Patent GB-849827-A (GB)
Title: Improvements in circuit elements for electronic data storage systems or the like
IPC: G11C11/22
Filed: 1957-01-15
Abstract: 849,827. Circuits of multi-stable dielectric elements. INTERNATIONAL BUSINESS MACHINES CORPORATION. Jan.15, 1957 [Jan. 17, 1956], No. 1464/57. Class 40 (9). [Also in Group XXXVI] A bi-stable circuit element comprises a crystal of material capable of existing in a first and second crystalline state at a particular temperature and in which transition between said first and said second crystalline state can occur in response to changes of electric field intensity, and means for maintaining the temperature of said crystal substantially constant at said particular temperature. According to the Specification barium titanate can exist in three different crystalline forms depending upon its temperature, the crystals being orthohombic below about 5‹ C., tetragonic between this temperature and about 120‹ C. (the Curie temperature) and cubic above the Curie temperature, and whereas at room temperature barium titanate exhibits the familiar rectangular hysteresis loop, Fig. 1, near the lower structural transition temperature three loops, Fig. 13, (4‹ C.) occur and near the upper structural transition temperature two loops, Fig. 4b (121 ‹ C.) occur. Of these hystereses loops those occurring in the 1st and 3rd quadrants are accompanied by a change of crystal structure. Thus at 121‹ C., Fig. 4b, at which temperature barium titanate has a cubic crystal structure, as a steadily increasing electric field is applied to it the polarisation increases steadily until point h is reached, when it rises rapidly to point k, the rise being accompanied by a rapid change in structure of the barium titanate from cubic to tetragonic. As the electric field is steadily decreased, the polarization steadily decreases to point e where it changes rapidly as the barium titanate returns from the tetragonic form to the cubic form. A similar cycle occurs at 4‹ C., Fig. 13, the rapid rise in polarisation at point h being accompanied by a change of structure from orthohombic to tetragonic and the rapid decrease at point e being accompanied by a change of structure from tetragonic to orthohombic. According to the Specification as an excursion around any one of these loops involves changes in crystal structure, the variations in the electric field required to produce such an excursion are precisely defined, and therefore, continues the Specification, by applying a suitable biasing voltage Eb, to the barium titanate bi-stable operation can be achieved without the defects of " hysteresis loop-decay " (or fatigue) and " hysteresis walking " both of which are present in the known devices employing the centrally disposed hysteresis loop. A further advantage also accrues from the use of these loops in that since the slopes of the upper and lower portions of the loops differ appreciably the effective capacitance of a barium titanate capacitor when in state c, is different from that, when in state d, and thus a non-destructive read out can be obtained by varying the applied voltage within the range Ep to Ef, Fig. 4b, or Eo to Et, Fig. 13. A circuit Fig. 10 employing a barium titanate capacitor 50, maintained at 121‹ C. by a thermostatically controlled oven 70 is described, which can function either as a single-bit digital data store having non-destructive read out or as an and gate, or as a trigger circuit. The barium titanate capacitor 50 has a steady biasing voltage Eb, Fig. 4b, applied to it by a battery 72, and can be set in state d, Fig. 4b, by depressing a key 75 or in state c by depressing a key 80. A continuous indication of the state of the barium titanate capacitor is available at a terminal 113 for the duration of the period for which a key 102 is depressed, depression of this key applying inductively a high frequency voltage to a coil 100 arranged to resonate with the capacitor when in its d state (the voltage being less in amplitude than Ec, Fig. 4b), and the voltage developed across the condenser as a result of this being amplified, rectified and smoothed, and passed to terminal 113. When the circuit is used as a trigger circuit the key 102 is maintained depressed. The trigger is then turned on by depressing key 75 and turned off by depressing key 80.