Compositions – Light transmission modifying compositions – Inorganic crystalline solid
Reexamination Certificate
2002-03-06
2003-11-25
Tucker, Philip (Department: 1712)
Compositions
Light transmission modifying compositions
Inorganic crystalline solid
C423S594200
Reexamination Certificate
active
06652780
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to iron-doped lithium niobate crystals having an increased ratio between the trivalent iron to divalent iron contained therein.
Lithium niobate crystals are known as electro-optic materials that are useful as holographic recording media which have fairly good sensitivity. This sensitivity, as well as diffraction efficiency, can be greatly improved by doping such crystals with iron, as has been disclosed in U.S. Pat. No. 3,703,328 to Glass, et al.
In such applications, it is generally advantageous to maintain a minimum fraction of iron ions (Fe) in the divalent state (Fe
+2
), in order to achieve an optimal holographic write sensitivity, and a relatively larger function of Fe ions in the trivalent state (Fe
+3
), in order to minimize photoconductivity and hence, minimize sensitivity to self-erasure effects when large numbers of holograms are time-sequentially stored within a common volume of lithium niobate. By maximizing write sensitivity and minimizing erase sensitivity, more high-efficiency holograms can be stored and greater information densities achieved within a given volume of iron-doped lithium niobate (Fe:LiNbO
3
).
Problematic with such iron-doped lithium niobate crystals, however, is the difficulty in producing such crystals wherein the iron portions contained therein exist predominantly in the trivalent state. More specifically, significant difficulty arises in oxidizing the divalent iron ions normally contained within such crystals, insofar as conventional oxidizing agents and techniques fail to thoroughly penetrate such crystals and hence oxidize the iron contained therein. Further difficulties arise in forming such crystals having a large size, namely greater than 1.0 cubic centimeter.
While certain processes are known in the art that are effective in oxidizing substantially all of the divalent iron ions present to trivalent ions, such processes suffer from other drawbacks. Generally, most prior art processes require heating such crystals to extremely high temperatures for prolonged periods of time. Such heating, which is typically carried out at temperatures between 800° C. and 1100° C., can cause the formation of lithium triniobate (LiNi
3
O
8
), which, as is known to those skilled in the art, causes light to scatter and thus renders the crystal useless in holographic applications.
Such prolonged heating further is known to cause a portion of the lithium present to diffuse out of the crystal. Such diffusion, which is known to begin to occur at approximately 900° C., causes light-absorbing color centers to form within the crystal. The light-absorbing properties of these centers dramatically limit the ability of such crystals to function properly in holographic applications, especially insofar as such color centers are known in the art to absorb light having a wavelength around 532 nanometers, which, for system reasons, is near-optimal with regard to recording holographic data.
Accordingly, there is a need in the art for an efficient, more thorough process for oxidizing iron-doped lithium niobate wherein the trivalent to divalent iron ion ratio is greatly enhanced. There is also a need in the art for a process for oxidizing iron-doped lithium niobate such that the iron ions present may be substantially oxidized from the divalent state to trivalent state that avoids the formation of unacceptable levels of lithium tri-niobate and further substantially minimizes the diffusion of lithium from the lithium niobate.
BRIEF SUMMARY OF THE INVENTION
The present invention specifically addresses and alleviates the above-identified deficiencies in the art. More specifically, the present invention is directed to a process for oxidizing a portion of the divalent iron ions present in a crystal of iron-doped lithium niobate to trivalent state, and is particularly effective in achieving [Fe
2+
]/[Fe
3+
] ratios which are less than or approximately equal to 0.01. The process of the present invention further allows for the divalent iron ions present in a heavily iron-doped lithium niobate crystal (i.e., greater than 0.145 mol present of ion) to be oxidized while allowing the host crystal to retain its holographic recording properties (i.e., a minimum concentration of Fe
2+
). In this regard, the process of the present invention prevents or otherwise substantially reduces the formation of lithium tri-niobate within and diffusion of lithium from the host crystal. The present invention further includes iron-doped lithium niobate crystals that are relatively large, and in particular, have a length, width and depth of at least 1.0 cm or greater.
The process comprises the initial step of protonating the iron-doped lithium niobate. Such protonation may be achieved by exposing the crystal to steam at a temperature of approximately 600° C. As is known to those skilled in the art, the application of an electric field may further enhance the protonation of the iron-doped lithium niobate. The crystal is thereafter placed in a pressure chamber and immersed in an oxygen-containing atmosphere at a pressure preferably between 10 and 100 atmospheres. While in such environment, the crystal is then heated from room temperature (approximately 30° C.) to 950° C. at a rate not to exceed 25° C. per minute. The crystal is then maintained at the 950° C. temperature for approximately 50 hours and then is cooled to room temperature at a rate not to exceed 50° C. per minute, and preferably at approximately 25° C. per minute. The pressure chamber is then vented and the crystal removed. The crystal is then de-protonated by, for example, heating the crystal to between 160° C. and 240° C. and simultaneously applying an appropriate electric field.
It is therefore an object of the present invention to provide a process for oxidizing iron-doped lithium niobate such that the iron ions contained therein exist predominantly in the trivalent state, and preferably such that the concentration of divalent iron ions to trivalent iron ions is less than or approximately equal to 0.01.
Another object of the present invention is to provide a process for oxidizing iron-doped lithium niobate that effectively converts a substantial portion of the divalent iron ions present in the crystal to trivalent iron ions while minimizing the formation of lithium tri-niobate therein.
Another object of the present invention is to provide a process for oxidizing iron-doped lithium niobate that effectively converts a substantial portion of the divalent iron irons present in the crystal to trivalent iron ions while minimizing the diffusion of lithium from the iron-doped lithium niobate, and thus minimizing the formation of light-absorbing color centers within said iron-doped lithium niobate.
A still further object of the present invention is to provide a crystal useful for recording and reading holograms consisting of a single lithium niobate crystal having a length, width and depth of at least 1.0 cm or greater that is doped with greater than 0.145 mol percent of iron wherein at least fifty percent of the iron present exists in the trivalent state.
REFERENCES:
patent: 3703328 (1972-11-01), Glass et al.
patent: 3799642 (1974-03-01), Phillips et al.
patent: 3932299 (1976-01-01), Phillips
patent: 4052119 (1977-10-01), Williams et al.
patent: 5904912 (1999-05-01), Kitamura et al.
patent: 197512 (1975-12-01), None
Putzka et al., Mossbauer Spectroscopy of Single Crystal LiNbO3: Fe(111), Appl. Phys A 29, pp. 1-7, (1982).
Northrop Grumman
Stetina Brunda Garred & Brucker
Tucker Philip
LandOfFree
Process for oxidizing iron-doped lithium niobate does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for oxidizing iron-doped lithium niobate, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for oxidizing iron-doped lithium niobate will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3184116