Amorphous Oxides Prepared from Organometallic Compounds Part2. Structure Evolution in Sodium Borosilicate Gels

Transcription

1 Memirs f the Faculty f Engineering,Okayama University.VI.23. N.1. Nvember 1988 Amrphus Oxides Prepared frm Organmetallic Cmpunds Part2. Structure Evlutin in Sdium Brsilicate Gels Akiyshi OSAKA*, Mtkazu YUASA**, and Yshinari MIURA* and Katsuaki TAKAHASHI* (Received September ) SYNOPSIS Sdium brsilicate gels f cmpsitins similar t that f Vycr R glass like 80Si02.15B203.5Na20 (wt%) were prepared frm hydrlysis and plycndensatin f metal alkxides under the HCl catalysis. Variatin f specific surface area and prsity with temperature indicated that clsed pres were pened belw C, and cllapsed abve C after the prsity reached a maximum value arund C. The structural evlutin was examined due t IR spectra and a phase separatin in the gels was bserved. The slubility f the gels int 1N-HCl was studied as a functin f the treatment temperature. 1. INTRODUCTION In the previus paper(1) we dealt with gelatin time and thermal behavir f the sdium brsilicate gels f cmpsitins similar t Vycr R glass. In this paper, we will examine the variatin f specific surface area and prsity as well as their IR spectra as a functin f the heat treatment, based n which the structure evlutin will be discussed f the gels under treatment. The slubility f the gels after varius stages f the treatment is examined and interpreted with respect t phase separatin in the gels. * Department f Applied Chemistry ** Nw at Sekisui Chemical Industry, Ltd. 39

2 40 Akiyshi OSAKA, Mtkazu YUASA, Yshinari MIURA, and Katsuaki TKAHASHI 2. EXPERIMENTAL 2.1. Preparatin The cmpsitins f the gels were i) 80Si02.(20-X)B203.xNa20 [wt%l (0<x<20) ii) 7SSi02.(2S-x)B203.xNa20 [wt%l (0<x<20) The starting materials were the use f tetraethyl rthsilicate (TEOS), trimethyl brate and sdium methylate. HCl was emplyed as the catalyst. Amng several values f the mixing mlar ratis H 2 0/TEOS and HC1/TEOS adpted previusly(1), the fixed ratis H 2 0/TEOS=1 and HC1/TEOS=1/100 have been taken fr preparing the gels (methd I). Althugh the gels have als prepared frm the mixtures f the alkxides and acetylacetne withut water (methd II), discussin in the present paper will be fcused n the gels thrugh methd I, unless therwise nticed. Full hydrlysis and plymerizatin f the starting slutins need much water than added, hence the deficient amunt fr that is supplied frm the air. The detail f the preparatin prcedure is described in the previus paper(1). (1) L :::J... C L a E (1) I- 5 h Saking Temperature 300 C-650 C Fig. 1. The heat-treatment schedule fr the sdium brsilicate gels. Time Figure 1 shws the heat-treatment schedule. After the heating at a rate f SOC/h up t 1 SOOC, the dried samples were kept fr Sh at 1 SOOC in rder t drive ff water and slvents adsrbed n the gel surface. The temperature was subsequently raised at a rate f SOC/h t the saking temperature, ranging frm t 6S0 0 C at which temperature the samples were saked fr varied duratin f time. After thse treatments, the gels were let cled in the furnace by cutting ff the furnace pwer.

3 Structure El'lutin in Sdium Brsilicate Gels Prsity and thermal shrinkage Apparent density f as-prepared and heat-treated gels was measured by the Archimedes methd using EtOH as the immersing liquid and a silica glass blck as the reference. Apparent prsity represents the vlume fractin f pen pres in a bulk prus slid. The gels were dried fr a day at C in an ven. Befre the weighing in EtOH the pieces f the gel were immersed in EtOH fr 10 t 20 min. After the immersin in EtOH the gels were quickly taken ut, and excess EtOH n the gel surface was wiped ff. The apparent prsity then was btained by weighing the gels whse pen pres were filled with EtOH. Specific surface area was measured fr the pulverized gels by means f the BET methd with using N 2 gas. 2.5,...,. M E u- M.O. glass Si02"158203'5Na20 5hr Temperature (OC) Fig. 2. Apparent density f 80S-15B-5N gel as a functin f the saking temperature. Density f a melt-quenched brsilicate f the same cmpsitin is als pltted. 3. RESULTS AND DISCUSSION 3.1. Prsity and specific surface area Figure 2 shws the variatin f the apparent density f a gel f cmpsitin 80Si02.15B203.5Na20 (80S-15B-5N) pltted as a functin f the saking temperature. The density f the gels little changes belw

4 42 Akiyshi OSAKA. Mlkazu YUASA. Yshinari MIURA. and Kalsuaki TKAHASHI M Eu 'E cr IUIIl a.c: a.c1j Cl Time (hr) -0 80S-l5B-5N gel 450 C 15 Fig. 3. Apparent density f 80S-15B-5N gel saked at C as a functin f the saking time C and is abut 3/4 times as dense as that f the melt-quenched glass f the same cmpsitin, indicating the presence f islated clsed pres as much as 20% in vlume in the 80S-15B-5N gel. The density f the gels slwly increases abve C and is almst equal t that f melt-quenched glass after they are heated at C fr 5 h. The apparent density, hwever, shws little dependence n the time f saking as indicated in Fig. 3. Figs. 4 (a) and (b) indicate the apparent density fr the gels f cmpsitins i) 80Si0 2.(20-X)B I Ii I I- 0 i /},... 0 A 0 E [J Cll, , A A 0.. '0 0 Cll... :l: /}, M.O.glass 0 M.O.glass 1: 4000C 0400 C(J) C (I) C Z a. c. /},400OC a. A 400 C m ) a. ct C (n ) ct C 75SiOt(25- x )Bz03'xNaz0 80SiOt(20 -x )B 2 0),5NazO. too 5 hr hr I I x (.,.) x (.J') C.. (a) Fig. 4. Apparent density f gels f cmpsitin (a) 80Si0 2.(20-x) B203.xNa20 and (b) 75Si0 2. (25-x)B XN 2 0 prepared thrugh methds I and II and treated at varius temperatures. (b) I

5 Structure Et'lutin in Sdium Hrsiliratl' Gels 43 xna20 and ii) 7SSi02.(2S-x)B203.xNa20 (O<x<10wt%), respectively. They were heated fr S h at temperatures frm C t 600 C. The triangles indicate the plts fr the gels thrugh methd II. The trend that apparent density fr methd II is larger than that fr methd I shws prus slids f less prsity is btained by the slwer gelatin reactin caused by retarded supply f water nly frm the air. The apparent density increases with increase in the saking temperature and appraches that f the melt-quenched glass represented by the pen squares. Figure S shws the prsity f the gel f cmpsitin BOS-1SB-SN as a functin f the saking temperature. Prsity f the gels starts increasing with the saking at 4S0 C. It is therefre suggested that a part f the clsed pres in the gels are pened due t the heattreatment. After reaching a maximum, prsity suddenly decreases after the saking abve 4S0 C. The specific surface area f the gel has been pltted in Fig. 6 as a functin f the saking temperature, 40 80S ioz150:3" 5Na20 gel 5hr ,..., 300 CI f!!. -20 III Cl.10 -N E 200 "- III Q.I III $ " :::J lfl Si02 15B203" 5Na20 3 hr 0'----L.-...L----I---l.--.L J Temperature (OC ) Fig. 5(left). Prsity f BOS-15B-5N gels as a functin f the temperature f saking fr Sh. Fig. 6(right). Specific surface area f the gels as a functin f the saking temperature.

6 44 Akiyshi OSAKA, Mtkazu YUASA, Yshinari MIURA, and Katsuaki TKAHASHI indicating a temperature dependence similar t that fr the prsity. The specific surface area increases up t C and then decreases rapidly with the saking temperature. It has already been indicated(1) that the gel shrinks gradually in the saking treatment belw abut C and that an appreciable amunt water is driven ff belw C. Since nly a little lss f the surface area is fund arund C the vlume fractin f the pres is nt significantly reduced during the initial stages f the pre cllapse. The fact that the surface area becmes maximum after the treatment at C is prbably related t pening f the clsed pres. The small shrinkage has been fund(1) with lss f water n heating the gels between C and C, which fact is an indirect evidence fr cndensatin amng adjacent Si-OH and B-OH grups n the pre walls, as shwn later n the basis f IR spectra in Fig. 8. The cndensatin f these grups n ne wall with thse n an ppsite wall f the micrpre causes the pre cllapse. This may ccur lcally t a limited degree at lwer temperature, which explains the small amunt f shrinkage that is bserved belw C. The pre size distributin has been measured by the Hg methd. Althugh the heated gels have the surface area as large as m 2 /g, the vlume fractin f the pres frm 100A t 10 4 A in diameter is nt s large as t explain the large surface area. On this basis, it is suggested that a cnsiderable fractin f the pres is belw 40A I:! S00e SOQe Fig. 7. Variatin f prsity fr 80S-15B-5N gels saked at 400, 450 and C is shwn as a functin f the saking time. The value fr the dried gel is represented as "n h.t."

7 Structure Evlutin in Sdium Brsilicate Gels 45 in diameter and is nt measurable by the Hg methd. The mechanism f the pre cllapse is nt understd at this stage, but Thge et al.(2) is against the viscus flw mechanism because f the lw activatin energy (abut 5 kcal/ml) fund fr the brsilicate gel. Figure 7 shws the effect f heat-treatment time n the apparent prsity f the 80S-15B-5N gel heated at 400, 450, 500 C. The prsity f the gels increases and reaches a maximum value after the saking fr abut 10 min regardless f the temperature. After the maximum, the prsity fr the gels heated belw C decreases and reaches a cnstant value, whereas that fr the gels heated at C decreases cntinuusly t almst null (abut 3% prsity). Such change can be interpreted as fllws: At first the pre wall has a number f Si-OH and B-OH grups inside as well as utside fr which the cndensatin takes place causing the clsed pres t be pened. Then the pre wall shrinks itself due t further cndensatin amng these active -OH grups inside the wall since they can easily find the cunterpart in the vicinity. This causes the decrease in the prsity. The difference in the final prsity after the saking as lng as 5h depending n the treatment temperature may cme frm the difference in the size f the cllapsed pres: that is, the larger pres are left withut cllapse at lwer temperature but they are much less in number IR spectra f the brsilicate gels Figure 8 shws the IR spectra fr a sdium brsilicate gel f cmpsitin 80S-15B-5N heat-treated fr 5h at varius temperatures, tgether with that f the melt-quenched glass f the same cmpsitin which was heat-treated fr 20 h at 700 C, the immiscibility temperature f the glass. The spectra f the gels befre the heat-treatment have strng bands at 470, 810, 930, 1050 and 1400 cm- 1. The difference in the spectrum between the melt-quenched glass befre and after the heat-treatment is characterized by the presence f a weak band arund 930 cm- 1 attributed t the defrmatin vibratin f the Si-O-B bnds. This band becmes weak with increasing treatment temperature. A new band at abut 680 cm- 1, due t the B-O-B bnds, appears fr the gels treated abve 450 C. The prfile fr these gels is very similar t that f the melt-quenched glass. Mrever, the intensity f the bands becmes well-defined with increasing the treatment temperature, indicating the structure evlutin in the gel. Cmparisn f the spectra fr the gels befre and after the heat-treatment indicates that a band arund 1600cm- 1 due t the O-H grups is weak but still

8 46 Akiyshi OSAKA. Mtkazu YUASA. Yshinari MIURA. and Katsuaki TKAHASHI RT Fig. 8. Infrared spectra f 80S-1SB-SN gels treated at varius temperatures. Thse fr silica glass and meltquenched brsilicate glass f the same cmpsitin are als indicated Wavenumber ( cm-1 ) fund. It is suggested accrdingly that the gel derived brsilicate glass experiences the phase separatin when it is saked at 6S0 0 C fr Sh Slubility f the gels t acid After dried at C fr 1 day in the ven and saked at varius temperatures fr Sh, the gels were saked in 10ml 1N-HCl slutin fr 1 day. Then the samples were washed with an excess amunt f distilled water and dried at C fr 1 day. The weight f the gels befre and after the acid-treatment was measured. Figure 9 shws the effect f heating temperature n the weight lss f the 80S-1SB-SN gel due t leaching by 1N-HCl slutin. The weight lss increases up t abut 10 percent when the gels are heated belw C. The samples are cracked after the acid treatment due t capillary pressure f water immersed in the pen pres f the gel. The lss decreases when the gels are treated abve C - SOOOC, and reaches less than 1%. A

9 Structure Et'lutin in Sdium Hrsilimte Gels 47 slight increase is seen in the weight lss fr the gel treated abve C. The dependence f the weight lss, shwn in Fig. 9, is very similar t that f the apparent prsity and surface area. It is reasnable that the gels f larger specific surface area shuld suffer frm the greater attack f acid than the gels f less surface area. Thugh the gels are phase separated after the saking at higher temperatures, the leached lss is much smaller than that expected frm I ,------,---r-----,.--..,-- r--,..., -10 l/l l/l.2.c 5 0'1 80Si02,15B20:3'5Na20 5 hr Temperature ( C ) Fig. 9. Weight lss f 80S-1SB-SN gel as a functin f the temperature f the heat treatment fr Sh after leaching in 1N-HCl fr Sh. the Na20 and B cntent in the 80S-1SB-SN gel, indicating that the phase-separated gels are resistant t acid. This suggests that an amrphus sdium brate phase is separated frm the gel r gel-derived glass, whereas the phase separatin is nt an entangled type but a drplet type. The leaching hence takes place nly at the surface f the glass. Such a pssibility might be negligible as the structure is different between the melt-quenched glass and gel-derived glass, and the sdium brate phase separated frm the gel is mre alkali resistant than that separated frm the melt-quenched brsilicate glass f the same cmpsitin. 4. CONCLUSION Sdium brsilicate gels f cmpsitins near the Vycr R glass,

10 48 Akiyshi OSAKA. Mtkazu YUASA. Yshinari MIURA. and Katsuaki TKAHASHI 80Si02.15B203.5Na20 (wt%) have been prepared and their physical prperties such as prsity, specific surface area, and IR spectra have been measured. The 80S-15B-5N gel has a maximum prsity (35%) when it is heated at C. It becmes pre free and is phase-separated after the treatment at C. Subsequent treatment causes precipitatin f cristbalite. Frm the IR spectra it is cncluded that the pre cllapse begins with the frmatin f B-O-B bnd frm the cndensatin f B-OH bnds. The very small leaching lss f the phase separated gel-derived glass suggests the drplet-type phase separatin but nt the entangled-type ne which is the usual case f the meltquenched glass. ACKNOWLEDGMENT Financial supprt f the Grant-in-Aid fr Scientific Research frm the Ministry f Educatin, Science and Culture, Japan (Grant # ) is gratefully acknwledged. REFERENCES [1] A. Osaka et al.: Memirs Faculty. Eng., Okayama Univ., 23(1988). The preceding paper. [2] N. Thge et al.: J. Nn-Crystal. Slids, 63(1984) 95.