Patent GB-753618-A (GB)
Title: Forming electromechanically sensitive ceramic bodies
IPC: C04B35/46
Filed: 1954-03-15
Abstract: 753,618. Piezo-electric elements. CLEVITE CORPORATION. March 15, 1954 [March 18, 1953], No. 7449/54. Class 40 (8). [Also in Group V] Relates to the use of ceramic bodies in electromechanical transducing devices and consists in forming a body of electromechanically-sensitive ceramic material by a method which comprises (i) applying a liquid coagulant to a thin unrefractory support having apertures between its marginal edges, (ii) causing contact between the support and a dispersion of electromechanicallysensitive ceramic raw material to coagulate a coating of ceramic material which covers the support to a predetermined thickness and fills the apertures therein, (iii) heating the coated support with firing of the coating and elimination of the unrefractory support to leave a non-composite coherent body of electromechanically sensitive material with surface portions joined by connecting portions. In the example described each unrefractory support comprises six nylon threads 14, each thread being a twist of three filaments and wound on a spool 12. Eighteen spools are shown, mounted on a board 11, and the threads are unwound from them by the traction of rollers 36, 37 driven by a motor'38. The threads pass through a guide 13 comprising slots or holes in a horizontal row, between rollers 19, 21 which may be moved relatively to adjust their grip on the threads and which dip into coagulant in a bath 16, through spaces formed by serrations in the edge of a plate 45, Fig. 2, at the bottom of a container 27, through a layer of mercury and then through a dispersion of barium titanate resting on the mercury in the container and over a guide roller 34 to the rollers 36, 37. Each group of six threads 14 passes through a corresponding group of six indentations in the serrated edge, with several unoccupied indentations between, with the result that the threads converge and form groups which become coated in the container 27 to form elongated structures 40-42. The coating material in these structures is dried by heat lamps 43. As the threads pass the edge of the plate 45 they pass across the edge of a felt strip 48 which dips into a trough 47 containing water and are thereby moistened, this felt strip also pressing the threads into the indentations. The threads have already been coated with coagulant from the bath 16, a suitable coagulant being an aqueous solution of ammonium pentaborate, as described in Specification 663,424. The dispersion in the container 27 is replenished constantly from a reservoir 32. A suitable dispersion comprises a slurry containing comminuted barium titanate and a small proportion of other oxidic materials, which act as fluxes during the subsequent firing, dispersed in water. A dispersing agent is added, comprising sodium liquor sulphonate and an aqueous solution of polyvinyl alcohol which is coagulated or gelled from the solution with the dispersed material as a binder to add strength. The ends of the roller 21 are mounted in brackets 23 and the ends of the roller 19 in brackets 22, and an adjusting screw 24 alters the distance between the brackets and so adjusts the pressure between the rollers. The pressure of a brake 26 on the roller 21 may also be varied, to adjust the tautness of the threads under the pull of the rollers 36, 37. The layer of mercury at the bottom of the container 27 acts as a seal and prevents the dispersion from running down the threads and causing premature coagulation and lumps in the structure. The structures descending from the rollers 36, 37 are cut into lengths which are placed in a furnace which fires the ceramic coating material and burns out the nylon support, resulting in bodies such as that shown in Fig. 3, in which surface portions 52, 53 are joined by connecting portions 54. Electrodes may be introduced in the holes 56 by allowing a liquid suspension of conductive particles to be drawn into them by capillary action, and a similar suspension may be painted on the outer surfaces. Polarizing potential may then be applied, as described in Specifications 753,617 and 753,619 to result in a bender transducer device. More spools of thread may be used, to the limit when all the indentations in the serrated plate are filled with threads -and a single structure is formed from which short lengths are cut. Instead of a large number of threads a few flat strips of a suitable material, e.g. calendered parchment paper, may be used, with a suitable spacer instead of the plate 45, resulting in a ceramic body with a cross-section as in Fig. 5. Instead of a few separate strips, e.g. three, one strip, Fig. 6, may be used, with bridging portions 87 that obviate a spacer. The ceramic structure is cut after drying but before firing at lines 90, 91, 92, 93, located by notches left after coating due to indentations 88 in the strip. This form of strip gives a ceramic body with a cross-section as in Fig. 5. If a strip is used perforated all over its area a ceramic body as in Fig. 8 results, and if the strip is wide enough shapes may be inscribed in the surface of the structure before firing and cut or stamped out after firing. Instead of a continuous strip process a form may be dipped successively into coagulant and dispersion, kept flat by hanging a weight at one end, if necessary, and the form need not then be a strip but of any shape, e.g. circular, with tabs which may be broken away to result in gaps for the escape of burnt material and for the introduction of electrode material. Specification 663,423 also is referred to.