Patent GB-665452-A (GB)
Title: Method and apparatus for making artificial piezoelectric elements
IPC: H01L41/253
Filed: 1948-02-29
Abstract: 665,452. Piezo-electric crystals. TECHNICAL ASSETS, Inc. Feb. 29, 1948 [April 8, 1944], No. 6353/48. Class 40 (viii). Artificial piezo-electric elements are made by suspending par. ticles of a piezo-electric material in a liquid, semi-liquid or paste, orienting the particles and then separating the particles from the fluid while maintaining the orientation and forming them into a solid mass. By orienting before the solidification is commenced the forming of the element described in Specification 665,451 is facilitated and speeded and a greater concentration of piezo-electric material in the final element is obtained. In the arrangement shown in Fig. 1 the piezo-electric particles mixed in the suspending fluid (e.g. paraffin) in the upper cavity are oriented by means of an electric field applied to supports 9 and electrode 131 and synchronized mechanical vibrations from vibrator 13. The oriented particles together with an inactive binder also in the mixture are then deposited on a filter element 1 supported on a finely perforated sheet 6 backed by a coarser grid 7. When filtration is complete the deposit is solidified by polymerizing the binder. Vibrator 13 may be of a piezoelectric or fluid-operated type the electric current or fluid power being introduced through shaft 14. Filtration may be speeded by pressure applied at inlet 9 or outlet 12. A static pressure and a constant electric field may in some cases be used instead of the oscillating pressure and field though if the particles are thermo-electric the pressure may be unnecessary as polarization of the particles may be effected by heating or cooling. In an alternative apparatus, Fig. 2 (not shown), the top 3 of the cavity is flexible and vibrator 13 is fixed to its outside surface. In another modification, Fig. 3 (not shown), a rubber membrane divides the cavity transversely, the liquid being introduced below it and compressed air above. The liquid may alternatively be extracted in a centrifuge (Fig. 5) having an outer wall electrode 38 lined with a material 40 on to which the particles from the mixture fluid in cavity 39 are to be deposited. An inner elastic wall 42 acts as the second electrode and is vibrated by fluid within this wall under the action of a ram (not shown). The ring of piezo-electric material formed may be used in that shape or opened out and cut into slabs. In a further alternative, separation of the suspended particles is effected by sedimentation, Fig. 4 (not shown). Various methods of unifying the particles into a solid mass are suggested. The residual liquid after filtration may be solidified as by drying or cooling. Alternatively the deposit may be thoroughly dried and a binder may be introduced as a liquid, or where a binder is mixed in the deposit this may be solidified by polymerizing or melting and cooling, under pressure, if necessary. With high melting-point crystals (e.g. quartz or tourmaline) a low coefficient of expansion glass binder may be used and with tourmaline, amorphous quartz is suggested. In some cases the crystal material itself may be fused. Transfer to the solidifying apparatus is facilitated if the filtrate is bound temporarily with melted wax or resin which may later be evaporated off or a colloid may be used which does not pass through the filter but which readily disappears during the heating phase of solidifying. Orientation is better maintained before solidification if the particles are rolled spherical before being mixed with the fluid. Specification 311,055, [Class 40 (v)], also is referred to.