US3470100A - Growth of piezoelectric bismuth oxide - Google Patents

Description

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. 1 ,5 tinitcd i 3,470,100 GROWTH OF PIEZOELECTRIC BISMUTH OXIDE Albert A. Ballman, Woodbridge, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York No Drawing. Filed Jan. 25, 1966, Ser. No. 522,840

Int. Cl. H01v 7/02; B01j 17/00; C04b 35/00 US. Cl. 252-629 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to a technique for the growth of single crystal bismuth trioxide (Bi203). More particularly, the present invention relates to a technique for the growth of optically active bismuth trioxide manifesting high piezoelectric activity.

Thus far, it has been concluded that bismuth trioxide exists in four crystallographic modifications, namely, (a) a stable low temperature monoclinic form designated oc-Bi O (b) a metastable body-centered cubic form designated 'y-Bi O (c) a tetragonal form designated {i-Bi O and (d) a single cubic form. Although some interest has been generated in these compositions, the literature has heretofore been totally silent with regard to the electrical properties thereof, so negating its potential in the electronics industry.

In accordance with the present invention, a technique is described for the preparation of single crystal Bi O manifesting optical activity photoconductivity and high piezoelectric activity, so suggesting its use in electrical optic devices, electromechanical transducers or in acoustic amplifiers. The inventive technique involves growing from a high purity melt comprising Bi O and small quantities of at least one oxide selected from among the dioxides of germanium and silicon or the oxides of gallium, titanium and aluminum. The resultant single crystal composition is of good quality, evidences electromechanical coupling coefficients in excess of 25 percent and is found to belong to the rare point group 23.

An important aspect of the present invention lies in the use of specific melt compositions, for example, those containing critical proportions of Bi O and additive oxide, hereinafter designated MeO for convenience. In the growth of single crystal Bi O as described, it is essential that the mol ratio of Bi O to MeO be within the range of 3:1 to 12:1. The preferred range is from 5:1 to 7:1, an optimum being found to correspond with the approximate mol ratio of Bi O /MeO of 6: 1.

It has been determined that the use of mol ratios of Bi O /MeO less than the noted 3:1 minimum results in an increased incidence of bismuth compounds other than the desired Bi O Similarly, studies on the growth of Bi O have revealed that the use of mol ratios in excess of the 12:1 maximum result in the formation of the on form 0f Bi203.

In accordance with the inventive technique, it has been determined that the desirable physical and electrical properties discussed hereinabove can successfully be generated in Bi O obtained by any prior art procedure. However,

:ented Sept. 30,1969

it has been found essential to utilize high purity starting materials. Thus, it has been found that the Bi O source material must evidence a minimum purity of 99.9 percent. Similarly, the oxide materials employed in the practice of the present invention must necessarily evidence a purity of at least 99.9 percent. Studies have revealed that slight deviations from these minimum impurity levels result in failure to generate either piezoelectric or optical activity in the resultant Bi O With regard to the oxidic materials found suitable in the practice of the present invention, it has been found that at least one compound from among germanium dioxide (GeO silicon dioxide (SiO gallium oxide (Ga O titanium oxide (TiO and aluminum oxide (A1 0 is required to obtain the desired properties in Bi O Examples of the application of the present invention are set forth below. They are intended merely as illustration and it is to be appreciated that the processes described may be varied by one skilled in the art without departing from the spirit and scope of the invention.

The examples are in tabular form for convenience and brevity. Each set of data in the table is to be considered as a separate example since each set of data was obtained in a separate process. The procedure followed in the examples is as follows:

A mixture of the starting materials, obtained from commercial sources, was weighed into a platinum crucible and heated to a temperature of the order of 935 C., the melting point of the mixture. Heating was effected by coupling the crucible with an RF induction heater. The crucible,

together with its contents was then permitted to attain a temperature of 935 C. at which point the charge was entirely liquid. Next, a 30 ml. platinum wire was inserted into the melt. The Czochralski technique of pulling crystals from the melt was then employed to grow Bi O to one-half inch diameter at a growth rate of one-half inch per hour. In order to obtain preferred orientation single crystal Bi O obtained in this manner was subsequently employed in seed crystal form and Bi O grown upon the seed. The data set forth below is based upon Bi O grown upon seed crystals.

After cooling, the crystals were tested qualitatively for piezoelectric activity by means of the well known Giebe- Scheibe test. Thereafter, optical activity was determined by passing light of a fixed frequency (white light) through a given thickness (1 mm.) of Bi O and bringing it to extinction. Following, rotation was effected to extinction again and the rotation for a given thickness measured.

Photoconductivity was determined by connecting the output from the piezoelectric detector (employed in the Giebe-Scheibe test) to an oscilloscope, resonant frequency of the Bi O crystal being evidenced by multiple peaks. Following, the crystal was exposed to a white light source, so resulting in the loss of the peaks and indicating a change in conductivity of photoconductivity.

Finally, the electromechanical coupling coefficient of the Bi O was measured, that is, the degree of efficiency of the piezoelectric body in transforming electrical energy to mechanical energy was determined. In order to determine the coupling coefficient a slice in disk form was taken off a Bi O crystal normal to the growth direction and electroded by applying silver paste to the major faces of the disk and firing by conventional techniques. Following, the electroded crystal was studied by scanning the frequency spectrum in search of a strong resonance point. The coupling coefficient was determined by observing the frequency shift from resonance to anti-resonance and computing it by means of well known formulae.

TABLE I Coupling B1 0 Purity, MeO Purity, CoeIIicient, Optical Photo- Example (grams) Percent (grams) Percent Product Giebe-Scheibe Percent Activity conductivity 2,795.66 99.9 Geog-104.59 v- 99.9 B1203 25 22lmn1.

2,795.66 99.9 Gu m-187.44 99.9 B1203 25 22,mm.

2,795.66 99.9 Bios-60.09 A. 99.9 131 0 25 22/mm.

+ :positive.

What is claimed is:

1. Single crystal, optical active piezoelectric composi- 1 References Cited Kroger: Some Aspects of the Luminescence of Solids, 1948, page 264.

Penkov et al.: Effect of Impurities on the Nuclear Quadrupole Resonance Spectra of the aand 7-M0dlfic2ltion of Bi O Soviet physics, solid state, v01. 7, N0. 1,

July 1965, pages 145-147.

Gattow et al.: Ceramic Abstracts, page 2381', 1965.

15 =TOBIAS E. LEVOW, Primary Examiner R. D. EDMONDS, Assistant Examiner US. Cl. X.R. 252-501 @2 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: No. 3, WO,lOO Dated September 30, 1969 Invent fl Albert A. Bellman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, column 3, line 10, change "optical" to --optically--.

SIGNED ANu SEALED DH; 2 3 19$ (SEAL) Atteat:

EdwardMFletcher, Jr.

WILLIAM E. SGHUYLER, JR. Attestmg Officer Oomissioner of Patents