WO2015181066A1

WO2015181066A1 - Transparent spinel ceramics and methods for the production thereof

Info

Publication number: WO2015181066A1
Application number: PCT/EP2015/061369
Authority: WO
Inventors: Thomas Hutzler; Andreas Krell
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2014-05-27
Filing date: 2015-05-22
Publication date: 2015-12-03
Also published as: KR20170013317A; CN106414367A; KR102066530B1; CN106414367B

Abstract

The invention concerns the field of technical ceramics and relates to transparent spinel ceramics, which can be used, for example, as protective ceramics, and to methods for the production thereof. The problem addressed by the invention is to specify transparent spinel ceramics that contain no visible defects, to the extent possible, and that have an in-line transmittance of more than 82% in wavelength ranges of infrared light, measured between 1000 and 2500 nm, for specimen thicknesses of > 3 mm. This problem is solved by means of spinel ceramics that are transparent in wavelength ranges of infrared light, composed of sintered magnesium aluminum spinel having an average microstructure grain size of < 10 µm and having in total at most 0.5 mass percent of homogeneously distributed additives of calcium and/or strontium and/or barium, which are present at a concentration, in each case expressed as an oxide, of 0.005 to < 0.2 mass percent CaO and/or 0.005 to < 0.5 mass percent SrO and/or 0.005 to < 0.5 mass percent BaO.

Description

Transparent spinel ceramics and process for their preparation

The invention relates to the field of technical ceramics and relates to transparent spinel ceramics, as used, for example, for applications with increased mechanical stress, for. As a protective ceramic, or with increased optical quality requirements, e.g. for optical devices, and methods of making the same.

There has long been a desire for a transparent ceramic, which combines the advantageous properties of a ceramic, for example, in terms of hardness and strength, with the greatest possible transparency. So far, however, this has not succeeded to the desired extent despite prolonged and intensive efforts of the experts. In addition, the results achieved are often not comparable due to very different definitions of the desired criteria, so that the actual level of knowledge on transparent ceramics is at least disorderly and unclear.

Accordingly, terms will be explained and defined below as they apply within the scope of this invention and also with reference to the cited prior art. The terms "transparent" and "translucent" are not clearly delineated in the prior art. For this reason, transparent materials are to be understood as meaning those which exhibit an in-line transmission of more than 50% at thicknesses of> 1 mm and visible wavelengths around 600 nm.

For the assessment of the clear transparency (transparency) of materials, the "true" in-line transmission (RIT) to be measured for the exclusion of scattered light from the detected intensity with a narrow aperture angle of approx for example, in EP 1053983 A (Yamamoto et al.) or by Apetz et al., J. Am. Ceram., Soc., 86 (2003) 480, the size responsible: the difference between RIT and that with conventional spectrometers, according to Apetz an in-line transmission is inherently particularly large at shorter (leading to higher stray losses) wavelengths, as well as over the entire visible range, while this metrological distinction in the infrared range does not matter.

In the case of absent or extremely low light absorption, the transmission for optically homogeneous materials, such as glass or for single crystals, only of the material-specific, the refractive index n specific reflection R _s = ((n-1) / (n + 1)) ² limited to each front and back. The resulting theoretical maximum value of the transmission T _max is for highly transparent substances with consideration of the multiple reflection T _max = (-R) with R = 2R _S / (1 + Rs) or T _max = (1 -R _S ) ² for materials of low transparency that is, negligible multiple reflection; T _max is about 86.9% for MgO-Al ₂ O ₃ spinels whose refractive index in the visible range is 1.712 to 1.736.

As a particular advantage of very high RIT for a flexible, adapted to the respective applications design of the transparent products, it should be noted that when approaching the theoretical maximum T _max the influence of thickness fades, while conversely, any larger scattering losses naturally increase with the thickness of the light-scattering material and the transparency is then guaranteed only for very thin components.

Thus, such a thickness influence is a criterion that considerable light scattering - ie low transparency - is present and possibly high Transmittance values go back to a too large, the measurement of the true in-line transmission impossible aperture angle.

Apart from possible absorption, RIT is reduced in the passage of light through the microstructure of sintered polycrystalline ceramics mostly by the following processes compared to T _max :

1 . diffuse scattering of pores (depending on size and number of pores), and

2. especially in non-cubic ceramics, such as corundum (a-Al ₂ O ₃ ), additional light scattering by birefringence at each transition of the light beam from one to the next crystallite of the microstructure.

The scattering losses must therefore be kept low in all sintered ceramics by the lowest possible residual porosity of <0.1%, preferably <0.01%, and by pore sizes that are as small as possible, as the wavelength of the light.

Since only the former scattering mechanism occurs in cubic sintered ceramics, their transparency, unlike ceramics with birefringent (non-cubic) crystallites, is not subject to any direct grain size influence. This situation, which differs depending on the crystal structure, makes it understandable why cube-type crystal lattice ceramics are preferred for transparent applications and harder or even stronger non-cubic ceramics, such as only translucent sintered-corundum or tetragonal ZrO 2 ceramics with larger thicknesses, play only a minor role play: the most advanced, ie the finest-grained and porenärmsten sintered corundum (trigonal) have so far at thicknesses> 1 mm RIT> 70%, with tetragonal ZrO2 ceramics have even lower transparency, while for example MgO-Al ₂ O3 spinel (cubic ) repeated measurements between 80 and 85%, even at higher thicknesses and thus closer to the above theoretical maximum, were given.

However, most prior art cubic transparent ceramics known in the art tend to show lower levels of hardness, scratch resistance, and strength. Improvements of these mechanical properties are only possible by way of reduced grain size of the sintered structure. An example of this is known from US Pat. No. 7,247,589 A (Krell et al.), In which developments of this type have hitherto been limited by the fact that such finely crystalline sintered structures have medium particle sizes Grain sizes <10 μηη or even <1 μηη are feasible only at low temperatures. Below certain sintering temperatures, however, this comes into conflict with the above-mentioned requirement for minimum residual porosity or high relative sintering densities> 99.9% or preferably> 99.99% and then leads to fine-grained microstructure but with reduced transparency or even visible defects, often in the form of incompletely sintered subregions of the microstructure.

In addition to various efforts to optimize (also pressure-assisted) sintering regime, such. EP 0332393 A (Shibata et al.) has also attempted to solve the problem of dense sintering of transparent spinel ceramics by the use of finer and finer nanopowders (J. Zhang et al., J. Phys. D: Appl. Phys. 42 (2009) 5, 2002-2006) or by changing the stoichiometry (EP 1873129 A, Sasame et al.). However, the optical quality has remained unsatisfactory so far especially in the field of visible light.

Special attention has therefore been devoted for a long time to the optimization of sintering-promoting and as far as possible also temperature-reducing and, in this way, grain-growth-reducing doping. Most well-known for the preparation of translucent spinel ceramics are the world-wide experiments with liquid-phase-forming dopants, in an early example with 0.25 mass% CaO (R.J. Bratton, J. Ceram Soc 57 (1974) 7, 283-286). The production of a translucent spinel ceramic with in-line transmission <40% for 0.37-0.95mm thin samples in the visible range is described. Later developments intensified the effect of liquid-phase sintering, in particular with LiF doping (US 201 1/0028303 A. Villalobos et al.), Which, however, typically leads to the formation of visible defects (Gilde et al., J. Am. Ceram ) 10, 2747). Presumably these defects can be minimized in the context of such liquid phase sintering, but can not be avoided.

This is the background of increasing efforts for doping, which are to perform for transparent spinel as possible in the context of pure solid phase sintering or minimizing the occurrence of liquid phases similar, which causes the well-known for decades MgO doping in the sintering of Al ₂ O ₃ ceramic: on the one hand a promotion diffusion and thereby dense sintering atomic action of a solid solution of parts of the doping in the lattice of the main phase of the ceramic, on the other hand, but also a reduction of their grain growth by second phase particles in the grain boundaries, as described by Peelen (JGJ Peelen, Dissertation, TH Eindhoven, 1977). In view of the combination of high transparency with the finest possible graininess of the microstructure as a prerequisite for improved mechanical properties, the following examples should be mentioned here in particular.

K. Tsukuma, J. Ceram. Soc. Japan 1 14 (2006) 10, 802-806 achieved a significant reduction in sintering temperature with a doping of 0.015-0.150 mass% B2O3, but gives transmittance values of 83% at 600-650 nm wavelength for only 1 mm thin samples so that only a limited transparency improvement is to be expected. Statements on the frequency and size of remaining visible defects are not made, however, such defects are to be suspected, if the lowering of the sintering temperature similar to the case of the above LiF doping based on the suspected formation of B ₂ O 3 addition of local liquid phases.

According to EP 21 12127 A1 (A. Ikesue), very high transmission values close to the theoretical limit for 10 mm sintered spinel specimens are known which are obtained by hot isostatic pressing (HIP) at high temperature (1780 ° C.) with addition of MgF ₂ / AIF ₃ codoping were generated. However, the real cause of the achieved transparency is unclear: despite years of effort, the authors have failed to reproduce the results disclosed in the patent application. Also, no successful adoption of this doping by other authors has become known.

According to FR 0334760 B1 (P. Bergez et al.) An addition of 0.5 -5 wt .-% MgO is known, which is to be added as possible in the course of the synthesis of spinel powder and does not represent actual sintering doping in the above sense, as the MgO excess should only compensate for the known effect of evaporating MgO from the spinel lattice at high sintering temperatures. However, numerous publications are known which, even without such an excess of MgO, achieve a similar transparency of about 80% at 600-650 nm wavelength for 3 mm thick samples, as in FR 0334760 B1.

A particularly broad range of dopant additives to spinel is from Sarkar et al., Ceram. Int. 29 (2003) 1, 55-59, but without reference to transparent ceramic qualities. Additions of T1O2, V ₂ O _5, Cr ₂ O 3 and B2O3 were described in their effect on the reactive sintering of MgO / Al ₂ O 3 mixtures, with only ΤΊΟ2 showed a positive effect and the effect on the sintering vorkalzinierter spinel remains unknown. The literature review cited in this paper also addresses Y2O3 and other rare earth dopants as well as MnO2, but also without reference to the preparation of transparent ceramics.

While on the one hand TiO2 dopants to sinter spinels were discussed again and again, the sole effect of ΤΊΟ2 for satisfactory transparency seems inadequate. For example, according to FR 2917404 B1 (Bernard-Granger et al.), Co-doping of T1O2 with at least one further additive to be selected from the group of ZrO2, CaO and MgO is known. This doping should then universally improve the transparency of very different ceramics of MgAl ₂ O ₄ spinel, cubic ZrO 2 or also Y-Al-garnet (Y ₃ Al ₅ O 12). This universal effectiveness could indicate the formation of a liquid phase, which would, however, be associated with a risk for the above-mentioned disadvantages of the liquid-phase sintering of transparent ceramics. The spinel embodiment calls for a fairly high in-line transmission of 84.8%, but for a sample that is only 1.3 mm thin, so that here, too, a rather limited transparency improvement can be assumed. Statements on the frequency and size of remaining visible defects are not made here either.

A similar attempt at co-doping to improve dense sintering has been made according to Tsai, et al., Mater. Be. Closely. B177 (2012) 13, 1 133-1 137, by combining T1O2 and C0CO3, but without showing a clear advantage of this doping.

The effect of ZrO 2 dopants on the reactive formation of spinel from MgO / Al ₂ O 3 mixtures and on the densities of sintering was reported by J. Kim, Ph.D., without reference to light transmission. Dissertation, Case Western Reserve University, 1992. Thereafter, ZrO 2 dopants reduce the sintering rate of the spinel and, in the case of Al-rich compositions, promote the formation of Al 2 O 3 precipitates. Both are contrary to the goal of improved in-line transmission and the minimization of visible defects.

One of the earliest doping studies for translucent spinel ceramics was reported by RJ Bratton, J. Am. Ceram. Soc. 57 (1974) 7, 283-286 with 0.25% CaO. Addition carried out, but could achieve no transmission> 40% in the visible wavelength range, the material remained translucent. Later, Huang et al., J. Am. Ceram. Soc. 80 (1997) 12, 3237-3241, the effect of CaCO ₃ on the dense sintering of spinel as a co-doping to LiF, but without the objective of transparency. Our own experiments with> 0.2 mass% CaO doping showed that in this way the sintering temperature can be lowered and quite fine-grained transparent spinel microstructures can be formed, but at the expense of a true in-line transmission RIT and reduced by several percent with the formation of visible defects in the volume of the transparent ceramic. The partially dendritic structure of these defects suggests that even such CaO dopants achieve their temperature-lowering effect by forming local liquid phases - in conflict with the necessary single-phase defect-free transparent ceramics, in which excretions of foreign phases with different refractive indices are only tolerated if their size is small compared to the shorter wavelengths of the light spectrum.

From the alkaline earth group of the Periodic Table of the Elements, so-called "BAM" phosphors of the formula BaMgAli ₀ Oi _{7 are} known, which are usually doped with europium or strontium (US Pat. No. 561 1959 A). The existence of such compounds says nothing about a possible effect of Ba, Sr or Eu as a sintering additive to Mg-Al spinel ceramics. And if, according to DE 10 2012 220 257 A1, a transparent spinel ceramic is known which may contain, inter alia, one of the oxides of europium, barium or combinations thereof or of 10 other elements listed without justification, the required transparency, however, results in grain growth of> 10 μιτι is bound and also for any of the dopants methods, effects or results are given, this publication also discloses no teaching for the production of a fine crystalline spinel ceramic high transparency.

The lack of orientation of the developers in the search for sinter-promoting doping also illustrates US 7799267 B2, where there for the special technology of "Tape Casting Slurry Materials for Manufacture of Transparent Ceramic Materials" the production of transparent spinel ceramics, as well as transparent Y-Al garnet , Ce2O3, Y2O3, SC2O3 and LU2O3 exclusively by means of TEOS additive (tetraethyl orthosilicate, also tetraethoxysilane) is described in On the one hand, however, it is noted that, as "sintering aids", "sintering aids" can potentially also be used as "sintering aids", such as oxides of titanium, oxides of zirconium, barium oxides, calcium oxides, magnesium oxides, Strontium oxide, boron oxide and mixture thereof ", which could actually be anything - while facing this potential breadth of possibilities on the other hand are real sintering temperatures, those at 1700 ° C in the spinel example (without reported transparency) and even higher temperatures for other ceramics are discouragingly high, exclude the production of finely crystalline microstructures and still allow only limited transparency: Presumably due to the high contents of organic additives required for the film casting described in US 7799267 B2 81.20 - 81 .27% for a 2.7mm as the only transmission result thick YAG ceramic indicated at 600-650 nm wavelength.

Although the literature thus describes sintering-promoting dopants for spinel ceramics from almost all groups of the periodic table, it has not yet been possible to identify which doped ions promote the achievement of high transparency while largely avoiding visible defects, and this at the lowest possible sintering temperature for realizing fine-grained microstructures with high mechanical parameters. Also, the literature for magnesium-aluminum spinel lacks similar comprehensive data e.g. to the amount of solid solubility of individual elements of the periodic table as a function of temperature and atmospheres, as has been known for AI2O3 for decades.

In view of the stated limited previous achievements of the prior art, it is an object of the present invention to provide transparent spinel ceramics which contain no visible defects or, in the case of defects with sizes> 20 μιτι, these highest at a low frequency <300 / cm ³ , and with sample thicknesses> 3 mm in wavelength ranges of infrared light, measured between 1000 and 2500 nm, have an in-line transmission of more than 82%.

Another object of the present invention is to provide transparent spinel ceramics that do not have visible defects or, in the case of Defects with sizes> 20 μιτι contain these highest in low frequency <300 / cm ³ , and at specimen thicknesses> 3 mm visible in wavelength ranges of light, measured between 600 and 650nm, a true ("real") in-line transmission RIT> 80%.

And another object of the present invention is to provide methods for producing such transparent spinel ceramics at as low sintering temperatures as possible.

The objects are achieved by the invention specified in the claims. Advantageous embodiments are the subject of the dependent claims.

The spinel ceramics according to the invention, which are transparent in wavelength ranges infrared light, consist of sintered magnesium-aluminum spinel having an average grain size of <10 μιτι and with a maximum of 0.5 Ma .-% of homogeneously distributed additives of calcium and / or strontium and or barium, in a concentration, in each case expressed as oxide, of 0.005 to <0.2% by mass of CaO and / or 0.005 to <0.5% by mass of SrO and / or 0.005 to <0.5 Ma .-% BaO are present.

The spinel ceramics according to the invention, which are transparent in wavelength ranges of visible light, also consist of sintered magnesium-aluminum spinel with an average grain size of <10 μm and with a maximum of 0.3% by weight of homogeneously distributed additives of calcium and / or strontium and / or barium, in a concentration, in each case expressed as oxide, of 0.005 to <0.2 mass% CaO and / or 0.005 to <0.3 mass% SrO and / or 0.005 to <0.25 Ma .-% BaO are present.

Advantageously, the average grain size of the sintered magnesium-aluminum spinel <5 μιτι, advantageously <2.5 μιτι, more preferably <1 μιτι.

Also advantageously, for spinel ceramics which are transparent in wavelength ranges of infrared light, a maximum of 0.3 mass% of homogeneously distributed additives is present. Likewise advantageously, for spinel ceramics which are transparent in wavelength ranges of visible light, a maximum of 0.2% by mass of homogeneously distributed additives is present.

Further advantageously, as an additive, calcium is present in a concentration expressed as CaO of from 0.01 to 0.1 mass%.

Also, advantageously, for spinel ceramics that are transparent in wavelength ranges of infrared light, as additives, strontium and / or barium in a concentration, expressed as SrO and / or BaO, of 0.01 to 0.4 mass% of SrO and / or 0.01 to 0.4% by mass of BaO, with a maximum of 0.3% by mass of additions of strontium and / or barium being present.

It is also advantageous for spinel ceramics, which are transparent in wavelength ranges of visible light, as additives strontium and / or barium in a concentration, expressed as SrO and / or BaO, from 0.01 to 0.2 mass% of SrO and / or 0.01 to 0.15 wt .-% of BaO are present, with a maximum of 0.2 wt .-% of additions of strontium and / or barium are present.

The spinel ceramics according to the invention, which are transparent in wavelength ranges of infrared light, show at thicknesses of> 3 mm at least in the range of infrared wavelengths, measured between 1000 and 2500 nm, an in-line transmission of> 82%, advantageously> 84%, more advantageously > 85%.

The spinel ceramics according to the invention, which are transparent in wavelength ranges of visible light, show at thicknesses of> 3 mm, at least in the range of visible wavelengths, measured between 600 and 650 nm, an RIT of> 80%, advantageously> 82%, more preferably> 84%.

It is advantageous if the spinel ceramics according to the invention contain no visible defects. It is also advantageous if the spinel ceramics according to the invention contain visible defects greater than 20 μm only with a frequency of <300 / cm ³ , advantageously with a frequency of 50 to 150 / cm ³ .

And it is also advantageous if Vickers hardness HV10 of> 12.5 GPa are available.

In the method according to the invention for producing transparent spinel ceramics which are transparent in wavelength ranges of infrared light, the addition of a maximum of 0.5% by weight of additives of calcium and / or strontium and / or barium is in a concentration, in each case expressed as oxide , from 0.005 to <0.2 mass% CaO and / or 0.005 to <0.5 mass% SrO and / or 0.005 to <0.5 mass% BaO in the form of undissolved Ca, Sr and / or Ba-containing compounds and / or Ca, Sr and / or Ba-containing compounds dissolved in water and / or Ca, Sr and / or Ba-containing compounds dissolved in organic solvents to give magnesium-aluminum spinel Powder realized, the additives are distributed homogeneously and then sintered the materials to a transparent spinel ceramic.

In the process according to the invention for producing transparent spinel ceramics, which are transparent in wavelength ranges of visible light, the addition of a maximum of 0.3% by weight of additives of calcium and / or strontium and / or barium is in a concentration, in each case expressed as oxide , from 0.005 to <0.2 mass% CaO and / or 0.005 to <0.3 mass% SrO and / or 0.005 to <0.25 mass% BaO in the form of undissolved Ca, Sr and / or Ba-containing compounds and / or Ca, Sr and / or Ba-containing compounds dissolved in water and / or Ca, Sr and / or Ba-containing compounds dissolved in organic solvents to give magnesium-aluminum spinel Powder realized, the additives are distributed homogeneously and then sintered the materials to a transparent spinel ceramic.

Advantageously, the addition of the additives during the slip or offset treatment of the ceramic, or as part of an impregnation or infiltration of the additives in open pores of a dry ceramic body, or by ion exchange between wet magnesium-aluminum spinel bodies and inventive Ca, Sr and / or Ba-containing solutions realized.

With the solution according to the invention, it is possible for the first time to specify transparent spinel ceramics having mean particle sizes <10 μm, which contain no visible defects or, in the case of defects with sizes> 20 μm, these at most in low abundance <300 / cm ³ , and those in sample thicknesses> 3 mm in wavelength ranges of infrared light, measured between 1000 and 2500 nm, have an in-line transmission of more than 82%.

This is achieved by transparent in wavelength ranges infrared light spinel ceramics consisting of sintered magnesium-aluminum spinel with an average grain size of <10 μιτι and with a maximum of 0.5 Ma .-% of homogeneously distributed additives of calcium and / or strontium and / or barium, in a concentration, in each case expressed as oxide, of 0.005 to <0.2% by mass of CaO and / or 0.005 to <0.5% by mass of SrO and / or 0.005 to <0.5 Ma. -% BaO exist.

Furthermore, it is possible for the first time by the inventive solution to provide transparent spinel ceramics with average grain sizes <10 μιτι that contain no visible defects or defects in the case of defects> 20 μιτι these highest in low frequency <300 / cm ³ , and the sample thicknesses > 3 mm in wavelength ranges of visible light, measured between 600 and 650 nm, have a true ("real") in-line transmission RIT> 80%.

This is achieved by light visible in wavelength ranges transparent spinel ceramics, the sintered magnesium-aluminum spinel with an average grain size of <10 μιτι and with a maximum of 0.3 wt .-% of homogeneously distributed additives of calcium and / or strontium and / or barium, in a concentration, in each case expressed as oxide, of 0.005 to <0.2 wt.% CaO and / or 0.005 to <0.3 wt.% SrO and / or 0.005 to <0.25 Ma. -% BaO exist. And likewise, the solution according to the invention makes it possible for the first time to specify a method for producing such transparent spinel ceramics at the lowest possible sintering temperatures.

This, in turn, is achieved for spinel ceramics which are transparent in wavelength ranges of infrared light, by a total of at most 0.5% by weight of additives of calcium and / or strontium and / or barium in a concentration, in each case expressed as oxide, of 0.005 to <0.2 Ma .-% CaO and / or 0.005 to <0.5 Ma .-% SrO and / or 0.005 to <0.5 Ma .-% BaO in the form of undissolved Ca, Sr and / or Ba-containing compounds and / or in water-dissolved Ca, Sr and / or Ba-containing compounds and / or dissolved in organic solvents, Ca, Sr and / or Ba-containing compounds are added to magnesium-aluminum spinel powder, the additives homogeneously distributed and then sintering the materials into a transparent spinel ceramic.

For visible in wavelength ranges light transparent spinel ceramics, the production of such transparent spinel ceramics at the lowest possible sintering temperatures is achieved by a total of at most 0.3 wt .-% of additives of calcium and / or strontium and / or barium in a concentration, each expressed as oxide, from 0.005 to <0.2 mass% CaO and / or 0.005 to <0.3 mass% SrO and / or 0.005 to <0.25 mass% BaO in the form of undissolved Ca, Sr and / or or Ba-containing compounds and / or water-dissolved Ca, Sr and / or Ba-containing compounds and / or dissolved in organic solvents, Ca, Sr and / or Ba-containing compounds to magnesium-aluminum spinel powder are added, the additives are homogeneously distributed and then the materials are sintered to a transparent spinel ceramic.

The additives have different upper limits of the concentrations to be used, which are due to the different atomic weights, so that similar molar concentrations are already available with lower mass addition of e.g. Ba can be achieved as in the case of Sr or Ca.

Slightly different upper limits of the additives to be used are also to be considered if the spinel ceramics are intended only for infrared use or In addition, they should also have high transparency in the area of visible light.

The selective addition of the additives is particularly important according to the invention, since the elements Ba, Sr and / or Ca can already be present in the form of impurities in the starting materials for magnesium-aluminum spinel powder or in the commercial spinel powder to be sintered. In this case, however, the added elements for the sintering of the magnesium-aluminum spinel can not be effective because these elements are already incorporated in the crystal lattice of the starting materials and thus unreactive.

Only the actual targeted addition of the proportions of Ba, Sr and / or Ca leads to the inventive transparent spinel ceramics.

Under magnesium-aluminum spinel, referred to in this invention as spinel shall continue to be understood in the context of this invention, any composition of the type MgO Xai ₂ O3, with x values that are within the homogeneity range of the known phase diagrams. In addition, the sintered spinel ceramics according to the invention contain additives and / or dopants and may also contain small precipitates of MgO or Al ₂ O ₃ fractions not dissolved in the spinel lattice or impurities dissolved or not dissolved in the spinel lattice, as long as their content is so low. that the light scattering or absorption caused thereby does not prevent the transparency to be achieved according to the invention.

For the purposes of the present invention, sintering is to be understood as any densification of powder-produced bodies determined by diffusion processes without or with the application of pressure and without restriction to specific atmospheres or vacuum.

Infrared wavelength range according to the invention, a wavelength range in the lower to medium infrared wavelength range, such as between about 800 to 6000 nm to be understood.

Under visible wavelength range according to the invention, a wavelength range of about 350 to 750 nm to be understood.

By homogeneous distribution of additives in the spinel ceramic is to be understood that by the uniform distribution of the additives in the spinel ceramics an equality of the chemical composition and, consequently, the physical properties over the entire spatial extent of the spinel ceramic is realized.

In principle, several of the additives according to the invention of calcium, strontium and barium may also be present together in the spinel ceramics. However, in order to avoid or minimize to a minimum transmission-reducing foreign phase precipitations and the formation of visible defects in the volume of the spinel ceramics according to the invention in wavelength ranges of infrared light, an upper limit of the sum of all additives of a total of 0.5 mass%, advantageously of 0, 3% by mass, to be observed. In order to avoid as far as possible or to minimize transparent spinel ceramics in the volume of visible spinel ceramics according to the invention which are visible in wavelength ranges, an upper limit of the sum of all additives of a total of 0.3 mass%, advantageously of 0, in order to minimize transmission-reducing foreign phase precipitates and the formation of visible defects. 2% by mass, to be observed.

For physical reasons, the spinel ceramics which are transparent according to the invention in wavelength ranges of visible light also fulfill the transparency criteria of the spinel ceramics which are transparent according to the invention in wavelength ranges of infrared light. On the other hand, transparent spinel ceramics that are transparent in wavelength ranges of infrared light can only meet the transparency criteria of the spinel ceramics that are transparent in wavelength ranges of visible light, provided that the criteria of proportions of additives for the spinel ceramics that are transparent in wavelength ranges of visible light are met.

For the purpose of avoiding or minimizing any liquid phase formations, the spinel ceramics according to the invention advantageously contain no further additives or dopings of elements outside the group of alkaline earth elements. Naturally, raw material and processing-related impurities in the extent of up to about 0.1% by mass can often not be avoided and can be present in the spinel ceramics according to the invention. If, for example, moldings doped in this way were produced from spinel powders with about 30 m ² / g of specific surface, by means of additives according to the invention based on Sr or Ba compounds with unchanged processing of the powder, pressureless sintering up to relative densities of 96-98%. the theoretical density with closed porosity and subsequent hot isostatic final compression (HIP) the sintering temperature only insignificantly changed compared to a spinel ceramic according to the prior art, for a <0.2 Ma .-% CaO doped spinel ceramic according to the invention by about 50 - 100 ° C lowered. Although with a Sr or Ba-containing addition of the spinel ceramic according to the invention, the necessary sintering temperature may even be slightly increased (about 1525 ° C. compared to 1510 ° C. in the case of undoped spinel ceramic), at least two surprising effects are also present with these additives: the average texture grain sizes all inventively doped spinel ceramics (determined as the 1, 56 times the average chord length of conventional line intersection analysis) are typically <10 μιτι, preferably between 0.3 and 1, μιτι μιτι, often in the narrower range of 0.4 to 0.7 μιτι. Together with the almost complete densification of the transparent spinel ceramic, these finely crystalline microstructures lead to high hardnesses, measured for example as Vickers hardness HV10 (at 10 kg or 98 N test load of the indenter) between 12.5 and 15 GPa, often around 14 to 14, 5 GPa, lie. Although similar mean grain sizes and hardnesses can be achieved by means of the Bratton 1974 known higher Ca doping, but then accompanied by the above lower RIT values and a whitish, caused by light scattering turbidity thicker components (eg 4 mm thickness is after DIN5036-1 certain turbidity of such ceramics ia> 3%). The dopants according to the invention avoid this disadvantage and, given a thickness of polished samples> 3 mm for wavelengths in the range between 600 and 650 nm, combine true in-line transmission RIT> 80% with a particularly low content of visible defects> 20 μιτι of <300 / cm ³ . These are predominantly smaller incompletely sintered microstructures. Depending on the type and level of doping their frequency is often in the range of 50-150 / cm ³ , but in any case below 300 / cm ³ , compared to a much higher frequency in undoped spinel ceramics according to the prior art from the same raw material at analog Processing. Although the visibility of defects also depends on the respective analysis method used for detection, according to the invention visible defects should always have dimensions greater than 20 μm.

Higher than the inventive concentrations of additives are to be avoided because they lead to light-scattering precipitates of foreign phases in the spinel, which then reduce the in-line transmission and may also form visible defects. On the other hand, concentrations of additives above the lower limit of the invention are required for a significant effect of the additives.

The additives according to the invention can be added to the spinel powder both as oxides CaO, SrO and / or BaO, as well as in the form of other Ca, Sr and / or Ba-containing compounds, ie also as carbonates, nitrates or else. It is important in each case, on the one hand, compliance with the concentrations of the invention expressed as oxide, as well as a good spatial homogenization of the additives with the spinel powder. This homogenization can be carried out, for example, by a common, particularly thorough dispersion milling of the spinel powder with the addition in a ball mill, as has been known for decades, inter alia, for the homogenization of MgO dopants in Al ₂ O ₃ ceramics. On the one hand, this homogenization step ensures the uniform presence and sintering effect of the doping at the microscopic level throughout the sintered body and, on the other hand, prevents the formation of light-scattering foreign phase precipitates, such as might be the result of local agglomeration of the additives.

The process conditions for the preparation of the transparent spinel ceramics according to the invention are the process steps known per se for the production of spinel ceramics. They include the dispersion and homogenization of the starting materials in an aqueous suspension or in a suspension produced based on organic solvents, for example by grinding, drying of the suspension, for example by freeze-drying or spray granulation and the subsequent shaping, for example by pressing. Furthermore, it includes the subsequent heat treatment, for example, consisting of debindering at temperatures <1000 ° C and sintering in an oven and additional hot isostatic pressing or hot pressing depending on the starting powder used in each case at temperatures between 1200 ° C and 1850 ° C. The addition of the additives to Ca, Sr and / or Ba compounds takes place according to the invention in the first process step of the dispersion and homogenization of the starting materials.

The particular advantage of the solution according to the invention over the prior art is that concrete details of additives with regard to type and amount and in particular with regard to the total amount are made in connection with an upper limit for the mean grain size, exclusively to magnesium-aluminum spinel powders be added and according to the preparation of the invention lead to spinel ceramics, which have very special and very high transmission values in different wavelength ranges and no visible defects or only with a low frequency.

The invention will be explained in more detail below with reference to several exemplary embodiments.

example 1

A MgAl ₂ O suspension of 29.8% by weight spinel powder (S30CR, Fa. Baikowski), 69.6% by weight of deionized water and 0.6 Ma .-% dispersing aid Dolapix CE64 (Zschimmer & Co.) dissolved therein. Black) was added after 2 hours Deagglomerationsmahlung in the laboratory attritor 0.1 Ma .-% CaO powder (Merck).

After addition of 0.6% by weight of polyvinyl alcohol (Mowiol 4-88, Zschimmer & Schwarz) and 2.0% by weight of glycerol, based on the mass of spinel powder, and intensive homogenization by means of an agitator, the suspension was dried ,

The dried and sieved granulated offset powder was pre-pressed into circular discs of 30 mm diameter and 9 mm thickness for the purpose of shaping and then cold isostatically densified (CiP). After 2 hours of thermal debinding in air at 800 ° C, the moldings were sintered at 1450 ° C for 2 h in air and subsequently to achieve transparency at 1420 ° C for 15 h hot isostatic (HiP) in argon atmosphere re-pressed. The temperatures were determined as a result of a separate optimization of the heat treatment processes specifically for this offset powder. For differently doped offsets other temperatures are required.

The density of the resulting transparent disks was> 99.9% of the theoretical density. The mean microstructure grain size (determined as 1.16 times the measured mean chord length) was 0.68 μm.

The Vickers macrohardness HV10 measured on ceramographically prepared sections was HV10 = 14.0 GPa.

The disks were ground plane-parallel to thicknesses of 3 and 5 mm and polished on both sides to a surface roughness R _z <0.03 μιτι.

The true in-line transmission (RIT, spectrophotometer Gigahertz LCRT-2005-S) measured on these wheels at 640 nm wavelength was 84.7% for 3 mm thick disks and 84.5% for 5 mm thick disks.

The in-line transmission measured on these disks by means of a spectrometer of the type Spectrum 400 (Perkin-Elmer, Waltham, MA, USA) at 2000 nm wavelength was 89% for 5 mm thick disks and thus achieves the same within the scope of the measurement accuracy Wavelength expected theoretical transmission.

To characterize the visible defect population above a size of 20 μιτι a 12 mm x 12 mm large sample detail was taken with a high-resolution scanner. The visible defects were counted on an approx. 20x enlarged graphic image of this section. In this case, a defect concentration of 246 / cm ^{3 was} determined. Example 2

A MgAl ₂ O suspension of 29.8% by weight spinel powder (S30CR, Fa. Baikowski), 69.6% by weight of deionized water and 0.6 Ma .-% dispersing aid Dolapix CE64 (Zschimmer & Co.) dissolved therein. Black), after 2 hours of deagglomeration milling in the laboratory attritor, 50 ml of an aqueous solution of strontium nitrate was added. The concentration of this doping solution was selected so that the content of strontium oxide SrO based on the bulk of the spinel powder was 0.125 mass%.

The dried and sieve-granulated offset powder was pre-pressed for the purpose of shaping into circular disks with a diameter of 30 mm and a thickness of 9 mm and then cold-isostatically recompressed (CiP). After 2 hours of thermal debinding in air at 800 ° C, the molded body was sintered at 1525 ° C for 2 h in air and subsequently to achieve transparency at 1530 ° C for 15 hours hot isostatic (HiP) in argon atmosphere re-pressed.

The density of the resulting transparent disc was> 99.9% of the theoretical density. The average grain size (determined as the 1. 56 times the measured mean chord lengths) was 0.61 μιτι.

The disc was ground plane-parallel to a thickness of 4 mm and polished on both sides to a surface roughness R _z <0.03 μιτι. The true in-line transmission (RIT) measured at this slice at 640 nm wavelength was 85.3%. The measurement was carried out with the spectrophotometer Gigahertz LCRT-2005-S.

Using the Varian Cary 4000 spectrometer (Varian Inc. Mulgrave, Vic. Australia) at 640 nm wavelength, the value of total forward transmission with TFT = 86.4% was measured. The according to ASTM D 1003-00 from the respective total and in-line Transmission data determined for the turbidity (difference of TFT and RIT, divided by TFT, multiplied by 100) was 1, 3%.

The in-line transmission of the same disk measured in the infrared wavelength range at 2000 nm using a spectrometer of the Spectrum 400 type (Perkin-Elmer, Waltham, MA, USA) was 86.3%.

To characterize the visible defect population above a size of 20 μιτι a 12 mm x 12 mm large sample detail was taken with a high-resolution scanner. The visible defects were counted on an approx. 20x enlarged graphic image of this section. In this case, a defect concentration of 1 1 1 / cm ^{3 was} determined.

Example 3

A MgAl ₂ O suspension of 29.8% by weight spinel powder (S30CR, Fa. Baikowski), 69.6% by weight of deionized water and 0.6 Ma .-% dispersing aid Dolapix CE64 (Zschimmer & Co.) dissolved therein. Black), after 2 hours of deagglomeration milling in the laboratory attritor, 50 ml of an aqueous solution of strontium nitrate was added. The concentration of this doping solution was selected so that the content of strontium oxide SrO related to the mass of the spinel powder was 0.3125 mass%.

The dried and sieve-granulated offset powder was pre-pressed for the purpose of shaping into circular disks with a diameter of 30 mm and a thickness of 9 mm and then cold-isostatically recompressed (CiP). After 2 hours of thermal debinding in air at 800 ° C, the molding was sintered at 1470 ° C for 2 h in air and subsequently to achieve transparency at 1470 ° C for 15 h hot isostatic (HiP) in an argon atmosphere nachgepresst. The density of the resulting transparent disks was> 99.9% of the theoretical density. The mean texture grain size (determined as the 1.56 times the measured mean chord lengths) was 0.36 μm.

The Vickers macrohardness HV10 measured on ceramographically prepared sections was HV10 = 14.3 GPa.

The disks were ground plane-parallel to thicknesses of 4 mm and polished on both sides to a surface roughness R _z <0.03 μιτι. The true line transmission measured at this slice at 2000 nm wavelength using a Spectrum 400 Spectrometer (Perkin-Elmer, Waltham, Mass.) Was 86.6%.

To characterize the visible defect population above a size of 20 μιτι a 12 mm x 12 mm large sample detail was taken with a high-resolution scanner. The visible defects were counted on an approx. 20x enlarged graphic image of this section. In this case, a defect concentration of 85 / cm ^{3 was} determined.

Example 4

A MgAl ₂ O suspension of 29.8% by weight spinel powder (S30CR, Fa. Baikowski), 69.6% by weight of deionized water and 0.6 Ma .-% dispersing aid Dolapix CE64 (Zschimmer & Co.) dissolved therein. Black), after 2 hours of deagglomeration milling in the laboratory attritor, 50 ml of an aqueous barium nitrate solution was added. The concentration of this doping solution was selected so that the content of barium oxide BaO based on the mass of the spinel powder was 0.125 mass%.

After addition of 0.6% by weight of polyvinyl alcohol (Mowiol 4-88, Zschimmer & Schwarz) and 2.0% by weight of glycerol, based on the mass of spinel powder, and intensive homogenization by means of an agitator, the suspension was dried , The dried and sieve-granulated offset powder was pre-pressed for the purpose of shaping into circular disks with a diameter of 30 mm and a thickness of 9 mm and then cold-isostatically recompressed (CiP). After 2 hours of thermal debinding in air at 800 ° C, the shaped body was sintered at 1520 ° C for 2 h in air and subsequently to achieve transparency at 1520 ° C for 15 hours hot isostatic (HiP) in argon atmosphere re-pressed.

The density of the resulting transparent disks was> 99.9% of the theoretical density. The mean microstructure grain size (determined as 1.16 times the measured mean chord length) was 0.59 μm.

The Vickers macrohardness HV10 measured on ceramographically prepared sections was HV10 = 14.1 GPa.

The disks were ground plane-parallel to thicknesses of 4 mm and polished on both sides to a surface roughness R _z <0.03 μιτι. The true in-line transmission (RIT) measured on these wheels at 640 nm wavelength was 84.5%. The measurement was carried out in each case with the spectrophotometer Gigahertz LCRT-2005-S.

Using the Varian Cary 4000 spectrometer (Varian Inc. Mulgrave, Via, Australia), the total forward transmission TFT = 85.5% was measured at 640 nm wavelength. The value of turbidity (difference between TFT and RIT divided by TFT multiplied by 100) determined according to ASTM D 1003-00 from the respective total and in-line transmission data was 1.5%.

The in-line transmission of the same disk measured in the infrared wavelength range at 2000 nm using a spectrometer of the Spectrum 400 type (Perkin-Elmer, Waltham, MA, USA) was 86.8.

To characterize the visible defect population above a size of 20 μιτι a 12 mm x 12 mm large sample detail was taken with a high-resolution scanner. The visible defects were counted on an approx. 20x enlarged graphic image of this section. In this case, a defect concentration of 1 16 / cm ^{3 was} determined. Example 5

A MgAl ₂ O suspension of 29.8% by weight spinel powder (S30CR, Fa. Baikowski), 69.6% by weight of deionized water and 0.6 Ma .-% dispersing aid Dolapix CE64 (Zschimmer & Co.) dissolved therein. Black), after 2 hours of deagglomeration milling in the laboratory attritor, 50 ml of an aqueous barium nitrate solution was added. The concentration of this doping solution was selected so that the content of barium oxide BaO related to the mass of the spinel powder was 0.3125 mass%.

The dried and sieve-granulated offset powder was pre-pressed for the purpose of shaping into circular disks with a diameter of 30 mm and a thickness of 9 mm and then cold-isostatically recompressed (CiP). After 2 hours of thermal debinding in air at 800 ° C, the molding was sintered at 1490 ° C for 2 h in air and subsequently to obtain the transparency at 1490 ° C for 15 hours hot isostatic (HiP) in an argon atmosphere nachgepresst.

The density of the resulting transparent disks was> 99.9% of the theoretical density. The mean texture grain size (determined as the 1, 56 times the measured mean chord lengths) was 0.38 μιτι.

The disks were ground plane-parallel to thicknesses of 4 mm and polished on both sides to a surface roughness R _z <0.03 μιτι. The true line transmission measured at this slice at 2000 nm wavelength using a Spectrum 400 spectrometer (Perkin-Elmer, Waltham, MA, USA) was 86.7%. To characterize the visible defect population above a size of 20 μm, a 12 mm × 12 mm sample section was taken with a high-resolution scanner. The visible defects were counted on an approx. 20x enlarged graphic image of this section. In this case, a defect concentration of 61 / cm ^{3 was} determined.

Claims

claims

1 . Spinel ceramics, which are transparent in wavelength ranges of infrared light, consisting of sintered magnesium-aluminum spinel having an average grain size of <10 μιτι and with a maximum of 0.5 Ma .-% of homogeneously distributed additives of calcium and / or strontium and / or Barium, in a concentration, in each case expressed as oxide, of 0.005 to <0.2 wt.% CaO and / or 0.005 to <0.5 wt.% SrO and / or 0.005 to <0.5 Ma. % BaO are present.

2. spinel ceramics which are transparent in wavelength ranges of visible light, consisting of sintered magnesium-aluminum spinel with an average grain size of <10 μm and with a maximum of 0.3% by weight of homogeneously distributed additives of calcium and / or strontium and or barium, in a concentration, in each case expressed as oxide, of 0.005 to <0.2 wt.% CaO and / or 0.005 to <0.3 wt.% SrO and / or 0.005 to <0.25 Ma .-% BaO are present.

3. Transparent spinel ceramics according to claim 1 or 2, wherein the mean grain size of the sintered magnesium-aluminum spinel <5 μιτι, advantageously <2.5 μιτι, more preferably <1 μιτι.

4. Transparent spinel ceramics according to claim 1, in which a total of at most 0.3% by mass of homogeneously distributed additives are present.

5. Transparent spinel ceramics according to claim 2, in which a total of at most 0.2% by mass of homogeneously distributed additives are present.

6. Transparent spinel ceramics according to claim 1 or 2, wherein as an additive calcium in a concentration, expressed as CaO, of 0.01 to 0.1 Ma .-% is present.

7. Transparent spinel ceramics according to claim 1, wherein as additives strontium and / or barium in a concentration, expressed as SrO and / or BaO, of 0.01 to 0.4% by mass of SrO and / or 0.01 to 0.4% by mass of BaO are present, with a maximum of 0.3% by mass of additions of strontium and / or barium being present are.

8. Transparent spinel ceramics according to claim 2, wherein as additives strontium and / or barium in a concentration, expressed as SrO and / or BaO, from 0.01 to 0.2 mass% of SrO and / or 0.01 to 0.15 wt.% Of BaO are present, with a maximum of 0.2 wt.% Of additions of strontium and / or barium being present.

9. Transparent spinel ceramics according to claim 1, which show thicknesses of> 3 mm at least in the range of infrared wavelengths, measured between 1000 and 2500 nm, an in-line transmission of> 82%, advantageously> 84%, more preferably> 85% ,

10. Transparent spinel ceramics according to claim 2, which exhibit a thickness of> 3 mm, at least in the range of visible wavelengths, measured between 600 and 650 nm, an RIT of> 80%, advantageously> 82%, more preferably> 84%.

1 1. Transparent spinel ceramics according to claim 1 or 2, which contain no visible defects.

12. Transparent spinel ceramics according to claim 1 or 2, the visible defects greater than 20 μιτι with a frequency <300 / cm ³ included.

13. Transparent spinel ceramics according to claim 12, which contain visible defects greater than 20 μιτι with a frequency of 50 to 150 / cm ³

14. Transparent spinel ceramics according to claim 1 or 2, in which Vickers hardness HV10 of> 12.5 GPa are present.

15. A process for the preparation of transparent spinel ceramics according to claim 1, wherein the addition of a maximum of 0.5 wt .-% of additives from calcium and / or strontium and / or barium in a concentration, in each case expressed as oxide, of 0.005 to <0.2 mass% CaO and / or 0.005 to <0.5 mass% SrO and / or 0.005 to <0 5% by weight of BaO in the form of undissolved Ca, Sr and / or Ba-containing compounds and / or Ca, Sr and / or Ba-containing compounds dissolved in water and / or Ca dissolved in organic solvents, Sr- and / or Ba-containing compounds to magnesium-aluminum spinel powder is realized, the additives are homogeneously distributed and then the materials are sintered into a transparent spinel ceramic.

16. A process for producing transparent spinel ceramics according to claim 2, wherein the addition of a maximum of 0.3 wt .-% of additives of calcium and / or strontium and / or barium in a concentration, in each case expressed as oxide, from 0.005 to < 0.2% by weight of CaO and / or 0.005 to <0.3% by weight of SrO and / or 0.005 to <0.25% by weight of BaO in the form of undissolved Ca, Sr and / or Ba-containing Compounds and / or dissolved in water, Ca, Sr and / or Ba-containing compounds and / or dissolved in organic solvents of Ca, Sr and / or Ba-containing compounds to magnesium-aluminum spinel powder is realized, the Additions are homogeneously distributed and then the materials are sintered into a transparent spinel ceramic.

17. The method of claim 15 or 16, wherein the addition of the additives during the slip or offset treatment of the ceramic, or as part of an impregnation or infiltration of the additives in open pores of a dry ceramic body, or by ion exchange between wet magnesium aluminum spinel bodies and Ca, Sr and / or Ba-containing solutions according to the invention is realized.