### 2. METHODS AND PROCEDURES

Table 1 shows the glass compositions prepared in the present study, and the glasses were prepared by a conventional melt-quenching method. The raw materials of reagent grade $ Na_{2}CO_{3} $ , $ B_{2}O_{3} $ , $ SiO_{2} $ , $ Na_{3}PO_{4} $ and $ TiO_{2} $ were mixed thoroughly, and melted in a Pt crucible at $ 1400^{\circ}C $ for 30 min. By pressing the melts with iron plates, glass plates were obtained. Differential thermal analyses (DTA) were carried out to determine the glass transition and crystallization temperatures, $ T_{g} $ and $ T_{x} $ .

Table 1. Nominal compositions (mol%), $ T_{g} $ and $ T_{x} $ (°C) of the samples.

No.	$ Na_{2}O $	$ B_{2}O_{3} $	$ SiO_{2} $	$ P_{2}O_{5} $	$ TiO_{2} $	$ T_{g} $	$ T_{x} $
1	10	40	50	-	-	469	-
2	10	40	47	3	-	450	-
3	10	40	46	3	1	426	-
4	10	40	47	3	3	420	-
5	10	40	44	3	5	412	663
6	10	40	37	3	10	412	549

To induce phase separation, the glasses were heat-treated at 500 - 660°C for 16 h. After the heat treatment, the glasses were immersed in nitric acid of 1.0 N for 24 h at 90°C to dissolve $ B_{2}O_{3} $ -rich phase. The insoluble residues were collected with vacuum filtration, and after being rinsed in water and dried in an oven, silica-rich glasses were recovered.

X-ray diffraction (XRD) measurement with a Cu-Kα radiation was carried out to confirm the crystallization. Chemical compositions of the specimens, such as the glasses before phase separation, the solutions and insoluble residues after immersion in nitric acid were determined by inductively-coupled plasma (ICP) emission spectroscopy and x-ray fluorescence (XRF) spectrometry. Microscopic texture of the glasses after phase separation was observed by scanning electron microscope (SEM). $ {}^{29} $ Si, $ {}^{11} $ B, and $ {}^{31} $ P MAS-NMR measurements were performed with Varian Unity Inova 300 spectrometer to investigate the local structures around Si, B, and P, respectively. For the $ {}^{29} $ Si measurements, 0.2 mol% of $ Fe_{2}O_{3} $ was added to the glass batches in order to shorten the relaxation time. The respective NMR measurement conditions of $ {}^{29} $ Si, $ {}^{11} $ B and $ {}^{31} $ P measurements were as follows: magnetic field = 7.05 T, frequency = 59.6 MHz, 96.2 MHz and 121.4 MHz, sample spinning speed = 5.0 kHz (all), pulse duration = 2.5 μs, 0.6 μs and 3.3 μs, repetition time = 1.0 s, 1.0 s, 5.0 s, chemical shift second standard = poly dimethyl siloxane (PDMS), $ BPO_{4} $ and $ NH_{4}H_{2}PO_{4} $ . Infrared absorption spectra were recorded on an FT-IR spectrometer in the 4000 - 400 cm $ ^{-1} $ range using KBr pellets containing powdered glasses.

### 3. RESULTS AND DISCUSSION

SEM photographs of the heat-treated glasses are shown in Fig. 1. Typical textures associated with spinodal decomposition are commonly observed in the presence or absence of $ P_{2}O_{5} $ or $ TiO_{2} $ . In the DTA curve of the glasses No. 5 and No. 6, an exothermic peak due to crystallization was observed (See $ T_{x} $ in Table 1). As shown in Fig. 2, anatase-type $ TiO_{2} $ crystal was precipitated in the heat-treated glasses with $ TiO_{2} $ content $ \geq 5 $ mol%.

Fig. 1. SEM photos of glass surfaces heated at $ 600^{\circ} $ C for 16 h. (a) No. 2, (b) No. 5