lmli;lll Journal of Pure & App lied Ph ys i c~

Vol. :1<.1 . Aui-!ust 2001. pp. 5 14-524

Chemical resistance of some irradiated ceramic-glazes F M Ezz-Eidin

Nati onal Centre for Radi ati on Research and Technology. Nasr-City. Cairo. Egypt

and

W M Nageeb

National Research Centre. Dokki . Cairo. Egypt

Received I November 2000: accepted I 0 Apri I 200 I

The effect of neutrons irradiation on the chemical durability of some ceramic-glazes wi th different compos i tion~ h a~

hccn in vcs ti !:(ated with part icular attention to the change in the corrosion rate. Di fferent factors have been studied such as i-! l;tt.C composition. irradiation dose. concentration of the leach ing solu tion. immersion time and temperatu re of leaching solution . It was found thm zircon improved the chemical durabi l ity. Corrosion rate increased with increasing concentration of immersion so lution up to 2N HNO, then decreased. also it increased with increasing immersion time and ra ising the temperature of leach ing so lut ion. The amount of the si li ca released in so luti on showed a maxi mum weight loss percent up to 21\J H 0 ,. ;111dthcn decreased dramatica lly with increasing the concentration of the tested solut ion. T he corrosion results have been exp lained in relati on to the hyd rati on. di ffusion and selecti ve dissoluti on. The density increases with successive increase of zirconium ox ide content reachi ng the highest value of 2.59 g/cm-' . The induced defects obtai ned on irrad iating a glaze. ei ther in the sur face or in the bu lk. have been assumed to result from neutron irradiation which decreases both the chemical durability and the density.

I Introduction

Ceramic coatin gs are fo rmul ated to be res istant to ma ny reagents. inc luding hot water, ac ids. a lka li s and most if not a ll o rganic medi a. The onl y reagent for whic h these mate ri a ls cannot be considered is hydro flu oric ac id 1

• Surface s ites in g lazes are inc reas ing ly occupi ed by a lka li and di valent ions and the transport o f these i on s~ through the leached layer is the rate contro lling fac tor fo r corros ion in g laze. It has bee n reported -' that corros ion w ill occur in a strongly bas ic medium o nl y when a water molecul e can pene trate the network to conve rt the Si-0- to S i-OH. Si lica is the most important ox ide4

used in ceramic and glass industri es. The refore, g lazes a re a network of si li ca tetrahedra to whi ch the other compone nts have been added as modi f iers . Horic ox ide can he used as the primary f lu x in a ~laze or a:-- an auxili a ry fl ux. It is an ox ide of strong flux in g power. comparab le to that of lead ox ide or sod ium ox ide. Because it is a network-forming ox ide. it genera ll y contributes to a lowerin g of the ex pansio n of the g laze, whi ch makes it useful in preve ntion of c razin g. T he primary limitati on on the use of bor ic ox ide is its effect on the durability of the _? laze . In sod ium boros ilicate g lazes . additi ons of

bori c ox ide up to 12 wt % improve the durab ili ty. Above that, however, the durability rap idl y deteri orates 1

•

It is genera ll y accepted that large doses of fas t neutrons are required to introduce noticeable damage to the struc ture of quartz. The fact that a lmost a ll fo rms of s ilicon diox ide a re transformed to optically isotropi c, g lass li ke mate ri a l w ith virtua ll y identical density, the rmal expans io n and absence of X-ray di ffraction patte rn led to the conc lu sion that neutron irrad iation produces a new phase of s ilicon diox ide which is indepe ndent of the o ri g ina l phase before irradiati on . T hi s new phase was named metamic t phase. adapting the earlie r des ignation fo r mine ra ls di sorde red hy radi ati on ' .

For some vitreous mate ri a ls, part icul arl y those conta ining boron, the absorpt io n fo r the rma l ne utrons w ill be quite hi gh, s ince the capture cross­secti on of the 1118 isotope is about 4000 barns. Afte r neutron capture the boron decays accordin g to the eq uati on:

10 B I 7 . 4 . 5 + 0 n --j ~ Lt + 2 He + 3.3MeV ... (I)

EZZ-ELDIN & NAGEEB: CERAMIC-GLAZES 5 15

From thi s reacti on. one mi ght ex pect : (a) change or co llapse of the structure of the matrix because of the conversion of some of the boron to lithium, (b) formati on o f helium gas because of pickup of an e lectron pair by the a lpha particle. and (c) intense ioni zati on within the glassy phase by 3.3 MeV of energy carried off by the lithium nucleus and the :.tlpha parti c le.

The objec ti ve of the present work is to determine what effec t neutron bombardment might ha,·c o n g laze'- . A lso the effec t of a variety of aqueou:-- so luti on concentrati ons at diffe rent temperatures and for various immersion times on the corros ion behavior of g lazes, was studied . The stud y o f those factors and the ir correlation to the durab i I ity is the correc t approach to prepare more suitable g lazes for us ing in the industrial applica ti ons and a lso probabl y for encapsulation of nuclear wastes.

2 Experimental Details

2.1 Glaze Preparation

The frit batch which is ultimatel y a sodium­boro-s ilicate glaze was compounded using albite as a source of soda and zircon as an opacifying mate rial together with the usual glaze component and the weight percent is given in Table I . The batch was me lted in a frittin g rotating kiln of 1/2 ton capaci ty. the maximum temperature being around

1320°C as measured by the optical pyrometer. The duration of thorough me lting was around 5 hrs . The homogeneous cubes with dimension I em' were

prepared by casting the g laze batches, the composi ti ons of cubes a re given in Table 2.

T he g laze batches molecular compositions as given in Table 3 were melted individuall y in

pl atinum c rucib les at 1050°C for 2.5 hr and were stirred frequ entl y for complete ho mogeneity until reasonably free from air bubbl es . From thi s me lt.

slahs w ith dimensions I x I em. and with constant thi ckness (0 .2 e m) were shaped in a sta inless steel mo ld and se lected on the basis of uniformity of dimen\ion:-- and freedom from bubbles. The specimen~ were annealed in an e lectric furnace at 6000C and afte r soaking for I hr. These were left to coo l slow ly over-night at a rate of 20"C/hr.

2.2 Neutron Irradiation

Plate samples of the glaze of I x I em diameter and with constant thickness were placed in front of the neutron beam and placed inside a specially designed source collimator. For neutron irradiation a 5 Ci-185 Gbq- 24 1Am-Be neutron source. manufactured by Schlumberger (Germany) was used . The neutron yield from the source has a value of 0 .86 x I 07 neutron/sec. Two differe nt methods were used for neutron dosimetry: the first is the Rem meter type NM-2 neutron monitoring (N uclear Enterprise Ltd), while the second using CR -:W dosimeter and the chemical etching technique.

As a result of the interaction of a-particl e:-­emitted from the 241 Am isotope with beryllium. neutrons are emitted with mean e nergy of

approximately 4 .25 MeV. and it was assumed that == 23 % of the neutrons had energies below I .0 MeV and the mean energy of these neutrons was 0.4 MeV .

The 241 Am-Be source used in the experimental work has the following characteristics :

Neutron dose equivalent rate. I m from source

I I 0 IJ.Sv/hr ( I I m rem/hr). Gamma-ray dose

equivalent rate, I m from the source = 125 ~J.sv/hr

( 12.5m rem/hr).

241 Am isotope has a half life time of about 432 years although this isotope decays by emitting alpha particles of about 5.5 MeV, these particl es are

followed by y-rays in the 40-60 ke V region in the majority of the disintegrations. The effect of irradiating the glaze with neutron irradiation o n the weight loss behaviour was assessed by subjecting

the g laze samples to following doses: 0 I = 5.5 x 10 1 ~, 02 =II X 1012, 03=22 X 101\ 04 = 33 X 10" and 05 = 44 x I 0 12 n/cm2

, at approximately 25 C.

2.3 Leaching Measurement

Glaze samples (rectangular slabs) were leac hed in polyethylene containers for various times in 120 ml of nitric acid solution of varying concentrations .

The procedure of chemical leaching studi es can be summari zed as fo llows:

( I) The g laze cubes were accurately weighed (±

I%), placed in a 250 ml polyethy le ne beaker and 120 ml of I ,2,4 or 6N HN03 for each gram of un­irradiated g laze and 4N HNO , for each gram of irradiated g laze was added to completely cover the

) lh INDIAN J PURE & APPL PHYS. VOL 39. AUGUST 2001

~laze specimens: (2) The glaze spec imens were left in the <JC id so lutions for 2,4.8 or 12 hr in a regulated

water bath at 25°C (±0.5°C) or 90°C for un­irradiated and irradiated glaze specimens; (3) The g laze specimens were washed, dried and reweighed after eac h time interval (the weight error was

±0 1°/r ): (4) The SiO~ released in the solution was determined calorimetricall y with the molybdate mcrhorl ''.

2.4 Density Measurements

The density or the samp les were measured by the Archimedes method using xylene as the immersion liquid .

3 Results

The three glaze samples studied ( 1-3) were leac hed in aqueous nitric acid solutions of varying

G1

0·20

concentrations ( 1.2,4 and 6N) at 25 and 90 C for various periods of time. The results obtained are represented graphically as percentage weight loss (at 25 and 90'C) versus the normality of the acid in Figs I and 2 respectively. From Figs I and 2, one may conclude that all samples loose weight continually with time. On increasing the acid concentration, the loss increases up to 2N HNO, then decreases . With regard to g laze composition. the percent loss decreases as the mount of zircon increases in the sample. Furthermore. rai sin g of te mperature leads to an increase in the weight loss of glaze samples leached by an acid of definite concentration for a fixed period of time . Fig. 1 reveals the amount of silica in ppm. that is dissolved from the investigated un-irradiated glazes, as a function of HNO, concentration for 12 hr at 90'C. The data show that the release of si lica in the

G2 G3

~12hrs :;:

~8hrs 0 ~

/-----_., ._.12 hrs

;;,1 ·;:; ~

~ ... C)

<::..

0-12 ~4hrs

0-04 ~Zhrs

0-00 0 2 4 6 8 0

Nonnality

/ ~-6 8hrs

~4hrs ~-zhrs

4 6

' Normality

8 0

~12hrs ~6hrs

_........___ . - t, iYs

,...... ----- 2 h-s

2

NnrmalitY

8

Fi ~ I - Dependence of the % weight loss on the normality of the nitric acid used in leaching glaze samples 1-3 at 2)°C for different periods

3

12 hrs ~•12tvs ~ - -------.. 8 hrs

~4hrs

---------------- 2 hrs oL--'--~--_.__ __ ...._ __ ....J

0 2 I. 6 0 2 4 6 0 2 4 6 Not·tnali-ty Nu•·nutlit)'

Fi~ 2- Dependence or the o/r weight loss on the normality of the nitric acid used in leaching glaze samples 1-3 at 90°C for different periods

EZZ-ELDIN & NAGEEB : CERAMIC-GLAZES 517

40

32

-:::. :::. 24

"'0 <:)

"-

<lJ

"E N 16 0

en

8

1 ~-------

Normalitv 4 6

Fig. 3 - Released Si01 (ppm) for un-irradiated glaze as a Ju nction nr acid normality for l.eaching time 12 hr at 900C

solution increases for all glazes up to 2N HN03 and

then dramaticall y decreases with stronger HNO, acid so luti o n. The effect of irradiation on the

durability of g laze samples 1-3 was studied by me; tsu rin g the pe rcent weight loss as a function of ne utron influences for samples in 4N HNO, at 90"C

for various periods of times. The results are

represented graphically in Fig. 4. The durability of the g laze samples decreases with the increase of neutron fluences (Fig. 4) . Also. iinear relations are

observed for shorter immersion periods. e .g. 2 and 4 hr. At higher periods. an irregular increase in weight loss with the increase of neutron fluences is observed. Fig . .'i shows the effect of composition and neutron irradiation at different fluences on the dens1tv of g lazes G I-G3 at 2.S'C. The density of gla ze~ in c rease~ as the amount of zircon increases (1- ig . .'i) . C hanges ~n density of glaze G I and G2 have the same trend upon irradiation in which a slight decrease occurred at first stage of irradiation. Subsequent irradiation fluences , however. resulted

in <I gradual increase in density, reaching maximum

"' "' ~ .c ~ -~

~ ... = Ill u 1.. 11 ~

"' "' ..s ... .c ~

"Qj :3: ..... = Ill u ... Ill ~

"' "' 0

5

4

3

2

0

4

3

2

0

3

~ 2 ~

"Qj

!!: ... r:: Ill ... 1..

Gl

12 2 0 10 20 30 40 X10n/cm

Neutron fluence (n/ cm 2)

G2

0 10 20 30 40 X1CfntcrA Neutron fluence (n/ cm1)

~ 0

12 2 0 10 20 30 40 X10n/cm

Neutron flnence ln/cm 2)

Fig. 4 - Neutron lluence dependence of % weight loss or glaze samples G I-G3 leached in 4N HN03 at 90°C for · • - :! hr. • - 4hr. ~ - 8 hr and ~- 12 hr

.'iiX INDIAN J PURE & APPL PHYS, VOL 39. AUGUST 200 1

at :n X I 0 ': n/cm2. After that, a noticeable decrease

in density at 44 x I 0 ' ~/cm' . With regard to glaze G3.

a noticeabl e decrease in density at 5.5 x 1()1 ~ n/cm2

is observed followed by a considerab le increase,

reaching maximum at 22 x 1012 n/cm2, then

dec reases aga in at 33 x 1 0 '~ n/cm' . At neutron

flu ence of 44 x I ()' 2 n/cm2, a sharp increase in

density is observed.

4 Discussion

Glaze-cerami c materials are cons idered polycrystalline so lids containing a res idual glassy phase ;111<.l possess a va lu ab le combination of the fa vourab le properti es of both g lasse. and ceramics7

Dissolut ion of vitreous materials is a complex process w hi ch de pends on so luti o n conditi ons (pH,

2 60 G3

2-48

0 10 20

degree of concentrati on. etc.) as we ll as the compositi on and microstructure of the vitreous materials . The easi ly released components 1 n solutions are the basic oxides consisting of the monovalent and di va lent cations wh ich occupy modifier pos itions in the structurex. S di um re mova l in vo lves the ion exchange mechani sm with H' or H,O+ from leaching solution and is accompanied with the easnly soluble borate phase .

As the number of surface s ites occupied by W or (H ,O•) decreases (i .e . with inc reas ing quantity of a lka li ions detached or re leased). the tran sport o f a lkali ions through the leached layer is retard eel because the boundary concentration is increased. thus increasing the act ivation energy for both diffusion of a lka li ions and extract ion of Na~O.

30 12 40 X10 : 1 n/cm

1

Neutron fluence (n/ cm 2)

Ft!,! . .'i - Dcrcndcncc of the density of glaze sa m!11 es 1-3 at ::sec on the neutron ll uenc.::~

EZZ-ELDIN & NAGEEB : CERAMIC-GLAZES 511)

Table I - Frit compositions in weight percent

Borax Boric acid A lbite Zinc oxide Dolomite Pen!:~ -

hydrate 11 .-11 X 27.71 12.026 0.78 3.634

Tahir 2- Glaze compositions in weight percent

Gla! L' Glaze compositions in weight per cent notat1nn

Gl G2 G~

Frit 95 90 X5

Clay 5 5 5

Zircon 0 5 10

It has been shown that2 an intermediate step is nece~ sary for water to penetrate into the vitreous mat eri a l network . When most of the surface groups arc dissoc iated. water can no longer be absorbed : penetrati on 1nto the network is thereafter retarded. At that moment the dominatin g corrosion mec hani sm is nuc leophilic attack of the Si-0-Si bonds by OH· ions.

4.1 Hydrolysis in Silicate Networks

One way to look at glazes is to view them as a network of silica tetrahedra to which most of the other materi als have been added as modifiers. Glaze disso lution can he modeled as a combination of ion­ex chan ge and matrix dissolution reaction. The reaction zone. as ori ginall y described by Douglas and EI-Shamy" for silicates has been determined to conta in two react ion zones. One zone occurs at the leached layer solution iPiterface where equilibrium is considered to be between the matrix surface sites and the ions in solution . The second zone occurs at or ncar the leached layer glaze interface where counter-i on exchange occ urs. The relative size of the two zone~ in g laze exposed to the identical co nditi on~ vari e~ according to the type of glaze, e .g. h i ~h l :, durab le g lazes have thicker glaze leached l;~ w r zones and thinner leached-layer solution zones. With increasing time or solution stress, the di sso lution rate is modified by formation of hydrated amorphous surface layer and/or metal precipitation of secondary metal hydroxides and/or metal silicate complexes "'.

Calcium carbonate Zircon Sand Sod ium carbonate

8.992 13.929 18.78 2.722 8

The react ions at the glaze surface-so lution interface have been described in terms of two chemical reactions:

(a) Cations are released into the solution as a result of ion exchange with protons from the solution :

=Si-OM + H20 ~ =Si-OH + M· +OR . . . (2)

where M is a monovalent metal cati on.

(b) Silica is released into the so lutt on as the sil oxane bonds in glaze matri x are attacked by the hydroxyl ions from the solution:

=Si-0-Si= +OR ~ =Si-OH + =Si-O· . . . (3)

=Si-O· + H20 ~ =Si-OH +OR . . . (4) •

A non-bridging oxygen , designated as NBO or =Si-O· is formed in reaction given in Eq . (3) . Thi s NBO can interact with water to form additi onal silanol bonds on the glaze surface and . thereby. release another hydroxy l ion . In turn . this hyd roxyl ion can serve as the source of hydroxy l for fu rther breaking of more sil oxane bond s on the glaze surface via reaction given in Eq . 0). An increase in the acti vity of the hydroxy l ion in the so luti on will. therefore, favor increased removal of s i I ica and suppression of reaction given in Eq . (2) as the activity of the hydrogen ion concentrat ion is decreased.

It is, therefore, proposed that "' 11 strong ac ids accelerate condensation react ions and cause restructuring of not only the s ilica ske leton but also the colloidal depo its in the pores of leac hed glaze surface and hence increases the durab ility o r the glaze samples, as is deduced in Figs I and 2. T he drop in the di ssolved s ilica as shown in Fig. 3 at normality 4 and 6 of the acid soluti on may be ascribed to the polymerizati on of the hydrated microporous si lica produced upon l eachin ~ . The

INDIAN 1 PURE & APPL PHYS , VOL 39, AUGUST 2001

inc reased [W[ is expected to disrupt the Si-OH bond followed hy a condensation reaction :

=St-OH + HO-Si= ~ =S i-0-Si= + H20 .. . (5)

4.2 Hydrolysis of Borosilicate and Borate Networks

In the case of preparing glazes containing boron components. the seq uences of corrosion behavior ~1rc ex pec ted to he somewhat similar in borosilicate and borat e glasses. In neutral or basic so lutions the primary reaction leading to the hyd rolys is of borate bonds mvo lves the nucleophilic attack of OR (or water) on neutral trigonal boron sites :

As desc ribed above for si licate sites, nucleophilic attack on borate sites requires an expansion in coordinat ion number around the boron to allow fo r the formation of a B-OH bond. Trigonal boron sites can readily form tetrahedral sites, and arc thu s susceptible to nuc leophilic attack . Anionic tetrahedral sit es are already coordinatively ~ ar uratcd. and are resistant to nucleophilic attack . In !'act. di~ sol u tion studies show that Si-0-B bonds in vo lving tetrahedral boron si tes are almost as re~ i s t~1nt to hydrolysi s as Si-0-Si bonds' 2 and the h vdro l v ~ t ~ or such bonds probably invol ves nucleophilic attack on Si rather than on B.

For horosi I icate glasses and glazes of equivalent compml!ion. another important reacti on leading to the t'nnnarion of silanols is the hyd rol ys is of Si -0-B bonds. In ac idi c so luti ons. hydrolysis of Si-0-B and B-0 -B bonds (w ith either trigonal or tetrahedral borons l involves the electrophili c attack of protons on brid ging oxygen atoms :

Reaction given in Eq. (7) exp lains why boron lc< ichlllg ts promoted tn acidic solutions and why ac tdi c so lutions promote the conversion of four-fold boron sites to three fold sites. the reaction given in

Eq. (7) has been observed in zeolites containing AI and B 13• Moreover. Bunker et a/.14 suggest that, the pH below wh ich the bridging oxygen is protonated and subsequently hydrolyzed for both Si-0-B and B-0-B bonds is near 4 .

4.3 Role of Modifier Cations

Alkali and alkaline earth ions associated with the un-polymerized glaze (SiO,)~ tetrahedral network are considered to be re leased to the sol ution hy ion-exchange with protons W (or wi th hydronium ions H30 +). Addition of alkaline earth ions generall y decreases glaze durability. but much less than alkali s. Hence, substi tuting alkaline earth for alkali on a mole-for-mole basts will be to improve the durability.

The effect of ZnO on acid resistance ts somewhat ambiguous.Its solubility is less than that of alkali s and alkaline earths, but more than that of Si02 or similar oxides. It was found that ' ~ . below pH 7, ZnO is a deleterious constituent. However. in the alkaline region. it is a beneficia l one. The isoactivi ty points for HZn02- and ZnO, 2 near pH 13. Hence ZnO can be a desirab le additton in the pH 9 to 12 range .

The presence of the Zn 2+ is assumed to depress

the passage of the alkali ions from the glaze into the solution and it appears that the sodium ions are liberated only after the breaking of the silica network. This may be due to the ability of zinc oxide to form network forming groups and the sod ium ions are bound to these groups to achieve electrical neutrality15

• Lewi s et a/.21' attributed the

effect of zinc to a surface enrichment of this element as insoluble silicate. because alkali •va:-­preferably leached out. In our samples the percentage of ZnO could be too low to allow the formation of a considerable quantity of surface

Table 3- Molecular compositions of glazes

C!:J/<' Ch icles (molecular equivalents) R20 , group . R02 group nnt:t lion J ~ ,() and RO grou p or oxides at unity

Na,., O CaO MgO ZnO B203 AI203 Zr02 Si02 (j I 0.3'J2 0.482 O.OR4 0.042 1.301 1.164 0.3'J2 2.32R C ' ,_ 0.3X1J 0.4X5 O.OR5 0.041 1.307 l. t67 0.433 2. 511 (i ~ 0 . .3 'J2 0.4lt? 0.082 0.043 1.306 t.l72 0.545 2.631

EZZ-ELDIN & NAGEEB : CERAMIC-GLAZES 521

laye r of zinc silicate on the glass surface. Thi s protecttve layer was al so suggested by Wicks et al 1

".

Grambow 1; assumed that the leaching rate of

zinc containing glasses is controlled by a phase with so lubility similar to that of Zn(OH)2• Tait and Jensen 1s suggested that the zinc ions might act as inhibitors of the diffus ing process. Thi s fact can be justified by considering that alkali cations break the S i-0-S i network through the introduction of no n­bridgi ng oxyge n s ites (e .g . Si-0- a' ). On the contrary a multipl y charged ion as Zn2+, larger and les :-. mobil e. can block the non-bridging oxygen sites .

Add ition of alumina is particularly beneficial in inc reas ing acid res istance of glasses and glazes. In the presence of alkali and/or alkaline earth oxides, a lumina takes over a function similar to boric oxide. formin g te trahedra coordinated by oxygen ions taken from the alkalis and alkaline earths 1

". The only c ircumstances when alumina is not beneficial ucc m when large amounts a re used. sufficient to fo rce some AI+' from 4 to 6 coordination :

The precipitation reaction is written as follows:

. . . (8)

where M+' is a trivalent cation, an amorphous AI(OH) , is the hydrated product. Subsequently, glazes whi ch are known to have AI+' in tetrahedral coordinati on on leaching. the res idual moles of Si02

arc assumed to be hydrated to H2Si0, as in the fo ll ow ing equation :

... (9)

The tetravalent oxides of the type R02 have found spec ia l interest as g laze components because of a property that has long been recognized ; their favourable influence on the chemical resistance of glasses and g lazes211

•

The primary effec t of small additions of zirconi um oxide is to improve the alkali resi stance . Eve n small concentrati o n (2 %) yie ld s significant improveme nt in both ion-exc hange and alkaline durability . A lthough the hydrati on of Zr02 is energet ica ll y very favourable, so luble ionic species li ke Zr0 2

+ and Zr4+ occur onl y be low pH 7. The other so lu ble spec ies is HZrO,, which predominates

at t'H:::: 1721. Dasc noticed that , subst itution of Si02

by ZrO~ makes the glass not o nl y more durable but

also less sens itive to changes in the pH of the medium.

It has been shown that in thi s connection , the use of Zr02 is most favourable. The explanation for thi s lies in the formation of zirconium hydroxide or hydrated zirconia, which are relatively insoluble species and probably offer a very high activation barrier for the diffusion of the other ionic spec ies through it. Thi s layer must be dense to have a protecti ve effect. The reaction of Zr02 in acidic medium may be presented as follows :

. .. ( I 0 )

The results obtained and illustrated in Fig. I closely agree with the theoretical support menti oned before, considering that additions of zircon oxide can take part in the glaze structure as a network former and network modifier, the Zr4+ occupancy of Si~+ sites in the glaze structure introduces a weaker Zr-0 bond than Si-0 bond and more non-bridging Zr-0 bonds than Si-0 bonds a re formed . Thi s leads to an increase of bridging oxygen atoms per sili con atom and consequently the glazes in the presence of Zr4+ will result in more packed structure w ith a higher durabi I ity .

4.5 Effect of Temperature

The effect of temperature (Figs I and 2) is of a

special interest with regard to accelerated tests o f chemical durability. The reaction temperature does not only alter the kinetic reaction but also dictates the rate controlling mechani sm of corrosion . At 90°C, corrosion rate is controlled by tota l di ssolution , while at 25°C selective leachin g is the rate controlling. The solubility of s ilica in so lution (Fig. 3) is quite low at 25°C, hence at thi s temperature silica acid is relativel y insoluble and thereby forms a layer of hydrated silica on the surface. As R20 and RO are leached into solution, the s ilica layer thickens . At 90°C, the hydrated silica can enter the solution as silica acid. Likewise, R20 and RO are continuously leached into soluti on. The re lative rate at which these two processes occur dete rmines the ex tent of leaching observed in F ig. 2 .

4.5 Effect of Irradiation

The observed tendency for the irradiati on to decrease the durability of the glaze may be re lated to the effects of ionization results mainl y from the

INDfAN J PURE & APPL PHYS. VOL 39, AUGUST 2001

pa~sage of ~ and y-ray. th rough the mate ri a l and fro m the initi a l s lowing down of e nergeti c di spl aced atom-::: .

In the beginning of the radiation the majority of neutrons w i II he captured c lose to the g laze surface caus in g: the surf<:lce <:l rea to ex pand o r contract.

"'·hil c the centre remain s unaffected. In thi s connection it is inte res ting to note that g lazes which h;J \ ' t' heen compactecl hy annea ling before ex pos ing 111 Irradiation , radia ti on will dec rease its density (ex panded ). Thi s may be taken as indication that the rad iat ion d isturbed g laze struc ture.

Accordin gly the resu lts g iven in Fi g. 4 can be disc ussed as fo ll ows: ne utro n irradiation may lead to damage. displ acement and mi gratio n of the mobil e (non-network) cati ons. These de fects may lead to acce le rat io n of leaching process . Inc reas ing irradi a ti on dose may lead to acce le rati o n of leaching proces-: resu lted in inc reasi ng the so lubility of the ~ l;1ze~ These nbse rvati ons are consistent with phase separat ion be 111 g the pn mary reason fo r the ob~erved c he mi cal durab ili ty results .

T he phase microstruc ture is an important fac tor 111 these changes. M ic rostructures of phase­se par;lted vi treous mate ri a ls fa ll into three categor ies is ex pl a ined by Tomozawa23

.

t a J Both phases have interconnected . continuous ~ lruc t u res. whe n the ir vo lume proporti ons are arprox 1111ate ly equa l: (b) The more durabl e phase t u >. u ; dl ~ SiO .- ric h J is d ispe rsed in a cont inuous mai n !\ of the les>. durable phase (usua ll y rich in H,O . and glass-mod ify ing compone nts): (c) The less durab le phase is d ispersed in a continuous matrix of the mnre durab le phase. T his is the con verse of ty pe h.

T he durabi I ity o f a g laze usua ll y dete riorates upon phase-separat ion to a type a o r b mi c rostructure. hut is prac tica ll y unc hanged when a ty pe c mic rostructure is fo rmed . T hi s is ex pected for cnmpo -; it io n<.. near the s ilica-ri ch end of a ti e line .

'L' Ui rPn IJT;.~diat \ O II may he lp in the deve lop ment of a pha:-c -~eparated microstructure in whi ch the less dura h il' CRO+R,O+B,O ,)- ri ch phase may be d ispe rsed a~ d iscrete partic les wi th in a more d urable il rO - + SiO, ). T he degree of improvement in durahi lny is st ron gly dependent on Z r0 2 content. The leac hing rate of Zr-conta inin g g lazes is

contro lled by a phase w ith solubil ity s imil a r to that Z r(OH)2, zirconium ions act as inhibito r of the diffusing process . Thi s fac t can j ust ified by considering that alka li cations break the S i-0 -S i network through the introductio n of non-bridg ing oxygen s ites (e .g. Si-0-Na+). On the contrary. a multiple charged ions as Zr"+ are larger and less mobile and can bl oc k the non-bridg ing oxygen s ite ~.

4.6 Density

In genera L the additi on of species that enter the inte rstices of the vitreous network tend to inc rease the densi ty by reduc ing the free vo lume. and additions of othe r ion s to g laze w ill a lter the density in proportio n to the mass of the added ions re lat ive to that of the ions a lready present in the g laze. Thus, the observed ra ising in g laze de ns ity, (Fi g. 5), may be corre lated to the inc rease in the amount of zircon introduced . M oreove r, increasing zircon crysta llinity from G I to g laze G3 as we ll as the presence of monocl inic zirconi a 111 G3 may parti c ipate in the increase of density. T he -;light decrease in density in G laze 2 at firs t stage of irradiation may be re lated to the dec rease in c rysta llinity of the zircon phase and/or the formati on of point defects within the g laze matri x. Moreover, zircon amorphization as a resul t of neutron irradiation leads to dec rease in its density. A lso, swelling and consequentl y the decrease in density in irradiated me ta ls were attributed tn inte rstiti a ls and defects suc h as d is location loops and to condensatio n of vacanc ies into voids2~ .

Whereas. the subsequent increase in density at

ne utron fl ue nces of II x I 0 12 n/c r 1 may be re lated to c rystallization of zi rcon and/or recombinati on of poi nt defects within the g laze matri x. The fo rmation of a-quartz in G I and G 2 starting at ne utron f lucnce

of 22 x I 0 1: n/cm" and the propagati o n in its

crysta llin ity may result in a more compact struc ture thus ra is ing the de nsity of the g lazes . O n the o the r hand. the decrease in density upon irrad iatio n to

neutron fluence of 44 x I 0'" n/cm~ o f G I. to a val ue somewhat lowe r than the un-irradi atecl one. and or G2, to a va lue somewhat lowe r than the un­irradiated sample, may be attributed to the di sappearance of a-quartz as a result of its amorphizatio n in G I and to a morphi zati on o f a­q uartz togethe r with the dissoci atio n o r zircon c rystalline phase into zirconia and silica . In

EZZ-ELDIN & NAGEEB: CERAMIC-GLAZES

addi ti on, point defect may have an effect Ill

dec reasin g the density at high fluences.

With regard to G3. the decrease in density at the first irradiation dose may be due to the decrease in crysta llinity as a result of amorphizati on of zircon phase and/or zi rcon di ssoc iation into monoclinic Zirconi a and vitreous sili ca as well as the formation of point defects wi thin the glaze matrix . The ~ uh-;eque nt inc rease in density at the second and third closes ma y be interpreted on the basis of propagati on in the crystallinity of zircon phase, in add ition . to the formation of a-quartz and the propagati on in crystallinity of monoc linic zirconia a .~ we ll a~ of a-quartz. respecti ve ly. At neutron flu ence of :n x 10 '~ n/cm2 s li ght increase in zircon crystallinity accompanied by the propagation of a­quartz crystallinity and a sharp decrease in the crv-;r;dlinity of zirconia phase. thi s may be Interpreted to the dec rease in density at thi s fluence. Roth the increase in crystallinity of zi rcon phase and Zirconi a phase. regardl ess nf the di sappearance nf a -quartz. may be responsible for the sharp mcrease in density at neutron fluence of 44 x I 0 " n/cm2

•

5 Conclusion

There are two processes operative in the aqueous corrosion of a vitreous surface; lOll­

exc han ge process and bulk dissolution of the glaze suri'ace . For ioni c exchange and diffusion in glaze. two -; tcps must occur:

( I l Ioni c honds between the re leased cation and anionic non-bridging oxygen Si-0- must be broken: and . (2) The re leased cations must have unobstructed pathways to migrate between different sites in the glaze structure. Hydration of glaze du ring leac hing results in the protonation of Si-0- to form sil ano l SiOH group. ·

Leached surfaces of Al20 , contam1ng glazes cou ld there fore be assumed to contain. in additi on tn =Si -OH and Si-0- groups. A!(OH), and A I 0 2- in ;untlLI!li s wl11ch woul d depend on pH and decreases tilL corrosion. Sim il arit y, the leac hed surfaces of 7.rn, conta ining glazes would contain =-Si-OH.=Si-0 -. and proportions of Zr(0H)4 and HZrO,-, depending on the pH. A dynamic equil ibrium which would determi ne the fracti on of surface sites occu pied would thus be set up at the interfaces

between the leached layer and soluti on. Thi s equilibrium is controlled by the concentrations and solvation energies of the ions in so lution and at the interface. The M + (alkali ) ions were not included here because these ions would simpl y rematn on sites near groups such as AIO~· , HZrO, and =SiO for electrical compensation .

A more like ly explanation fo r the effect o1

irradiation process is based on the assumption th at the effect of both neutrons and accompanied gamm<t irradi ati on lead to structural damages espec iall y tP the arrangement of the three d imensiOnal netwo:·k together with the creation of small ioni c spec ies and large molecular islands, thus fac ilitating the migration of the mobile (non-network ) cati ons. These defects enhance the leaching process. The random behaviour in chemical durability or density of glaze samples after bein g irradiated can he ascribed to the difference in crystallinity of the glazes. Moreover. the corrosion process is affect'ecl by varying the temperature or time of ex po .~ure

during the leaching process.

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