Method for bonding together hollow glass spheres

Abstract: A method for bonding together adjoining hollow glass spheres to form an acoustic matching member for a transducer comprises subjecting a mixture of adjoining spheres and a solution of a heat curable resin and a volatile organic solvent to a first temperature to drive off the solvent and then to a second temperature to cure the resin. This causes the spheres (6) to bond together at their points of contact but otherwise voids (7) are formed between the spheres (6).

Description: The present invention relates to a method for bonding together hollow glass spheres to form a transducer. DE-A-3431620 relates to an ultrasonic transducer comprising a piezoelectric element, a loading ring and an impedance matching member all bonded together, wherein the member consists of hollow glass balls bonded together with an epoxide material, the mass ratio of balls to epoxide being so selected that any cavities necessarily present between the balls are at least largely filled up. DE-A-3622780 relates to a heat insulating, translucent bonding element in which the element contains hollow glass balls which are bonded together by a thin layer of a polymer adhesive in such a way that gaps remain between the balls. GB-A-1530347 relates to a transducer comprising a layer of material, an oscillator element secured to said layer and operable to be oscillated at a resonant frequency of said element; and an annular loading element enclosing said oscillator element and secured to said layer, the thickness of said layer being equal to a quarter of the wavelength in the material of the layer corresponding to said frequency or an odd integral multiple of said quarter and the density and mass of said loading element being greater than, respectively, the density and mass of said layer. GB-A-2225426 discloses a method for bonding together hollow glass spheres to form an acoustic matching member for a transducer. In the method the spheres, e.g. the microspheres or bubbles, are placed in a mould to form a matrix or bed which is heated to a temperature where the glass softens and is then compressed to reduce the overall volume occupied by the spheres so that adjoining spheres bond together as by sintering. The temperature conditions and the compression conditions are so controlled as deliberately to cause the formation of voids between those sphere surfaces which have not bonded together because of the spherical shape of the spheres. It is important that a high percentage of the matrix is occupied by voids so as to produce a low acoustic impedance (3 x 10 5 or lower) to provide acceptable acoustic coupling between a gas and the piezo element of the transducer. It is also important that bonding between adjoining spheres is substantially complete. If too many touching spheres are not bonded to their neighbours the impedance becomes unpredictable and varies with acoustic intensity. Unfortunately with the bonding method described in GB-A-2225426 it has been found that certain spheres remain unbonded to their neighbours, particularly those in the centre of the matrix and distant from the source of compression during sintering. Furthermore it has been found that in use spheres may unbond from their neighbours. It is therefore an object of the present invention to provide a method for making a transducer where the degree of unbonding described is reduced while the voids are retained. According therefore to one aspect of the present invention, we provide a method of forming a transducer comprising pouring a mixture of adjoining hollow glass rigid spheres (6) and a solution of heat curable resin and a volatile organic solvent into a mould, placing a piezo element (1) on top of the mixture, subjecting the mixture to a temperature sufficient to drive off the solvent and cure the resin thereby causing the adjoining spheres (6) to bond together, the size and packing density of the spheres (6) and the concentration of the resin in the solvent being so chosen that voids (7) are otherwise left between external surfaces of the spheres (6) away from points of contact and subjecting the mixture to a compressive force to urge the element (1) towards the mixture to compress the mixture and to adhere to the mixture after curing, characterised in that the size of the spheres (6) ranges between 20 and 130µm, the packing density of the spheres (6) lies within a range of between 10 and 100% of fully close-packed, the ratio of the resin by weight to the solvent in the solution lies within the range 1:1 to 1:10 and the temperature is above the boiling point of the solvent. Preferably the mould and the element are both cylindrical. Suitably before the mixture is poured into the mould, a layer (5) to serve as a protective front face for the transducer is placed in the mould, the layer (5) subsequently adhering to the mixture when the resin is cured. Conveniently, the glass is C-glass. Preferably, the resin is an epoxy. Suitably, the solvent is acetone. Conveniently, the layer is a disc (5). Preferably, the layer is a cup. An embodiment of the invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a perspective view from the side of a transducer, Figure 2 is a general side view, Figure 3 is a view along lines A-A of Figure 2, Figure 4 is an amplified view of the matching member of the transducer, and Figure 5 is a further amplified view of the microstructure of the circled portion of the matching member of Figure 4. Referring to Figures 1 to 4 the transducer comprises a piezo element, for example, of type PZT4 or PZT5A, in the form of a PZT cylinder 1 with electrical connecting wires 2, 3 (Figure 1), an acoustically matching member 4 for the cylinder 1 and a protective layer for the member 4 in the form of a thin disc 5 (see Figures 2 to 4). The matching member 4 which is in the form of a cylinder affixed to one end face of the PZT cylinder 1 has one of the wires 2 connected to its circumferential wall while the other wire 3 is connected to the other end face of cylinder 1. As shown in Figure 5, the matching member 3 comprises a close packed matrix of glass bubbles or microspheres 6, adjoining bubbles 6 being bonded together by a resin in the region of points of contact, while voids 7 are deliberately formed between the external surfaces of the bubbles 6. The bubbles 6 are of the type A16/500 manufactured by 3M and known as C-glass. The bubble size ranges between 20 and 130µm, average size 60µm. The resin is an epoxy. A typical resin is ER1040 made by Design Resins Ltd. This is a two-part high temperature curing epoxy. The disc 5 is of polyimide (trade name Kapton) which may be 0.07mm thick. Instead of a disc a cup-shaped layer may be used to offer side as well as end protection. Instead of a plastics material a metal could be used as the protective layer if thin enough e.g. aluminium. The transducer is manufactured in the following way. Firstly, the resin is dissolved in a suitable solvent such as acetone. The ratio of the resin by weight to the solvent in the solution lies within the range (1 to 10) to (1 to 1) for example, 1:2. The viscosity of such a solution may be little more than the viscosity of the solvent, for example in the range 0.3 to 10 centipoise. The glass bubbles are then added to the solution to "absorb" all the solution i.e. until the final resin/solution mix is stiff with no visible liquid content. The packing density of the bubbles at this stage approaches the close-packed state. The protective disc is then inserted through the open end of a known type cylindrical mould which may be of silicone rubber until the disc rests on the base thereof. The mould has the same internal diameter as the external diameter of the PZT cylinder. A quantity of the bubble/solution mixture is then transferred by means of a spatula say into the mould leaving an uppermost cavity into which is next inserted the PZT cylinder. A reasonably constant pressure is then exerted on the cylinder by means of a known spring loaded plunger to urge the cylinder towards the bubble/solution mix to compress it. The mould is then heated to a temperature above the boiling point of the solvent in order to drive it off. In the case of acetone solvent this may be 120°C. Then the mould is heated to a temperature sufficient to cure the resin which in the case of the two-part epoxy described may be the same 120°C or up to 180°C while the pressure is still applied. This procedure provides a transducer of the type previously described with a cylindrical matching member located between a PZT cylinder and a protective disc, both of which are affixed by the resin to the member.

Claims: The resin causes the spheres to bond to each other at their points of contact but otherwise voids are left between the spheres. In this way the matching member may form a ¼ wavelength acoustic coupling for the transducer if of the correct length for the PZT cylinder. A method of forming a transducer comprising pouring a mixture of adjoining hollow glass rigid spheres (6) and a solution of heat curable resin and a volatile organic solvent into a mould, placing a piezo element (1) on top of the mixture, subjecting the mixture to a temperature sufficient to drive off the solvent and cure the resin thereby causing the adjoining spheres (6) to bond together, the size and packing density of the spheres (6) and the concentration of the resin in the solvent being so chosen that voids (7) are otherwise left between external surfaces of the spheres (6) away from points of contact and subjecting the mixture to a compressive force to urge the element (1) towards the mixture to compress the mixture and to adhere to the mixture after curing, characterised in that the size of the spheres (6) ranges between 20 and 130µm, the packing density of the spheres (6) lies within a range of between 10 and 100% of fully close-packed, the ratio of the resin by weight to the solvent in the solution lies within the range 1:1 to 1:10 and the temperature is above the boiling point of the solvent. A method as claimed in claim 1, characterised in that the mould and the element (1) are both cylindrical. A method as claimed in claim 1 or claim 2, characterised in that before the mixture is poured into the mould, a layer (5) to serve as a protective front face for the transducer is placed in the mould, the layer (5) subsequently adhering to the mixture when the resin is cured. A method as claimed in any of the claims 1 to 3, characterised in that the glass if C-glass. A method as claimed in any of the preceding claims, characterised in that the resin is an epoxy. A method as claimed in any of the preceding claims, characterised in that the solvent is acetone. A method as claimed in any of claims 3 to 6, characterised in that the layer is a disc (5). A method as claimed in any of claims 4 to 7, characterised in that the layer is a cup.

Inventors: GILL MICHAEL JOHN, GILL, MICHAEL JOHN
Applicants: BRITISH GAS PLC, BG PLC
Application year: 1991
Main classification: C03C27
IPC codes: C03C27, H04R17, G10K11, C04B26, G01S7, B06B1