TiO_2 を多量に含む BaO-SiO_2-TiO_2 系ガラスの耐アルカリ性  [in Japanese] Alkaline Durability of TiO_2-rich BaO-SiO_2-TiO_2 Glasses  [in Japanese]

標題の系の融液はNa2O-BaO-Al2O3-TiO2系の融液に比べて繊維状ガラスに成形しやすく, BaOの一部をNa2O, CaO及びSrOで, TiO2の一部をZrO2で置換した組成の融液は, これを白金ノズルから連続的に引き出す方法によっても繊維状ガラスに成形することができた.標題の系のガラス粉末2gを2N NaOH水溶液100mlに浸漬した時の重量減少率は, ガラス中のTiO2/SiO2モル比が増大するにつれ低下し, TiO2/SiO2>0.75以上の組成のガラスの重量減少率はNa2O-SiO2-ZrO2系のG 20ガラスのそれよりはるかに小さかった. しかし, その繊維5本を同じ液に浸漬した時の直径減少量はG 20ガラスのそれよりかなり大きかった. BaOの一部をCaOまたはSrOで置換すると直径減少量が減少し, G 20ガラスのそれより低いガラスも得られた. BaO-SrO-SiO2-TiO2系のガラスの侵食速度は浸漬時間が長くなるにつれ低下した. また侵食速度は, NaOH水溶液にあらかじめ少量の同組成のガラス粉末, あるいはSrO, BaOまたはTiO2の水和物を添加しておくと著しく低下した. NaOH処理後の同ガラスの表面にはTiO2に富んだ層が認められた. これらの結果に基づき, この系のガラスのアルカリ侵食機構を推察した.

(1) TiO2-rich melts in the BaO-SiO2-TiO2 system were formed into plates (1-3mm thick) and fibers (60-300μmφ) by pressing between two steel plates and by drawing up with the tip of a Pt rod, respectively. The compositional region from which clear glass plates and fibers were obtained are shown in Fig. 2. Automatic drawing of fibers through a Pt orifice was difficult for the above compositions, but became feasible by substituting Na2O, CaO and SrO for BaO and ZrO2 for TiO2, partially (Fig. 1).(2) 2g of grains (297-500μmφ) converted from the plate glasses of the BaO-SiO2-TiO2 system was immersed in a 100ml of 2N NaOH aqueous solution at 95°C for 18h. The weight losses due to corrosion of the glasses having TiO2/SiO2 mole ratio higher than 0.75 were less than that of G 20 glass (1Li2O⋅11Na2O⋅1Al2O3⋅71SiO2⋅16ZrO2, wt%). On the contrary, however, diameter reduction of the fibers of the same compositions, were much larger than that of G 20 glass, when 5 pieces of them (250-300μmφ×5mm) were immersed in the same solution (Fig. 3).(3) The diameter reduction of the fibers of the BaO-SiO2-TiO2 system was decreased by partial substitutions of CaO or SrO for BaO (Figs. 4 and 5). The diameter reduction of fibers of S4-15Sr glass (15SrO⋅15BaO⋅30SiO2⋅40TiO2, mol%) could be further supressed by addition of a small amount of S4-15Sr glass grains or powdered chemicals of Sr(OH)2⋅8H2O, Ba(OH)2⋅8H2O and/or TiO2⋅1.5H2O to the solutlon (Fig. 6, Table 3). Presence of a TiO2-rich thin layer on the surface of S4-15Sr glass fibers was confirmed by electron probe microanalysis after the fibers were immersed in the NaOH solution (Table 4).(4) On the basis of the above results, alkali corrosion process of the TiO2-rich BaO-SiO2-TiO2 glasses was suggested as follows: The NaOH solution near the surface of the glasses would firstly be saturated with TiO2 dissolved from the glasses because of its extremely low solubility, leading to formation of a thin TiO2-rich layer on the glass surface. Ba2+ ions dissolved from the glasses would be adsorbed by the TiO2-rich layer because of their high affinity to TiO2. The TiO2-rich layer thus formed would suppress further dissolution of BaO and SiO2 from the interior of the glasses.