N e w s a n d V i e w s

Pore s i z e c o n t r o l (1) Pore s i z e c o n t r o l (2) Workers a t Kyoto Un/versity, J a p a n have been looking a t ways to control the pore s/ze In s /I /ca-based ox/de gels. The gels, which have pro-range Interconnected pore morphologies, a re p repa red b y the spmoda l decomposi t ion of alkoxy-der/ved gel forming solut ions and it h a s been found tha t solvent exchange t r ea tmen t s on the wet gels can dete~mlne the pore s/ze range In the dried s ta te .

Incorporat ion of var ious water-soluble organic polymers In the hydrolysis a n d polycondensat ion of alkoxy~lmne unde r wa~er-rich condit ions leads to the format ion of gels with p~n-rAn~e Interconnected pores. The macropore s t ruc tu re of the gels can be regulated by changing the solubflfly of the polymers or the polymerizat ion ra te of the s/I/ca, bo th of which Influence the spinodal decompos/ t ion behavlour at the gel forming stage. Under acld-catalyzed a n d water-poor conditions, with addit ion of form~mlde, hydrolys/s of alkoxysflane resu l t s In gels which can have pore s/zes going from several n m u p to the pm range.

Further Information.from.. Dr Kazuki Nakanlshl, Department of lndustrlal Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606-01, Japan.

In the USA, a r e sea rch group hm~ been fabricat ing ceramic m e m b r a n e s wi th ~ o r e d pore size a n d dis tr ibut ion b y a me thod which entAtl~ the Incorporat ion of ca rbon fibres a s a fugitive phase m the ceram/c m a t r l ~ The fugitive ca rbon fibre phase Is subsequen t ly removed b y oxidation and the resul t Is oriented microchanne ls of well-controlled dimensions In the m a t r ~

The US t eam h a s been under t ak ing bas/c s tudies on the feaslbfllty of the technlque us ing polyacrylomtrl le (PAN) b a s e d fibres In a Pyrex glass m a t r l ~ The m e c h a n t s m a n d control of the ox/dation process for the fibre r e m o v a l / s still a n a rea tha t requlres work, b u t it bAR b e e n conf i rmed tha t on removal the resu l t an t mlc rochanne l s represen t the original ca rbon fibre size (about 5 ~m In diameter) and shape. Mlcrochannel alze a n d s h a p e can be tailored by proper fibre selection a n d m/crochanne l orientat ion

P o r o u s g lass m e m b r a n e A porous glass m e m b r a n e p repa red f rom sodlum borosfl/cate glass containing ZrO2 and CaO - - composl t ion (wt%) $IO2 (53), B203 (24.5), Na20 (5.5), Ca<:) (8), ZrO2 (6),/11203 (3) - - is being character /zed at the, Government Industr ia l Research I n s t i t u t e / n Osaka, J apan . To p roduce the m e m b r a n e the borate- r lch p h a s e is leached by aqueous solution of 1N HNOs for 48 hou r s a t 98 "C. The resul t Is a median pore d iameter of 3.0--3.5 nm. Surface modification t r ea tmen t s a re cur ren t ly being e ~ m l n e d and tes ted for their effect on the separa t ion of gaseous CO2 from a 5 0 / 5 0 m/x ture of CO2 a n d N2.

Two t r ea tmen t s look promising: the first Involves dlppL-.g the glass m e m b r a n e In a n aqueous n/ t ra te solution of po tass inm, magnes /um, silver and ch romlum and a subsequen t bak /ng a t the decomposl t ion t empera tu re s of the nl t ra te salts; In the second, the m e m b r a n e Is d ipped tn xylene and subsequen t ly b a k e d In a reducing a tmosphere . Measu remen t s of permeabt l / t ies and CO2 separa t ion factor were m a d e a t t empera tu re s f rom 30 to 400°C.

Although In general bo th var iables decrease wlth Increased t empe ra tu r e there is a beneficial effect with the second t r ea tmen t which ma in t a in s a good separa t ion factor even a t the higher tempera tures .

Further Information frorrc Dr Tetsuo Yazawa, Government Industrial Research Institute, Osaka Mkiorigaoka l-chome, Ik~da-shl, Osaka 563, Japan. Tel +81 727 51 8531.

(a) Mb~ostructure of a PAN-fibre~Pyrex composite after wddativn of the carbon flbres at 793 K; (b) A higher magnylcation vtew of individual ~crochannels.

Membrane Technology No. 29 3