Abstract:
A device and method for maintaining the volume of a liquid contained within a cavity between two substrates to something equal or nearly equal to that of the volume of the cavity. A particular application is a liquid crystal display (LCD), in which a liquid crystal (LC) material is contained within a cavity between two flat display substrates. The device serves to minimize the volume differential between the liquid and the cavity caused by a change in temperature of the display, such that the formation of bubbles within the liquid is substantially or completely prevented. In so doing, the device essentially eliminates thermally-induced defects that would otherwise be visible to the user.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to structures in which a volume between two substrates forming a panel is filled with a liquid. More particularly, this invention relates to liquid crystal displays of the type used with computer equipment, and provides a method and device for maintaining a desired level of liquid crystal material between two substrates forming a display panel so that bubble formation does not occur as a result of changes in volume of the liquid and/or the cavity between the substrates, as would occur as a result of temperature fluctuations.  
           [0003]    2. Description of the Prior Art  
           [0004]    Liquid crystal display (LCD) panels are made in a variety of ways, including flat panel displays used in portable computers. Flat panel displays typically comprise two flat substrates spaced apart to define a cavity therebetween in which a liquid crystal (LC) material is contained. Stringent requirements must generally be placed on the quality of displays used in the computer industry in order to find acceptance in the marketplace. For example, small defects in an LCD can visually be very disturbing and easily detected by the viewer. One known problem with flat panel displays is associated with temperature fluctuations, which can be quite considerable. For example, a computer and its flat panel display may be stored at temperatures well below −20° C., then utilized in an environment where the display may operate at a temperature of 40° C. or more. With such fluctuations, the LC material and the cavity in which the LC material is contained will expand and contract. Because liquid display materials have a greater coefficient of thermal expansion than the substrates of the display, the excessive contraction of the LC material relative to its cavity can have the effect of creating vacuum spaces or bubbles in the LC material. If there is undue expansion, the problem is less severe though excessive volume expansion of the LC material may damage the cavity or cause poor definition due to the ensuing incomplete polarization of the panel light upon switching of individual pixels.  
           [0005]    The expansion/contraction problem is especially likely to occur with the so-called one drop fill (ODF) method of the type disclosed in U.S. Pat. No. 5,263,888 to Ishihara et al., in which relatively rigid spacers are used to separate two flat substrates. Such spacers cannot be readily compressed to allow the substrates to move toward each other, which would decrease the volume of the cavity containing the LC liquid to compensate for a decrease in the volume of the LC material as a result of a temperature drop.  
           [0006]    From the above, it can be seen that it would be desirable to provide a solution to the problem of visual defects that occur in a LCD flat panel display as a result of fluctuations in the temperature of the display.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a device and method for maintaining the volume of a liquid contained within a cavity between two substrates to something equal or nearly equal to that of the volume of the cavity. In this manner, the formation of voids or bubbles within the liquid is substantially or completely eliminated or minimized. If the liquid is a liquid crystal (LC) material between two flat display substrates, the invention essentially eliminates thermally-induced defects that would otherwise be visible to the user.  
           [0008]    The present invention generally provides means for minimizing the volume differential between an LC material and a display panel cavity in which the LC material is contained. One approach provided by the invention is to deliver additional LC material to the cavity and receive LC material from the cavity as required in response to changes in temperature. For this purpose, a reservoir is provided from which the LC material is dispensed and accumulated as required to compensate for the differential volume expansion of the LC material and display cavity. Means are preferably provided for reducing the volume of the reservoir in response to a temperature drop, thus forcing some of the LC material within the reservoir into the panel cavity to compensate for the contraction of the LC material within the cavity that occurred as a result of the temperature drop. Similarly, if there is an expansion of the LC material within the panel cavity, the means associated with the reservoir allows the excess volume of LC material to flow from the panel cavity into the reservoir. Another approach of the invention is to provide a panel cavity with a controllable variable volume. With either embodiment, though the ambient temperature of the display fluctuates, the panel cavity remains substantially filled with the LC material, so that voids and bubbles are avoided.  
           [0009]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 schematically represents the interior of a flat panel display equipped with a reservoir containing an LC material in accordance with a first embodiment of this invention.  
         [0011]    [0011]FIG. 2 is a cross-sectional view of the flat panel display of FIG. 1.  
         [0012]    [0012]FIGS. 3 and 4 show the operation of the reservoir of FIG. 1 in response to temperature fluctuations.  
         [0013]    [0013]FIGS. 5 and 6 schematically represent reservoirs for containing, dispensing and accumulating an LC material in accordance with two additional embodiments of this invention.  
         [0014]    [0014]FIG. 7 schematically represents the interior of a flat panel display in accordance with a fourth embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIGS. 1 through 7 show various embodiments of this invention for maintaining a desired fill level for an LCD material contained within a flat panel LCD of a type known in the art, such as flat panel LCD&#39;s used as the displays for portable computers, TV monitors, desktop computer displays, cellular phones, personal digital assistants (PDA&#39;s), global positioning systems (GPS) and avionics. In the three embodiments represented by FIGS. 1 through 6, the flat panel display is equipped with a small reservoir containing LC material that can be delivered to or received from the display, according to the temperature of the display. In contrast, the embodiment of FIG. 7 provides an LCD whose internal cavity has a controllable variable volume. While schematically represented and described in reference to LCD&#39;s for computer displays, those skilled in the art will recognize that the advantages of this invention can be applied to a variety of other applications.  
         [0016]    In the embodiment of FIGS. 1 and 2, a flat panel display  10  is represented as being composed of two flat substrates  12  that define a cavity  14  therebetween. The cavity  14  is filled with an LC material  20  of any suitable type known in the art, such as a fluorinated liquid crystal available under the name ZLI5080 from Merck &amp; Co., Inc. The substrates  12  can be formed of a variety of materials, typical examples of which include glass materials such as 1737 from Dow Coming. The substrates  12  are spaced apart along their perimeters by edge members  16  and within their interiors by a number of rigid spacers  18 . The edge members  16  enclose the cavity  14  along the perimeters of the substrates  12 , and therefore are formed of a suitable structural material that is compatible with the LC material  20 . The spacers  18  contact both of the substrates  12 . As known in the art, the spacers  18  are distributed within the cavity  14  to maintain a fixed spacing between the substrates  12 . For this reason, the spacers  18  are typically formed of a relatively rigid polymeric material, such as a polyimide, though other materials could be used. The manufacture and assembly of the display  10 , including its materials and the manner in which the cavity  14  is filled with the LC material  20 , are known to those skilled in the art, and therefore will not be discussed in any detail here.  
         [0017]    As shown in FIG. 1, the display  10  includes a reservoir in the form of a thin tube  22  mounted to one side of the substrates  12 . The tube  22  is shown as being an integral portion of the side member  16 , though it is foreseeable that the tube  22  could also be formed as a discrete component that is mounted to the display  10 . The tube  22  contains an LC material  24 , preferably identical to the LC material  20  within the panel cavity  14 . Consequently, suitable materials for the tube  22  include the same materials used to form the edge members  16  of the display  10 , though other materials could foreseeably be used. The LC material  14  within tube  22  directly communicates with the LC material  20  within the panel cavity  14  through a small passage  26 , shown as being located at the bottom of the display  10  and tube  22 , though other locations are possible. The tube  22  is equipped with a small diaphragm  28  formed of a compliant material and located near the top of the tube  22 . The diaphragm  28  may be in the form of a cap to the tube  22 , as generally represented in FIGS. 3 and 4. Suitable materials for the diaphragm  28  include rubber or another thin flexible material, such as an elastomer sheet material. According to the invention, the diaphragm  28  is caused to deflect inward into the tube  22  (and allowed to move outward away from the tube  22 ) by way of a contact actuator that is responsive to temperature changes, such as a bimorph spring  30  shown in FIGS. 3 and 4. A suitable spring  30  is formed of two strips of different materials, which are swaged, welded or otherwise joined together. The strip materials are chosen to have different coefficients of thermal expansion so that a sufficient change in temperature will cause the spring  30  to deflect. Suitable material combinations are well known in the art, and include strips of tungsten and copper, copper and tin, and iron and aluminum. In FIG. 3, the spring  30  is at a null position corresponding to a temperature at which compensation is not required for the thermal contraction of the volume of LC material  20  within the cavity  14 . The situation depicted in FIG. 3 may correspond to the highest ambient temperature intended for the display  10 , in which case the LC material  20  has expanded to its maximum volume within the cavity  14 . In contrast, FIG. 4 shows the diaphragm  28  deflected into the tube  22  by the spring  30  in response to a temperature drop, causing a portion of the LC material  24  within the tube  22  to be forced into the panel cavity  14 . By appropriately sizing the tube  22 , the degree to which the spring  30  deflects with respect to temperature can be calibrated to dispense an amount of LC material  24  from the tube  22  so that the volume of LC material  20  within the cavity  14  remains essentially constant over a suitable range of temperatures, e.g., about −20° C. to about 70° C., with the result that the formation of voids or bubbles within the LC material  24  is eliminated or at least minimized. On the other hand, if the LC material  20  within the cavity  14  expands as a result of a temperature rise, the pressure applied by the spring  30  on the diaphragm  28  would be reduced or completely eliminated as a result of the spring  30  deflecting away from the diaphragm  28 , allowing the extra volume of LC material  20  from the cavity  20  to flow into the tube  22  via the passage  26 .  
         [0018]    [0018]FIG. 5 shows a spring or hydraulic actuator  32  as an alternative type of device for operating the diaphragm  28  of FIG. 1. The actuator  32  may be controlled by a computer (not shown) to which the display  10  is connected, in response to a signal generated by a temperature sensor  34 . Other possible actuators include piezoelectric elements, springs and other temperature-sensitive biasing elements known in the art.  
         [0019]    [0019]FIGS. 6 and 7 illustrate further embodiments for achieving the temperature compensation effect of this invention. In FIG. 6, an LC display  110  includes a reservoir in the form of a thin channel  122  located along one side of the display  110 . The channel  122  is fluidically connected by a passage  126  to a panel cavity  114  defined within the LC display  110  by two glass substrates  112 . The passage  126  is shown as being located midway up the edge member  116 , though the passage  126  could be located elsewhere on the display  110 , including the lower corner of the display  110  as done with the embodiment of FIGS. 1 and 5. The channel  122  is shown as being defined within a discrete component that is attached to a side member  116  of the display  110 . At least a portion of the outer containment wall  128  of the channel  122  is formed of a flexible material, allowing the wall  128  to serve as a diaphragm that can deflect inward and outward relative to the channel  122  to maintain a substantially constant volume of LC material within the cavity  114  over a desired temperature range. Finally, FIG. 7 represents an LC display  210  having one edge member  216  equipped with a flexible diaphragm or wall portion  228 . In contrast to the previous embodiments, the panel cavity  214  of FIG. 7 has a variable volume that is controlled with the wall portion  228  to substantially coincide with the volume of LC material within the cavity  214 . The wall  128  and wall portion  228  of FIGS. 6 and 7 may be deflected with any of the contact actuators  30  and  32  shown in FIGS. 1, 3,  4  and  5 , as well as any other suitable type of device or effect. For example, it may be possible for the wall  128  or wall portion  228  to be sufficiently flexible to sense a volume change within its cavity  114  or  214  and compensate for such a change by means of atmospheric pressure exerted on the wall  128  or wall portion  228 , in which case a discrete actuating device would not be required.  
         [0020]    By compensating for the loss of volume of LC material within an LCD panel cavity as a result of thermal contraction, the present invention prevents or at least minimizes the formation of voids or bubbles within the LC material that would otherwise occur if the material has a coefficient of thermal expansion that is sufficiently lower than the materials from which the LCD substrates, edges and spacers (e.g.,  12 ,  16  and  18 , respectively, of FIGS. 1 and 2) are formed, and the display is subject to sufficiently great temperature variations. The volume of LC material that must be dispensed from the tubes  22  or channel  122  of FIGS. 1, 5 or  6  to compensate for a differential volume of the corresponding LC material and panel cavity is relatively small. The total LC content of a flat display panel is typically less than one milliliter, such that only a fraction of one milliliter is required to be contained within the tube  22  or channel  122  to compensate for fluctuations in the amount of the LC material within the cavity  14  or  114 . As a result, a practical size for the tubes  22  and channel  122  is an inner diameter of about one millimeter and a length of about fifty millimeters, with lesser and greater lengths being foreseeable.  
         [0021]    While various other factors may be involved, the difference in thermal expansion between the LC material  20  and the spacers  18  is likely to be the primary factor in the formation of voids and bubbles within the LC material  20  during fluctuations in temperature. The effective volume of the cavity  14  at a temperature “T” may be denoted as V(T), based on: 
           V ( T )= A ( T )× H ( T ) 
         [0022]    where A(T) is the effective surface area of one of the substrates  12  within the cavity  14  at temperature T excluding the cumulative area of the spacers  18 , and H(T) is the height of the spacers  18  at temperature T. Changes in temperature will cause variations in H(T) and in A(T) as follows: 
           H ( T )= H   0 (1+α 1   ΔT ) 
         [0023]    and 
           A ( T )= L   0 (1+α 2   ΔT )× W   0 (1+α 2   ΔT ) 
         [0024]    where H 0  is the height of the spacers  18  and L 0  and W 0  are the length and width, respectively, of the cavity  14 , with all subscript parameters taken at a particular reference temperature, generally the temperature during filling of the cavity  14 . α 1  is the linear thermal coefficient of expansion of the spacers  18 , α 2  is the linear coefficient of thermal expansion of the substrates  12  (e.g., glass), and ΔT is the temperature difference between the liquid crystal fill temperature and the ambient temperature in question. There may be cases where α 1  is approximately equal to α 2 , such as when the materials for the spacers  18  and the substrates  12  are the same.  
         [0025]    Bubbles will form when the volume V(T) of the cavity  14  is sufficiently greater than the volume of LC material  20  within the cavity  14 . The volume of the LC material  20  is designated as W(T), and is temperature dependent based on a thermal coefficient of volume expansion, β. The volume W(T) of the LC material can be expressed as 
           W ( T )= W   0 (1+βΔ T ) 
         [0026]    Unless compensated for, bubbles form within the LC material  20  when a temperature change (ΔT) occurs such that the volume V(T) is greater than W(T). Accordingly, the present invention specifically compensates for the volume differential, V(T)−W(T)=ΔV to eliminate bubble formation.  
         [0027]    An example follows using typical panel (cavity) dimensions, the following thermal expansion coefficients, and assuming a temperature drop (ΔT) of 40° C. from the original temperature at which the cavity  14  was filled.  
         [0028]    α 1  (polymer)=20×10 −6 /° C. (linear coefficient)  
         [0029]    α 2  (glass)=7.6×10 −6 /° C. (linear coefficient)  
         [0030]    β 1  (LC material)=670×10 −6 /° C. (volume coefficient)  
         [0031]    L 0 =25.4 cm  
         [0032]    W 0 =20.3 cm  
         [0033]    Using the above values, the shrinkage in volume of the LC material  20  as compared to the volume of the cavity  14  is calculated to be about 7.2×10 −3  cm 3 . While this volume differential (ΔV) may appear to be inconsequential, bubbles that can develop in the LC material  20  are often on the order of about 4×10 −6  cm 3 . As a result, as many as two thousand bubbles may form from a temperature drop of 40° C., which is sufficient to cause major viewing or visual defects in the LC display  10 . Though differing in certain respects, each of the embodiments of this invention is adapted to compensate for smaller and larger volume differentials, for the purpose of preventing the formation of bubbles over the entire operating temperature range of the display  10 .  
         [0034]    While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, appropriate materials could be substituted for those noted, or various types of actuating devices could be substituted for those disclosed. Accordingly, the scope of the invention is to be limited only by the following claims.