WO2002004370A1

WO2002004370A1 - Sol-gel process for the production of high dimensions dry gels and derived glasses

Info

Publication number: WO2002004370A1
Application number: PCT/EP2001/007027
Authority: WO
Inventors: Lorenzo Costa; Massimo Sparpaglione
Original assignee: Novara Technology S.R.L.
Priority date: 2000-07-10
Filing date: 2001-06-21
Publication date: 2002-01-17
Also published as: ITMI20001546A1; ATE427919T1; BR0112326A; ITMI20001546A0; DE60138288D1; CA2410557A1; ES2324593T3; US7216509B2; RU2278079C2; KR100501759B1; BR0112326B1; PT1320515E; EP1320515B1; AU2001266086A1; KR20030017617A; EP1320515A1; IT1318617B1; JP4035440B2; JP2004502632A; CA2410557C

Abstract

It is described a sol-gel process that allows to obtain dry gels, and possibly the corresponding dense glassy bodies, having higher dimensions compared to similar known processes.

Description

ω N> N> l-¹ o cπ o cπ o Cπ

MX_n + n H₂0 -> M (OH) _n + n HX ( I )

The thus obtained mixture, that may be a solution or a colloidal suspension, is defined as sol;

- polycondensation of M-OH groups according to the reaction:

M-OH + M-OH -> M-O-M + H₂0 (II)

with formation of an oxidic polymer, defined gel, that takes the whole volume originally occupied by the solution. This phase is defined gelation;

- drying of the gel obtaining a dry gel formed of a porous monolithic body, having apparent density (weight divided by the geometric volume of the monolithic body) comprised between about 1/20 and 1/3 of the theoretical density of the corresponding non-porous oxide; drying may be realized by controlled evaporation of the solvent, obtaining a body defined in the field as "xerogel", or by supercritical extraction of the solvent, obtaining a so-called "aerogel"; as given above, the dry gel may find industrial applications as such, or it can be submitted to densification by thermal treatment, obtaining a glassy body of theoretical density.

Sol-gel techniques have been studied since long for the replacement of traditional techniques in the industrial production of glasses because, allowing a better control of any process parameters, these make possible obtaining higher purity or metastable compositions, that cannot be obtained passing through a melting phase.

Sol-gel processes for producing glassy bodies, either of silicon oxide alone or of mixed oxides, are described in many patents. Patents US-A-4, 324, 576 and US-A-5, 076, 980 describe sol-gel processes wherein alkoxides are employed ω ω r M μ> μ> cπ o Cπ o cπ o Cπ s: μ- TJ n tr Q. CD 3 CO M TJ M . to o a. CO rt to o rt A μ- l-i rt Ω Ω rt o (-1- CD σ CD

SD 3 ^< Φ d μ- SD μ- d SD 3 μ- μ- tr Φ O O o Hi t μ-¹ 3 φ Φ SD CD tr tc O H o Φ CO

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;v rt O Ω 3 φ o μ- o Φ O φ μ- SD rt <! rt ω d tr to H CO 1 t μ- 3 H tr Ω TJ 3 H 3 Ω μ- H φ tr φ μ- H to CO μ- 3 φ μ- φ o μ- μ- μ- J

Φ Cϋ A r t 3 rt Φ H l-i

H 3 Φ μ- 3 to 3 φ D φ rt to o 3 3 φ 3 3 φ >< rt N μ- Φ Φ ω Φ φ

Ω 3 O A μ- SD CD Φ μ- 3 Φ Ω rt rt Φ to M SD A α TJ rt rt Ω o Φ μ- 3 o o O rt 3 H o _^ rt φ 3 C σ μ- rt d rt H tr ri o • d tr Ω • • to 3 Hi Mi o 3 μ- O X μ- CO H o £D μ- Φ o o o φ SD Hi H φ σ μ- to SD co rt CD H rt 0 3 rt O μ- rt μ> o to d Ω 3 H to

3 CO ω H φ TJ rt TJ 1 0 • φ tr 3 H 0 μ- 3 tr rt φ A M Φ CO 3 0 μ- μ- μ- μ- o X ^■ : tr TJ to H rt Φ Ω 3 O Φ SD to CD CO Φ μ- rt μ- H φ o v-> o Ω Hi rt φ 1 O O O 3^* 3 O CD Hi to TJ μ- rt J CD CO 3 Φ tr μ" rt to

CD μ- H¹ o H O μ- <! μ- C Mi Φ φ 3 TJ CQ SD TJ rt Φ Φ " O μ- tr o tr Ω 3 CD Φ ιQ to Ω φ 3 rt A rt μ- A o " H Φ to H X Ω Φ 0

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3 H TJ ψ CD M O rt α-» 3 CD t-i φ O μ- o Φ Hi 3 μ> H H s CO μ- O

TJ H Ω O μ- H SD w 00 rt SD ^ o 3 H 3 d 3 CD l-i tc d o Φ o μ- a 3 Ω

Φ • o tr 3 μ- n_ι _^ 3^* H¹ Hi H Hi to ω SD 3 CO O Φ μ¹ Hi CΛ " Φ SD

H α CΛ 3 Φ Ω 3 1 Φ ? μ- O iQ Φ O 3 μ- α * 3 <! to ^< Φ _^ CD Ω rt

P d μ- o D O φ > o 3 0 Φ CO iQ to o d CD CO ι-i 3 rt SD μ- rt 3 Ω O H 3 to 1 CΛ X CD rt H C Ω <! Φ ι-i φ CD o μ- H CD Φ tr CO o tr Φ t μ- O μ- - π 1 l-i μ- Φ • Ω CD φ SD rt μ^< d Hi to Φ 3 Φ Φ 3

Φ Φ -i + 3 H" o o 00 SD tr TJ 3 O 3 tr rt tr ?v Φ . to l Φ Φ μ- μ- rt σϊ 1 s: Φ tr S μ- rt Mi μ- O Φ o CD CD o CΛ 3 O rt CD f rt Ω α SD o J-» σ CO φ CD O rt H μ- A O IX tr Hi rt rt μ- O Hi $, O

M rt 3 CD Φ CD CD l-i _^ SD 3 H μ- Φ 3 3 rt φ μ- ^ o o φ 1 l-i tr

SD 3^* o Ω H rt CO σi Ω CD to Φ Φ 3 CD SD o ω tr α μ- d CD rt . — . Φ rt o

Φ Ω to TJ μ- 00 ? H Φ O • l-i rt 3 l-i μ- Φ tr Φ ιQ rt μ- μ- M o 3" to φ CO o o r 3 0- CO Φ ¹ <3 H O A N CD CO tr r 3 SD 1 Ω Φ Φ

3 rt o H ≤: μ- A μ- σ O Φ TJ CD 3 SD φ P- <l Φ K) o Φ o O

SD l-i μ« A 3 to ( > o H o M o Hi Ω φ 3 • μ- φ SD H I x rt rt tc TJ o

3 CD α. μ- Φ Φ Hi Ω ¹ Hi CD "_^ H O rt rt μ- Hi 3 H O μ- tr 3^{^} ω § H !X! φ 3 3 M • o - tπ rt μ- Ω φ H μ- Φ Ω H Hi *_* Φ rt _^ Φ 0 ^-^ O O μ- φ tr A Φ *_* H 5 o rt tr Φ O rt to Ω O l-i 3 μ- rt tr Φ X _^ 3 0- Q.

- 3 •< M >τ) s 3 CD tr Φ ω 3 tr _"* Φ 3 to TJ Ω rt CD rt s: to Hi

SD "< I-¹ d Φ

CD s: ^ Φ α Φ H Ω φ CO o H d Hi 3^* 3 - μ- O Ω

(D 3 O H" μ- H SD CΛ o Φ rt μ- S SD μ- O o μ- μ- Φ H . Φ rt Mi

Ω Φ SD M rt o r-¹ rt 1 TJ Ω Mi 3 CO tr Φ CO tr to Cπ " CD μ- O f H rt ^ tr A tr H o rt O SD Φ 1 Ω rt d μ^j rt rt 3 d μ- 3 Φ • Φ O μ- _^-_^ tr Φ CD O h

Φ Φ o Φ 1 Φ CO A h μ¹ rt Φ rt H φ *_* Φ ιQ to H Φ 3 3

H π o H Φ J TJ 3 X π <! rt Φ SD to l-i H Φ H to A Φ SD μ- CD α SD

CD α H H CD μ- μ- (D μ- H 3 rt 0 rt 0 μ- s: tr φ 3 to H 0 tr CD H rt tc H Hi 0 H Ω > o ^><: Φ μ- rt Hi φ C rt 3 μ- μ- o rt " Φ CD

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Cπ o cπ o cπ o Cπ

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CD H ^ Φ 3 3 CD Hi Φ g Φ (D o Ω 1 α A Φ Φ SD μ- r-¹ Φ σ tr o μ- SD • μ- CD rt CD rt A Ω d tr Φ φ H Φ 3 3 σ SD Cπ l-i Φ 3 CD rt o 3 o φ X SD 3 CD tr H (D * Φ 3 φ Ω rt l-i CD M Φ rt 1 ^ μ> Φ to μ- μ- rt d to * tr H d A SD φ J O 3 a. Ω 3 O 3^{^} tr μ- A rt SD O N3 to - Φ o 3 1 rt φ •< 3

H t tr H o rt tr ■^■ Ω Ω φ tr μ- 3 Ji. (a •< to 3 TJ TJ rt Φ a a ffi μ- ^ ^ Hi μ- H o H tr • ^►< 3 O Φ μ- r-> hi tr ) TJ O Φ o d

Ω CD N A α μ- o Φ O O Ω CD H φ *< M Cfl Hi rt rt ^ o CD 1 hi Hi tr H i μ>

O rt CD Φ hi 3 TJ 3 3 X to 3 O rt SD ^ CO μ- O O 0- rt CO o 0 rt TJ to

3 σ o iQ o o μ- SD Φ μ- 3 μ- H to oo rt 3 O rt d _"» Ω hi CD rt •

TJ <i SD hi o co N 3 rt TJ o § σ 0 00 rt l-i Φ rt tr Ω hi φ μ- tr

H CD Φ rt •< TJ Φ Hi Φ TJ O H O 3 o < M tr o rt tr φ rt CD SD to μ- 3 Φ d Q

Φ μ- CD tr CO α hi μ* CD O d 3 a. d d TJ tr Φ μ- μ- 3 CD 3 Φ 3

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CO rt Φ tr α d φ Φ 3 3 3 3 J Ω o Φ φ 3 <! φ rt CD A SD tr CD hi D φ μ- o • d μ- n hj A SD SD μ- rt tr to CO μ- φ •< φ rt 3 >< Φ

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H o Hi SD tr Ω 3 3 * ¹ SD H 3 CO 3 o to μ" tr O μ- tr hti Φ 3 tr

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H tr Φ 3 H

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number of parameters, such as reagents concentration and timing of the different phases.

From the reading of the cited patents, and in particular the last one, it results clear that the most critical phase in a sol-gel process is gelation. During this phase in fact the microstructure of the gel is formed, and as given above the mechanical resistance of such a gel and the possibility of submitting it to subsequent treatment of drying and densification avoiding that this develops strains or breaks depend from this microstructure and from distribution of porosity in the gel.

These problems are the worse the bigger is the dry gel to be produced. Particularly, it is commonly acknowledged in the field that tridimensional gels where any of the three lateral dimensions exceed about 2 cm are impossible to produce in practice, apart from the production of unique samples and on a laboratory scale. In the following, with the definition of big size gels it will be meant gels having all of the three lateral dimensions greater than about 2 cm.

Object of the present invention is to provide a sol-gel process that allows to overcome the drawbacks of the prior art, and in particular to provide a sol-gel process that is simple and that allows the production of big size gels with good yield and at low cost, that may then be submitted to densification treatments for the production of the corresponding glassy bodies.

This object is obtained according to the present invention with a sol-gel process comprising the operations of:

- preparing a composition A comprising pyrogenic silica and an aqueous solution of an acid, such that the molar ratio H₂0/Si0₂ in composition A is equal or lower than 20 and its pH is equal or lower than 1.5; - preparing a composition B comprising silica and an aqueous solution of a base not containing metallic cations, such that the molar ratio H₂0/Si0₂ in composition B is comprised between 6 and 40 and its pH is comprised between 10.5 and 13;

- forming a composition C by mixing composition A and composition B in such a ratio that the molar ratio between silica from composition A and silica from composition B is comprised between about 1:2 and 3:1, the pH of composition C is comprised between 1 and 5 and the molar ratio H₂0/Si0₂ is comprised between about 5 and 15;

- allowing gelation of composition C;

- substituting water present in the gel pores and in the vessel where the gel is contained with an non-protic liquid miscible with water;

- drying the gel by evaporation or supercritical extraction of the non-protic liquid.

The process of the invention may optionally be followed by the operations necessary to densification of the dry gel to yield a glass.

The inventors have found that by using silica powder only - as precursor material, initially subdivided into a strongly acid composition and a strongly basic one that are then united in suitable ratio, it is possible to obtain dry gels with microstructural characteristics that are superior to those of gels obtained by using alkoxides; these microstructural characteristics are reflected in the mechanical characteristics of the gels, than can be obtained in the form of bodies in which all of the three lateral dimensions are greater than about 2 cm. For the preparation of composition A it is only possible to use as starting material silica of the pyrogenic kind, such as e.g., "Aerosil OX-50" of the firm Degussa-Huls AG. This composition is prepared by mixing silica with an acid in aqueous solution.

Molar ratio H₂0/Si0₂ in composition A is lower than 20, as higher ratios would result in an excessive dilution of the final composition C, such that to make difficult, if not impossible, to form a gel from this latter composition.

The lower limit of molar ratio H₂0/Si0₂ in composition A is not strictly fixed and it's determined from silica concentration and pH of composition B, because it must be guaranteed that composition C has a molar ratio H₂0/Si0₂ comprised between 5 and 15 and pH comprised between 1 and 5. As composition B may be prepared with a very high molar ratio H₂0/Si0₂, it follows that composition A may have, for the same molar ratio, very low values.

Depending on the molar ratio between water and silica, composition A may look like a powder or a liquid phase. In particular, at molar ratios H₂0/Si0₂ below about 0.5 the thermodynamically stable form of composition A is such that the aqueous solution of acid is present as microscopic droplets whose surface is covered with a layer of pyrogenic Si0₂ particles, making composition A looking as a dry powder; when molar ratio H₂0/Si0₂ is over the previously given limit, a gradual transition takes place until when, for values of the molar ratio H₂0/Si0₂ higher than about 1, composition A is stable as a suspension of silica in the aqueous solution of the acid.

The pH of composition A must be lower than 1.5. In case of molar ratios H₂0/Si0₂ comparatively high in composition A, the value of pH can be directly measured with a pH-meter, while in the case of very low ratios as stated above composition A has the look of the a powder, in which case CO r r μ>

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Φ rt Φ to TJ 3 CD Hi μ- SD SD μ> o d o d o Φ hj tr rt o o CO μ- to o t Ω rt d Ω CD rt tr _^ tr 3 rt o o 3 3 > Hi CD φ

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Composition B is basic due to addition of a base that doesn't contain metallic cations, such as ammonium hydroxide or a aqueous solution of organic amines such as monomethylamine, dimethylamine, ethylendiamine. The pH of composition B is comprised between about 10.5 and 13. At pH values lower than 10.5 the hydrolysis rate of silica is practically negligible, while pH values higher than 13 would require too high amounts of acid in composition A, without offering advantages in silica hydrolysis in composition B. For the objectives of the invention, composition B has preferably a value of pH slightly higher than 10.5.

Composition A and B prepared as hereinabove described are mixed to form composition C, in ratios such that the molar ratio between silica coming from composition A (referred to in the following also as Si0₂ (A) ) and silica coming from composition B (referred to in the following also as Si0₂(B)) is comprised between about 1:2 and 3:1. At values of the ratio Si0₂ (A) /Si0₂ (B) below about 1:2 the process productivity becomes low, while at ratios higher than 3:1 the final dry gel results fragile.

Moreover, composition C must have a molar ratio H₂0/Si0₂ comprised between 5 and 15: at lower ratios there are difficulties in obtaining homogeneity in composition C with negative consequences on the mechanical characteristics of the final gels, while at higher ratios there are difficulties in gelation.

Finally, composition C must have a pH value comprised between 1 and 5 and preferably between 1.5 and 3. At pH values lower than 1 it is necessary to use a great amount of base to bring sol to gelation and the process has thus little efficiency, while at values higher than 5 gelation is practically immediate at the moment of mixing of compositions A and B, so that it is not possible to have homogenization of these two. In the indicated pH range, gelation rate is higher the higher is pH.

It is preferable that gelation does not take place too quickly after mixing of compositions A and B, such that there is time to reach pH homogeneity throughout composition C; a gelation of composition C not too fast after its formation also allows evaluation and adjustment of parameters such as Si0₂ concentration or pH. As a consequence, it is preferable to operate in such a way that composition C at the moment of its formation has a pH comprised between about 1,5 and 3; once it's been verified the obtainment of a homogeneous composition C, pH may be then raised to a value comprised between 4 and 5 by slow addition and under stirring of a base, e.g. NH₄OH, thus allowing rapid gelation.

Any operations so far described for the preparation of compositions A, B and C may be carried out under ultrasound stirring, that favors dispersion and homogenization of suspensions or solutions of silica in aqueous medium.

Conditions given above for composition C, taken together, are used to define exact formulation of compositions A and B.

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The gel resulting from the solvent exchange can be dried by normal evaporation, possibly placing it in a container with a wall permeable to the solvent vapors, capable to avoid a too high evaporation rate so as to reduce at a minimum the risk of breaks in the gel. Preferably, drying of the gel is carried out supercritically. In this case, it is possible to carry out supercritical extraction of the non-protic liquid with which water has been replaced. It may however be preferable to carry out a further exchange of the non- protic liquid of the previous exchange with an organic compound, liquid in its turn, showing lower values of supercritical pressure and temperature, of which supercritical extraction is carried out. The preferred solvent for this purpose is ethyl acetate, having critical temperature (Tc) and critical pressure (Pc) respectively equal to 250.4 °C and 37.8 bars. Other liquids suitable for the aims of the invention, and their critical constants, are:

Methyl propionate Tc = 257.4 °C Pc = 39.3 bars

Propyl formiate Tc = 264.9 °C Pc = 40.1 bars

Ethyl propionate Tc = 272.9 °C Pc = 33,0 bars

Propyl acetate Tc = 276.0 °C Pc = 32.9 bars

Methyl butyrate Tc = 281.0 °C Pc = 34.3 bars

n-pentane Tc = 196.6 °C Pc = 33.4 bars

n-hexane Tc = 234.2 °C Pc = 29.9 bars

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composition B comprising silica in a basic aqueous solution, adding 222 g of pyrogenic silica "Aerosil 0X- 200", slowly and under mechanical stirring, to 1 liter of a solution of NHOH of concentration 0.3 M in a glass flask. After silica addition has been completed, the sol is stirred for a further half a hour and then brought at a temperature of about 0 °C placing the flask in a laboratory refrigerator. The whole volume of composition A is united to composition B in a polypropylene beaker having diameter of about 15 cm, placed in a bath refrigerated at 2 °C. The mixture is maintained under vigorous stirring during five minutes; it is thus formed composition C, that. is brought to pH 4 through addition of ammonia of concentration 0.05 M. Stirring is then interrupted, the beaker is extracted from the refrigerating bath and its content is poured into four cylindrical teflon dies of diameter 9 cm, reaching a filling height of 4 cm in each die, and then the temperature of the sol is allowed to naturally raise. Gelation requires about 3 hours. After gelation the wet gels are subjected to two exchanges of the liquid in the pores and in the container, by substituting first water initially present with acetone, and then this with ethyl acetate. The thus obtained wet gel is supercritically dried in an autoclave with inner volume of 9 liters wherein it is realized pre-pressurization with nitrogen at 50 bars.

Supercritical drying is carried out by bringing the inner of the autoclave at 280 °C and 50 bars. A dry gel of cylindrical shape of diameter 9 cm and height 4 cm is extracted from the autoclave. The dry gel has good mechanical strength and it appears homogeneous and with no defects at visual inspection.

EXAMPLE 2

A dry gel specimen produced in example 1 is subjected to a densification treatment for the obtainment of a silica glass, according to the following schedule: - heating from room temperature to 500 °C during 6 hours in an air flow, and keeping at these latter conditions during 8 hours;

- heating from 500 °C to 800 °C during 3 hours, and keeping of the last reached temperature for 2 hours in a flow of helium containing 5% by volume of anhydrous HC1;

- heating from 800 °C to 1350 °C during 3 hours in a helium flow, and keeping of the last temperature for 10 minutes;

- natural cooling down to 300 °C, and extraction of the specimen from the oven.

The obtained sample does not show defects, has density of 2.2 g/cm³ and has good transmittance in the visible, UV and near IR range.

Claims

1. Sol-gel process for the production of tridimensional dry gels having all of the three lateral dimensions higher than about 2 cm, comprising the operations of:

- preparing a composition A comprising pyrogenic silica and an aqueous solution of an acid, such that the molar ratio H₂0/Si0₂ in composition A is equal or lower than 20 and its pH is equal or lower than 1.5;

- preparing a composition B comprising silica and an aqueous solution of a base not containing metallic cations, such. that the molar ratio H₂0/Si0₂ in composition B is comprised between 6 and 40 and its pH is comprised between 10.5 and 13;

- allowing gelation of composition C;

- substituting water present in the gel pores and in the vessel where the gel is contained with an non- protic liquid miscible with water;

2. Process according to claim 1 wherein the molar ratio H₂0/Si0₂ in composition A is lower than about 0.5.

3. Process according to claim 1 wherein composition A is prepared with an aqueous solution of an acid chosen between HCl and HN0₃ having pH lower than 1.

4. Process according to claim 3 wherein said solution is a solution containing 32% by weight of HCl in water.

5. Process according to claim 3 wherein said solution is a solution containing 65% by weight of HN0₃ in water.

6. Process according to claim 1 wherein the molar ratio H₂0/Si0₂ in composition B is comprised between about 8 and 30.

7. Process according to claim 1 wherein composition B is prepared with an aqueous solution of a base non containing metallic cations.

8. Process according to claim 7 wherein the base is ammonium hydroxide.

9. Process according to claim 7 wherein the base is chosen among monomethylamine and/or di ethylamine and/or ethylendiamine.

10. Process according to claim 1 wherein, at the moment of its formation, composition C has a pH comprised between about 1.5 and 3.

11. Process according to claim 10 wherein the pH of composition C is raised before gelation at a value comprised between about 4 and 5.

12. Process according to claim 1 wherein composition C is formed by mixing compositions A and B having a temperature lower than 10 °C.

13. Process according to claim 12 wherein said temperature is about 0 °c.

14. Process according to claim 12 wherein the temperature of composition C is raised, before gelation, at a value comprised between room temperature and 50 °C.

15. Process according to claim 1 wherein said non-protic liquid is chosen among low molecular weight ketones and tetrahydrofuran.

16. Process according to claim 15 wherein said non-protic liquid is acetone.

17. Process according to claim 1 wherein, before supercritical extraction, said non-protic liquid is substituted from gel pores and container where the gel is contained with a liquid organic compound chosen among methyl propionate, propyl formiate, ethyl propionate, propyl acetate, methyl butyrate, n-pentane and n-hexane.

18. Process according to claim 17 wherein the gel is dried by supercritical extraction of said liquid organic compound.

19. Process according to claim 1 further comprising a thermal treatment for the densification of the dry gel to the corresponding glass.

20. Process according to claim 19 wherein said treatment comprises the following phases:

- treatment in an oxygen or air flow from room temperature up to a temperature of about 500 °C.

- treatment up to about 800 °C in an atmosphere of chlorine, hydrogen chloride, carbon tetrachloride or one of these gases diluted in an inert gas;

- treatment up to the temperature of densification to glass in a flow of an inert gas or of an inert gas containing little percentages of oxygen.

21. Process according to claim 20 wherein the inert gas is helium.

22. Silica dry gels produced according to any of the claims from 1 to 18.

23. Silica glasses produced according to any of claims from 19 to 21.