Transparent Ceramics

Field of the Invention

The present invention relates to poreless ceramics which have excellent optical transmission properties.

Description of the Prior Art

Generally, poreless ceramics have excellent thermal, electrical, chemical and optical properties. Transparent ceramic materials can be used in the fabrication of, for example, crystron tubes, power tubes, antenna windows, integral circuit substrates, high pressure gas-discharge tubes, rocket nozzles, and the like. Currently, many transparent ceramic materials such as Al.sub.2 O.sub.3, MgO, Y.sub.2 O.sub.3, BeO, Gd.sub.2 O.sub.3, LiAl.sub.5 O.sub.8, CaO, ThO.sub.2, MgAl.sub.2 O.sub.5, (Pb,La) (Ti,Zr)O.sub.3 and the like, are known. However, the conventional transparent ceramic materials all have problems concerning their dielectric constants and thermal constants. In addition, the conventional ceramic materials have to be fabricated under hot pressing conditions or special sintering atmospheres and at high sintering temperatures of about 2000.degree. C. Therefore, the cost of these materials becomes expensive because of the necessity of indispensible, complicated facilities and because of difficulties in manufacturing.

A need, therefore, continues to exist for a ceramic material which is inexpensive to prepare and which possesses good optical transmission characteristics and dielectric properties.

Accordingly, one object of the present invention is to provide poreless ceramics having good optical transmission characteristics over a wide wavelength range and excellent dielectric properties.

Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a transparent, complex oxide ceramic having the formula:

wherein Me' is at least one element selected from the group of Ca, Ba, and Pb; Me" is Nb and/or Ta and x is 0.01 to 0.50.

The oxide compound of the present invention is a transparent binary oxide of the perovskite-type having the formula:

wherein Me' is at least one element selected from the group of Ba, Ca and Pb, and Me" is Ta and/or Nb. This composition can be prepared from mixtures of raw materials by conventional ceramic fabrication techniques.

The transparent ceramic materials of this invention may be prepared in the following manner. The raw materials such as SrO, BaO, CaO, PbO, Li.sub.2 O, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5 are accurately weighed out in prescribed amounts, are mixed in a ball mill and calcined at 900.degree.-1100.degree. C.

It is most desirable that the raw materials be of high purity. Suitable raw materials include such metal compounds as the hydroxides, carbonates and oxalates all of which can be converted to the corresponding oxide upon heating. The calcined powder is then pulverized. The powder thus obtained is mixed with a binder, such as water or polyvinyl alcohol, and the mixture is shaped under a pressure of about 0.5-2 tons/cm.sup.2 into, for example, a plate having a diameter of 20 mm and a thickness of 1 mm. The plates are then sintered under temperature conditions of 1150.degree.-1450.degree. C. over a period of 2-4 hours, preferably under an oxygen atmosphere.

The composition of the present transparent ceramic material is defined by the formula:

wherein Me' is at least one element selected from the group of Ca, Ba and Pb; Me" is Ta and/or Nb, and x is 0.01-0.50. The preferred transmission levels are obtained over a compositional range for x of 0.01-0.50. When x .ltoreq. 0.01, the ceramic compositions cannot be sintered to a sufficient density, and when x .gtoreq. 0.50, a second phase is produced.

Having generally described the invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limited unless otherwise specified.

Various mixtures of SrCO.sub.3, PbO, CaCO.sub.3, BaCO.sub.3, Li.sub.2 CO.sub.3, Ta.sub.2 O.sub.5 were weighed out so as to obtain a series of materials having the formula (Sr.sub.1-x Me'.sub.x) (Li.sub.1/4 Me".sub.3/4)O.sub.3, wherein x is 0-0.60. The components of each composition were mixed well in a ball mill. Each powder mixture was calcined at 1050.degree. C. for 1 hour, and the resulting mass was pulverized into a powder. Then, each powder sample was mixed with polyvinyl alcohol as a binder, and each mixture was compressed into the shape of a plate of 20.sup..phi. x 1 mm at 1 ton/cm.sup.2. The green samples obtained were sintered at 1250.degree. C. for 2 hours in an electric furnace.

By this technique 161 transparent ceramic plates including control samples were prepared. The density (P%), and the transmission characteristics (transparency or transmittivity) T (%) at 700 m.mu. of each of the plates were measured. Both sides of the measured plates were polished to a mirror finish. After the transparency measurement, a silver electrode was baked on each side of the sample plates. The permittivity (dielectric constant) .epsilon. characteristics of each plate were measured at a frequency of 1 MHz as indicated in Table 1. The transmission characteristics of the present ceramic are superior to the transmission characteristics of conventional Al.sub.2 O.sub.3 ceramic.

The present transparent ceramic composition has many advantages when used in the following applications.

(1) It can be used as an optical element for flash lamp tubes and devices which use infrared rays, because the present ceramics exhibit excellent transparency to visible rays and infrared rays. Moreover, the present ceramics can be produced more easily and less expensively than the conventional Al.sub.2 O.sub.3 ceramics.

(2) The present ceramics are used in the substrates of high quality electronic parts which can be applied to integral circuit substrates, for example, if the particular ceramic has excellent electrical properties. Because the present ceramic materials are poreless materials, it is evident that a substrate can be provided with a flat surface which has a roughness less than about one tenth of the wavelength of the light. Both sides of a transparent ceramic substrate can be used as a substrate for complicated circuit devices.

(3) The transparent electro-optic ceramics of the present invention can be used in microwave frequency devices. It is possible to miniaturize circuits by use of the present high dielectric permittivity and low loss ceramics.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the scope or spirit of the invention as set forth herein.