DD213514A1 - PROCESS FOR PREPARING CALCIUM FLUORIDE CRYSTALS FOR OPTICAL PURPOSES - Google Patents

Abstract

The invention relates to the production of calcium fluoride single crystals for optical purposes without interfering absorptions, Truebungen, Verfaerbungen or other Inhomogenitaeten that have an extremely low birefringence and therefore are suitable for optical components with the highest polarization optical requirements. In the production of calcium fluoride monocrystals by growth from the melt under vacuum with the addition of lead II fluoride, the essence of the invention is that the subsequent annealing process takes place in a lead II fluoride atmosphere. The process according to the invention can be used both for brittle crystal blanks and for optical crystal components which have already been shaped up to the polishing process.

Description

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Title ;;, Method of Making Calo? Uoride Single Crystals for Optical Purposes.

Application of the Invention ι

The invention relates to the production of calcium

Γ fluoride - '(eaF2) " ^is suitable for optical purposes without disturbing absorptions in the entire spectral range from UV (125 nm) to IR (9000 nm), without thermal and mechanical stresses and free of particle precipitation, turbidity or other inhomogeneities or discolorations In particular, it is possible with the aid of the invention, CaF ₂ " ² * ¹¹ ""

  '' ·. 'to produce crystals with extremely low stress birefringence and from the single crystals the optical components satisfying the highest polarization requirements'.

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Since calcium fluoride has a very high permeability for a very large wavelength range of the electromagnetic spectrum, which extends from the far UV to the near IR, CaFg-EiBk ^ istaile have as op-

2Ö technical medium of great technical importance. In addition, due to its favorable dispersion properties, CaP _{2 is} an important material for corrected high-performance optical systems, especially in microscopy. Therefore, the technical methods for the production of CaPg single crystals are developed in many ways and

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verbesseri; Service. Hauptziolrichtiing of the known technical solutions was the purification of the saum used reaching urian or synthetic calcium fluoride raw materials and ensuring the purity and optical homogeneity during the crystal growth from a CaFp-

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Sp is aware of a process for growing molten vacuum optical CaFg mncrystals under vacuum, in which up to 2 % lead II fluoride (PbFg) is added to the GaPgi melt for crystal growth; to avoid clouding of the CaFg 2 crystals by hydrolysis products due to residual traces of water in the crystal substance or from the culture of growth (U.S.P. 2,498x186, J. Opt. Soc. Amer., 731 (1949)) PbF ₂ addition leaves no contamination in the grown crystal, the Mach parts of the method consist in particular in that in the CaF ^ crystals again turbidity by precipitates of oxides or oxide fluorides form when the crystals annealed after cultivation to reduce stresses become, whereby a relatively strong light scattering is caused. In addition, the crystals thus produced are not free of stress after tempering, so that they are easily broken during mechanical processing. In particular, they have a considerable stress birefringence, so that they can not be used for polarization optical purposes.

These deficiencies can not be eliminated with another known method, which consists in the thermal pretreatment of natural, crystalline GaFg raw material in a fluorine atmosphere, optionally in the subsequent crystal growth phase, an addition of PbF ₂ to the growing melt takes place (ÜS Pat .173; SU copyright 324 219).

The prior art also includes a process for the preparation of colorless CaFg crystal for

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optical purposes, wherein the crystal growth from the melt takes place in the presence of an atmosphere of cobalt-III or manganese-IIlfluorid. Optionally, the melt batch also additives of PbFg, PbO or Pb ^ O ^ or lead or vanadium

The deficiency of this method is that while the crystals are colorless in the visible part of the spectrum, they do not completely eliminate interfering absorptions in the UV and IR regions become. To reduce or eliminate stress or stress birefringence, this method is not suitable in the form described.

Also known is a method in which CaF ₂ crystals are grown and annealed in an HF-containing He atmosphere (J, Appl. Phys., 2482 (1963)). The main drawback consists in the very high technical apparatus Aufτ 'wand and the extensive environmental and occupational safety measures, due to the chemical and physical properties of hydrogen fluoride. Despite sophisticated control and regulation technology for compliance with the crystal growth parameters and the conditions for the tempering process, blistering due to HF inclusions in the finished crystal can not be ruled out. Although annealing the grown crystal to remove stress immediately after crystal growth in the growing furnace itself, without exposing the fresh crystal to atmospheric moisture or the influence of oxygen in the air, is also a reduction in stress birefringence below 10-30 nm / cm unreachable. In addition, the UV transmission deteriorates by building up absorption at 200 nm.

It is also known to make this process more efficient by supplementing the purification operation by means of an ion exchange process or by repeated recrystallization in a He / HF atmosphere with a partial pressure ratio of H ₂ O: HF <10 " ⁵ (US Pat. 3,649 * 552; 3 * 935,302).

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However, the Hachteil is to further increase de, s ^ ijec cally-appa ^ ative and "ffieß- ^ u ^ ci r African effort in gleichzeititei ^ ^ ung SrEBh d§4? The process steps without ^D ih Höezug ^ au ^ to be achieved by the optical-optical properties ^^ 'VörbeWserujigeii ·, ".-'"''. Ϊ '.' ^: '-'':' ^:: '-.'' · -'.;: "- '. '·.'.

The aim of the invention is the j ^ Jiajsh Eixe the known V.erfahren for the production of GAPP ~ Sinkristallen for purposes optical to • 4ieeeiMgßÄ "; .. JPie ^-; persecuted ingbe # oridW & ^r ea ^ fi ^ dung. ^'! & - ^ optics

Produce components of shaping, machined CaP ₀ - * single crystals, which derive the highest polarization optical requirements / »· ' ^ν ---"': ~ α ^: · ::, ι :: ':,: -:.: ^_..,:; .- · '...'.

Explanation of the nature of the invention; The invention has the task "of both the crystal growing ^process and, after the shaping process, of the Gap2 ^{sinker s} ^ ^{a le} ^ ⁸ / ώ ^

like la. ' B · Turbidities and ilclits.tr.öRenae; ^ s.iB ^ MWung.en,.

as well as to eliminate mechanical thermal damage and to prevent their re-formation in the annealing process known from the prior art, without negatively affecting the other optical properties. ,: ·. Ζ ': ··.:: '. ' : ' ... ,': ... '.... . /. ':.''.:''' _:

At the same time, the task of reducing stress birefringence in CaPg monocrystals and in shaping CaPo optical components to values of = 1 nm / cm is associated with this.

The object of the invention is furthermore aazuge- aa process for the preparation of O.aPg'-Siiikristallen for optical purposes without increased procedural complexity. 'ben · /. '·. Λ "'' ·. '-.-; .. · _^..:.':;;;;;

The object is achieved according to the invention by a process for the preparation of calcium fluoride CGaGy single crystals for optical purposes by growth from the melt in the

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Vacuum while adding 0.2 to 2 wt _e "% lead II fluoride (pbpg) and subsequent annealing dissolved» which is characterized in that the annealing during the heating-up period "of the adjoining, fmr a period of 2 to 3 hours held constant temperature of 1200 ⁰ C and the following cooling period down to a temperature of 900 ⁰ C in a! PbFg ^ atmosphere of 10 to 1 Torr.

The method according to the invention can be used both for melt-grown crystal blanks and for crystal components which have already been shaped up to the polishing process »

The invention is described with reference to an exemplary embodiment.

In a graphite or ffiolybdenum crucible, for example, 20 CaPg crystal blanks or already processed crystals (eg 20 μm 0.10 mm thick) which were grown according to the Stockbarger or Bridgman method and a double-stranded billet of between 10 and 25 nm / cm are used The crucible is then placed in a high-vacuum system with molybdenum heating, which guarantees a homogeneous temperature distribution in the working zone, so that no temperature gradients can act on the crystals. The crucible becomes closed with a lid, on the inside of which a container is screwed which contains the PbPp analyte necessary for the formation of the PbFp tempering atmosphere in an amount which is sufficient, within the heating time, the period of the maximum maximum temperature and to evaporate continuously during the cooling period up to a temperature of 900 ° C * To form the PbPg-At mosphäre in the crucible and in the rest of the high-vacuum system, the threads of the screw connections of the crucible lid and the PbPg container on the crucible lid are designed so that the PbPp steam can pass through the turns,

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so that separate evaporation channels are unnecessary.

After the crucible prepared in this way has been placed in the high-vacuum system, it is evacuated for 1 hour until the vacuum has reached a value of 10-torr. Thereafter, a linear heating is carried out from room temperature to 1200 ° C. within 6 hours. Thereafter, the annealing batch 2 is carried out A constant PbF ₀ atmosphere of 10 to 1 Torr is formed. The subsequent cooling is carried out at 100 K / h to 900 ° C. and further to room temperature at a rate of 15 K. / Hr · The accuracy of the temperature control must be at least - 2 K possible ·

As a result of the described method, GaF ₂ crystals are present, which have a stress birefringence of... 1 nm / cm, which at the same time have no disturbing absorptions in the entire spectral range from UV (125 nm) to IR Θ0 nm) and which are free of thermal and mechanical stresses, discolorations, turbidity or other inhomogeneities are * They are for the highest optical requirements, in particular polarization optical. '. ' ^: '"'''. -:, "''.' , ^:; - _: '' ^: - ^: '·; -

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Claims (1)

-V- ' ^: · Λ. ":"'. ' . '' - ^Ί -. '..:. : '''. . ' 'y' / .. '. ^ν Invention claim: Process for the preparation of calcium fluoride (GaF ₂ ) single crystals for optical purposes by cultivation
from the melt in vacuo with the addition of 0.2 to
2 wt.% Lead II fluoride (PbPg) and subsequent tempera, characterized in that the annealing during the heating period, the adjoining held for a Bauer 2 to y hours constant temperature of 1200 ⁰ O and the following Abkiihlperiode to below: su a temperature of 900 ⁰ G in a PbPg atmosphere of ΙΟ ** · to
·: 1 Torr occurs ·.; - ':'':'."'.-'.'^; :: ^{/ /.} ''] ':. ' / '.' · .. ''_;