Electro-optic ceramic materials

Abstract

The present invention provides a product and manufacturing method for electro-optic ceramic material having the composition (A(1-y)Ay).sub.1-XLnxM.sub.(1-2X/5)O3 wherein 0<x<0.1; 0<y<1; A and A are independently, alkali metals; Ln is a lanthanide metal; and M is a transition metal. The present invention provides a product and manufacturing method for an electro-optic device that is operable at room temperature and the properties of which are tunable by an applied external electric field.

Claims

1. A ceramic material having the composition:
(A.sub.(1-y)A.sub.y).sub.(1-x)Ln.sub.xM.sub.(1-2x/5)O.sub.3 wherein 0<x<0.1; 0<y<1; A and A are independently, alkali metals; Ln is a lanthanide metal; and M is a transition metal.

2. The material according to claim 1, wherein Ln is a trivalent lanthanide metal.

3. The material according to claim 1, wherein M is a pentavalent transition metal.

4. The material according to claim 1, wherein 0.04x0.07.

5. The material according to claim 1, wherein 0.4y0.6.

6. The material according to claim 1, wherein the material has an ABO.sub.3 type crystal structure.

7. The material according to claim 1, wherein the material has the composition:
(Na.sub.0.5K.sub.0.5).sub.(1-x)La.sub.xNb.sub.(1-2x/5)O.sub.3 wherein 0<x<0.1.

8. A method for fabricating a ceramic material comprising the step of contacting alkali metal starting materials, a lanthanide metal starting material, a transition metal starting material and oxygen according to the composition:
(A.sub.(1-y)A.sub.y).sub.1-xLn.sub.xM.sub.(1-2x/5)O.sub.3 wherein 0<x<0.1; 0<y<1; A and A are independently, alkali metals; Ln is a lanthanide metal; and M is a transition metal.

9. The method according to claim 8, wherein Ln is a trivalent lanthanide metal.

10. The method according to claim 8, wherein M is a pentavalent transition metal.

11. The method according to claim 8, wherein 0.04x0.07.

12. The method according to claim 8, wherein 0.4y0.6.

13. The method according to claim 8, further comprising a step of forming a solid solution, and calcining the solid solution at a temperature range of 750 C. to 900 C., wherein the calcination is performed for a duration of 2 hours to 6 hours.

14. The method according to claim 8, further comprising a step of compacting of the solid solution, and sintering the solid solution at a temperature range of 1000 C. to 1350 C., wherein the sintering is performed for a duration of 2 hours to 12 hours.

15. An electro-optical device comprising the ceramic material as claimed in claim 1 or fabricated according to claim 8.

16. The device according to claim 15, further comprising two electrodes on either side of the ceramic material.

17. The device according to claim 15, wherein the optical properties are tunable by applying an external electric field, wherein the tuning comprises increasing the transparency of the material at wavelengths between 750 nm and 2800 nm.

18. The device according to claim 15, wherein the device is operable at room-temperature.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and, not as a definition of the limits of the invention.

(2) FIG. 1 is an optical photograph taken with a disc of the (K.sub.0.5Na.sub.0.5).sub.0.95La.sub.0.05Nb.sub.0.98O.sub.3 (5 mol % La-doped KNN) ceramic material overlayed on top of a representative picture.

(3) FIG. 2 is a graph showing the X-ray diffraction spectra of the (K.sub.0.5Na.sub.0.5).sub.(1-x)La.sub.xNb.sub.(1-2x/5)O.sub.3 ceramic material when 0<x<0.1.

(4) FIG. 3 is a schematic diagram showing the set-up used to evaluate the electro-optical property of the electro-optical device comprising 5 mol % La-doped KNN ceramic material.

(5) FIG. 4 is a graph showing the IR properties of the electro-optic device comprising the 5 mol % La-doped KNN ceramic material at different voltages.

(6) FIG. 5 shows scanning electron microscopy (SEM) micrographs comparing the surface morphology of (a) un-doped KNN and (b) 5 mol % La-doped KNN ceramic materials.

(7) FIG. 6 is a graph comparing the IR properties of the 5 mol % La-doped KNN ceramic material, ZnO and natural solar radiation.

DETAILED DESCRIPTION OF DRAWINGS

(8) FIG. 1 is an optical photograph taken with a disc of the (K.sub.0.5Na.sub.0.5).sub.0.95La.sub.0.05Nb.sub.0.98O.sub.3 (5 mol % La-doped KNN) ceramic material (101) overlayed on top of a representative picture. This photograph shows that the 5 mol % La-doped KNN ceramic material is transparent, as it transmits light in order for the object behind it to be distinctly seen. The density of this ceramic material was measured to be 99 percent of the theoretical density. The transparency remained unaffected by extreme environments such as high or low temperatures and high or low pressures.

EXAMPLES

(9) Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1: Fabrication of the Electro-Optical Devices

(10) The (K.sub.0.5Na.sub.0.5).sub.0.95La.sub.0.05Nb.sub.0.98O.sub.3 (5 mol % La-doped KNN) ceramic material was prepared by conventional solid state ceramic preparation techniques. The 5 mol % La-doped KNN ceramic material was prepared by the mixed-oxide method. The starting materials K.sub.2CO.sub.3, Na.sub.2CO.sub.3, La.sub.2O.sub.3 and Nb.sub.2O.sub.5 were weighed according to the chemical formula. The compounds were mixed and planetary milled for 4 hours at 300 rpm using ethanol as solvent and a mixture of 5 mm and 10 mm diameter zirconia balls The powder weight to ball media ratio was kept at 1:3. as the milling media to form the (K.sub.0.5Na.sub.0.5).sub.0.95La.sub.0.05Nb.sub.0.98O.sub.3 solid solution powder. The solid solution powder was then calcined at 800 C. to 850 C. for 4 hours to 6 hours in air to decompose the carbonates and form the perovskite structure. The milling step was repeated to homogenise the powder and also to reduce the average particle size. The powder was dried in an oven at 120 C. for 2 hours then put inside a custom made high carbon steel die punch and uniaxially pressed for 2 min at 500 MPa to obtain pellets with approximately 10.0 mm diameter and 1.3 mm thickness. The removal of the binder was carried out at 500 C. for 1 hour at a ramp rate of 2 C. per minute from room temperature. Sintering was carried out at 1170 C. for 3 hours to 10 hours in air at a ramp rate of 2 C. to 5 C. per minute. The temperature was then ramped down at a ramp rate of 3 C. per minute. The formed 5 mol % La-doped KNN ceramic material had a high density; that is, a density of about 99 percent of the theoretical density.

Example 2: Crystal Structure of the Electro-Optical Devices

(11) FIG. 2 is a graph showing the X-ray diffraction spectra of the (K.sub.0.5Na.sub.0.5).sub.(1-x)La.sub.xNb.sub.(1-2x/5)O.sub.3 ceramic when 0<x<0.1. The filled square boxes indicate where peaks of X-ray diffraction spectrum appear as a perovskite phase. At all values of x measured, that is, when x=0 (202), x=0.01 (204), x=0.02 (206), x=0.04 (208), x=0.05 (210), x=0.07 (212) and x=0.1 (214), the compositions appear to be single phase. Only minor shifts are observed in the peak positions indicating small changes in the lattice parameters. Further, the similarity of the diffraction spectrum to that of perovskite suggests that similarly to perovskite, the 5 mol % La-doped KNN ceramic material also has an ABO.sub.3 type crystal structure.

Example 3: Evaluation of the Electro-Optical Property

(12) The set-up used to evaluate the electro-optical property of the 5 mol % La-doped ceramic material is depicted in FIG. 3. The wavelength of light employed in this experiment ranged from 190 nm to 3200 nm, covering the deep UV-region to the far IR region. The experiment was carried out at room temperature and the incident light was non-polarized. An electrical field was applied across the 5 mol % La-doped KNN ceramic material (404) through indium tin oxide electrodes (402) and (406) deposited on either side of the 5 mol % La-doped KNN ceramic material (404) by magnetron-sputtering at room temperature.

(13) The observed IR properties of the 5 mol % La-doped KNN ceramic material is depicted in FIG. 4. Light with wavelengths shorter than about 750 nm (in the UV-Vis range), was found to be completely absorbed by the 5 mol % La-doped KNN ceramic material. However, the transmittance was observed to increase as the wavelength of the light increased, until it was observed to sharply decrease at about 2750 nm. Effectively, the 5 mol % La-doped KNN ceramic material was found to have a transparency window in the near IR region. Furthermore, the transmittance of light within the near IR transparency window was found to be tuneable by an external electric field, while maintaining the strong absorbance outside the transparency window. This could be done by applying a bias on the indium tin oxide (ITO) electrode pairs (402) and (406). The tuneability of transparency by applying an electric field or a bias across the electrodes is due to the change in the bifringence in the ceramic material. This in turn changes the unit cell structure and the geometrical bonding arrangements of the atoms within the material, in a reversible process called electrostriction. Beyond a certain threshold of electrostriction, the material may undergo electrical breakdown. In tuning the transmittance of the transparency window, the transmittance was found to increase as the externally applied voltage was increased. Maximum transmittance was observed when a voltage of +12.50 kV/cm was applied, where a 40 percent transmittance was observed at a wavelength of approximately 2250 nm.

(14) The inventive ceramic material is anisotropic and bifringent. Therefore, the polarization in such materials depends on both the direction and the magnitude of the externally applied electric field. The observed maximum transmittance is believed to be due to a change in the refractive index resulting from the maximum polarization in the materials at a particular electric field that is applied.

Comparative Example 1: Comparison Between Un-Doped KNN and 5 mol % La-Doped KNN Ceramic Materials

(15) FIG. 5(a) shows the SEM micrograph showing the surface morphology of un-doped KNN ceramic material sintered at 1080 C. for 2 hours. The cubical grain sizes are in the range of 10 to 20 microns and are large. In addition, isolated pores (301) are observed which is the likely cause of poor ceramic density.

(16) FIG. 5(b) in contrast shows the 5 mol % La-doped KNN ceramic material sintered at 1170 C. for 3 hours. The high transparency observed in the 5 mol % La-doped KNN ceramic material may be attributed to the removal of pores during the sintering process. The presence of La may have contributed to the acceleration of the grain growth. The grains in the 5 mol % La-doped KNN ceramic material was found to be uniformly distributed and the grain boundaries were observed to be much shorter than that observed in un-doped KNN ceramic material. The average value of the grain boundaries in 5 mol % La-doped KNN ceramic material was found to be about 30 to 50 nm while that of the un-doped KNN ceramic material was found to be about 1 micron.

(17) Further, the density of the 5 mol % La-doped KNN ceramic material was found to be higher than that of the un-doped KNN ceramic material. 5 mol % La-doped KNN sintered for 3 hours and 10 hours was found to densities of 97 percent and 99 percent of the theoretical density, respectively. In contrast, the un-doped KNN was found to have a density between 70 percent to 89 percent of the theoretical density. Since the transparency largely depends on the presence of pores in the sintered body, removal of pores in the 5 mol % La-doped KNN ceramic material caused the increase in transparency.

(18) It should be noted that the highest transparency was observed when x=0.05, that is for the 5 mol % La-doped KNN ceramic material with the formula (K.sub.0.5Na.sub.0.5).sub.0.95La.sub.0.05Nb.sub.0.98O.sub.3.

Comparative Example 2: Comparison Between Natural Solar Radiation, ZnO Semiconductor and 5 mol % La-Doped KNN

(19) The IR properties of the 5 mol % La-doped KNN (602), ZnO semiconductor (604) and natural solar radiation (606) in the same wavelength range as Example 3 are compared in FIG. 6. ZnO was chosen as a reference as it is a commonly used transparent semiconductor, which can easily be fabricated into a conductive transparent ceramic or a conductive transparent film. FIG. 6 shows that compared to the ZnO semiconductor, the 5% La-doped KNN is more effective at absorbing wavelengths in the UV-Vis region while transmitting wavelengths in the near IR region of natural solar radiation.

Applications

(20) The disclosed ceramic material comprises La which may improve the densification of the device, leading to better electro-optic properties.

(21) The disclosed ceramic material comprises La in a specified amount which may decrease the overall charge neutrality of the final product.

(22) The disclosed ceramic material may have superior electro-optic properties such as transparency, large and rapid electro-optic response and a wide window of transparency.

(23) The disclosed ceramic material may have a transparency window in the near IR range.

(24) The disclosed ceramic material comprises La which may improve the densification of the device, leading to more facile fabrication of the material.

(25) The disclosed ceramic material may be fabricated using conventional solid-state ceramic synthesis techniques which are not suitable for fabricating conventional KNN-type ceramics.

(26) The disclosed ceramic material may be fabricated at lower temperatures than conventional devices.

(27) The transparency and light-transmittance properties of the disclosed electro-optical device may be tuned by applying an external electric field.

(28) The disclosed electro-optical device may be electrically turned on and off.

(29) The disclosed electro-optical device may act as an electrical switch.

(30) The disclosed electro-optical device may be operable at room-temperature.

(31) The disclosed electro-optic device may lead to cost-savings as it is low-cost to both operate and manufacture than conventional KNN-devices.

(32) Accordingly, the disclosed electro-optic device may be used in numerous applications, including but not limited to, infrared inspection windows (viewports) for IR thermography, UV absorbing materials, optical coating and filter for radiation in the UV to mid-IR range, tuneable optical filters, light shutters, light modifiers and in colour rendering.

(33) Other applications include the use of the disclosed electro-optic device in green buildings, bio-medical applications, food packaging and as a moisture barrier.

(34) In green building applications, the disclosed electro-optic device may be used to tune the inner brightness of a building by applying electric bias across windows coated or made with the disclosed ceramic material.

(35) In food packaging applications, the disclosed electro-optic device may be used to package foods which are sensitive to light. The disclosed ceramic material may be used to limit the transmittance of light through the packaging and allow the food to be stored longer.

(36) It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.