ES2204291B2 - Procedure for obtaining monocrystalline fibers of alpha-alumina doped with transition metals, such as fibers or ruby, sapphire and others with semi-precious characteristics - Google Patents

Abstract

The procedure is for obtaining monocrystalline fibers of alpha-alumina doped with transition metals, such as fibers of ruby, sapphire and others with semi-precious characteristics. Starting with crystalline fibers of alpha -alumina (corundum) obtained from aluminum and quartz, the chemical composition of which is modified with the incorporation of transition metals (Cr,Fe,Ti,V), the fibers acquire new properties in all ways - color, hardness and chemical resistance.

Description

Fiber obtaining procedure α-alumina monocrystallines doped with transition metals, such as ruby, sapphire and other fibers semi-precious features.

Fiber obtaining procedure α-alumina monocrystallines doped with transition metals, such as ruby, sapphire and other fibers semi-precious characteristics obtained from fibers α-alumina crystals (corundum), these last obtained from the reaction of aluminum and silica in controlled atmosphere ovens.

The chemical composition of crystalline fibers of α-alumina is modified with the addition of doping elements (Cr, Fe, Ti, V) to the system. In this way the resulting doped α-alumina fibers acquire new properties: especially shape, color, hardness, mechanical resistance, etc. Highlight in the procedure that the incorporation of doping elements occurs simultaneously to fiber growth.

The production of alumina fibers for use in multiphase materials, known as composites, He has been the subject of numerous studies in the area of materials advanced. Α-alumina or corundum fibers  (α-Al 2 O 3) are resistant to high temperatures (melting point 2040 ° C), have high modulus elastic and have great thermal and chemical stability (Das. Gopal, Thermal Stability of Single Crystal and Polycrystalline Fibers and 85% Al 2 O 3 - 15% SiO 2 Fibers, Ceramic Engineering & Science Proceedings, Sep. 01, Vol. 16, No. 5, pp. 977-991 (1995)).

As an example, when using fibers of α-Al 2 O 3 in combination with others materials are obtained composites with mechanical properties similar to those of steel, but only one third of its density. These advanced materials also have a great application potential in the aerospace industry (M. Touratier and A. Béakou, on the Mechanical Behavior of Aluminum Alloys reinforced by Long or Short Alumina fibers or SiC Whiskers. Laboratoire Génie de Production-ENIT, Composites Science and Technology vol. 44, pp. 369-383 (1992)) and (T. L. Dragone and W. D. Nix, Steady State and Transient Creep Properties of an Aluminum Alloy Reinforced with Alumina Fibers. Department of materials Science and Engineering, Stanford University, Stanford, Metall Act. Mater. Vol. 40, No. 10, pp. 2781-2791 (1992)).

In the present invention a procedure to obtain corundum fibers containing small quantities of transition metals, in a doping process that It gives the fibers new properties. The interest of these changes resides in that the variations of color, of hardness, of reactivity, so ... they can be very beneficial for Some applications of the fibers.

For example, the average length of a set fiber is important for the mechanical properties of composite, since the greater the length / diameter ratio, known as "aspect ratio" of a set of fibers, the greater the reinforcing effect achieved ("Ceramic Matrix Composites "K.K. Chawla, Ed. Chapman & Hall, (1993)), so control over features is always interesting morphological of the fibers produced.

The applications of these new fibers of doped corundum could be from the merely decorative, to materials for corrosion protection. For example the sheets thin alumina doped with metals have been used for multitude of applications ("CVD of Chromium-doped Alumina "Ruby" Thin Films ", Bradley D. Fahlman and Adrew R. Barron, Chem. Vap. Deposition, 7, No. 2, 62-66, (2001)).

Table 1 shows a summary of the varieties of crystalline aluminas known as semiprecious, with its usual name and the transition metal (or metals), with the corresponding valence, conferred by the new properties.

TABLE 1

Impurity Color Name Cr 3+ Red Ruby Fe 2+, 3 +, Ti 4+ blue Sapphire Cr 3+, V 3+ Green Emerald Oriental Cr 3+, Ti 4+, Fe 2+ Violet Amethyst Oriental Fe 3+ Yellow Topaz Oriental

We comment below some of the features that accompany color in gems or materials semi-precious

In general, semi-precious materials (whether natural or artificial gems) have special properties Optics: characteristic brightness, luminescence (by fluorescence or by phosphorescence), pleocroism, dispersion, refraction, birefringence, etc. Any of these properties can provide new applications for metal doped fibers of Transition.

For example, Ti produces an increase in mechanical resistance of crystalline surfaces. Doping with Cr 3+ gives rise to rubies that can be used in the manufacture of lasers at the wavelength of 694 nm. Rubies they also have applications in bombardment damage sensors of ions, in pressure sensors, in devices to detect fonons, ... In addition, it produces improvements in the interface metal-ceramic due to transfer phenomena loading

Get these materials with properties Special presents various difficulties. While in the systems Organic dyes are very abundant (more than 5000), in ceramic systems these elements are only about 30, included within transition metals and rare earths. In particular, in ceramic systems, red always results insufficient or unstable at high temperatures ("Chromium -based Ceramic Colors "F. Ren, S. Ishida, N. Takeuchi, and K. Fujiyoshi, Ceramic Bulletin, Vol 71, No. 5, 759-764 (1992)).

Transition metal ions produce various colors, depending mainly on the binding forces present in the crystalline network where these ions are distributed chromophores

For example, the increase in content in Chromium causes an increase in alumina network constants, causing color changes from purple to red, reaching Even to green. The introduction of tetravalent ions gets intensify the field that acts on the chromophore ion, observing the transition of the green hue, typical of Cr 3+ in a weak field, at the typical red hue for the same ion in a strong field ("Synthesis of new colored ceramic pigments red by the Pechini method ", R. Pavlov, V. Blasco, E. Cordoncillo, P. Escribano and J.B. Carda, Bol. Soc. Esp. Ceram. Glass, 39, [5], 609-616, (2000)). The ruby requires for its formation the presence of chromium. In color, in addition to the amount of Cr 3+, influences the presence of other elements dyes, such as iron, that darkens the tone enlarging it.

Sapphire, which contains Fe 2+, 3+ and Ti 4+, It also requires adequate crystallization conditions. In addition to the characteristic blue colors there are also Sapphires such as green and yellow (due to presence Fe 3+), purple and violet (with Fe 2+, Ti 4+ and Cr 3+), and roses (with small amounts of Cr 3+). In the Sapphires, the presence of Faith and You give a blue appearance and blue-green alumina ("Defect Clustering and Color in Fe, Ti: α-Al 2 O 3 ", Anthony R. Moon and Mathew R. Phillips, J. Am. Ceram. Soc. 77 [2] 356-67 (1994)).

The proposed invention to obtain fibers doped corundum monocrystalline is based on adapting a method for the production of corundum fibers, modifying it substantially so that during fiber growth the transition metals that constitute the doping Based on the crystals of α-Al 2 O 3, the addition of small amounts of these transition metals (primarily Cr, V, Ti y Fe) allows to obtain a great variety of species crystalline

Spanish patent ES2146506 describes a method of obtaining crystalline alumina fibers by deposition Solid Liquid Steam from silica and aluminum metal, ("Obtaining α-Al2 O3 fibers by VLS for use in composites ". V. Valcárcel. Thesis publication on CD-ROM. ISBN: 84-8121-750-6;" Novel ribbon shaped \ alpha-Al_ {2} 3 fibers ", V. Valcárcel, A. Pérez , M. Cyrklaff, and F. Guitián. Advanced Materials 10, No. 16, (1370-1373), 1998.; "Development of single crystal? -A_ {2} 3 fibers by vapor-liquid-solid deposition (VLS) from Aluminum and powdered silica "V. Valcárcel, A. Souto, and F. Guitián Advanced Materials, 10, No. 2, pp. 138-140, (1998)). Subsequently, in the patent P200100703 ("Method of obtaining crystalline fibers of α-alumina (corundum) from aluminum and silica (quartz), in controlled atmospheres containing metal gases" V. Valcárcel, et al ) describes a procedure to increase the production of fibers deposited by VLS.

In these patents a procedure is described which uses pieces of Al deposited on silica, inside of closed chamber furnaces, with controlled atmospheres. Be They use temperatures between 1300ºC and 1550ºC. Under those conditions, α-alumina fibers are produced monocrystalline, which appear on the aluminum after each thermal cycle.

These fibers grow following the process physicochemical called Steam- Liquid-Solid or also VLS. In this process, the gaseous species (Al 2 O, AlO, SiO, Al and Si) that will constitute the fiber material dissolves in a liquid drop, saturating it, and starting the precipitation of alumina.

The invention consists in carrying out the growth of the fibers in an atmosphere in which ions of the coexist transition metals with the gaseous species described in the previous paragraph. In this way these ions are deposited simultaneously with alumina during fiber growth,  slightly altering the link energies of the network of the alumina, and acting as chromophores agents.

Process

The procedure proposed herein invention describes several alternative systems for obtaining of corundum fibers doped with transition metals. For get the doping substances into the structure of the corundum is used a procedure for obtaining fibers of alumina contributing the doping elements to the oven atmosphere at the same time that the fibers are growing, and at a rate adequate that allows control over how and how much they are modified the properties of pure corundum fibers.

The essential problem solved is to provide methods that solve the contribution of transition metals suitable with the appropriate valence, at the same time as the fibers alumina are growing without altering the deposition mechanism vapor-liquid-solid (VLS). Almost all these metals are high melting point (see Table 2), for so much is a difficult task to evaporate them simply by temperature.

TABLE 2

Metals Melting point Point from ºC Boiling ºC Cr 1857 2672 Faith 1535 2750 You 1660 3287 V 1890 3380

Table 2 shows a selection of the metals used in this procedure. As we know, the temperatures for fiber growth following the process VLS range from 1300ºC to 1550ºC, which are lower than that appear in Table 2, and therefore it is difficult for these metals are in the atmosphere of the furnace used for the corundum fiber growth. In addition, metals do not They have the proper ionization state for doping.

To get doping with the ions of the Table 1 we propose:

1.- Use of precursors in the growth furnace of fibers

To obtain the doped fibers, uses a VLS deposition procedure in atmosphere furnaces controlled. This deposition is altered by introducing metals of transition in the oven atmosphere, so that the fibers grow with a doping that gives them special properties.

The first alternative is to generate the doping ions by placing a precursor material directly on the same oven where the fibers are growing. We understand by precursors those substances capable of providing the ions metallic necessary for doping during the growth of fibers In general, to achieve doping with a metal ion concrete may be used as a precursor to the metal itself but also an oxide or a salt containing it.

As a first example it is possible to use the metal oxides as these can be reduced to their state metallic by reaction with Al (g), given the great affinity that this element presents by oxygen, one of the largest of the periodic table. But it is also possible to use the salts since these can be separated into their constituent ions by dissolution, through high temperatures, through electric fields, etc. It is also possible to use other compounds, including Some organic

When the production of metal ions of transition from the chosen precursors requires the same conditions (atmosphere, temperature, etc.) than those existing in the oven interior, precursors can be placed directly in the vicinity of the raw materials they generate the fibers (aluminum and silica).

When these precursors are placed in the inside the fiber growth chamber, and the oven is heated to growing temperature, these precursors generate the necessary ions for doping the fibers of corundum

2.- Use of zones at different temperatures

Sometimes, to generate the metal ions the mentioned precursors need a different temperature than the temperature at which the fibers of α-alumina that are intended to dop (1300-1550 ° C). The first alternative that we propose is to use systems capable of providing different temperatures in different parts of the oven, using ovens where it is possible to establish different zones with temperatures different. The second alternative is to use a camera insulated where the precursor is heated to a temperature specific and then dragged into the chamber where the fibers of alumina are growing. For this transport, the most appropriate is employ a stream of argon. As a third alternative, it is possible perform a specific heating of the precursor using a heat focusing system, for example a laser.

3.- Use of ion implantation techniques

There are many applications for which they have developed techniques that achieve ion production Metallic through strong electric fields. All these techniques they are useful in the fiber doping process. From In this way, the necessary ions can be generated by techniques of  CVD, sputtering, etc. Also in this case it can be separated into two alternatives, considering separately the case that you are techniques are performed inside the oven where the fibers are produced, Or outside. In the latter case, the species generated must be dragged afterwards, for example through a stream of Argon, to the zone of fiber growth.

4.- By dissolution of the substances in a suitable medium

Many of the precursors described are soluble in aqueous, acidic or basic media (generally acidic). One time dissolved, ionic separation of the precursor compound occurs. If a gas stream is bubbled through the acid, then Argon example, this drags a few parts per million of the liquid, transporting metal ions to the chamber where they grow the fibers This liquid carryover can also be carried out. through a Venturi, an atomizing device that allows greater control over the amount of liquid that is introduced into the chamber from the oven.

We describe below as starting from a oxide, alumina fibers doped with metals of transition. The choice of the appropriate oxide results from the greatest importance for the formation of doped fibers because many of these oxides are also refractory, as described in Table 3, and therefore remain stable until high temperatures, unless they react with any of the species present in the oven, which are basically Al2O, A1O, SiO, Al and Si.

TABLE 3

Metals Melting point Point from ° C Boiling ° C Cr 2 O 3 2435 4000 Fe_ {O} {3} 1565 2750

The chemical reactions between species Sodas present in the oven and oxides can be very diverse. Without intending to make an exhaustive analysis of all possibilities, we describe them differently according to doping  alleged.

Considering the sapphires (which need iron in alumina structure), all reactions are relevant producing Fe (gas), or Fe (liq) which, by pressure of steam, will again result in Fe (gas). We will consider the following possibilities:

one

Considering the rubies (which need Cr), the possibilities are smaller, the most appropriate being the use of CrO_ {3}. The CrO 3 has a low melting point (196 ° C), and It also decomposes at 400ºC, losing oxygen and generating Cr + 3.

2

Keep in mind that any rust in the which metal is present with a valence lower than necessary will not provide the desired properties to alumina. However, if the oxide (like CrO_ {3}, in which we have Cr + 6) has a valence greater than necessary, it can be reduced by the Al present in the oven atmosphere, generating the Cr + 3, necessary for the formation of rubies. Confirming by Direct calculation through redox potentials.

       \ dotable {\ tabskip \ tabcolsep # \ hfil \ + # \ hfil \ + # \ hfil \ tabskip0ptplus1fil \ dddarstrut \ cr} {
 Cr 6+ + 3e - \ leftrightarrow Cr 3+ \ + E = 1.10V \ + en
acidic medium Cr 6+ + 3e - \ leftrightarrow Cr 3+ \ + E
= -0.12V \ + in basic medium \ cr Al 3+ + 3e -
 \ leftrightarrow Al \ + E =
-1.706V \ + \ cr}
    

In the case of the rest of the metals selected, some of the reactions are shown below involved, justifying the appearance of metal ions suitable in the oven atmosphere, by reaction of the oxides with gaseous species present in the growth chamber of fibers

3

Example 1

A mixture is arranged in an alumina crucible intimate finely ground silica powder, below 63 um, with 10% nickel powder with particle size also <63 \ mum. Small ones are deposited on this mixture 4 mm diameter aluminum wires, buried approximately up to half its thickness (Figure 1). Said Figure 1 shows schematically the assembly described below, to produce fibers doped with Cr.

The vessel type alumina crucible is introduced into a controlled atmosphere oven, which has an alumina chamber connected with seals by one of its ends to an argon gas source whose flow is maintained constant at 0.2 L / min during the rising temperature ramp. On the other end, a bubbler filled with water is placed that allows to ensure an Ar pressure greater than 1 atm inside from the oven chamber. The crucible is heated to a temperature of plateau of 1550ºC with a ramp of rise of 10ºC / min. So the gas flow is cut, keeping the inside of the oven tight and stable temperature for 3 hours. It subsequently cools to  10 ° C / min

At the same time a test tube is prepared with 2 g of CrO 3, placed in a bath at a temperature of 250 ° C and connected at its end to the tube that carries the gas to the oven. The CrO_ {3} is liquefied and, by Venturi effect, some molecules are dragged into the oven. Going through a temperature zone> 400 ° C, the CrO 3 decomposes. Subsequently, upon reaching the oven chamber where the fibers Cr 6+ reacts with the Al (gas) present causing Cr 3+ that enters the drops of the VLS growth. Following this procedure they appear in the crucible Generalized red ruby fibers (Figure 2).

Example 2

A mixture is arranged in an alumina crucible intimate finely ground silica powder, below 63 um, with 10% nickel, 10% Fe 2 O 3 and 10% TiO_ {2}, also <63 µm. On this mixture they are deposited small 4 mm diameter aluminum wires buried approximately to half its thickness.

Figure 3 shows a diagram of one of the mounts used for the production of sapphires. A crucible of ship type alumina is introduced into an oven of controlled atmosphere. This oven has an alumina chamber connected with hermetic seals by one of its ends to a argon gas source whose flow is kept constant at 0.2 L / min during the rising temperature ramp. On the other end is place a bubbler full of water that ensures a Ar pressure inside the oven chamber slightly greater than 1 atm. The crucible is heated to a temperature of plateau of 1550ºC with a ramp of rise of 10ºC / min. Then Cut off the gas flow by keeping the inside of the oven tight and stable temperature for 3 hours. It subsequently cools to 10 ° C / min After a thermal cycle, intense fibers appear Sapphire blue color in the crucible (Figure 4).

Example 3

Figure 5 represents a diagram of the assembly. In an alumina crucible is arranged an intimate mixture of silica finely ground (<63 µm) with 5% nickel powder with particle size also <63 µm. About this mixture deposit small aluminum wires 4 mm in diameter buried approximately halfway to its thickness. The crucible alumina type boat is introduced into a furnace controlled atmosphere. This oven has an alumina chamber connected with hermetic seals by one of its ends to a argon gas source whose flow is kept constant at 0.2 L / min during the rising temperature ramp. On the other end is place a bubbler full of water. The crucible is heated until a plateau temperature of 1550ºC with a rise ramp of 10 ° C / min. Upon reaching said plateau temperature, the gas flow keeping the inside of the oven tight and stable temperature for 3 hours. It subsequently cools to 10 ° C / min.

At the same time, Cr acetyl acetonate is placed (Cr (AcAc) 3), or a stoichiometric mixture of Cr (AcAc) 3 with Al (AcAc) 3, in the inside of a chemical vapor deposition chamber (CVD). This chamber is located in the path of the gas stream incoming into the alumina oven so that the ions generated by CVD are dragged into the oven. In this way the Ar is used both for the CVD process and for the transport of ions to the oven

At atmospheric pressure, it has been found that the Optimum temperature is about 195ºC. In the case of using the Mixture of Cr (AcAc) 3 with At (AcAc) 3 this temperature means that the Cr (AcAc) 3 is dissolved in Al (AcAc) 3 liquid. Following this procedure, Ruby fibers appear widely in the crucible.

Claims (9)

1. Procedure for obtaining monocrystalline α-alumina fibers doped with transition metals, such as ruby, sapphire and other semi-precious fibers characterized by obtaining pure α-alumina fibers, by means of a VLS (Steam) mechanism -Liquid-Solid) generated by reaction of silica powder with aluminum metal in an oven, with the incorporation of transitional metallic species in the atmosphere of said furnace. 2. Process for obtaining doped α-alumina monocrystalline fibers according to claim 1, characterized in that when the incorporated transition metal species is Cr 3+, ruby fibers are obtained. 3. Process for obtaining doped α-alumina monocrystalline fibers according to claim 1, characterized in that when the transition metal species incorporated are Fe 2+, 3+ and Ti 4+, fibers are obtained Sapphire 4. Process for obtaining doped α-alumina monocrystalline fibers according to claim 1, characterized in that when the incorporated transition metal species are suitable mixtures of Cr 3+, V 3+, Fe 2 +, 3 +, Ti 4+, fibers of semi-precious crystalline varieties are obtained. 5. Process for obtaining doped α-alumina monocrystalline fibers according to the preceding claims, characterized in that the metallic transition species in the atmosphere can be generated from metals, their oxides, salts, or organic compounds containing them. . 6. Method for obtaining doped α-alumina monocrystalline fibers according to claim 5, characterized in that the atmospheres with transition metals are generated from the substances, described in the preceding claim, by dissolution in a liquid, acidic medium or basic, this liquid being transported to the oven by bubbling a gas through it, by evaporation, or by atomization of said liquid and subsequent entrainment. 7. Process for obtaining doped α-alumina monocrystalline fibers according to claim 5, characterized in that the atmospheres with transition metals are generated from the substances described in claim 5 by reaction with the gaseous species existing during growth VLS of α-alumina fibers. 8. Method for obtaining doped α-alumina monocrystalline fibers according to claim 5, characterized in that the atmospheres with transition metals are generated from the substances described in claim 5 by reaction at high temperature decomposition. 9. Process for obtaining doped α-alumina monocrystalline fibers according to claim 5, characterized in that the atmospheres with transition metals are generated from the substances described in claim 5 by CVD, vapor deposition, or by ion implantation techniques.