ITRM20120612A1 - METHOD FOR THE CREATION OF TRANSPARENT RARE-EARTHEN CERAMICS - Google Patents

Description

DESCRIPTION

â € œMETHOD FOR MAKING TRANSPARENT RARE EARTH-BASED CERAMICSâ €

The present invention relates to a method for making transparent rare earth based ceramics.

Due to their characteristics, ceramic materials have been the subject of great interest in many technical sectors for several years.

In the laser field, transparent ceramic materials are studied for their use as active elements. A particular role is played by power lasers with large active elements and in particular very high energy pulsed lasers. While the high powers of continuous lasers are conveniently achieved with simple and efficient fiber lasers, this is not possible for pulsed sources of great energy. In fact, the high peak irradiations that occur in the active fiber in the pulsed regime (in particular in the case of pulses below 10 ns) cause non-linear phenomena such as Stimulated Brillouin Scattering which lead to limitation in optical extraction or even to the breaking of the fiber.

Among the pulsed high-energy applications of solid-state lasers, the laser ignition of pellets of solid deuteriotritium mixtures for Inertial Confinement Nuclear Fusion plays an important role. This is a rapidly growing sector of technological research which could undergo a sharp acceleration in the next few years. Many laboratories involved in this research are investigating laser amplifier schemes with an architecture based on large active elements. In this context, transparent ceramic technology is a key element.

Traditionally, single crystals obtained with the Czochralski method (of stretching) are used for low power laser applications. The technique consists in growing a single crystal from a melt: a crystalline germ is placed in contact (on the surface) of the melt held at a constant temperature just above the melting point. The germ is gradually pulled out of the spindle and slowly rotated; the melt cools and solidifies on the germ and forms an elongated crystal with the same crystallographic orientation as the original germ.

The production of crystals according to the Czochralski method has the disadvantage related to a very high cost and the impossibility of obtaining crystals of arbitrary geometry. Furthermore, composition gradients are unavoidable and high dopant concentrations are not possible.

An alternative to crystals is the use of transparent ceramics obtained by sintering submicronic powders. Such ceramics have advantages both from a functional point of view and from an economic point of view.

For the application of a transparent ceramic as an active component in lasers, an optical transparency is required, which implies a narrow distribution of the size of the grains, a very low porosity in the sintered product (indicatively less than 1 ppm) and a minimum size of the grain boundary phase.

The advantages obtained by replacing the single crystal with the ceramic are:

â € ¢ Lower production cost. For example, it may take 4-6 weeks to grow YAG crystals by Czochralski's method; single crystals can only be produced in iridium crucibles, and the cost of each single crystal increases exponentially with its size.

â € ¢ Larger component sizes. This is a considerable advantage, since the size of the active component is closely related to the laser power.

â € ¢ Greater concentration of the dopant element and possibility of multiple doping. For example, the polycrystalline YAG offers the possibility to considerably increase the dopant element concentration (up to 9% of Nd), making it possible to obtain much higher powers. Furthermore, it is easier to combine different dopant elements, obtaining innovative lasers.

To date, for the sintering of transparent ceramics, there are sintering procedures in "vacuum only" and other patented methods that provide for three stages of sintering, which we will define as "HIP vacuum". In particular, the "vacuum HIP" methodologies provide a first stage of sintering (or pre-sintering) in vacuum, a second stage of high pressure sintering (Hot Isostatic Pressing, HIP), for example at 3000 bar in a graphite oven, and a third stage of reoxidation of the carbonaceous species introduced under oxygen pressure. While the "vacuum only" methodologies produce products of limited transparency, except for the use of extremely expensive powders because they are characterized by particular characteristics of reactivity, surface area and particle size, the "vacuum HIP" methodologies appear limited due to their complexity and high costs of the necessary installations.

Although, as reported above, the realization of the transparent ceramic by sintering today constitutes an advantage compared to the formation of the single crystal with the Czochralski method, nevertheless the need was felt to have a sintering process capable of breaking down the production costs, while ensuring high levels of optical quality of the transparent ceramic produced.

It has been surprisingly and unexpectedly found by the Applicants a method for the production of transparent ceramics by reactive sintering, which allows to obtain better optical characteristics compared to the known art relating to sintering in "vacuum only", without having to resort to extremely expensive ceramic powders or particularly complex and expensive systems as in the case of the "vacuum HIP" method.

The object of the present invention is a method for manufacturing transparent rare earth-based ceramics whose essential characteristics are reported in claim 1, and whose preferred and / or auxiliary characteristics are reported in claims 2-7.

For a better understanding of the invention, some embodiments are reported below for illustrative and non-limiting purposes with the aid of the attached figure which shows an absorption spectrum of a ceramic made according to the method of the present invention, one absorption spectrum of a ceramic made with the vacuum + HIP method and an absorption spectrum of a single crystal.

Below is an example of the realization of 2 mol% Nd-YAG (Nd0.2Y2.98Al5O12) transparent

- Preparation of the precursor mixture of ceramic powders - Approximately 300g of a mixture of precursor powder of mixed oxides have been prepared for the preparation of transparent Nd-YAG.

They were weighed:

- 139.171g of alumina, title 99.19% w / w,

- 182.044g of yttria, title 99.22% w / w,

- 6,714g of neodymium oxide, ground and dried by freeze-dry, titre in Nd2O385,62% w / w

In particular, Baykowski Baikalox SM8 alumina was used (surface area 10m <2> / g, d90 = 700nm, d50 300nm, purity compared to other elements 99.99), Stark yttria grade C (surface area 10-16m <2> / g; d90 = 2500 nm; d50 = 900 nm, purity with respect to the other elements 99.95%), and neodymium oxide of Cerac (325 mesh, purity with respect to the other elements 99.999%).

The ratio (Nd + Y) / Al is slightly greater than the stoichiometric one and is between 0.6015 and 0.6018 (compared to a theoretical of 0.6000, as foreseen by the stoichiometry Nd0.2Y2.98Al5O12), obtained with an excess of Nd.

To the powders were added:

- 240g of water, in which they have been solubilized: 1.63g of CE64 (dispersant and fluidifying agent by Zshimmer and Schwarz, 0.5% w / w with respect to the weight of the dry powder), 6.53g of PVA 15000 (2% w / w compared to dry powder), 0.98g of citric acid (pH corrector), 18 drops of iso-octyl alcohol or Contraspum (as antifoam).

- 1.68g of TEOS, pre-hydrolyzed under heat for 30 minutes in 8g of ethanol, with the addition of 3.90g of 0.01N HCl, under magnetic stirring;

- 530g of 3-4mm alumina grinders (GMHDA-4mm, Mattech Corporation, Cincinnati, Ohio).

The resulting mixture was wet ground for 20 hours, stirring. After the first 5 hours of grinding, another 0.49g of citric acid was added to correct the pH which, varying during the grinding, would negatively affect the rheological characteristics of the suspension.

After 20 hours of grinding, the suspension was filtered on a 25 micron sieve and the consumption of the alumina grinding bodies was checked for the calculation of the stoichiometry corresponding to the precursor powder produced, possibly making the necessary corrections. The powder obtained was subjected to freeze-dry (freezing of the powder with liquid nitrogen and drying under vacuum). Subsequently, the powder was passed through a 250 micron sieve and stored in a sealed container. The granulated powder was pressed by isostatic pressing at 1500 bar to obtain tablets or discs. Isostatic pressing can be preceded by uniaxial low pressure pressing, without significant changes in the final transparency characteristics of the ceramic. The tablets or discs obtained were subsequently subjected to a heat treatment at a temperature between 800 and 1200 ° C in air, in order to carry out the combustion of the organic additives used to produce the suspension and to allow forming. The artifacts made in this way were stored in a dryer, or vacuum packed, to protect them from humidity and CO2 until sintering. This precaution is advisable due to the tendency of rare earth oxides to form basic carbonates.

- Sintering phase of 2 mol% Nd-YAG - The articles were loaded into the furnace prepared for sintering. After loading the samples, the furnace chamber was subjected to a washing procedure with inert gas (generally with helium) aimed at removing the air before heating. After preheating at 1100-1300 ° C between 1 and 20 hours and reaching a vacuum of at least 10 <-5> mbar, a heating ramp was performed at a speed between 500 and 2000 ° / h up to reaching a temperature between 1700 and 1800 ° C. The heating was assisted by a pumping system capable of guaranteeing, upon reaching the isothermal temperature, a dynamic vacuum of at least 10 <-4> mbar, with a rapid vacuum recovery up to 10 <-5> mbar. The use of a fast heating ramp has the purpose of guaranteeing a better degassing of the pre-sintered product and a more homogeneous distribution of the low melting eutectics which act as sintering aid.

In the case considered, as per literature indications, the lowest melting eutectic that is formed is a neodymium silicate. The experimental data detected is, with equal vacuum, an improvement of the optical quality with the ramp speed. During this phase, called "reactive sintering", the reaction of aluminum oxides, yttrium and neodymium takes place to form the 2 mol% Nd-YAG phase (Nd0.2Y2.98Al5O12, doped with partial substitution of Y <3+> with Nd <3+>). After a period of 15 hours (generally between 5 and 35 hours), nitrogen or another inert gas was introduced into the oven chamber, reaching a pressure of 10 bar in the chamber itself. Under the pressure conditions of 10 bar and without modifying the previously set temperature (temperature between 1700 and 1800 ° C), the products were subjected to the sintering treatment for a further time of 25-75 hours. In this phase the porosity of the transparent ceramic material is reduced, leading to a transparent product. In all these sintering phases, the oven temperature was measured with a type C or D thermocouple, with hafnia powder insulation and tungsten outer sheath, placed a few centimeters from the sample.

At the end of the treatment, the ceramic samples were treated at a temperature of 1400 ° C (generally between 1300 ° and 1600 ° C) in static air for 3 hours to decolor them and subsequently they were subjected to polishing, using discs, cloths and diamond pastes, up to 1 micron in fineness. The last stage of polishing, before optical measurements, is that which involves the use of colloidal silica.

In this way, discs with a diameter of up to 100 mm and pads with a diameter of 10 mm and a thickness of up to 15 mm were produced.

In order to carry out vacuum and pressure sintering in a single furnace, it must have characteristics that conventional furnaces do not have. In fact, conventional high vacuum furnaces for temperatures over 1700 ° C and higher vacuum of 10 <-5> mbar must have all the hot parts in tungsten (screens and resistors), which is the only metal that does not sublimate in the above conditions thanks to its high melting point (3422 ° C). In order to work both in high vacuum and under pressure, it was necessary to have an oven that guaranteed thermal insulation not only in vacuum conditions, but also in conditions of presence of gas and transmission of heat by convective motions. It was therefore necessary to insert a layer of ceramic felt, resistant to high temperatures, between the outermost metal screens.

- Experimental tests on the ceramics produced - Through absorption spectroscopy it is possible to verify the presence of impurities (through the search for anomalous peaks), analyze the cross sections of the various atomic transitions of the active medium under examination (analyzing the intensity relative of the lines of the measured spectrum) and any modifications of the atomic orbitals (from the position and width of the observed lines). In the case of Nd-YAG, in the region 700-900 nm, the three bands are attributable, from left to right, to the transitions 4I9 / 2 â † '4F7 / 2 4S3 / 2, 4I9 / 2 â †' 4F5 / 2 2H9 / 2 and 4I9 / 2 â † '4F3 / 2, of the Nd <3+> ion. By measuring the absorbance it is also possible to highlight the presence of porosity, a source of scattering phenomena. On the pads produced according to the process described above, maximum transmittances of 99% with respect to the theoretical have been reached.

The figure shows the absorption spectrum of the ceramic obtained with the method of the present invention and having a stoichiometry of 2 mol% Nd-YAG (a). For comparison, the absorption spectra of a ceramic having the same stoichiometry but made with the vacuum + HIP technique (b) and of a single crystal with stoichiometry 0.6 mol% Nd-YAG (c) have been obtained.

From figure 1 it can be seen how the absorption spectrum relating to the ceramic obtained by the method of the present invention is equal (and, therefore, superimposable) to that of the ceramic obtained by the vacuum + HIP technique. Furthermore, by comparing the absorption spectrum of the ceramic obtained by the method of the present invention with that of the single crystal, it can be seen that the transparency characteristics of the former are verified.

The spectra shown in figure 1 were recorded by means of a Perkin-Elmer lambda 900 spectrophotometer, on ceramic and crystalline samples of equal thickness. Non-transmitted radiation is reflected, absorbed or scattered by the sample. The reflected radiation is easily calculated with Fresnel's laws. In the spectral regions in which there are no transitions allowed, the radiation diffused by the sample can be quantified (by difference from the value given by Fresnel's laws); in this way the radiation absorbed in the area of the transitions of interest is obtained by difference.

The observations and the transmittance measurements have shown that if the pressure phase exceeds 75 hours, the optical quality of the artefact worsens. Furthermore, it has been found that for the same time as 25 hours of vacuum sintering, the presence of the pressure stage following the vacuum stage significantly improves the optical quality.

On identical pads with a diameter of 10 mm, 15 mm thick, we pass from a transmittance of 95% with respect to the theoretical, with a sintering of 15 + 25h in vacuum only, to transmittances greater than 99% with respect to the theoretical, with a sintering 15 hours in vacuum followed by 25 hours at 0.5 bar relative pressure. A transmittance value of 99% also made it possible to carry out a â € œlaser generationâ € experiment: the 15 mm â € œrodâ € was inserted as an active medium in an optical cavity and pumped in an â € œend-pumpâ € configuration. with 808nm radiation generated by laser diodes. This produced 80 W of optical radiation with a conversion efficiency of 20%.

Tests for improving transmittance with pressure, up to a pressure of 10 bar, were performed on 50 mm discs and, therefore, more than representative of the result on large-sized artifacts. It was thus verified that increasing pressure values (equal to the other parameters and in particular of the ceramic mixture) allow increasing values of transmittance, in particular it was possible to pass, on the same ceramic mixture, from attenuation per unit of length of propagation of 0.26 cm <-1>, with treatments under pressure at 1 bar, at a value of 0.16 cm <-1>, with treatments under pressure at 10 bar.

The method object of the present invention offers the great advantage of producing ceramics with high optical characteristics, without thereby compromising the productivity in terms of costs and complexity of the methodology used.

In fact, the method object of the present invention requires the use of a single furnace and, therefore, of a single sintering phase in which the only modification is related to the single pressure, which remains below 100 bar, i.e. in the range pressure defined Gas Pressure Sintering (GPS) and not HIP.

The method object of the present invention, by carrying out all the sintering in a single metal furnace, allows both to significantly reduce production costs and to reduce the sources of pollution that could compromise the transparency characteristics of the material.

Another advantage of the method object of the present invention relates to the preparation of the powder for subsequent reactive sintering, which involves wet mixing starting from low-cost commercial submicronic powders, and the use of water as a solvent, with dispersing additives. and for the control of the pH, with additions made at the beginning and in the middle of the mixing, in order to optimize the rheological characteristics and guarantee the maximum solid content, with advantages on the powder obtained in the granulation phase.

Reactive sintering leads to the creation of the grains of the thermodynamically more stable phase, by reaction of the homogeneously mixed phases in a stoichiometric ratio, thus making the weight of some characteristics of the submicronic starting powders much less critical (such as the surface area and the granulometry ). In this way, the only critical parameters are purity, homogeneity of the mixing and the stoichiometric ratio.

Furthermore, the powder was prepared using an excess of the elements that form the most volatile eutectics. In this case compounds rich in neodymium, compared to stoichiometry. These eutectics act as sintering aid, distributing themselves on the grain boundary in the initial stages of heating in vacuum, without however causing the formation of grain boundaries, which would be detrimental to the optical properties. Such evidence has been verified in the case of the use of Nd in the YAG, in combination with a small amount of silica, typically used as a sintering aid.

Another preferred characteristic of the method object of the present invention concerns the drying by freeze-drying of the suspension, after the grinding / homogenization operation. Other powder drying systems (such as spray-drying) or other forming systems (such as slip casting) used as intermediate steps for obtaining the pre-sintered product can also benefit from the advantages of the sintering methodology identified, in order to accurately control the stoichiometric ratio between the oxides.

The method object of the present invention is, therefore, capable of producing transparent ceramics with high optical characteristics, the destination of which, in addition to that of the active elements in power lasers, can be found in the aeronautical sector, or as windows for high temperatures. , LEDs, scintillators, bulbs for lamps and also for aesthetic uses such as costume jewelery.

Claims (10)

CLAIMS 1. Method for making transparent rare earth-based ceramics, comprising a step of preparing a ceramic presintered by forming a mixture of oxide powders and a sintering step in which said ceramic presintered is subjected to a treatment thermal; said method being characterized in that said sintering step consists in subjecting the ceramic presintered product to a continuous heat treatment at a temperature between 1700 and 1800 ° C; said continuous heat treatment comprising a first sub-phase in which the ceramic pre-sintered is subjected to a pressure lower than or equal to 10 <-5> mbar for a time between 5 and 35 hours and a second and subsequent sub-phase in which the ceramic pre-sintered is It is subjected to a pressure between 0.5 and 100 bar for a time between 25 and 75 hours. 2. Method for the production of transparent rare earth based ceramics according to claim 1, characterized in that said first sub-phase lasts a time comprised between 10 and 20 hours. 3. Method for making transparent rare earth based ceramics according to claim 1 or 2, characterized in that the lowest fluxing rare earth oxide is present in the precursor powder in stoichiometric excess. 4. Method for making transparent rare earth-based ceramics according to one of the preceding claims, characterized in that the temperature of said heat treatment is reached with a ramp speed of between 300 and 2000 ° C / h, and a minimum vacuum of 1x10 <-4> mbar. 5. Method for producing transparent rare earth-based ceramics according to one of the preceding claims, characterized in that said step of preparing a precursor powder comprises in succession a mixing operation of oxide powders, a freeze-drying operation and a forming operation. 6. Method for the production of transparent rare earth based ceramics according to claim 5, characterized in that said forming operation is carried out by pressing and dewaxing or by the succession of slip casting, drying and dewaxing. 7. Method for making transparent rare earth-based ceramics according to claim 1, characterized in that it comprises a decolorization step in which the ceramic samples obtained from the sintering step are treated between 1300 and 1600 ° C in static air or fluent for between 1 and 10 hours. 8. Transparent rare earth-based ceramic characterized in that it has been made according to one of the preceding claims. 9. Rare earth-based transparent ceramic according to claim 8, characterized in that it is YAG, or YAG doped with Neodymium or other rare earths. 10. Active element for high power laser characterized in that it is a rare earth-based ceramic made with the method according to one of claims 1 to 7.