals were precipitated in MgO-Al 2 O 3 -SiO 2 -TiO 2 (MAST) glass by one-step heat treatment (HT). The effects of P 2 O 5 on MAST glass forming and crystallization behavior were investigated. Furthermore, the structure and surface morphology of the precipitated crystals were evaluated.

Experimental section

Glass synthesis

The chemical composition of the studied glass was (60. (≥99.0%), MgO (≥99.5%), TiO 2 (≥99.0%), Na 2 CO 3 (≥99.0%), Li 2 CO 3 (≥99.0%), and Na 2 HPO 4 (≥99.0%), which were melted in 90% Pt/10% Rh crucibles (covered with a Pt lid) in an electric furnace (Carbolite BLF 1800 furnace) at 1620 °C for 3 h. Then, the melt was quenched by pouring onto a copper plate and immediately pressing by a second plate to maximize the cooling rate. After cooling down to room temperature, as-quenched glasses were heat-treated with different crystallization temperatures from 610 °C to 890 °C for 2 h.

Characterization

Differential scanning calorimetry (DSC) data were collected with 40 mg of J o u r n a l P r e -p r o o f powder specimen using a Simultaneous Thermal Analyzer (STA 449 F1, Netzsch) in the temperature range of 40 ℃-1000 ℃ at a heating rate of 10 K/min under a constant flow of nitrogen gas. The glass transition temperature T g and crystallization temperature T p were determined from DSC curves.

The crystalline phase of glass and glass ceramics was observed by X-ray diffraction (XRD, PAN-alytical X'Pert Pro; Cu K α : 40 kV, 40 mA; 2θ=5°-80°; scanning speed=10°/min; step size = 0.01°/s). The powder sample was used for XRD test.

A laser confocal microscopic Raman spectrometer (DXR, Thermo Electron, USA) equipped with a 532 nm laser was utilized for structural analysis of glass samples operating at 100 mW. The spectra were acquired on the clean surface of the quenched glass from 200 cm -1 to 1200 cm -1 at room temperature.

Images obtained from a field emission scanning electron microscope (SEM, Hitachi S-4800, 5 kV) were collected on unpolished glass-ceramic samples coated with ~ 4 nm thick platinum. Before spraying platinum, the glass-ceramics were etched with 2 Vol.% HNO 3 solution for 30 s and cleaned by ultrasonic bath. A Talos F200S scanning transmission electron microscope (STEM) operating at 200 keV was used to understand the compositional and microstructural evolution in transparent glass ceramics. The point resolution in STEM mode was 0.25 nm. The specimens for STEM analysis were prepared by using a multifunctional beam ion thinning apparatus.

(Gatan PIPS 695). Sample processing is achieved by bombarding the sample with J o u r n a l P r e -p r o o f accelerated ions to sputter the surface atoms, with specimen thickness of 20 nm.

The Vickers hardness (H V ) was obtained using a microhardness Vickers tester (Q10A+, QNESS, Austria) in air atmosphere at room temperature. For block samples, polished surfaces were exposed through embedding the samples into the resin and careful grinding. Using an indentation load of 1.96 N for a dwell time of 10 s, H V is the average value of the test results over 10 times.

Results

DSC analysis

DSC curves of these three samples are shown in Fig. 1. T g increased from 580 ℃ to 638 ℃ with the increase of P 2 O 5 concentration. For the P 2 O 5 -free glass, two distinct crystallization peaks at 667 and 801 ℃ can be observed. With the increase of P 2 O 5 concentration, the integrated area of the first crystallization peak decreased, and then disappeared. Meanwhile, the second crystallization peak vanished when the concentration of P 2 O 5 was 2 mol%. When the concentration of P 2 O 5 reached 4 mol%, a new crystallization peak at 735 ℃ emerged.

X-Ray powder diffraction results

Fig. 2 presents the XRD-patterns of as-quenched glasses heattreated with different crystallization temperatures for 2 h. As shown in Fig. 2A, weak diffraction peaks corresponding to Mg(Ti 2 O 5 ) crystals (PDF: 89-6944) appear when the P0 glass J o u r n a l P r e -p r o o f heat-treated at 730 ℃ for 2 h. With the increase of HT temperatures, the peak intensity of these diffraction peaks increased. When the HT temperature reached 850 ℃, Mg 2 (SiO 4 ) crystals (PDF: 89-5130) can be observed. For sample P2 (2 mol% P 2 O 5 ), the same crystallization behavior can be found at 730 ℃ (Fig. 2B). With the increase of temperatures, a new diffraction peak (2θ=23°) appeared at 770 ℃, whereas the other two new peaks (2θ=27° and 38°) appeared at 810 ℃. Finally, Mg(Ti 2 O 5 ) (PDF:89-6499) and LiMgPO 4 (PDF:84-0342) crystals co-existed in the P2 glass ceramic. For sample P4 (4 mol% P 2 O 5 ), the crystallization behavior was totally different (Fig. 2C). Some diffraction peaks (2θ=21.91°, 25.61°, 30.25°) initially increased and then decreased, whereas some other peaks (2θ=20.89°, 23°, and 32.69°) increased with the increase of HT temperatures. Interestingly, phase transformation occurred, corresponding to the transformation of Ti 5 O 9 crystals into rutile crystals (TiO 2 , PDF:87-0710). Meanwhile, the diffraction peaks of LiMgPO 4 crystals suddenly increase with the appearance of TiO 2 crystals.

Microstructure

Raman spectroscopy was adopted to analyze the quenched samples and obtain the structural characteristics of the samples. Fig. 3 shows the Raman spectra of the three samples in the range of 200-1200 cm -1 . Four broad vibration bands appeared at 472, 585, 895 and 1070 cm -1 for the P0 samples. When the concentration of P 2 O 5 reached 2 mol% (P2), the four vibration bands still existed, but the intensity of the vibration band at 1070 cm -1 was weakened and a new Raman band appeared at 973 J o u r n a l P r e -p r o o f cm -1 . When the concentration of P 2 O 5 reached 4 mol% (P4), the Raman band at 1070 cm -1 disappeared and the one at 973 cm -1 was further enhanced with a new sharp peak at 959 cm -1 . Such a change of vibration band at 973 cm -1 and 1070 cm -1 associating with P 2 O 5 will be fully expounded in the Discussion section.

Figs. 4A-4C, Figs. 4D-4F, and Fig. 5 show the surface morphology after corrosion of the P0, P2, and P4 glass-ceramics prepared at different temperatures.

When the P0 sample was isothermally heat treated at 610 ℃ for 2 h, the microstructure of P 2 O 5 -free heat-treated glasses (Fig. 4A) concured with their XRD data showing a typical homogeneous amorphous characteristic.As shown in Fig. 4B, dense granular crystals appeare in sample P0 at 730 ℃. When the HT temperature reached 890 ℃ (Fig. 4C), the plate-like crystals appeared in the P0 sample.

As shown in Fig. 4D, holes of different geometries, ranging from approximately 30 nm to 70 nm, are visible. In addition, small particles of approximately 15 nm can be seen as agglomerates at different locations of the sample. As shown in Fig. 4E, dense crystals appeared in P2 samples with crystal sizes of approximately 10 nm-20 nm when the HT temperature reached 730 ℃. As shown in Fig. 4F, the sample was corroded deeply, showing granular crystals with a short L/D ratio at 890 ℃.

As shown in Fig. 5, large holes appeared in the erosion morphology of all samples. After HT at 610 ℃, only a few small particles with a size of approximately 10-20 nm were found (Fig. 5A). When the HT temperatures increased to 730 ℃ and 770 ℃, dense particles appeared in the sample (Figs. 5B and5C). By contrast, the J o u r n a l P r e -p r o o f microstructure of the GC heat treated at 810 ℃ (Fig. 5D) showed some needle-like crystal phases. When the HT temperature was raised to 890 ℃ (Fig. 5E), the crystal dimensions grews to approximately 300 nm×20 nm; the large L/D ratio of needle-like crystals was uniformly distributed inside sample P4.

Based on the XRD and SEM results, the P4 sample heattreated at 890 ℃ for 2 h was further investigated for their microstructural evolution using TEM. Based on the STEM image (Fig. 6) recorded using a HAADF detector, two different crystals (spherical and needle-like crystals) appeared with bright contrast in the dark glass matrix. We performed EDS analysis of these two crystals and the glass matrix. The result shows that the Ti atoms aggregate in the needle-like crystals (Fig. 6, area 1), whereas the Mg and P atoms aggregate in the spherical crystals (Fig. 6, area 2). The glass matrix comprises Si, Al, and O (Fig. 6, area 3) with Si being the main constituent. Fig. 7 shows STEM image and the corresponding elements mapping of the P4 heattreated at 890 ℃ for 2 h. Evidently, Ti atoms were enriched at the positions of needle-like crystal phases. P and Mg atoms were enriched at the positions of spherical crystal phases. Moreover, Si and Al atoms were distributed homogeneously in the glass matrix. Fig. 8 shows the high-resolution TEM images and selected area electron diffraction (SAED) pattern of needle-like and cluster-like crystals, respectively.

Well-resolved lattice fringes with a spacing of 0.322 nm can be observed (Fig. 7A).

J o u r n a l P r e -p r o o f

The diffraction pattern of SAED depicted the (110) and (101) planes of rutile (Fig. 7B). For the spherical crystals, the HRTEM showed two distinct lattice fringes with a spacing of 0.248 and 0.344 nm, corresponding to the (311) and (201) planes of LiMgPO 4 (Figs. 7C and7D). These lattice spacing were obtained by measuring 10 consecutive lattice spacings, and then calculating on average. The bright ring in the SAED diffraction pattern indicated the amorphous nature of the glass matrix. Fig. 9 shows that the Vickers hardness of the P4 sample increases significantly with the increase of HT temperature. In particular, the Vickers hardness increased from 6.56 GPa to 7.14 GPa when the HT temperature increased from 850 °C to 890 °C.

Discussion

As the P 2 O 5 content in the sample gradually increases from 0 to 2 mol% and 4 mol%, T g gradually increases from 580 °C to 588 °C and 638 °C. When the P 2 O 5 content is relatively small in the glass, the degree of polymerization of the glass can be increased, resulting in an increase in T g . The crystallization enthalpy of the first crystallization peak (T p1 ) of the P0 sample is 5.95 J/g, which is larger than that of the P2 sample (3.06 J/g). Moreover, T p1 of the P4 glass vanished, indicating that the first precipitated crystals have been restrained with the addition of 4 mol% P 2 O 5 . As shown in Fig. 3, the band at approximate 472 cm -1 corresponds to the bending vibrations of the bridging oxygen bonds of SiO 4 tetrahedron. [START_REF] Jung | Crystallization control for remediation of an Fe t O-rich CaO-SiO 2 -Al 2 O 3 -MgO EAF waste slag[END_REF][START_REF] Mulder | Defect structures in silica glass[END_REF] Based on J o u r n a l P r e -p r o o f the previous report by Jung and Sohn, [START_REF] Jung | Crystallization control for remediation of an Fe t O-rich CaO-SiO 2 -Al 2 O 3 -MgO EAF waste slag[END_REF] Raman band at approximately 585 cm -1 is assigned to the [AlO 6 ]-octahedron. The bands located at approximately 895 and 1070 cm -1 correspond to Q [START_REF] Budd | Sintering and crystallization of a glass powder in the MgO-Al 2 O 3 -SiO 2 -ZrO 2 system[END_REF] Si (superscript refers to the number of bridged oxygen) and Q 3 Si , respectively. [START_REF] Mysen | Relations between the anionic structure and viscosity of silicate melts-A Raman spectroscopic study[END_REF] In the P4 sample, the band at 1070 cm -1 is not present because of the high concentration of P 2 O 5 . [START_REF] Mcmillan | A study of SiO 2 glass and supercooled liquid to 1950 K via high-temperature Raman spectroscopy[END_REF] Based on the previous report by Yadav and Singh, the characteristic symmetric of the orthophosphate groups is normally found at approximately 970 cm -1 . [START_REF] Yadav | A review of the structures of oxide glasses by Raman spectroscopy[END_REF] For sample P2 and P4, the band at 973 cm -1 corresponds to the characteristic vibration of PO 4 3-tetrahedra, representing the environment of phosphorus. [START_REF] Popović | Raman spectroscopic study of phase transitions in Li 3 PO 4[END_REF] The sharp band located at 959 cm -1 in P4 is attributed to LiMgPO 4 crystalline clusters, which was consistent with the XRD results. [START_REF] Ariane | Effect of P 2 O 5 and Al 2 O 3 on crystallization, structure, microstructure and properties of Li 2 O-MgO-Al 2 O 3 -SiO 2 -TiO 2 -ZrO 2 glass ceramics[END_REF] The Raman band corresponding to PO 4 3-evolves significantly with the increase of P 2 O 5 concentration.

As shown in

In this work, no evident diffraction peak in P0, P2, and P4 was observed after isothermal HT at 610 ℃ (Fig. 2). However, Figs. 4A, 4D and 5A shows that the hole size of erosion increases with the gradual increase of P 2 O 5 content. In general, the addition of P 2 O 5 in the silicate glass would result in phase separartion. The formation of these pits and holes can be attributed to the preferential attack of HNO 3 on the phosphate-rich phase with weak acid resistance. [START_REF] Liu | Self-limited growth of nanocrystals in phosphosilicate melts during cooling[END_REF] Combined with P-element mapping (Fig. 7), the phosphate agglomeration area and eroded pores are similar, further indicating that P 2 O 5 promotes phase separation. Based on the XRD pattern of the P4 sample (890 o C for 2h) and Scherer formula, the calculated average grain size of the LiMgPO 4 crystals in the sample is 56.5 nm, which is consistent with the result of the J o u r n a l P r e -p r o o f high-resolution image in the TEM (Fig. 7).

Evidently, two types of crystals co-exist in the amorphous glass matrix, one is a cluster-like crystal and the other is a needle-like crystal.Notably the needle-like crystals are randomly oriented in the glass matrix. Therefore, their maximum length can be observed only if a needle is by chance oriented along the needle axis. Cutting the needles in an arbitrary direction during TEM specimen preparation, leads to shorter fragments observed in projection. The extreme case occurs if a needle crystal is oriented perpendicularly to the cutting plane: then, the cross-section can be observed. [START_REF] Höland | Formation and crystal growth of needle-like fluoroapatite in functional glass-ceramics[END_REF] Based on the element mapping results, Ti are prinarily concentrated in needle-like crystals; P and Mg are primarily concentrated in cluster-like crystals, and Al and Si are primarily presented in the glass matrix. Based on the above mentioned analysis, the needle-like crystals are rutile TiO 2 , and the cluster-like crystals are LiMgPO 4 .

As demonstrated by Ryerson and Mysen, [START_REF] Ryerson | The role of P 2 O 5 in silicate melts[END_REF][START_REF] Mysnn | The structural role of phosphorus in silicate melts[END_REF] the role of P 2 O 5 on the structure of silicate glass depends on the degree of polymerization. From the point of view of crystal chemistry, the ionic field strength F can be described as follows, [START_REF] Mcmillan | Glass-Ceramics[END_REF] 2

Z F r  (1)
where Z and r are the valence and radius of ionic, respectively. P 5+ ion has a higher ionic field strength than Si 4+ ion (43.2 compared with 23.8). [START_REF] O'donnell | Effect of P 2 O 5 content in two series of soda lime phosphosilicate glasses on structure and properties -Part I: NMR[END_REF][START_REF] Oliveira | Effect of SiO 2 on amorphous phase separation of CaO-P 2 O 5 -SiO 2 -MgO glasses[END_REF][START_REF] Höche | Microstructure of SiO 2 -Al 2 O 3 -CaO-P 2 O 5 -K 2 O-F-Glass Ceramics. 2. Time dependence of apatite crystal growth[END_REF][START_REF] Chen | Investigation on phase separation, nucleation and crystallization in bioactive glass-ceramics containing fluorophlogopite and fluorapatite[END_REF] The high field strength of the P 5+ ion attracts more O 2-ions around, which relax and disturb the silicate glass network because of the various interstitial or modifying cations. [START_REF] Mirsaneh | Effect of P 2 O 5 on the early stage crystallization of K-fluorrichterite glass-ceramics[END_REF] J o u r n a l P r e -p r o o f

Simultaneously, P 5+ is a pentavalent network-forming cation, and electroneutrality is ensured only if one of the four oxygen ions around the P 5+ ion is doubly bonded to the central P 5+ cation. The double bond in the PO 4 tetrahedra is shorter in length than the other three single bonds (0.1424 nm compared with 0.1595 nm), which results in the asymmetry of the PO 4 tetrahedron and disturbs the bonding in the silicate network. In addition, Mg 2+ and Li + cations are likely to be included in the phosphate-rich phase because P 5+ ion has a higher ionic field strength and greater tendency to shielding.

Under this circumstance, phase separation likely occurs with P and Mg-enriched phase forming in the glass matrix. In the study of J. W. Cao et al., a similar phenomenon also appeared; the P and Mg-enriched phase was formed from the glass matrix, and Mg 3 (PO 4 ) 2 crystals were formed, which sreved as the nuclei to favour the cordierite precipitation. [START_REF] Cao | Effect of P 2 O 5 on evolution of microstructure of MgO-Al 2 O 3 -SiO 2 glass ceramics[END_REF] Moreover, similar phenomenon was reported in fluorapatite glass ceramics, in which Ca and P-enriched droplets are formed in the glass matrix.

  5-x) SiO 2 -11.5 Al 2 O 3 -x P 2 O 5 -8 MgO-4 TiO 2 -11 Na 2 O-5 Li 2 O (x=0, 2, 4, in mol.%, labeled as P0, P2, P4). The starting materials were analytical grade reagents, including SiO 2 (≥99.0%), Al 2 O 3

Fig. 2 ,

 2 all three samples have evident crystallization peaks in the XRD patterns when the samples were heat treated at 730 ℃ for 2 h. The precipitated J o u r n a l P r e -p r o o f crystals represented by the first crystallization peak in the DSC curve of the P0 and P2 samples are Mg(Ti 2 O 5 ). However, the P0 sample has sharper crystallization peaks than the P2 sample, indicating that the Mg(Ti 2 O 5 ) crystals in the P0 sample is easier to precipitate, which is consistent with the result of DSC. When the P4 sample was heat-treated at 730 °C for 2 h, the crystallization peaks of Ti 5 O 9 and LiMgPO 4 crystals appeared. When the HT temperature increased to 770 ℃, the intensity of the Ti 5 O 9 crystal devitrification peak in the P4 sample reached the highest. Then the intensity of the Ti 5 O 9 crystal crystallization peak gradually decreased with the increase of the HT temperature. When the HT temperature reached 890 ℃, the Ti 5 O 9 crystal crystallization peak completely disappeared. Meanwhile, when the HT temperature reached 810 °C, the crystallization peaks of TiO 2 crystals emerged and became stronger with the increase of HT temperature. Combined with the SEM test results of the P4 sample (Fig. 5), when the HT temperature reached 770 °C, a large number of granular crystals appeared in the sample. However, when the HT temperature increased to 810 °C, short needle-like crystals began to appear, coexisting with Ti 5 O 9 granular crystals. When the HT temperature reached 890 °C, most of the Ti 5 O 9 granular crystals disappeared and became needle-like TiO 2 crystals. The P4 sample undergoes crystalline transformation from Ti 5 O 9 crystals to TiO 2 crystals during the HT process.

  2+ and Li + preferentially moved toward the phosphorous-rich regions, forming spontaneous phase separation, in which the LiMgPO 4 was formed in the glass matrix. Thus, the cations decreased in the glass matrix and Ti-rich region. In this work, the needle-like rutile with a large L/D ratio was grown from the Ti-rich region in the sample with P 2 O 5 up to 4 mol% J o u r n a l P r e -p r o o f under HT at 890 ℃ for 2 h. Moreover, the large L/D ratio of needle-like crystals significantly increased the hardness from 6.08 GPa to 7.14 GPa. Similar to other fiber reinforced composites, this needle-like crystals provided a new strategy to improve the mechanical properties of glass ceramics.

Fig. 1

 1 Fig. 1 DSC curves of P0, P2, and P4 glasses.

Fig. 2

 2 Fig. 2 XRD patterns of (A) P0, (B) P2, and (C) P4 glasses heat treated at various

Fig. 3

 3 Fig. 3 Raman spectra of as-quenched P0, P2, and P4 glasses.

Fig. 4

 4 Fig. 4 SEM images of P0 and P2 glasses heat-treated at various temperatures for 2 h.

Fig. 6

 6 Fig. 6 TEM image and corresponding EDS elements analysis at needle-like crystal

Fig. 7 (

 7 Fig. 7 (A) TEM bright field micrograph and (B-F) corresponding EDS elements

Fig. 8

 8 Fig. 8 High resolution TEM images and selected area electron diffraction pattern of

Fig. 9

 9 Fig. 9 Vickers hardness of the P4 sample heat treated at different temperatures. The

  [START_REF] Moisescu | Crystallization of rod-shaped fluoroapatite from glass melts in the system SiO 2 -Al 2 O 3 -CaO-P 2 O 5 -Na 2 O-K 2 O-F[END_REF][START_REF] Höche | Microstructure of SiO 2 -Al 2 O 3 -CaO-P 2 O 5 -K 2 O-Fglass ceramics. 1. Needlelike versus isometric[END_REF] Based on the XRD results, with the increase of P 2 O 5 in the system, silicate crystals disappeared, phosphate crystals appeared, and titanate crystals were transformed into rutile crystals. With the increase of P 2 O 5 content, the amount of PO 4 in the glass network increases, and more cations are enriched in the P-rich phase. significantly increased from 6.08 GPa to 7.14 GPa.Therefore, with the increase of P 2 O 5 content, the number of PO 4 3+ in the glass increased. Consequently, its ability to compete for Mg 2+ , Li + , and other cations was enhanced. Finally, titanate crystal (MgTi 2 O 5 ) can not be observed when 4 mol% P 2 O 5 is introduced. The Ti 5 O 9 crystals appear simultaneously with LiMgPO 4 crystals and transform to rutile TiO 2 crystals at high temperature. Moreover, rutile crystals precipitate in the form of needle-like crystals with a large L/D ratio, and the length reaches more than 300 nm. Furthermore, the Vickers hardness of the P4 sample was significantly increased to 7.14 GPa. Meanwhile, needle-like rutile crystals with high aspect ratio were grown from the SiO 2 -Al 2 O 3 -MgO-TiO 2 system by adding appropriate amount of P 2 O 5 . Under high ionic field strength of P 5+ , Mg

	5. Conclusions
	In this work, as the P 2 O 5 content gradually increased from 0 to 4%, T g of the
	sample gradually increased from 580 ℃ to 638 ℃, The silicate crystals and titanate
	crystals in the sample disappeared successively, and the phosphate crystals and rutile
	crystals gradually appeared.
	When the sample with 4 mol% P 2 O 5 (P4) was heat treated at 890 ℃ for 2 h, phosphate

and rutile were separately formed in the P-rich and Ti-rich regions. When needle-like rutile crystals with a large L/D ratio appeared, the hardness of the P4 sample J o u r n a l P r e -p r o o f

Acknowledgement

This work is financially supported by the National Natural Science Foundation of China (61975156) and Hubei Natural Science Foundation (2020CFB641).

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.

Summary of Novel Conclusions

The needle-like rutile crystals with a large L/D ratio were grown from the Ti-rich region in the MgO-Al 2 O 3 -SiO 2 -TiO 2 glass. After precipatation of the rutile TiO 2 and LiMgPO 4 crystals, the hardness of glass was increased significantly from 6.08 GPa to 7.14 GPa.