The present invention relates to a moulding tool for moulding a ceramic green body in the production of at least one optical lens made of an opto-ceramic. The invention furthermore addresses a process for producing the lenses mentioned before. Also, the invention is directed to the use of a moulding tool for producing opto-ceramic lenses.
Transparent ceramics suited as materials for lenses are hereinafter referred to as opto-ceramics. Such materials have very convenient properties (refractive indices, dispersions) and contribute to an improvement of optical imaging systems. In special cases new imaging concepts are only possible with such new optical material options. Especially the possibilities of more compact construction of for example digital cameras or improved of simplified colour corrections (chromatical or apochromatical) or mentioned here.
The lenses that can be produced with the moulding tool according to the invention and by making use of the new and advantageous process claimed herein consist of opto-ceramics. Such materials are defined afterwards. The term “consist of” in the sense of this invention does not exclude the presence of remaining binders or other additives present in the final lens besides the single phase, poly crystalline material (see below). Such compounds may be present depending on what concise process was used for producing the respective lens.
An opto-ceramic is substantially a single phase, poly crystalline material based on oxides and having high transparency. Opto-ceramics are hence a subdivision of ceramics. Lenses made of opto-ceramics are therefore in certain applications preferred over conventional lenses made of glass. A prerequisite for the successful placement on the market is the supply of sufficient amounts of high quality lenses with good reproducibility at acceptable prices. The prices are geared to the prices of conventional lenses of glass.
A further aspect in providing cost-effective lenses from opto-ceramics is that these lenses should not only comprise a refractive function but also function reflectively, transmittively and diffractively.
Additionally, mechanical properties like for example supporting areas, adjoining the lens circularly on the sides and thereby allowing positioning of the lens in the carrier, are often needed next to the optical functions. Even more complex mould recesses that are placed onto a local part of the lens might be needed in order to make integration into an optical system possible (monolithic optical elements).
Today lenses made from opto-ceramics are usually produced as follows. First the powders of the basic material (mostly oxides) are produced (first step: powder production). The powders used are often of high purity and in the nanometre scale as far as their sizes are concerned. Then the powders are conditioned (second step: powder conditioning). This step comprises milling of the powders, homogenizing the powder mixture and drying the batch. This step is followed by the third step which is the moulding step, where a green body is formed, already determining the final shape of the lens, and if necessary certain additives are added that depend on the moulding method to be applied.
Afterwards the green body is—if necessary—dried and/or debindered (if necessary: fourth step: drying and/or debindering). In the fifth step, sintering, the grains that are in loose contact to each other after moulding and—if necessary—drying and/or debindering form fixed contacts by mass transfer and/or diffusion. Simultaneously an increase in grain size is achieved and open porosity is removed from the body. In the subsequent sixth step of hot isostatic pressing (HIP) the closed porosity within the grain boundaries is pressed out of the sintered body. The thus obtained body can if necessary be further processed in a further step (seventh step: thermic post-processing), a post-processing step in form of further annealing steps and/or profiles, in order to remove the oxygen vacancies or graphite impurities produced by the HIP process or reduce the intergranulary fine porosity.
Preferably lenses from the following opto-ceramics are produced by using the above-mentioned production process:                a) cubic garnets of the generic stoichiometry(1−m){(M1)3+z3(M2)5−z3O12}m{A}wherein z3 has a value in the range of from −1 to +1, m has a value of from near zero to less than 0.05, preferably less than 0.03 and further preferred near zero or equal to zero.        
M1 is selected from Y, La, Gd, Lu, Yb, Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm or a mixture of one or more of these elements, wherein the active lanthanides Ce, Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, and Tm are either present within the opto-ceramic in amounts of together up to 100 wtppm or at least 15 mol % relating to the amount of these oxides;
M2 is selected from one or more of the elements of the group III or IIIa of the periodic table of elements, preferably from Al, Ga, In, Sc or a mixture of two or more of these elements;
A is one or more of additional components, especially selected from SiO2, Na2O, MgO, CaO or TiO2.                b) zirconium oxide, preferably cubically stabilized zirconia, of the generic formula(1−m){z1[ZrO2]z2[HfO2](1−z1−z2)[X2O3]}m[A] or(1−m){z1[ZrO2]z2[HfO2](1−z1−z2)[MO]}m[A]wherein z1+z2 is less than or equal to 0.92 and preferably smaller or equal to 0.90, wherein z1 and z2 are more than or equal to zero, m is less than 0.10 and preferably less than 0.06, further preferred less than 0.03 and most preferred m is zero;        
X is selected from Y, Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or a mixture of one or more of these elements, preferably selected from Y, Yb and Lu or a mixture of two or all three of these elements and particularly preferred X is Y. M is selected from Ca and Mg;
and
A is one or more components, especially selected from SiO2, Na2O, MgO, CaO, or TiO2.                c) cubic sesquioxides like Y2O3, Lu2O3, Yb2O3, In2O3, Sc2O3 and/or isotype cubic mixed crystal phases of the above oxides with other oxides like Gd2O3, La2O3, ZrO2, HfO2 etc.        d) Mg—Al-spinels of the approximate formula MgAl2O4 and        e) Al-Oxinitrides of the generic formulaAl23−1/3XO27+XN5−X, wherein 0.429<X<2, for example Al23O27N5         f) perovskites        
It can be taken from the above explanation of the production process for lenses made from opto-ceramics that the third step of moulding is decisive for the final form of the lens, because the shape obtained within this process step is not changed considerably during subsequent process steps. The subsequent steps merely cause contraction and shrinkage of the green body obtained by the moulding step.
Furthermore, the moulding method used and, hence, also the applied mould decisively determine the later properties of the opto-ceramic as far as optical properties are concerned, as the moulding step decisively influences the conduct of subsequent sintering and/or drying/debindering.
As moulding method different methods are used that differ mainly by their moisture content. Casting (25-40% moisture) comprises the methods of slip casting, vacuum pressure casting, pressure slip casting or electrophoresis as well as centrifugal slip casting.
The term plastic moulding (15-25% moisture) means the methods of extrusion, squeezing, turning and free forming.
The methods of wet pressing, dry pressing, pounding and compacting by vibration belong to the pressing techniques (0-15% moisture). As further methods that are not classified according to the moisture content of the moulded material the moulding methods of hot casting (also referred to as low pressure die casting) or die casting can be used.
It is known from the prior art that lenses made of transparent ceramics can be obtained by moulding by pressing. For example, DE 101 95 586 T1 describes the production of special opto-ceramics with perovskite structure. Therein (see from page 15) “ . . . the ceramic powder material is processed with a binder to a predetermined shape such that a ceramic green compact is obtained . . . ”. In a subsequent burning step the green compact is preferably embedded in specific powder. The processing of the ceramic powder material to a predetermined geometry is done using a binder. According to exemplary embodiment 7 moulding is done especially by pressing at 2000 kg/cm2 (196 MPa) in order to obtain a pane-shaped green compact with a diameter of 30 mm and a thickness of 1.8 mm.
EP 1 701 179 A1 discloses the use of cubic garnets for optical elements in microlithography in wavelength region<200 nm. Particularly, this document discloses the production of mono-crystals. Moulding tools or processes are not mentioned.
Further US 2004/0159984 A1 describes the production of transparent Y2O3 ceramics by moulding methods like slip casting or die casting. The document mentions the production of complex geometries, however, without specifying the shape of the moulding tool in detail. Especially, the production of components with curved surfaces is not mentioned.
The process of slip casting for the production of transparent ceramics for laser applications based on pane-shaped specimen is described by Ueda (‘Scalable Ceramic Lasers for IFE Driver’. Institute for Laser Science, Univ. of Electro-Communications, Japan-US Workshop ILE/Osaka, Mar. 13, 2003).
The processes described in the prior art are not suited for the cost-effective production of transparent ceramic lenses having superior optical qualities mad. In order to achieve the desired optical effects, lenses comprise definitely curved surfaces.
According to “Optical System Design” by Robert E. Fischer, ISBN 0071314916-2 the mathematical description of lens surfaces is achieved by the following equation:
                    y        =                                            c              ⁢                                                          ⁢                              x                2                                                    1              +                                                1                  -                                                            (                                              k                        +                        1                                            )                                        ⁢                                          c                      2                                        ⁢                                          x                      2                                                                                                    +                                    a              1                        ⁢                          x              2                                +                                    a              2                        ⁢                          x              4                                +                                    a              3                        ⁢                          x              6                                +                                    a              4                        ⁢                          x              8                                +          …                                    (                  Equation          ⁢                                          ⁢          A                )            
Wherein y is the location on the optical axis, k is the conic constant, x is the perpendicular distance from the optical axis, c=1/R and R is the radius of the curve and a1, a2, a3, a4, . . . are the coefficients of the aspheric term.
Spherical lenses are distinguished from aspherical lenses as follows:
k=0 and a1, a2, a3 . . . =0 spherical lens
k≠0 aspherical lens,
wherein the curves can be distinguished as follows:
                k<−1 hyperboloid surface        k=−1 paraboloidal surface        −1<k<0 ellipsoid surface        k>0 flattened spherical        
These lens surfaces are according to the prior art obtained by extensive chemical milling, grinding and polishing processes from a bulk semi-finished part. The processing of these simply shaped bulk semi-finished parts is time consuming and cost-intensive. This is especially true, because many opto-ceramic systems, like for example the group of sesquioxides (Y2O3, Yb2O3, Sc2O3, Lu2O3) and/or mixed crystals of these, garnets (for example YAG Yttrium-Aluminum-Garnet), AlON (Aluminum Oxinitride), Al2O3, perovskites, spinels, cubically stabilized zirconium oxide or hafnium oxide, are very hard materials. Especially the chemical milling and grinding processes for the production of shapes with recesses are thus very cost-intensive.