ES2206035B2 - In situ sol gel precipitation of ceramic products comprises infiltration of a solid with a sol, and deposition, for final baking to increase strength - Google Patents

Abstract

In situ sol-gel precipitation of ceramic products consists of infiltration of a solid with a sol, for controlled deposition of the selected material in the cavities into which the sol has penetrated. The final treating of the products virtually eliminates pores and increases mechanical strength properties, e.g. in composites where the precipitated substance differs from the matrix material.

Description

Procedure for obtaining single-phase ceramic pieces with improved mechanical properties by means of sol-gel precipitation " in situ " of the material within biscuit preforms.

It is based on infiltrating a biscuit preform with a sun that penetrates the pores of said material. Through changes in pH, temperature, etc. it is possible to precipitate " in situ " this sun, thus depositing the chosen material inside the cavities where the sun had penetrated. The gel thus deposited must be a precursor of the same substance that composes the matrix. The material, to acquire its final properties, must be subjected to its final firing after infiltration.

The developed method allows the manufacture of single-phase parts with little residual porosity or, in other words mode, pieces with real density close to the theoretical values. How Consequently, the parts have less microstructural defects and more mechanical resistance

During the processing of parts obtained from from powdered raw materials can be considered several stages The first consists of the formation of the powder until Give it the desired shape. Subsequently this piece is subjected to a baking in an oven, which can be carried out in turn two stages. In the first, often referred to as "biscuit," they get dense pieces with enough strength for their manageability The object is usually to decorate or rectify the biscuit pieces. Finally the piece would be taken back to a oven, where the definitive sintering to a temperature slightly below the melting temperature of material considered.

Therefore, and in general, you can establish three basic stages for part processing obtained from powders: forming, biscuit and cooking definitive

Bubbles appear in the conformation stage, drying cracks, etc. and they are, in general, very difficult to avoid. All these irregularities constitute microstructural defects They disappear to some extent during cooking. But nevertheless, they are never completely removed, leaving a residual porosity which is usually detrimental to the mechanical behavior of the piece.

The proposed solution is to fill partially or totally defects in the biscuit piece and then subject them to final cooking. Pieces are thus obtained practically without pores and, as a consequence, with high mechanical properties, mainly resistance to bending and traction.

The filling material penetrates the defects of the preform, acting as a sealant made with a material of Same chemical composition as the piece itself. In this way achieves a drastic increase in mechanical properties (flexural strength, thermal shock resistance, etc.) of the elaborate piece. The large specific surface of the precipitate favors the elimination of microstructural defects and helps final densification of the piece, approximating the density to the value 100% theoretical.

This method is of direct application in pieces of alumina, notwithstanding that the same principle may work in A diversity of other materials. For this, it is only necessary that certain characteristics are given in the matrix to infiltrate, such as will be described later.

For various reasons pores may appear in the ceramics obtained by the usual processing more conventional. These pores act as initiating defects of cracks that are the main cause of decreasing properties mechanical This is because the cracks easily spread to through the material requiring much less energy for this the necessary to break a completely dense material.

To cite a few examples, we will say that during extrusion processing cracks appear due to pulp tensions during extrusion through the nozzle, especially in little plastic masses. Cracks may also appear due to the too rapid removal of compositional water or to the removal of methyl cellulose or other agent plasticizer, etc. In the case of injection molding also they use additives, necessary for their gelling capacity, such as agarose, which when removed leaves pores in the pieces. The pieces obtained by casting, one of the most widely used methods, they often have pores due to the loss of water from composition of the slip, or other various problems.

In principle, as a sinter is sintered material, there is a generalized contraction that tends to decrease the gaps or pores left in the previous processing. Without However, many ceramic materials are refractory and present slow and incomplete sintering processes, at least at usual working temperatures. In this way, even after the sintering, there are always remaining pores and the final density of the piece does not reach 100% of the theoretical value. This problem is Of particular relevance in the case of alumina.

The procedure described herein invention is based on the dissolution of the material that will serve as filling. Subsequently this solution infiltrates the pores of matrix. Changing the conditions of pH, temperature, etc. be get the precipitation inside the pores, filling in that defects mode and simultaneously obtaining other effects beneficial.

The processes used for the dissolution-precipitation of almost any substance They are widely described in the bibliography. However you shouldn't confuse the term sun, with the meaning of a true dissolution, although in general the term sol is used as abbreviation of both. To be specific, a sun refers to a suspension of colloidal particles with a certain degree of stability ("Ceramic and Glasses", Engineered Materials Hand book, Vol 4, ASM International, ISBN 0-87170-282-7, Lisa C. Klein, "Sol Gel Process", pages 209-214 and Robert D. Shoup "Sol Gel Processes", pags 445-452).

Of particular interest in this case are those that fit into the Sol-Gel methods (For example: L. C. Klein, "Sol-Gel Processing of Silicates" Ann. Rev. Mater. Sci. 1, 227-248 (1985), John D. Wright and Nico A.J.M. Sommerdijk "Sol Gel Materials, Chemistry and Applications ", Gordon and Breach Science Publishers, Amsterdam 2001, ISBN 90-5699-326-7, etc.). The sol-gel processes enable the preparation of ceramic materials of high purity and homogeneity start generally from metal alkoxides. They are based on training of a colloidal suspension from hydrolysis and condensation of said metal alkoxides.

Often, procedures have been described for the addition of nanoparticles during material processing (Hsien.Kuang Liu and Azar Parvizi-Majidi, "The Effect of Solid Particles Addition in Sol-Gel Processing of Ceramic Matrix Composites "Ceramic Engineering & Science Proceedings, Vol 13, No 9-10, [642-649], 1992). In this case the addition of nanoparticles are performed simultaneously to the processing of ceramic powders The present procedure insists that the gel is incorporated into a solid biscuit piece that allows necessary manipulation, which we call preform.

Methods have also been developed for preform infiltration by pressure by a gel previously precipitate (F. F. Lange and K. T. Miller, "Pressure filtration: Consolidation Kinetics and Mechanics ", Am. Ceramic. Soc. Bull., 66, [1498-1504], 1987; F.F. Lange, David, C. C. Lam and O. Sudre, "Powder Processing and densification of Ceramic Composites "Materials Research (MRS) Proceeding: Processing Science of Advanced Ceramics, 309-318, 1989). The pressure needed in this case to get infiltration is very high, due to the viscosity of the gel, being much simpler infiltrate with the liquid where the dissolution has been made and precipitate later, as proposed in this process.

They exist today, among others (Azar Nazeri, Eric Bescher, and John D. Mackenzie, "Ceramic Composites by the Sol Gel Method: A Review "Ceram. Eng, Sci. Proc 14, [11-12], pp 1-19, 1993), following procedures described to form ceramic composites from gels:

- mix two or more suns to form a homogeneous solution.

- dispersion of a solid phase, such as powders or fibers, inside a sun before gelation.

- impregnation of a porous gel with substances organic or inorganic, so that the gel particles remain coated by the chosen substance.

- infiltration or coating of fibers, laminates, or three-dimensional fiber fabrics by a low sun viscosity, which is subsequently gelled.

- the combination of several of the processes previous.

We propose precipitation in the invention within the gaps of the biscuit preform to be filled, taking advantage of the large specific surface of the material deposited (with particle sizes normally much smaller than one micron) to facilitate subsequent sintering. Preform biscuit is a solid piece that has been processed in such a way that has consistency and that the porosity it possesses is open, is say, with interconnected pores.

Process

The processes will be described below. designed for the infiltration of a porous material. Along the this procedure we will use the word preform with the following meaning: it is a dense piece that resists manipulation, but the treatment has not yet been given (usually thermal) definitive, which will lead to the piece definitive

Generally, these preforms are obtained either by pressing the dry powder, or from a paste or slip that is, powder to which a binder is added, generally water To process this paste or slip, the amount of water used is strictly controlled. By losing a certain amount of that liquid (either during casting, or by dried, etc.), the preform becomes solid again, allowing its subsequent manipulation

In any case, the preforms obtained both by dry powder processing such as pastes and slippers They have holes between the particles.

As stated, the idea is to fill the pores of a material with a liquid in which it has previously dissolved the substance that interests us. This liquid must contain a precursor of the same substance that composes the matrix. To achieve that the liquid can penetrate the pores the phenomena can be used of adsorption by capillarity. In this way, it is often enough with submerging the preform into the liquid, since this, by surface tension, enters the pores. It also can get empty, thus driving liquid penetration in the room. A pressure system that facilitates the liquid inlet, using a principle similar to that of mercury porosimeters, which use increasing pressures to get the mercury to gradually penetrate the smaller pores

Three conditions have to be given to make it possible to infiltrate the preform with the solution:

1- The preform must be compatible with the solvents used. A large part of the processes Sol-Gel involve the use of acids or bases strong that can attack the preform.

2- The preform must have the consistency enough. Logically, when a green preform is submerged (no biscuit) in a liquid, it loses its consistency, becoming again pasty. To address this problem, we proceed to biscuit of the piece. The biscuit consists of a calcination to low temperature, so that sintering starts, but not advances significantly. The biscuit preform can be submerged in the liquid without getting rid.

3- The preform must have porosity completely open, otherwise the liquid could not enter In the cavities This is achieved by reducing the necessary biscuit temperature. Temperatures too low of biscuit produce unmanageable preforms (they fall apart handling). Temperatures too high produce porosity closed, not refillable.

Once a stable preform is achieved, it is say, that it can be submerged, the infiltration with the liquid that has dissolved or suspended material. TO then the processes leading to the precipitation.

One of the ways to produce precipitation is the immersion of the infiltrated specimen in another liquid with a different pH. Thus, when changing the solubility product, the material precipitates inside the preform. Other methods to achieve this is through temperature changes (the higher the temperature, the higher the solubility product), evaporation (as liquid is lost, the crystals will begin to precipitate),
etc.

Example 1

Prepare an alumina slip in water with 83.3% solids content and a viscosity of between 1000 and 2000 cps The alumina used in this case has an average size of 0.5 µ particle.

0.5 cm cylinders are prepared by casting diameter and 6 cm in length using a mold of plaster. This plaster cast has been prepared with the relationship by weight gypsum / water = 1.67, so that for every 60 gr of water, 41.4 se evaporate during the mold drying process, which leaves a adequate residual porosity.

The alumina cylinders are then dried at 50 ° C for 24 hours. Later they are introduced in an oven to proceed to biscuit at 600ºC. The cake ramp is the next: 2ºC / min up to 600ºC, 2h of permanence at 600ºC and 3ºC / min up to room temperature

On the other hand, 30 g of acetate are dissolved aluminum in 150 mL of water, by continuous stirring at temperature ambient, for approximately 24 h. To facilitate the process of solution a slight amount of HCl is added to the water concentrated.

The cylindrical pieces of biscuit alumina are immersed in this liquid for 3-4 h. TO They are then taken out and immersed in NH3, where they are left for 1-2 h. In this step the precipitation of Al hydroxide particles inside the alumina pieces.

The bars are again dried for 10 h at 50 ° C It is then cooked in an oven following the following ramp: 5ºC / min up to 600ºC; 2ºC / min up to 1600ºC, residence time at 1600 ° C 2 h; 2ºC / min up to temperature ambient.

At 1200 ° C all aluminum hydroxides have lost its compositional water and the resulting alumina has transformed to alpha-alumina or corundum, the phase thermodynamically more stable, resulting in cylindrical pieces of alumina without defects.

Comparing the already cooked bars obtained without infiltration with those who underwent this process are observed The following differences:

- Non-infiltrated bars have traces of the small bubbles in the alumina slip (Figure 1), while the infiltrates are completely homogeneous, without remains of such bubbles.

- The degree of sintering is much higher in pieces that were biscuited and infiltrated, being at high resolution in the SEM that alumina crystals are perfectly linked together (Figure 2).

- The mechanical resistance of the pieces infiltrates reaches a value almost double with respect to the non-infiltrated, as seen in the following table. The apparent density of the infiltrated pieces at 600ºC is close to the theoretical 4 gr / cm3 (for the elimination of defects), and they have a smaller specific surface (greater sintering), both data congruent with the result of the resistance in flexion.

\ sigma_ {f} (resistance in flexion Apparent density Surface specific to three points) Cylinders alumina SIN 128 MPa 3.3 gr / cm3 0.33 m 2 / gr " in situ " precipitation * Cylinders alumina WITH 367 MPa 3.8 gr / cm3 0.03 m 2 / gr precipitation " in situ " ** * Calcined at 1600ºC ** Biscuits at 600ºC, with precipitation " in situ " and subsequently calcined at 1600ºC.

Example 2

450 g of silica (SiO2), ground, are prepared and sieving, thus selecting the fraction lower than 63 \ mum. By uniaxial pressing platelets of 20 by 10 cm are made, so that using a pressure of 100 tons, the platelets they acquire an approximate thickness of 1.2 cm.

Platelets are baked in an oven to 1000 ° C

500 cm3 of a silica sol are prepared made by tetraethylorthosilicate hydrolysis (TEOS), to which add HCl to improve solubility. For the gelation of this Sun will be used in a later phase alcohol. The proportions suitable are:

TEOS H2O Alcohol HCl one 4 0.5 0.06

Platelets are submerged in the sun during a time not less than 1 hour to ensure infiltration. It adds the alcohol, producing gelation. Platelets are removed and the excess surface gel is cleaned.

Complete cooking at 1400 ° C, a clear improvement in platelet microstructure is observed obtained. From these platelets, 1 cm sections of thickness, obtaining bars suitable for the determination of mechanical properties by bending. There is also a clear improvement in the flexural strength of the pieces when compared with others that have not been subjected to the treatment of infiltration.

In this case, control over the properties end pieces are achieved by modifying both the pressure of Pressed as the cake temperature. Given the specials properties of quartz, sometimes it is interesting to add a small portion of plastic clay that facilitates the work of pressing and enable the subsequent handling of platelets.

Claims (4)

1. Procedure for obtaining single-phase ceramic pieces with improved mechanical properties by means of sol-gel precipitation " in situ " of the material inside biscuit preforms, characterized by the following stages: a) choice of the piece to infiltrate; b) infiltration with a sun; c) precipitation " in situ " of the sun. The piece to infiltrate, stage a), can be any piece biscuit with the following characteristics: 1) open porosity; 2) chemical resistance to solvents used in the dissolution-precipitation process; 3) mechanical resistance to withstand the necessary handling. 2. Method according to claim 1, characterized in that the average size of the pores to be infiltrated is controlled by modifying the amount and size of the processing additives or through the biscuit temperature. 3. Method according to claim 1, characterized in that the infiltration, step b), is carried out by immersion of the piece in the liquid with the sun, or by capillarity, or by the use of pressure or vacuum systems. 4. Method according to claim 1, characterized in that the precipitation, step c) is achieved with changes in the pH of the medium, by evaporation of the solvent, or by changes in temperature.