Abstract:
A piezoceramic based atomizer for atomizing high viscosity liquid is provided. The atomizer comprises a storing tank as a reservoir for storing the liquid, a receiving tank connecting to the storing tank, a piezoceramic based nozzle for forming micro-droplets of the liquid, and an external ultrasonic oscillation circuit. The nozzle comprises a liquid absorbing medium with at least one sunk area at one end for pumping and delivering the liquid, a holder for holding the absorbing medium, a micro-perforated membrane whose one side is attached on the side of the liquid absorbing medium having the sunk area, and an annular piezoceramic adhered to the other side of the membrane. The nozzle is configured such that the sunk area can balance the feeding rate and atomization rate of the liquid as well as hold the back pressure underneath the micro-perforated membrane, facilitating forming micro-droplets of the high viscosity liquid continuously.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     There are no related applications 
     FIELD OF THE TECHNOLOGY 
     The present invention relates generally to a device and methods used for forming micro-droplets or mist of high viscosity liquids. More specifically, the present invention relates to a piezoceramic based atomizer and its application for high viscosity liquids nebulization. 
     BACKGROUND 
     In many applications, such as disinfection, odour generation and air humidification, liquids or fluids are needed to be dispensed into micro-droplets or aerosols through atomization or vaporization. Normally, for medical purpose, such devices may contain a heating unit or an air compressing pump. However, these devices are generally limited by the decomposition of the medicinal products due to the high temperature, or by the risk of introducing external contamination because of the direct contact of air with medicine, or by the concomitant noise. 
     To satisfy the special demands of delivering medicines by respiratory pathway, the piezoceramic based atomization technology, i.e., by vibrating a micro-perforated membrane attached to a piezoceramic, is developed by manufacturers these years. Briefly, according to this technique, the micro-perforated membrane contact with a liquid is vibrated at an ultrasonic frequency by an activated piezoceramic, and then the liquid is drawn onto the top surface of the membrane, thus forming a film thereon. The liquid film absorbs vibrational energy from the membrane with vibrating direction perpendicular to the surface, and forms standing waves. As the amplitude of the given vibrational frequency is increased, the waves become unstable, and finally collapse at some critical point. As a result, micro-droplets of liquid are ejected from the films directed normally to the membrane. 
     However, most of the commercially available piezoceramic based atomization devices are designed for water-based or low viscosity liquids. Actually, many medicinal liquids possess relatively high viscosity, ranging from several tens to more than 300 centipoises (cP), and may not be water-based. Unfortunately, these high viscosity liquids cannot be atomized by aforementioned devices, owing to the fact that the viscous liquid film on the surface of the membrane clogs the micro holes and thus blocking the atomization. Therefore, it would be desirable to develop an effective method that capable of forming micro-droplets from high viscosity liquids by piezoceramic based atomization. 
     SUMMARY OF THE INVENTION 
     Accordingly, the first aspect of the present invention relates to a device for forming micro-droplets from high viscosity liquids. In particular, the device of the present invention is a piezocermaic based atomizer. The piezoceramic based atomizer for high viscosity liquids according to one embodiment of the present invention includes: a liquid storing tank used for storing the liquid and a liquid level control system connected to said liquid storing tank for maintaining the height of the stored liquid to a certain level; a receiving tank connected to the control system, used for receiving the liquid from the control system; a nozzle holder fixed to the receiving tank; a specially designed liquid absorbing medium inserted into the nozzle holder in a manner that only one end contacts with liquid, used for pumping and delivering the liquid received by the receiving tank; an atomization unit comprising a micro-perforated membrane and an annular piezoceramic that is adhered, e.g., glued tightly, to the micro-perforated membrane, where the micro-perforated membrane communicates with the other end of the liquid absorbing medium and vibrates at ultrasonic frequency driven by the annular piezoceramic, thereby ejecting droplets of liquid; an external ultrasonic oscillation circuit used for generating an ultrasonic frequency signal to stimulate the annular piezoceramic to drive the micro-perforated membrane to vibrate at said ultrasonic frequency. 
     One characteristic of the present invention is that the liquid is guided to the rear surface of the micro-perforated membrane by said liquid absorbing medium, which enables the atomizer to operate equally at any angle from vertical to horizontal. Accordingly, the liquid storing tank can be located below, around or above the atomization unit. The liquid absorbing medium can be made of various materials and have different shapes, but is preferably cylindrical. A surface of the liquid absorbing medium is in direct contact with one side of the micro-perforated membrane of the atomization unit, characterized by forming a sunk area in said surface. The sunk area can be either flat or curved, and have one or several separate pits. 
     The micro-perforated membrane can be made of a variety of materials, such as plastic, silicon and ceramic, but is preferably made of metal, like stainless steel, titanium and aluminum. The membrane can be flat or forming a protuberance of a dome in the central area. The membrane can be in a thickness ranging from 20 um to 400 um. The micro-perforation of the micro-perforated membrane can be processed by electroforming, lasing or chemical etching, ranging from 1 um to 300 um in equivalent diameter and from one to several thousands in quantity according to the demand. The total cross-sectional area of the perforations of the membrane is preferably smaller than the cross-sectional area of the annular piezoceramic. 
     According to other embodiments of the present invention, a mist guiding tube is provided. The guiding tube is assembled to the nozzle holder, orienting the mist to any demanded direction. The shape of the guiding tube body and the outlet geometry thereof can be arbitrary. 
     Before explaining the embodiments in detail, it is to be understood that the present invention is not limited in its application to the details of construction and to the assembling sequence of the components in the following description or drawing illustration. It is also understood that the terminology used herein are for better understanding and description and should not be regarded as limiting. At the same time, since various modifications and structural changes may be made by those skilled in the art basing on the conception of the present invention, it is important to include all such modifications in the claims as they do not depart from the spirit and scope of present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present invention will become apparent from the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view for the arrangement of the piezoceramic based atomizer for high-viscosity liquids according to a first embodiment of the present invention. 
         FIG. 2  is an exploded view of the piezoceramic based nozzle according to the first embodiment of the present invention. 
         FIG. 3  is a cross-sectional view for showing the arrangement of the piezoceramic based nozzle with a mist guiding tube according to the first embodiment of the present invention. 
         FIG. 4  is a magnified view of the piezoceramic based nozzle illustrated in  FIG. 2  marked with dashed circle. 
         FIG. 5  is a cross-sectional view of a liquid absorbing media according to a second embodiment of the present invention. 
         FIG. 6  is a perspective view of a liquid absorbing media according to a third embodiment of the present invention. 
         FIG. 7  is a cross-sectional view of an atomizing membrane according to a fourth embodiment of the present invention. 
         FIG. 8  is a cross-sectional of a horizontally arranged piezoceramic based nozzle according to a fifth embodiment of the present invention. 
         FIG. 9  is a perspective view of a piezoceramic based atomizer for high viscosity liquids according to the preferred embodiment of the present invention. 
         FIG. 10  is a block diagram of the controlling circuit of the piezo ceramic based atomizer according to the preferred embodiment of the present invention. 
         FIG. 11  is a schematic diagram of the controlling circuit of the piezoceramic based atomizer according to the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments of the present invention will now be described in detail with reference to the figures, in which like reference numerals correspond to like parts throughout.  FIG. 1  shows the cross-sectional view for the arrangement of the high viscosity liquid piezoceramic based atomizer according to a first embodiment of the present invention. As seen, the atomizer includes a liquid storing tank  2  containing a specially designed valve  3 , which can be controlled by a spring  9 . When installed, the liquid in the storing tank  2  will flow out through an outlet  4  into a storing trench  6 , since the valve  3  is pushed up by a cylinder  8   a . The liquid will stop flowing out from the storing tank  2  till the exit of the liquid storing tank  2  is sealed by the liquid. The groove  7  functions as both a liquid level observing window and pressure balancing inlet. The liquid storing tank  2  can be refilled from a filling cap  1 . 
     The atomizer illustrated in  FIG. 1  also includes a receiving tank  12 , connecting with the liquid storing tank  2  by connecting a channel  11 . The liquid level in the storing trench  6  keeps the same as that in the receiving tank  12 . When the liquid is atomized by the atomizer, the liquid level in the receiving tank  12  will lower, and accordingly, the liquid level lowers in the storing trench  6 . When the liquid level falls below the exit of the storing tank  2 , the liquid will flow out and air will flow in from the outlet  4  until the pressure is balanced. 
     As illustrated in  FIG. 1 , the atomizer also includes a piezoceramic based nozzle fixed in the receiving tank  12 , which is the core part of the atomizer.  FIG. 2  is an exploded view of the piezoceramic based nozzle according to the first embodiment of the present invention. As seen, the nozzle includes: a hollow cylindrical holder  14  in which a cylindrical liquid absorbing medium  15  is inserted in the center  22 ; a piezoceramic based atomization unit  16  with two conducting wires  17   a  and  17   b  stacked on the liquid absorbing medium  15 , being disposed in such a manner that one side of the micro-perforated membrane  16   b  contacts directly with the top surface (atomizing surface) of the liquid absorbing medium  15 ; and an annular cap  18  assembled closely with the piezoceramic based atomization unit  16  and cylindrical holder  14  by means of screws  25 . By this structure, it is easy to replace the atomization unit  16  when necessary. 
       FIG. 3  shows the cross-sectional view of the piezoceramic based nozzle of the first embodiment according to the present invention. In the center area of top surface of the liquid absorbing medium  15 , there is a cylindrical sunk area or inward hollow space  15   a . The bottom surface of the liquid absorbing medium  15  is immersed into the liquid in the receiving tank  12 , for absorbing liquid from slot  13  and then delivering the absorbed liquid to the top surface thereof. The absolute height between the top surface of the absorbing medium  15  and the liquid level in the receiving tank  12  are dependent on the practical applications. The liquid absorbing medium  15  may be made of any material, such as inkjet foam, that can absorb and hold liquid. The piezoceramic based atomization unit  16 , comprising an annular piezoceramic  16   a  adhered to a micro-perforated membrane  16   b , can synchronously vibrate with the external ultrasonic oscillation circuit (not shown) and is configured for atomizing liquid into droplets. The size and geometry of the piezoceramic based atomization unit  16  and the liquid absorbing medium  15  can be varied according to different applications, and the atomization unit  16  may include several groups of ceramic and membrane(s) stacked in cascade. The vibrating frequency of the piezoceramic based atomization unit  16 , driven by an external ultrasonic oscillation circuit can range from 50 kHz to 5 MHz, is dependent on the viscosity of the liquid and/or the desired size of the droplets. The external ultrasonic oscillation circuit can produce an output amplitude ranging from 50 V to 500 V. 
     The nozzle illustrated in  FIG. 3  also includes a mist guiding tube  20  that functions to guide the mist to preferred direction and recycle those droplets with large sizes. Those droplets with large sizes will flow down along the mist guiding tube  20  when contacting with its inner wall, and then to the receiving tank  12  through a gap  21  as illustrated in  FIG. 1 . The exit direction of the mist guiding tube  20  can be arbitrary, from vertical to horizontal, according to certain embodiments of the present invention. 
       FIG. 4  is the magnified view of selected area as marked in  FIG. 3 . The materials for the micro-perforated membrane  16   b  can be metals, ceramics or even plastics, provided that they have enough strength under high-frequency vibration, but metal is preferred in this example. The micro-perforation of the micro-perforated membrane  16   b  can be processed by different means but preferably by laser and chemical or electro-etching. The quantity of the micro-holes  16   c  can range from one to several thousands, depending on the desired atomizing intensity, and the diameter of the holes can range from 1 um to 200 um in equivalent size. The metal-based micro-perforated membrane  16   b  is firmly glued onto the surface electrode of the annular piezoceramic  16   a  by conductive adhesive. As shown in  FIG. 4 , the upper electrode of piezoceramic  16   a  is connected to one output electrode of the external ultrasonic oscillation circuit through a conducting wire  17   a , and the metal-based micro-perforated membrane  16   b  is connected to the other output electrode of the ultrasonic oscillation circuit by another conducting wire  17   b.    
     One characteristic of the present invention is that the micro-perforated membrane  16   b  is connected to a liquid absorbing medium  15 , instead of contacting directly with the liquid. When the micro-perforated membrane  16   b  is getting in contact with the liquid absorbing medium  15  saturated with liquid, the liquid will wet the lower surface of the micro-perforated membrane  16   b  which is in direct contact with the liquid absorbing medium  15  and then penetrate from said lower surface to the upper surface (atomizing surface) of the micro-perforated membrane  16   b  through the micro-holes  16   c  because of the capillary effect, and then forming a thin film of the liquid. Upon stimulated by the ultrasonic oscillation circuit, the micro-perforated membrane  16   b  is vibrating at certain frequency, and transferring the vibrating energy to the liquid film, forming a standing wave therein. When the amplitude of the standing wave reaches a critical value, droplets of liquid are ejected from the liquid film in a direction normal to the atomizing surface of the micro-perforated membrane  16   b . The crictial value of the amplitude of the standing wave is relative to the voltage. That is, the higher the voltage is used, the larger is the amplitude. After atomization occurs, the liquid will continuously be transferred to the atomizing surface of the membrane  16   b  from the liquid absorbing medium  15 . With the consumption of liquid, the liquid levels in the receiving tank  12  and the storing channel  6  will be lowered simultaneously. The liquid levels can be controlled by the liquid in the storing tank  2  in the manner described hereinbefore. The liquid absorbing medium  15  also functions as an effective cooling system that dissipating the heat produced by the atomization unit  16 . 
     Another characteristic of the present invention is that a specially designed cylindrical sunk area  15   a  is incorporated into the liquid absorbing medium  15  and disposed at the interface between the liquid absorbing medium  15  and the micro-perforated membrane  16   b , making the atomizer of the present invention suitable for high viscosity liquids. In traditional piezoceramic based atomizer, the perforations in the micro-perforated membrane are always blocked by the viscous film of high viscosity liquids, owing to the fact that the liquid feeding speed is faster than atomizing speed. On the other hand, this phenomenon also leads to overheating of the piezoceramic based atomization unit  16  if said specially designed cylindrical sunk area  15   a  is absent. The cylindrical sunk area  15   a  in the liquid absorbing medium  15  is configured to control the liquid feeding speed to the atomizing surface of the micro-perforated membrane  16   b , as well as to keep the back pressure below the membrane. The micro perforations in the membrane  16   b  can be viewed as “capillary tubes”, which exert capillary force to pump liquids from the bottom to the top of the membrane. Because of the sunk area  15   a  being disposed underneath the perforations of the micro-perforated membrane  16   b , these “capillary tubes” cannot pump liquid directly from the liquid absorbing medium  15 . As a result, the liquid feeding speed is balanced with the atomization speed by said sunk area  15   a , and the overheating of piezoceramic based atomization unit  16  can be suppressed simultaneously. At the same time, the sunk area  15   a  can also hold the back pressure below the membrane, ensuring the ejection of droplets occurs on the opposite surface. These properties enable the atomizer of the present invention suitable for high viscosity liquids. The diameter and the depth of the cylindrical sunk area  15   a  can be varied with different applications and liquids. 
     In addition, the geometry of the sunk area or space is not limited to cylindrical, and the number of the sunk areas needs not necessary to be one.  FIG. 5  is a cross-sectional view of a liquid absorbing medium according to a second embodiment of the present invention. As seen, an arc-shaped sunk space  26   a  is introduced in an absorbing medium  26 . The dimension of the arc-shaped sunk area or space can be varied with the viscosity of the liquids and the properties of the atomizer  FIG. 6  is a perspective view of another liquid absorbing medium according to a third embodiment of the present invention, in which four separate cylindrical sunk areas  27   a  are introduced in an absorbing medium  27 . The number of the sunk area(s) is determined by practical applications. 
     Further, the shape of the micro-perforated membrane needs not to be flat.  FIG. 7  is a cross-sectional view of a micro-perforated membrane according to a forth embodiment of the present invention. An arched protuberance  28   a  is formed in the central of the micro-perforated membrane  28 . The phrase “arched protuberance” should be understood to mean that the geometry of the protuberance may be flattened in to a plateau. The dimension of the protuberance can be varied with different applications. Micro-perforations  28   b  are also disposed in the central portion of the micro-perforated membrane  28  and intercalated with the arched protuberance  28   a.    
     It is important to note that the direction of the nozzle can be arbitrary, varying from vertical to horizontal.  FIG. 8  is a cross-sectional view of a horizontally oriented nozzle according to a fifth embodiment of the present invention. It is shown that, by using a bent liquid absorbing medium  29 , the nozzle can be placed horizontally. When the vertical end of the bent liquid absorbing medium  29  is immersed in the liquid, it will absorb liquid and then transfer to the other end contacting with the piezoceramic based atomization unit  16 . Thus, as discussed hereinbefore, droplets of liquid can be ejected from the nozzle horizontally when stimulated by the external ultrasonic oscillation circuit. 
     Hereinafter, the operation of the piezoceramic based atomizer for high viscosity liquid according to present invention will be briefly described. 
       FIG. 9  is a perspective view of the piezoceramic based atomizer according to the preferred embodiment of the present invention. Before switching on the atomizer, the storing tank  2  is filled with high viscosity liquid from the filling cap  1 . The viscosity of the liquid used in this example is about 200 cP. The present invention is capable of atomizing liquid with viscosity of 300 cP or above. At first, the liquid will flow into the storing trench  6  and then to the receiving tank  12  through connecting channel  11 . After the liquid flows into the receiving tank  12 , the liquid absorbing medium  15  will absorb the liquid and transfer the absorbed liquid to the micro-perforated membrane (e.g.,  16   b  as shown in  FIGS. 3 and 4 ), and finally reaches an equilibrium in which the respective levels of liquid in receiving tank  12  and storing trench  6  are equal. A button switch  30  is used to trigger the atomizer. When the button switch  30  is turned on, the electric power is supplied to an ultrasonic oscillation circuit (not shown in  FIG. 9 ), generating an ultrasonic stimulating signal to the piezoceramic based atomization unit  16  through conducting wires  17 . Accordingly, the annular piezoceramic and the micro-perforated membrane are vibrated at ultrasonic frequency, and then ejecting droplets of the high viscosity liquids. The preferred oscillating frequency is 120 kHz for the present invention, and the outer and inner diameters of the annular piezoceramic are 16 mm and 8 mm, respectively. A LED  32  is used to indicate the working status and the battery capacity. And a mini-USB port  31  is installed for charging. 
       FIGS. 10 and 11  are the respective block diagram and the schematic diagram of the controlling circuit of the piezoceramic based atomizer according to the preferred embodiment of the present invention. As seen in  FIGS. 10 and 11 , the external controlling circuit (or the external ultrasonic oscillation circuit) of a piezoceramic based atomizer for high viscosity liquids is composed of 13 functional modules (as shown in  FIG. 10 ). The circuit can be powered by either internal battery or external DC source, and the detailed working principles are described as follows: 
     For external DC source: the external DC source  101  is divided into two branches, one enters the charging module  103  to charge the internal battery  113 ; the other enters the voltage regulator  102 , then flows through the source selecting switch  105  to the coupling boost transformer  111  and the voltage regulator  106  that power MCU  107 . When triggering the switch  109 , the MCU  107  receives a working instruction and outputs a 100 kHz pulse signal. The pulse signal is then received by the boosting coil of the transformer  111 , generating a 100 kHz high-voltage pulse with amplitude in the range of 50 V-500 V. The high-voltage pulse will stimulate the piezoceramic based atomization unit  112  to vibrate the piezoceramic at ultrasonic frequency, and then eject micro-droplets of liquids. 
     For internal battery: without DC source  101  input, the voltage of the internal battery  113  is firstly increased and stabilized to 5 V by the DC step-up module  104 . Then the above 5 V DC input is tuned by the source selecting switch  105 , flows to the coupling boosting transformer  111  and voltage regulator  106  that power the MCU  107 . When the switch  109  is triggered on, a 100 kHz pulse signal is generated by the MCU  107  and then received by the boosting coil of the transformer  111 , producing a 100 kHz high-voltage pulse. As described hereinbefore, the high-voltage pulse will stimulate the piezoceramic based atomization unit  112  and the piezoceramic is vibrated at ultrasonic frequency, as a result, the liquid is atomized. 
     The MCU  107  will drive the LED indicator  108  through internal checking of the working status of the battery. When low battery status is detected by the reset circuit  110 , a reset instruction is sent to MCU  107 , then making itself to reset and shut down. 
     Although several embodiments of the present invention have been described and illustrated in detail, it is understood that numerous modifications and redesigning will be easily made by those skilled in the art. Thus, the present invention is not limited to the certain embodiments described, and accordingly, all such modifications and equivalents are regarded as including within the scope of the appended claims. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful in atomizing a variety of liquids, especially high viscosity liquid. Because of this advantage, the atomizer of the present invention is applicable in different sections including medicinal, engineering and also other consumer products requiring atomization of high viscosity liquid and/or non-aqueous liquid. Some of the components or modules of the present atomizer are also applicable to the conventional atomizers to improve their performance.