HAL Id: jpa-00225485
https://hal.archives-ouvertes.fr/jpa-00225485
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
GEL PROCESSING
J.L Woodhead
To cite this version:
J.L Woodhead. GEL PROCESSING. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-3-C1-12.
�10.1051/jphyscol:1986101�. �jpa-00225485�
JOURNAL DE PHYSIQUE
Colloque C1, supplbment au n02, Tome 47, fkvrier 1986 page ci-3
GEL PROCESSING
J
.
L WOODHEADE n g i n e e r i n g Sciences D i v i s i o n , B u i l d i n g 4 2 9 , AERE H a r w e l l ,
G B - D i d c o t , Oxfordshire 0x11 O R A , U.K
R6sum6 - La technique des procdd6s gel a dtd classifide en quatre groupes:
sol-gel, gel-prbcipitation, gel-addition et lavoie alkoxide. Chacun de ces groupes prdsente un avantage pour la prbparation de matdriaux cdramiques particuliers. Des recherches concernant la structure des gels d'oxydes hy- dratds montrent qu'ils peuvent Ctre utilisss pour ddcomposer des sels de md- taux alcalins ou alcalino-terreux. Dans certains cas les produits acides de ddcomposition thermique peuvent rdagir avec le substrat de gel hydratd pour donner des oxydes non agglom6r6s qui forment alors des sels aqueux concentr6s.
Abstract - Gel processing technology has been classified into four groups:
Sol-gel, Gel-precipitation, Gel addition and the Alkoxide route. Each of the groups has merit for preparing specialised ceramic materials. Investigations into the hydrous oxide gel structures revealed they could be used to thermally decompose alkali and alkaline-earth metal salts. In some cases the acidic thermal decomposition products reacted with the hydrous gel substrate to give deaggregated oxides which readily formed concentrated aquo-sols.
During the early 1960s, it was found /I/ that concentrated aquo-sols of thorium oxide could be evaporated to glassy gels which gave oxide products of very high density when calcined at relatively low temperatures. This opportunity to obtain thoria of near theoretical density at 113J°C rather than up to 1 6 3 0 ~ ~ which was needed for non-gel processed thoria was a great advantage for the designers of remote handling facilities for re-processing radioactive, oxide-based materials. An equally important development which occurred at this time was the capability to convert the fluid apuo-sols into rigid gel particles in spherical form which were then heat-treated to give the very high density oxide microspheres of thorium oxide alone, and of urania-thoria and plutonia-urania ceramics. In addition to the advantages of high density at lower-than-usual temperatures, the new processes were essentially dust-free operations and out-of-size gels could be easily recycled by re-dispersing.
Very few ideas in chemistry are really new and the use of colloids and gels plays a part in many chemical processing operations. The fabrication of spherical particles by dehydrating aquo-dispersions with hot oil was well established in the catalyst field / 2 / and co-precipitation to provide homogeneity for mixtures finds widespread use. However, an opportunity to maximise the benefits of these individual
capabilities into a single process was not to be missed and new thinking on old ideas often leads to improved science and to new science-related products.
Thus the advantages found for the sol-gel processes used for nuclear ceramics
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986101
JOURNAL DE PHYSIQUE
i n i t i a t e d a wave of r e s e a r c h and development i n t o t h e s u b j e c t , and d u r i n g t h e n e x t t w e n t y - f i v e y e a r s a whole new t e c h n o l o g y h a s emerged 131.
The m a j o r o b j e c t i v e s a c h i e v e d u s i n g new p r o c e s s e s have been t o p r e p a r e s i n g l e and multi-component c e r a m i c p r o d u c t s w i t h : -
( a ) C o n t r o l of p a r t i c l e s i z e . ( b ) C o n t r o l of p a r t i c l e s h a p e . ( c ) Pre-determined p o r e s t r u c t u r e . ( d ) C o n t r o l of p r o d u c t d e n s i t y . ( e ) A h i g h d e g r e e of p u r i t y .
I n a d d i t i o n , t h e p r o c e s s e s h a v e been shown t o p r o v i d e economy i n e n e r g y by
e l i m i n a t i n g o r m i n i m i s i n g c r u s h i n g , g r i n d i n g and s i e v i n g o p e r a t i o n s and t o m i n i m i s e w a s t a g e by r e c y c l i n g . g e 1 w a s t e .
However, n o s i n g l e g e l p r o c e s s c a n p r o v i d e a l l t h e s e a d v a n t a g e s , and d i f f i c u l t i e s such a s i d e n t i f y i n g and o b t a i n i n g s u i t a b l e s t a r t i n g m a t e r i a l s f o r s o l p r e p a r a t i o n , s c a l e - u p problems and e x p e n s i v e i t e m s of equipment h a s r e s u l t e d i n t h e emergence of a t l e a s t f o u r s e p a r a t e g e l p r o c e s s e s . These a r e : -
( a ) S o l - g e l .
( b ) G e l - p r e c i p i t a t i o n . ( c ) G e l - a d d i t i o n .
( d ) The a l k o x i d e r o u t e o r t h e u s e o f o t h e r o r g a n o - m e t a l l i c p r e c u r s o r m a t e r i a l s . Each of t h e p r o c e s s e s h a s one o r more a d v a n t a g e s t h a t c a n b e u s e d t o t h e b e n e f i t of t h e p r o d u c t and a c o m b i n a t i o n of two o r even more of t h e b a s i c p r o c e s s e s c a n p r o v i d e a v e r y h i g h d e g r e e of p r o d u c t q u a l i t y f o r v e r y s p e c i a l i s e d a p p l i c a t i o n s . However, t h e r e i s a p r i c e premium t o be p a i d f o r s u c h s o p h i s t i c a t i o n , and t h e p r o d u c t v a l u e u s u a l l y d e t e r m i n e s t h e l e v e l of g e l p r o c e s s i n g t h a t c a n b e a p p l i e d . P e r h a p s t h e most e x p e n s i v e p r o d u c t we a t H a r w e l l p r e p a r e d u s i n g g e l p r o c e s s e s were s p h e r i c a l p a r t i c l e s of a n i s o t o p e e n r i c h e d y t t e r b i a ; e a c h 250 urn s p h e r e ( c a . 0 . 5 mgm) was v a l u e d a t $100 ( 2 . 0 x 10' p e r k g ) . N a t u r a l l y , f o r t h i s m a t e r i a l t h e c o s t of p r o c e s s i n g was v e r y i n s i g n i f i c a n t . A t t h e o t h e r end of t h e p r o d u c t v a l u e s c a l e , a l u m i n a and z i r c o n i a s o l s have been p r e p a r e d a t a c o s t o n l y f r a c t i o n a l l y g r e a t e r t h a n t h e c o m m e r c i a l l y - a v a i l a b l e s t a r t i n g m a t e r i a l . These s o l s c a n b e u s e d f o r c o a t i n g s o r b o n d i n g a g e n t s and t h e i n t r i n s i c a d v a n t a g e s of t h e s o l - g e l p r o c e s s i n g used t o e x e r c i s e c o n t r o l o v e r p r o p e r t i e s s u c h a s d e n s i t y and s u r f a c e a r e a .
A d d i t i o n a l g e l p r o c e s s i n g c a n be c a r r i e d o u t t o c o n v e r t i n e x p e n s i v e s o l - b a s e d f e e d s i n t o s p h e r i c a l g e l and o x i d e powders f o r u s e i n f l u i d i z e d b e d s o r a s f r e e - f l o w i n g powders i n p l a s m a - s p r a y i n g .
T h i s p a p e r w i l l d e s c r i b e t h e d i f f e r e n t g e l p r o c e s s e s and i n d i c a t e how t h e s e p a r a t e p r o c e s s e s c a n be u s e d t o p r o v i d e o r enhance t h e p r o p e r t i e s r e q u i r e d i n a p a r t i c u l a r p r o d u c t o r p r o d u c t a p p l i c a t i o n .
Some o t h e r a s p e c t s of H a r w e l l ' s r e s e a r c h e s i n t o t h e mechanism of s o l f o r m a t i o n w i l l a l s o be d i s c u s s e d w i t h s p e c i a l r e f e r e n c e t o multi-component c e r a m i c s y s t e m s . T h i s work h a s shown t h a t some v e r y i n t e r e s t i n g c h e m i c a l r e a c t i o n s o c c u r i n g e l s t r u c t u r e s d u r i n g low t e m p e r a t u r e h e a t t r e a t m e n t s .
I1 - . THE DIFFERENT - .- - - - - - - - - - - -- GEL - - - -- PROCESSES - - - - - - - - .
For t h e aqueous s y s t e m s c o n s i d e r e d h e r e f o r s o l - g e l p r o c e s s e s , t h e s o l i s d e f i n e d a s : -
"a d i s p e r s i o n of s o l i d p a r t i c l e s i n t h e l i q u i d medium i n which a t l e a s t one d i m e n s i o n of t h e p a r t i c l e s i s between 1 nrn and 1 pm".
An e s s e n t i a l p r e - r e q u i s i t e f o r t h e development and o p e r a t i o n of a s o l - g e l p r o c e s s i s
a s t a b l e , c o n c e n t r a t e d a q u o - s o l . The s h e l f - l i f e of t h e s o l must b e a d e q u a t e t o a l l o w t h e s u b s e q u e n t p r o c e s s i n g t o g e l s of t h e r e q u i r e d s h a p e and s i z e and t h u s c h a n g e s i n v i s c o s i t y o r any t e n d e n c y t o s e d i m e n t must be minimal.
Methods / 4 / f o r p r e p a r i n g t h e s o l s have b e e n d e s c r i b e d i n v a r i o u s j o u r n a l s and p a t e n t s and t h e most i m p o r t a n t t e c h n i q u e s a r e : -
- t h e t h e r m a l d e n i t r a t i o n of m e t a l s a l t s . - p e p t i z a t i o n of well-washed m e t a l h y d r o x i d e s .
- s o l v e n t e x t r a c t i o n o f t h e h y d r o l y t i c a c i d formed i n c o n c e n t r a t e d s o l u t i o n s of mf: t a 1 s a l t s .
- p e p t i z a t i o n of m e t a l h y d r o x i d e s formed from m e t a l - o r g a n o compounds.
L e s s well-known a r e methods s u c h a s : -
- r e a c t i n g t h e n i t r i c a c i d formed by h y d r o l y s i n g m e t a l n i t r a t e s a l t s w i t h formaldehyde: t h e v i o l e n t r e a c t i o n y i e l d s g a s e o u s p r o d u c t s and t h e t e c h n i q u e c a n b e u s e d t o p r e p a r e m e t a l s a l t s o l u t i o n s w i t h c a r e f u l l y c o n t r o l l e d N O , / M mole r a t i o s .
- h y d r o l y s i n g m e t a l s a l t s a t e l e v a t e d t e m p e r a t u r e s and p r e s s u r e s f o l l o w e d by removal of t h e h y d r o l y t i c a c i d and p e p t i z a t i o n of t h e m e t a l h y d r o x i d e w i t h w a t e r / 5 / .
The i m p o r t a n t o b j e c t i v e of a l l t h e s e p r o c e d u r e s i s t o i d e n t i f y and u s e a r e a d i l y - a v a i l a b l e s t a r t i n g m a t e r i a l and t o p r o c e s s i t t o a c o n c e n t r a t e d aquo s o l a t a c o s t f a v o u r a b l y r e l a t e d t o t h e e n v i s a g e d p r o d u c t v a l u e .
S u i t a b l y armed w i t h t h e s o l o r s o l m i x t u r e , we c a n now o p e r a t e a t y p i c a l s o l - g e l p r o c e s s which c a n b e r e p r e s e n t e d a s : -
+ SOL + GEL + O X I D E
MATERIAL
The A-B s t a g e h a s a n i m p o r t a n t b e a r i n g on t h e c o s t of t h e p a r t i c u l a r p r o c e s s , e . g . u s i n g a h i g h v a l u e r e a g e n t g r a d e s a l t i n a t h e r m a l d e n i t r a t i o n p r o c e s s i s v e r y e x p e n s i v e , whereas t o s u c c e s s f u l l y p e p t i z e a n i n e x p e n s i v e c o m m e r c i a l l y - a v a i l a b l e m e t a l h y d r o x i d e c a n y i e l d a v e r y c o m p e t i t i v e l y p r i c e d s o l - g e l p r o d u c t .
The s t e p B-C i s t h e s h a p i n g s t a g e . By u s i n g o r g a n i c s o l v e n t s t o d e h y d r a t e t h e s o l , h i g h q u a l i t y s p h e r i c a l g e l s c a n b e formed i n a wide r a n g e of s i z e s (1-1003 ~ m ) , o r by s u i t a b l e c o n t r o l of t h e s o l r h e o l o g y we c a n form t h e g e l a s f i b r e s o r e x t r u d a t e . A v e r y i m p o r t a n t p r o p e r t y of t h e g e l s formed by d e h y d r a t i n g t h e s o l s i s t h e i r a b i l i t y t o r e - d i s p e r s e i n w a t e r . Thus f o r many t r u e s o l - g e l p r o c e s s e s , o u t - o f - s i z e g e l p r o d u c t s c a n be r e - d i s p e r s e d i n w a t e r and r e c y c l e d .
The h e a t - t r e a t m e n t s t e p C-D i s t h e s t a g e a t which one of t h e most i m p o r t a n t f e a t u r e s of a s o l - g e l p r o c e s s m a n i f e s t s i t s e l f , i . e . c o n t r o l of p r o d u c t d e n s i t y . The p r i m a r y p a r t i c l e s i z e ( c r y s t a l l i t e s i z e ) and t h e d e g r e e t o which t h e p r i m a r y p a r t i c l e s a r e d e - a g g r e g a t e d have a marked e f f e c t on t h e d e n s i t y of t h e o x i d e p r o d u c t . The f o l l o w i n g t a b l e shows t h e e f f e c t of p a r t i a l l y and c o m p l e t e l y d e - a g g r e g a t e d ceriumIV h y d r a t e w i t h d i l u t e n i t r i c a c i d .
S i m i l a r b a t c h e s of w a t e r i n s o l u b l e ceriumIV h y d r a t e i n s l u r r y form were h e a t e d a t 9 0 ° C w i t h d i f f e r e n t c o n c e n t r a t i o n s o f n i t r i c a c i d . The c r y s t a l l i t e s i z e remained unchanged a t 70 2, b u t t h e n i t r i c a c i d t r e a t m e n t r e d u c e d t h e s i z e of t h e a g g r e g a - t e s , a l l o w i n g c l o s e r p a c k i n g of t h e p r i m a r y p a r t i c l e s t o o c c u r when t h e s l u r r i e s were d r i e d and c a l c i n e d .
The f o l l o w i n g r e s u l t s show how d i f f e r e n t g e l and o x i d e d e n s i t i e s c o u l d be o b t a i n e d
J O U R N A L DE PHYSIQUE
u s i n g t h e same s t a r t i n g m a t e r i a l and s i m i l a r c a l c i n a t i o n t e m p e r a t u r e s . Values r a n g e from v e r y p o r o u s t o a l m o s t t h e o r e t i c a l d e n s i t y .
Alumina, c e r i a , indium o x i d e , s i l i c a , t i t a n i a , t h o r i a and z i r c o n i a a r e m a t e r i a l s t h a t a r e a v a i l a b l e a s c o n c e n t r a t e d aquo s o l s s u i t a b l e f o r f a b r i c a t i n g t o r e a d i l y d e n s i f i a b l e g e l s and o x i d e s .
3 .
mole r a t i o - .- - .- - - - .-. .
N i l 0 . 3 2 0 . 5 3 1 .0
Gel p r e c i p i t a t i o n / 6 / i s u s e d t o d e s c r i b e t h e c o n v e r s i o n of aqueous s o l u t i o n s of m e t a l s a l t s e i t h e r s i n g l y o r a s m i x t u r e s i n t o a shaped g e l by a c h e m l c a l g e l a t l o n s t e p . E i t h e r i n t e r n a l o r e x t e r n a l g e l a t i o n p r o c e d u r e s c a n b e u s e d , and i f n e c e s s a r y , w a t e r - s o l u b l e polymers o r o t h e r a d d i t i v e s c a n b e i n c o r p o r a t e d i n t o t h e f e e d
s o l u t i o n s t o improve g e l s t r e n g t h and g e l - s h a p i n g p r o c e d u r e s .
T h i s d e f i n i t i o n i s v e r y wide and c o v e r s a r a n g e of t e c h n i q u e s and p r o c e d u r e s t h a t have b e e n d e v e l o p e d and u s e d t o p r e p a r e ceramic-based m a t e r i a l s . We a t Harwell h a v e made most u s e of t h e p r o c e s s t o p r e p a r e d e n s e , s p h e r i c a l p a r t i c l e s of mixed o x i d e s , e . g . (U,Th)O, and (U,Pu)O, f o r u s e a s t h e r m a l and f a s t r e a c t o r n u c l e a r f u e l s . Gel- p r e c i p i t a t i o n was c h o s e n s i n c e plutoniumIV and u r a n i u m l v s o l s a r e d i f f i c u l t t o p r e p a r e and t h i s p a r t i c u l a r p r o c e s s i s w e l l s u i t e d f o r t h e f a b r i c a t i o n of r e l a t i v e l y l a r g e , h i g h - q u a l i t y s p h e r i c a l p a r t i c l e s (100-800 urn).
_c_oncer_t_z_ationn~r.
s e t t l e d s l u r r y
- -- -- - - - -
203 g 1-I 375 420 526
I n o r d e r t o meet t h e s t r i n g e n t r e q u i r e m e n t s f o r t h e remote h a n d l i n g n e c e s s a r y f o r a - a c t i v e m a t e r i a l s , t h e p r o c e s s equipment h a s a t t a i n e d a h i g h d e g r e e of s o p h i s t i c a t i o n An o u t l i n e f l o w s h e e t f o r a t y p i c a l p r o c e s s i s shown below.
Ge 1 I . O g / c c 1.20 1.80 2.0
Metal n i t r a t e r s ) + Polymer, e . g . + M o d i f ~ e r , e . g . p o l y a c r y l a m i d e formamide --
J.
Tap d e n s i t y
o x - ~ ~ e - ~ ~ ~ o o - ~ C
- --- - - - -. -- .- - .- ..
1 . 1 5 g / c c 1 . 5 0 2.80 3 . 1
t o p r o v i d e t o p r o t e c t f e e d r h e o l o g y c o n t r o l from p r e m a t u r e and g e l s t r e n g t h g e l a t i o n
+
Feed J.
Gel a s s p h e r i c a l d r o p l e t s i n aqueous ammonia
i
Water-wash g e l t o remove s a l t s
i
Removal of w a t e r J.
H e a t - t r e a t m e n t t o remove polymer
J.
S i n t e r i n g
Many variations can be derived from this basic process. It is not only useful for preparing spheres; feed solutions can 'be converted to gel 'gravel' thereby providing an easy-to-filter and wash preciyitate (and eliminates the expensive and time consuming sphere-forming step).
Internal gelation procedures can be used when reagents such as hexamethylene tetramine are added to a cooled feed and when the liquid feed has been converted to the required shape or form, e.g. spheres or fibres, mild heat-treatment generates ammonia and produces a rigid gel. Photomicrographs of spheres and fibres prepared using gel precipitation processes are shown in Fig. 1.
Fig. 1 - Spheres (Nd,O,) and fibres (ZrO,) prepared using gel-precipitation processes
Disadvantages or limitations in the process stem from the somewhat uncontrolled nature of the precipitation step, i.e. the structure of the metal hydroxides formed when the feed is gelled with base is linked to the conditions that prevail at the time of the processing, i.e. feed concentration, composition, rate of mixing, temperature, etc. Segregation could occur with hydroxides from metal salts with widely different hydrolysis constants. The formation of ammonia-soluble species, e.g. nickel and cobalt amines can limit the range of materials and this woulo be an important constraint on the preparation ? f complex ceramic compounds such as ferrites or catalysts.
In recent years / 7 / there has been great interest in the use of metal-organo
reagents, namely alkoxide precursors that are referred to as sol-gel processes. The term gel processing is more relevant as a description of the procedures since in many of the reported preparative techniques sols as defined in this paper are conspicuous by their absence.
However, gel processes in which organo-metal compounds are hydrolysed have two important advantages:-
(i) Many metal alkoxides are liquids and can be purified by distillation and this property can be used to yield precursors of very high purity.
(ii) Metal alkoxides provides intimacy at the molecular level which is vastly
J O U R N A L DE PHYSIQUE
superior to any powder blending step and can be better than mixing colloidal dispersions.
Using the basic concept of preparing a hydrous gel by the controlled hydrolysis of metal-alkoxide precursors (either singly or as a mixture), a wide range of processes and products have been investigated for preparing ceramic powders, coatings and artefacts.
Application of the processes and products have been reported for zirconia-toughened alumina powders, doped titania powders, anti-reflectance coatings (by spinning, spraying and doping) and glass fibres and monoliths.
The benefits of purity and good mixing achieved by using organo precursors have to be set against the high cost of the starting materials and the hazards associated with the use of organic compounds. In addition, there is often little control of the state of aggregation of the hydrous oxide intermediates and the oxide products are usually low density powders. Whilst this itself is not always a disadvantage, e.g.
use as porous catalyst supports, an ability to control the crystallite size and the degree of particle aggregation would enhance the capabilities of this particular form of gel process.
(dl Gel Additi.:
Gel addition may be described as a gel process of convenience. Its first formally reported use was by Babcock and Wilcox 181 for preparing spherical particles of U02- PuO, ceramics. Well established sol-gel processes were available for preparing spherical gel particles of uraniumIV but preparing and processing plutoniumIV sols and gels presented many handling difficulties. These were minimised by preparing the uraniumIV gels and then contacting the single component urania gel with an aqueous plutonium solution to form a mixed gel. The gel was then dried and calcined to form the spherical particles of mixed oxide. The scope of the process was limited by the properties of the urania gel since only a relatively minor fraction of
plutonium could be adsorbed onto the gel.
However, the concept of the process is of great significance. Pre-formed gels can be doped or coated with other solutions or sols to form multi-component systems.
Hydrous oxide gels often have very strong ion-exchange andlor adsorptive properties and preparative procedures can enhance and/or introduce selectivity into the properties. Studies on hydrous gels as precursors to colloidal dispersions or for their use in non-dispersed forms has been of great value, not only for exploiting gel addition, but has been to the benefit of gel processing in the wider sense.
I11 - THE LOW TEMPERATURE HEAT-TREATMENT OF GELS -- - - --- - -. ---- - .- - -- ..- -
Glasses and complex crystalline oxides / 9 , 1 0 / have been prepared by using a sol as the key host material and then introducing the other components or their precursors either as sols or salt solutions. If salt solutions are used, great care must be exercised to ensure premature gelation or coagulation does not occur and impair the homogeneity of the system. For glass and glass-based materials, inorganic silica sols have been used as the key host material and the other components such as Na,O, L,i,O, A1203, CaO and B203 introduced as nitrate salts, or nitrate-based sols.
Typical feed dispersions contained up to 250 g 1-I of oxide equivalent and were stable to viscosity changes and precipitation for several hours. The sol-based feeds could be gelled by any of the well-established procedures, i.e. chemical gelation with ammonia gas either internally or externally or by dehydration procedures using alcohols or simply by tray drying in air to yield gel 'gravel'. The clear glassy gel products showed no evidence of segregation or coagulation during the gelation step.
When the silica-based air-dried gels containing the different salts were heat-treated
i n a i r , i t was d e m o n s t r a t e d t h a t t h e a l k a l i and a l k a l i n e e a r t h m e t a l n i t r a t e s a l t s decomposed a t a b n o r m a l l y low t e m p e r a t u r e s and by v e r y d i f f e r e n t mechanisms compared w i t h t h o s e o b s e r v e d when t h e same s a l t s were h e a t e d i n t h e a b s e n c e of t h e hydrous g e l s u b s t r a t e .
Duval 1111 r e p o r t e d t h a t sodium and p o t a s s i u m n i t r a t e s a l t s decompose a t 670 and 843°C r e s p e c t i v e l y , e v o l v i n g oxygen and y i e l d i n g t h e n i t r i t e s a l t s . F i g . 2 shows t h e r m o g r a v i m e t r i c t r a c e s of t h e d e c o m p o s i t i o n r o u t e we found f o r sodium and p o t a s s i u m n i t r a t e and f o r c a l c i u m n i t r a t e . To d e m o n s t r a t e t h e f o r m a t i o n of n i t r i t e , t h e r e s i d u e a f t e r c a l c i n i n g sodium n i t r a t e a t 850°C was l e a c h e d w i t h w a t e r and t h e a b s o r p t i o n s p e c t r a f o r t h e s o l u t i o n s o b t a i n e d measured between 250 and 450 vm. The s p e c t r a ( F i g . 3 ) showed t h e sodium n i t r a t e had p a r t i a l l y decomposed, p r o d u c i n g t h e band f o r n i t r a t e ( h max. 360 pm, E = 20) and f o r n i t r i t e ( A max. 310 vm, E = 7 . 2 ) . The c o n t i n u o u s l i n e shows t h e enhanced f i n e s t r u c t u r e s p e c t r u m c h a r a c t e r i s t i c of m o l e c u l a r n i t r o u s a c i d formed when t h e l e a c h s o l u t i o n was a c i d i f i e d .
Temperature O C
F i g . 2 - T h e r m o g r a v i m e t r i c a n a l y s i s c u r v e s f o r NaNO,, KNO, and Ca(NO,), s a l t s .
F i g . r e s i
I \ --- PARTIAL CONVE6 . N l l n n l t . . - - - - - - l u . . NITRITL: !SlON OF ENHANCED NITRITE S P E 7 R A THROUGH ACID1 FICATION
\
0 I !
250 3 0 0 350 LOO
3 - UV a b s o r p t i o n s p e c t r a of a q u e o u s s o l u t i o n s p r e p a r e d by d i .due formed a f t e r h e a t i n g sodium n i t r a t e a t 800-853°C.
s s o l v i n g t h e
J O U R N A L DE PHYSIQUE
F i g . 4 shows t h e r m ~ ~ r a v i m e t r i c a n a l y s i s c u r v e s f o r sodium n i t r a t e s a l t a d s o r b e d o n t o s i l i c a g e l . As t h e mole r a t i o of sodium n i t r a t e / g e l s u b s t r a t e was i n c r e a s e d , t h e d e c o m p o s i t i o n t e m p e r a t u r e i n c r e a s e d from a v e r y low v a l u e of 400°C a t a mole r a t i o of 0 . 1 t o a b o u t 900°C f o r s i l i c a g e l c o n t a i n i n g sodium n i t r a t e a t a mole r a t i o of 1 . 0 . However, when t h e r e s i d u e s formed a f t e r h e a t i n g t h e s a l t - d o p e d g e l s were l e a c h e d w i t h w a t e r , u n l i k e t h e s a l t s a l o n e , v i r t u a l l y n o n i t r a t e o r n i t r i t e c o u l d b e d e t e c t e d i n t h e r e s i d u e s . F i g . 5 shows t h e UV a b s o r p t i o n s p e c t r a of l e a c h s o l u t i o n s o b t a i n e d from r e s i d u e s a f t e r NaNO,/SiO, m i x t u r e s w i t h a mole r a t i o of 1 . 0 were h e a t e d a t d i f f e r e n t t e m p e r a t u r e s . U n l i k e F i g . 3 , which shows a m i x t u r e of n i t r a t e and n i t r i t e , F i g . 5 shows no n i t r i t e i s formed when t h e a l k a l i m e t a l s a l t s a r e decomposed i n a h y d r o u s g e l s u b s t r a t e .
F i g . 4 - T h e r m o g r a v i m e t r i c a n a l y s i s c u r v e s f o r NaNO, h e a t e d on a s i l i c a g e l s u b s t r a t e a t d i f f e r e n t NaNO,/SiO, mole r a t i o s .
- - - 629 -L50°C K A T TREATMENT
- 6 0 0 - 5 5 0 % HEAT TTEATMENT
F i g . 5 - U V a b s o r p t i o n s p e c t r a of aqueous s o l u t i o n s p r e p a r e d by l e a c h i n g t h e r e s i d u e formed a f t e r h e a t i n g NaNO, a d s o r b e d on h y d r o u s s i l i c a .
T h i s s u g g e s t e d t h a t when t h e n i t r a t e s a l t s a r e a s s o c i a t e d w i t h h y d r o u s g e l s t h e y i o n i s e , and when h e a t e d , t h e h y d r a t e d n i t r a t e a n i o n accommodated w i t h i n t h e h y d r o u s g e l s t r u c t u r e decomposes i n t h e t e m p e r a t u r e r a n g e a s s o c i a t e d w i t h
t h e r m a l d e c o m p o s i t i o n of n i t r i c a c i d . The i m p o r t a n c e of w a t e r i s d e m o n s t r a t e d by t h e even lower d e c o m p o s i t i o n t e m p e r a t u r e s f o u n d f o r sodium n i t r a t e a d s o r b e d o n t o a n a l u m i n a g e l . T h i s p a r t i c u l a r g e l was known t o r e t a i n i t s w a t e r t o a r e l a t i v e l y h i g h t e m p e r a t u r e .
These o b s e r v a t i o n s and r e s u l t s have v e r y i m p o r t a n t r a m i f i c a t i o n s f o r p r o c e s s
c h e m i s t r y . An a b i l i t y t o decompose a l k a l i and a l k a l i n e e a r t h n i t r a t e s a l t s c o m p l e t e l y and a t low t e m p e r a t u r e s i s of g r e a t v a l u e f o r t h e i n c i n e r a t i o n a n d / o r f i x a t i o n of r a d i o a c t i v e w a s t e . These e x p e r i m e n t s and c o n c l u s i o n s have v e r y i m p o r t a n t l i n k s w i t h f u n d a m e n t a l a s p e c t s of s o l - g e l t e c h n o l o g y and t h e c o n c e p t o f d e c o m p o s i t i o n of s a l t s on h y d r o u s g e l s h a s b e e n u s e d t o e n a b l e i n o r g a n i c s o l s t o b e p r e p a r e d from
i n t r a c t a b l e hydrous g e l s .
F o l l o w i n g t h e work on s a l t d e c o m p o s i t i o n u s i n g s i l i c a g e l s , o t h e r g e l s u b s t r a t e s were u s e d t o decompose a r a n g e o f m e t a l and non-metal s a l t s . The r e s u l t s were v e r y i n t e r e s t i n g , w i t h s c o p e f o r w i d e - r a n g i n g i n t e r p r e t a t i o n s of what happens t o i o n i s a b l e s a l t s when h e a t e d on h y d r o u s g e l s .
One p a r t i c u l a r r e s u l t was of g r e a t s i g n i f i c a n c e t o s o l - g e l t e c h n o l o g y . CeriumIV h y d r a t e i s u s u a l l y p r e p a r e d from c e r i u m 1 1 1 o r I V s a l t s o l u t i o n s by p r e c i p i t a t i o n w i t h a b a s e . The water-washed h y d r a t e h a s s t r o n g a d s o r p t i v e p r o p e r t i e s , and i n t h e a b s e n c e of u s i n g s p e c i a l d e - i o n i s a t i o n p r o c e d u r e s , s i g n i f i c a n t amounts of s a l t s
( u s u a l l y ammonium n i t r a t e ) a r e e n t r a i n e d i n t h e h y d r a t e .
H e a t - t r e a t m e n t s t u d i e s r e v e a l e d t h a t t h e e n t r a i n e d ammonium n i t r a t e s a l t s were decomposed by a r o u t e v e r y d i f f e r e n t from t h a t n o r m a l l y a s s o c i a t e d w i t h s a l t , i . e . t o w a t e r and n i t r o u s a c i d . We found NH, was e v o l v e d f o l l o w e d by n i t r i c a c i d and o x i d e s of n i t r o g e n . T h i s o b s e r v a t i o n was v e r y i n t e r e s t i n g and i n l i n e w i t h what we had found f o r a l k a l i - m e t a l n i t r a t e s a l t s i n s i l i c a g e l s u b s t r a t e s , i . e . t h e s a l t s were i o n i z e d i n t h e g e l and h e a t - t r e a t m e n t r e s u l t e d i n t h e a n i o n r e a c t i n g w i t h t h e w a t e r t o produce a c i d s p e c i e s . However, a f t e r decomposing t h e a d s o r b e d s a l t s , t h e h e a t - t r e a t e d c e r i a g e l s were found t o b e f u l l y d i s p e r s i b l e i n w a t e r and formed v e r y c o n c e n t r a t e d s t a b l e c e r i a s o l s ( u p t o 600 g 1 - I ) . V a r i o u s p e p t i z a t i o n and
d e a g g r e g a t i o n p r o c e d u r e s were known f o r p r e p a r i n g c e r i a s o l s t h a t r e q u i r e d a d d i n g a c i d t o t h e h y d r a t e . However, a f t e r d e a g g r e g a t i o n o r p e p t i z a t i o n , t h e e x c e s s a n i o n s a s s o c i a t e d w i t h t h e added a c i d were d e l e t e r i o u s t o t h e s o l .
I n t h e h e a t - t r e a t m e n t p r o c e d u r e / 1 2 / , t h e a c i d formed d u r i n g t h e ' a b n o r m a l '
d e c o m p o s i t i o n of t h e e n t r a i n e d s a l t i s v o l a t i l i z e d b u t i n s o d o i n g , d e a g g r e g a t e s t h e h y d r a t e s u b s t r a t e . The h e a t - t r e a t m e n t i s i n s u f f i c i e n t t o b r i n g a b o u t any s i g n i f i c a n t c r y s t a l l i t e s i z e g r o w t h , h e n c e a f t e r l o s s of t h e a c i d s p e c i e s t h e now d e a g g r e g a t e d h y d r o u s s x i d e s u b s t r a t e i s i d e a l l y s u i t a b l e f o r d i s p e r s i o n t o a h i g h l y c o n c e n t r a t e d s o l
T h e r e i s world-wide i n t e r e s t i n g e l p r o c e s s i n g a s a c a p a b i l i t y f o r p r e p a r i n g ceramic,^
w i t h c a r e f u l l y t a i l o r e d p r o p e r t i e s . The l a r g e i n v e s t m e n t of r e s e a r c h and development t h a t h a s been and i s s t i l l b e i n g a p p l i e d t o t h e s u b j e c t h a s g e n e r a t e d s i g n i f i c a n t t e c h n i c a l b e n e f i t t o c e r a m i c s p r o c e s s i n g and p r o d u c t s . However, t h e r e i s n o t h i n g m a g i c a l a b o u t s o l - g e l o r g e l p r e c i p i t a t i o n o r g e l a d d i t i o n p r o c e s s e s ; t h e y a r e a l l b a s e d on w e l l - e s t a b l i s h e d c h e m i c a l p r i n c i p l e s .
The s t r e n g t h of g e l p r o c e s s i n g l i e s i n i t s c a p a b i l i t y t o a c h i e v e s i g n i f i c a n t m a t e r i a l s economy, e n e r g y economies and t h e m i n i m i s i n g of d u s t and c h e m i c a l p r o c e s s i n g h a z a r d s .
I t i s t h e s e f e a t u r e s , t o g e t h e r w i t h t h e s o p h i s t i c a t i o n of t h e p r o c e s s e s t h a t c a n
JOURNAL DE PHYSIQUE
provide an overall cost benefit in addition to the undisputed technical advantages.
REFERENCES -
D.E. Ferguson, O.C. Dean and D.A. Douglas. The Sol-Gel Process for the Preparation and Remote Fabrication of Recycle Fuels. Third United Nations International Conference on the Peaceful Uses of Atomic Energy. Vol. 11, A/Conf ./27/p237 (1964).
Hoekstra, US Pat. 2620314.
P.A. Haas. Sol-Gel and Gel-Sphere Technology. Oak Ridge National Laboratory Review No. 1 (1984) .
J.L. Woodhead. Sol-Gel Processes to Ceramic Particles using Inorganic Precursors. J. Materials Education. Vol. 6, 5, 887, (1984).
C.B. Alexander and J. Bugosh. (DuPont Co.). US Pat. 2,294,628 (1961).
B. Stringer, P.J. Russell, B.W. Davies and K.A. Danso. Basic Aspects of the Gel-Precipitation Route to Nuclear Fuel. Radiochemica Acta, 36, 31, (1984).
Lisa Klein. Sol-Gel Glass Technology Update. The Glass lndus-iry, May 1982.
R.V. Carbon. The Preparation of Plutonia-Urania Fuels by a Sol-Gel Process.
Babcock & Wilcox 3714-10 (November 1969).
Rustum Roy. J. Am. Ceram. Soc. 52, 6 , (1956).
J.L. Woodhead, K. Cole, J.T. ~ a l t G , J.P. Evans and E.L. Paige. Science of Ceramics 12, St. Vincent, Italy (1983).
C. Duval. Inorganic Thermogravimetric Analysis. Elsevier Pub. Co., New York, (1963).
/12/ J.L. Woodhead and G. Raw. UK Pat. 2,075,478B (1983).
