Description
DEVICE, METHOD FOR PRODUCING A LIQUID CRYSTAL CELL, AND METHOD FOR OPERATING A DEVICE A device, a method for producing a liquid crystal cell, and a method for operating a device are specified.
Liquid-crystalline media have been used for many years in electro-optical displays (liquid crystal displays: LCDs) in order to display information. More recently, liquid-crystalline media have also been proposed for use in components for microwave technology, such as, for example, in DE 10 2004 029 429 A and in JP 2005-120208 (A).
DE 10 2004 029 429 A describes the use of liquid-crystal media in microwave technology, inter alia in phase shifters. Therein, liquid-crystalline media with respect to their properties in the corresponding frequency range have been discussed.
In order to improve the properties of microwave devices, attempts are constantly being made to develop novel compounds which enable such devices to be optimised. In particular, the loss in the microwave range must be reduced or the tunability (x) enhanced to improve the material quality (r1). In addition, there is a demand for an improvement in the low-temperature behaviour of the components. An improvement in both the operating properties and also in the shelf life has been addressed.
Especially in the field of microwave technology a very high clearing temperature of the liquid crystal is desired, as the liquid crystalline properties, i.e., the degree of order is temperature dependent. The so-called order parameter decreases with increasing temperature which can be comparatively high during operation of a device, for example an antenna. There is, however, a trade-off between a high clearing temperature and other liquid-crystal properties such as viscosity and the temperature range of a nematic phase.
Liquid crystal material with a high clearing temperature is often crystalline at room temperature or shows higher ordered smectic phases.
It is an object to provide an efficient device comprising a liquid crystal cell. Furthermore, a simple and efficient method for producing a liquid crystal cell shall be specified.
Additionally, an efficient method for operating a device shall be provided. -1 - -2 -
According to at least one embodiment, a device comprises a liquid crystal cell. In particular, the liquid crystal cell is configured or designed to modulate high-frequency radiation and/or visible and/or infrared light during intended operation of the device. In other words, the liquid crystal cell interacts with the high-frequency radiation and/or the visible and/or infrared light during operation.
Here and in the following, visible light is a term for electromagnetic radiation that has a wavelength in a range from about 400 nm to about 740 nm. Infrared light is a term for electromagnetic radiation having a wavelength in a range of between and including 780 nm and 3000 nm. Here and in the following, high-frequency radiation is a term for a range of radio-frequency electromagnetic waves, in particular radio waves, having a frequency above 3 megahertz (MHz), in particular between and including 3 MHz and 10 terahertz (THz), for instance between and including 3 MHz and 300 gigahertz (GHz), for example between and including 1 GHz and 30 GHz.
According to at least one embodiment, the device comprises heating means. The heating means are, for example, configured to heat the liquid crystal cell to a desired temperature. For example, the heating means is a resistive heating means, a microwave heating means, a liquid based heating system, warm air or warm gas, or infrared radiation. In particular, the device is a liquid crystal panel. Advantageously, the device can be operated independently of the ambient temperature of the device due to the heating means. In particular, ambient temperature is understood to be the temperature of a medium surrounding the device.
According to at least one embodiment of the device, the liquid crystal cell comprises a first substrate and a second substrate facing each other. In particular, a main extension plane of the first substrate is parallel to a main extension plane of the second substrate. The first substrate and/or the second substrate are, for example, transmissive to the electromagnetic radiation in the high-frequency radiation, visible light and/or infrared light.
In particular, the first substrate and/or the second substrate is transparent to visible electromagnetic radiation. For example, the first and/or the second substrate comprise or consist of a glass.
According to at least one embodiment of the device, the liquid crystal cell comprises a liquid crystal medium arranged between the first substrate and the second substrate. In particular, the liquid crystal medium is in direct contact with the first substrate and/or the -3 -second substrate. Alternatively, it is possible that further layers are arranged between the first substrate and the liquid crystal medium and/or the second substrate and the liquid crystal medium.
According to at least one embodiment of the device, the liquid crystal medium has a phase transition temperature from a first, non-nematic phase to a second, nematic phase. Here and in the following, the phase transition temperature is a defined temperature or a temperature range in which a phase transition from a first phase to a second phase occurs. In particular, in the nematic phase, the liquid crystal medium has liquid crystal properties. For example, the liquid crystal medium is a non-eutectic mixture. In particular, the phase transition temperature from the first, non-nematic phase to the second, nematic phase is a defined temperature. A defined temperature has, for example, a temperature range of at most 10°C, in particular at most 5°C, for example at most 2°C.
Here and in the following, a mixture is called eutectic when its constituents are in such a ratio to each other that it becomes liquid or solid as a whole at a certain temperature.
In particular, the liquid crystal medium comprises a mesogen or a mesogenic compound. The mesogen or mesogenic compound is a chemical compound which has a liquid crystalline phase. The liquid crystalline phase shows properties between those of a conventional liquid and those of a solid crystal. Here and in the following, liquid crystal medium, mesogenic medium or just medium is used as term for the liquid crystal medium. In particular, mesogens or mesogenic compounds comprise at least one calamitic or discotic mesogenic group. Here and in the following, "mesogenic group" means a group with the ability to induce liquid crystal phase behavior to the liquid crystal medium.
A calamitic group is a group which is rod-or board/lath-shaped. A discotic group is disk-shaped. A calamitic mesogenic group, in particular, comprises a mesogenic core consisting of one or more aromatic or non-aromatic cyclic groups connected to each other directly or via linkage groups, optionally comprises terminal groups attached to the ends of the mesogenic core, and optionally comprises one or more lateral groups attached to the long side of the mesogenic core, wherein the terminal and lateral groups are selected, for example, from carbyl or hydrocarbyl groups, polar groups like halogen, nitro, hydroxy, or polymerizable groups. -4 -
The term "carbyl group" denotes a mono-or polyvalent organic group containing at least one carbon atom which either contains no further atoms, such as, for example, -C=C-, or optionally contains one or more further heteroatoms, such as, for example, N, 0, S, P, Si, Se, As, Te, or Ge, for instance carbonyl. The term "hydrocarbyl group" denotes a carbyl group, which additionally contains one or more H atoms and optionally one or more heteroatoms, such as, for example, N, O, S. P, Si, Se, As, Te, or Ge.
According to at least one embodiment of the device, the liquid crystal cell comprises two electrodes for supplying an electric potential across the liquid crystal medium to drive the liquid crystal medium in a predetermined configuration. In particular, a predetermined configuration corresponds to a distinct orientation of the molecules in the liquid crystal medium. For example, by applying a distinct electric potential across the liquid crystal medium, the molecules in the liquid crystal medium align in a distinct orientation. Thereby, the orientation of the molecules can be dependent on the electric potential applied. It is also possible that the liquid crystal cell comprises more than two electrodes, for example four or six.
According to at least one embodiment of the device, the liquid crystal medium is in the first, non-nematic phase when the liquid crystal medium has a temperature below a predetermined operating temperature of the device. Here and in the following, the operating temperature is the temperature of the device which is reached during intended operation of the device. In particular, the liquid crystal medium is in the second, nematic phase when the liquid crystal medium has the predetermined operating temperature of the device. In particular, the operating temperature is above room temperature, which is for example 20°C to 30°C. For example, the operating temperature is between and including 30°C to 180°C, in particular between and including 50°C to 150 °C, for example between and including 60°C to 100°C or between and including 70°C to 90°C. In particular, the operating temperature is about 80°C.
According to at least one embodiment of the device, the heating means are arranged and configured for maintaining the temperature of the liquid crystal medium above the phase transition temperature during, in particular intended, operation of the device. In other words, the heating means are arranged and configured to heat and keep the liquid crystal medium in the second, nematic phase during intended operation of the device. For example, the heating means are arranged on the substrate of the liquid crystal cell. -5 -
Alternatively, the heating means are part of the first substrate and/or the second substrate. In other words, the heating means can be integrated in the liquid crystal cell.
According to at least one embodiment, the device comprises a liquid crystal cell and heating means, wherein the liquid crystal cell comprises a first substrate and a second substrate facing each other, a liquid crystal medium arranged between the first substrate and the second substrate, wherein the liquid crystal medium has a phase transition temperature from a first, non-nematic phase to a second, nematic phase, and two electrodes for supplying an electric potential across the liquid crystal medium to drive the liquid crystal medium in a predetermined configuration, wherein the liquid crystal medium is in the first, non-nematic phase when the liquid crystal medium has a temperature below a predetermined operating temperature of the device, and the liquid crystal medium is in the second, nematic phase at said predetermined operating temperature of the device, and the heating means are arranged and configured for maintaining the temperature of the liquid crystal medium above the phase transition temperature during operation of the device.
Advantageously, the device described herein allows a crystallization of the liquid crystal medium in the liquid crystal cell if the device is not in operation, such as during storage or in times the device is not being in use. Due to the heating means comprised by the device, the liquid crystal medium can be brought back from the first, non-nematic phase to the second, nematic phase. In other words, the heating means allows the liquid crystal medium to be brought back in a state in which the liquid crystal medium shows liquid crystal properties.
For example, the device is used in applications which require a temperature stabilized device. This can be explained as the operating temperature of the device can be chosen above a highest expected ambient temperature. For example, constant heating is provided by the heating means during operation of the device such that the temperature of the device is controlled and/or kept at the operating temperature of the device. A combined cooling and heating means are, in particular, not required. Thus, weight, cost, and complexity of the device are advantageously reduced.
Furthermore, as the device can be operated at a well-known temperature or at a well-known temperature range with the effort of merely heating, advantageously, the phase transition temperature of the liquid crystal medium from the first, non-nematic phase to the -6 -second, nematic phase can be selected in a way that, for example, an order parameter of the liquid crystal medium fits to the desired application. In particular, the liquid crystal medium is selected such that the phase transition temperature from the first, non-nematic phase to the second, nematic phase is slightly below the lowest operating temperature of the device. For example, then, the order parameter of the liquid crystal medium is expected to be high, and the liquid crystal medium possesses a high anisotropy.
Here and in the following, the order parameter of the liquid crystal medium describes a degree of order, such as alignment and orientation, of the liquid crystal medium, in particular in the second, nematic phase.
According to at least one embodiment of the device, the phase transition temperature of the liquid crystal medium from the first, non-nematic phase to the second, nematic phase is in the range of -20°C to 60°C, both inclusive, in particular in the range of -10°C to 60°C, both inclusive. In particular, the phase transition temperature is between and including 0°C and 60°C, for example between and including 20°C to 50°C.
Other devices comprise a liquid crystal medium, which have a phase transition temperature of the first, non-nematic phase to the second, nematic phase of at most -30 °C or -40°C. In particular, the liquid crystal medium in the other devices has a low temperature stability of -40°C or -30°C and several hundred hours. Due to the heating means, which allows a crystallization of the liquid crystal medium outside the operating temperature of the device, the low temperature stability of the liquid crystal medium as with the other devices is no longer a strict requirement.
According to at least one embodiment of the device, the first, non-nematic phase of the liquid crystal medium is a crystalline phase, a smectic phase, or a glass. In particular, in the crystalline phase, the liquid crystal medium is a solid, wherein the molecules in the liquid crystal medium are arranged in a highly ordered microscopic structure. For example, in the smectic phase, the molecules in the liquid crystal medium form layers that can slide over another. The layers in the smectic phase are ordered along one direction. In particular, the glass is an amorphous solid. The amorphous solid has, compared to the crystalline phase, no long-range order.
According to at least one embodiment of the device, the liquid crystal medium has a clearing temperature of at least 120 °C, in particular at least 140 °C, for example at least -7 - °C. In particular, the clearing temperature is between and including 180 °C and 240 °C. Here and in the following the clearing temperature is the temperature or temperature range at which a phase transition between a liquid crystalline phase, for example the second, nematic phase, and a third, isotropic phase occurs. The isotropic phase is, in particular, a phase of the liquid crystal medium in which the liquid crystal medium has properties of a conventional liquid phase. In other words, in the third, isotropic phase the liquid crystal medium has a random and isotropic molecular order, with little to no long-range order. Due to the high clearing temperature of the liquid crystal medium, the device is operable at high temperatures, for example between and including 50°C and 120°C.
According to at least one embodiment, the device is configured to operate at a temperature of at least 60 K, in particular at least 80 K, for example at least 100 K or at least 160 K below the phase transition temperature of the liquid crystal medium from the second, nematic phase to the third, isotropic phase. In particular, the phase transition temperature of the liquid crystal medium from the second, nematic phase to the third, isotropic phase corresponds to the clearing temperature. As the device is configured to operate at a temperature of at least 60 K below the phase transition temperature of liquid crystal medium from the second, nematic phase to the third, isotropic phase, is ensured that the liquid crystal medium does not enter the third, isotropic phase during operation of the device.
According to at least one embodiment of the device, the liquid crystal medium comprises one or more compounds of formula I wherein R1 denotes H, straight chain or branched alkyl having 1 to 12 C atoms or alkenyl having 2 to 12 C atoms, in which one or more CH2-groups may be replaced by -0--00--0- or 0-, where one or more non adjacent CH2-groups may be replaced by 0 and/or S, and where one or more H atoms may be replaced by F; or alternatively denotes a group RP, RP denotes halogen, CN, NCS, RF, RF-0-or RF-S-, wherein -8 -RF denotes fluorinated alkyl or fluorinated alkenyl having up to 9 C atoms, Z11, Z12 identically or differently, denote -CH=CH-, -CF=CF-, -CH=CF-, -CF=CH-, -CC-or a single bond, and denote a radical selected from the following groups: a) the group consisting of 1,4-phenylene, 1,4-naphthylene, 2,6-naphthylene, tetralin-5,8-diyl, tetralin-2,6-diyl, 9H-fluorene-2,7-diyl, phenanthren-2,7-diyl, 9,10-dihydrophenanthren-2,7-diyl, in which one or two CH groups may be replaced by N and in which one or more H atoms may be replaced by RL, b) the group consisting of trans-1,4-cyclohexylene, 1,4-cyclohexenylene, bicyclo[1.1.1]pentane-1,3-diyl, 4,4'-bicyclohexylene, bicyclo[2.2.2]octane-1,4-diyl, and spiro[3.3]heptane-2,6-diyl, in which one or more non-adjacent CH2 groups may be replaced by -0-and/or -S-and in which one or more H atoms may be replaced by F or alkyl having 1 to 6 C atoms, c) the group consisting of thiophene-2,5-diyl, thieno[3,2-b]thiophene-2,5-diyl, selenophene-2,5-diyl, dibenzofuran-3,7-diyl, dibenzothiophene-3,7-diyl, each of which may also be mono-or polysubstituted by RL, wherein RL on each occurrence, identically or differently, denotes F, CI, CN, NCS, SF5 or straight-chain or branched, in each case optionally fluorinated, alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy having 1 to 12 C atoms, X1 denotes NCS, F, CI, CN, fluorinated alkyl or fluorinated alkoxy each having 1 to 6 C atoms, fluorinated alkenyl or fluorinated alkenyloxy each having 2 to 6 C atoms, and n1 is 1, 2 or 3, preferably 1 or 2.
In particular, the compounds of formula I alone or in combination show a defined phase transition temperature from the first, non-nematic phase to the second, nematic phase.
Furthermore, the compounds of formula I are advantageously characterized by a high clearing temperature such that they can be applied in devices with a high operating temperature.
According to at least one embodiment of the device, the liquid crystal medium comprises one, two, three or more compounds of formula I in a concentration of at least 90 wt.%, in -9 -particular at least 95 wt.%, for example at least 99 wt.%. In particular, the liquid crystal medium consists essentially of one or more components of formula I. For example, the liquid crystal medium comprises exactly one component of formula I in a concentration of at least 90 wt.%, in particular at least 95 wt.%, for example at least 99 wt.%.
Other devices with liquid crystal cells without heating means are operated at around room temperature, which corresponds, for example, to a temperature between and including 20°C and 30°C. The liquid crystal media used therein have, in particular, a wide nematic phase, ranging from -40°C to -10°C on the low temperature side to up to 80°C to 150°C on the high temperature side. Outside this temperature range the liquid crystal medium is, for example, in the crystalline phase at lower temperatures or the isotropic phase at higher temperatures. For example, other the liquid crystal media used in other devices are eutectic mixtures. However, a crystallization temperature of eutectic mixture is not well defined and not only depending on temperature but also, for example depending on how long the liquid crystal medium is stored at low temperatures.
The not well defined crystallization temperature can reduce the efficiency of the device if the device is stored at temperatures low enough for the liquid crystal medium to crystallize. When the device with crystallized eutectic liquid crystal medium is brought back to higher temperatures where the liquid crystal medium was present in the second, nematic phase before crystallization occurred, it cannot be guaranteed that the second, nematic phase is fully restored. For example, parts of the liquid crystal medium, either in terms of location in the liquid crystal cell or certain components of the liquid crystal medium are still in the crystalline phase and others are in a non-crystalline phase.
Entering a non-nematic phase such as a smectic phase might be also possible.
The liquid crystal medium described herein, in particular if only one of the compounds of formula I is used in at least 90 wt.% in the liquid crystal medium, is, for example, a non-eutectic mixture. Thus, the liquid crystal cell can advantageously be operated at high temperatures while at the same time a storage at low temperature is possible. This can be explained as the liquid crystal medium can be brought back, in particular completely, into the second, nematic phase during operation of the device. Furthermore, a temperature range for the second, nematic phase does not have to be wide even though the device is operable at high temperatures. Thus, possible trade-offs of liquid crystal media having a wide temperature range for the nematic phase can be advantageously avoided.
In formula I, preferably IR1 denotes H, straight chain alkyl having 1 to 7 C atoms or alkenyl having 2 to 7 C atoms, Z11, Z12 identically or differently, denote -CH=CH-, -CF=CF-, -CC-or a single bond, X1 denotes NCS, F, CI, CN, CF3, OCF3, very preferably CN or NCS, and n1 is 1 or 2.
According to at least one embodiment of the device, the liquid crystal medium comprises a compound selected from the following group: independently denote
F \H -N N=
CH3 H3C where and alternatively denote
F 7-<"
)-NICS
F C5H1 rriN C5Hli
F
c3H7-\ \ \ C5H1---c} /\)--NCS \ * 44 \ /I= C51-311
-N
/H-C; \ / -\ e-\ C 51 -11 I -----< /.; ) k ?),^ /7.-CN F -\ \ Cs" ii---\\ 4, \^,,,, 6/ s....s /7' k", /7s.,-.o.., 14 / \ '''' \ ) ___tor
F \
C5H11-4 \)*-*-* \ *-- : :/ \'' /// 4---CN
PM F
5)13 _11 // / // \ \ 5 / NC-
Q
/ ____ cc= \ , , C5H C5H, 1-@ Y.,-4 J) = <,\, 1-NCS * ''' \ 44 _____I.PH3 i )--K,----; p \ c / \ r \'' 4/ C5.H.ii NCS /
F / -\
ir-NCS c 2 H 5
-NCS
i c3H7---Ac 4,----(\ ti-PN \ / \ ,. \.
F
F /
/ /-=', .4--=\
F
C51-411---S.^\ / //4 2 F /.__ \ j_<.., ,...=..\ ",.< CE.Hic<K\ "----K\ iii <.\.; ,r-., rNCS F. Cb.Hii---( / / C2H5 /-) \.* -C.5Hic \ / / / -) /C2ft 1/47- -1 2 -
F
CsFli NCS C5H11 According to at least one embodiment of the device, the the liquid crystal cell comprises a reservoir structure. In particular, the reservoir structure is configured to adjust an amount of the liquid crystal medium between the first substrate and the second substrate by having a cavity large enough to accommodate thermal expansion of the liquid crystal medium or, upon cooling, to provide liquid crystal medium to an area between the first substrate and the second substrate due to contraction of the liquid crystal medium. For example, the reservoir structure is arranged between the first substrate and the second substrate. Additionally or alternatively, the reservoir structure is in direct contact with the liquid crystal medium. Advantageously, the reservoir structure prevents the liquid crystal cell from damage if the liquid crystal medium changes its volume in dependence of the temperature. For example, the reservoir structure is at least one convexity which directly connects to the liquid crystal medium. In particular, the reservoir structure comprises a plurality of convexities.
According to at least one embodiment of the device, the liquid crystal cell comprises an alignment layer on the first substrate and/or the second substrate. In particular, the alignment layer is arranged between the first substrate and the second substrate. For example, the alignment layer induces a certain orientation of the molecules in the liquid crystal medium. In particular, the alignment layer comprises or consists of an organic material such as a polyimide. Alternatively a surfactant, additives, or an inorganic material such as SiO2 or A1203 can be used as a material of the alignment layer. Suitable additives to induce homeotropic alignment are disclosed for example in US 2013/0182202 Al, US 2014/0838581 Al, US 201 5/01 6689 0 Al and US 2015/0252265 Al. In particular, the surfactant, additives, and the inorganic material are characterized by a high stability in terms of chemical stability and mechanical stability.
According to at least one embodiment of the device, the heating means is a resistive heating means comprising a conductor. In particular, a material of the conductor is selected from the group consisting of silver, chromium, copper, platinum, indium tin oxide (ITO), nickel, molybdenum, aluminium, cadmium, and organic conductive materials, and conductive polymers. The resistive heating means has, for example, a mesh like structure. Alternatively, the material of the conductor is provided as a continuous layer. In particular, the heating means, for example the resistive heating means, provides heat to the entire liquid crystal medium in the liquid crystal cell. For example, if ITO is used in the resistive heating means, the device can advantageously be used for display applications as the ITO is transparent to electromagnetic radiation in the visible range.
According to at least one embodiment, the device further comprises a temperature sensor. In particular, the temperature sensor is configured to measure a temperature of the liquid crystal medium to generate an electric signal as a function of temperature. Additionally or alternatively, the temperature sensor is operable in conjunction with the heating means to maintain the temperature of the liquid crystal medium at a predefined temperature. For example, the electric signal is processed in the device and a further electric signal is send to the heating means which is controlled to maintain the temperature of the liquid crystal cell at the predefined temperature. Advantageously, the temperature sensor ensures that the liquid crystal cell is provided with such a heat that the predefined temperature is maintained during operation of the device or that the predefined temperature is reached before operation of the device. The temperature sensor may also prevent overheating and thus possible damage of the device and, in particular, the liquid crystal cell.
In particular, the device further comprises more than one temperature sensor. More than one temperature sensor allows local measurement of the liquid crystal medium. Together with a segmented heating means, this allows a local temperature control of the liquid crystal medium.
According to at least one embodiment of the device, the heating means is segmented. Advantageously, this allows for a selective heating of different regions of the liquid crystal medium. In particular, together with more than one temperature sensor, the temperature of the liquid crystal medium can be locally controlled.
According to at least one embodiment of the device, the liquid crystal medium is a tuneable dielectric material, in particular configured for use in high-frequency technology. Here and in the following, "tuneable" means that a property of the dielectric material can be adjusted. For example, the orientation of the molecules of the liquid crystal medium can be adjusted or tuned such that the molecules align in a distinct orientation. As the liquid crystal medium is a tuneable dielectric material, the liquid crystal medium can advantageously be applied to modulate high-frequency radiation and/or visible light.
According to at least one embodiment, the device is a liquid crystal based antenna element, a phase shifter, a tunable filter, a tunable metamaterial structure, a matching circuit, or a varactor. Here and in the following, a metamaterial is a material engineered to have a property that is not found in naturally occurring materials. Metamaterials are, for example, used as or in optical filters, radomes, or lenses for high-gain antennas.
According to at least one embodiment, the device is an optical component operable in the visible or infrared range of the electromagnetic spectrum. In other words, the device is configured to interact with visible or infrared light. In particular, the liquid crystal cell of the device is configured to modulate the visible or infrared light.
According to at least one embodiment, the device is a transmissive or a reflective spatial light modulator. In particular, a spatial light modulator is a device which is configured to impose some form of spatially varying modulation on a beam of light. For example, the spatial light modulator modulates the intensity and/or phase of electromagnetic radiation impinging on the spatial light modulator.
A microwave antenna array is specified. In particular, the antenna array comprises one or more devices as described herein. Thus, all embodiments, features, and advantages described in combination with the device also apply to the microwave antenna array and vice versa. If the antenna array comprises more devices as described herein, the antenna array can comprise devices having the same or different structures. Here and in the following, an antenna array is a set of multiple connected antennas which work together as a single antenna to transmit or receive microwaves or high-frequency radiation. In particular, microwaves are a form of electromagnetic radiation having a wavelength of between and including about 1 millimeter to about 30 centimetres. For instance, the antenna array transmits or receives high-frequency radiation having a frequency between and including 300 GHz to 10 THz. For example, the one or more devices comprised by the microwave antenna comprises a liquid crystal medium which is a tuneable dielectric material configured for use in high-frequency technology.
Furthermore, a method for producing a liquid crystal cell is specified. In particular, the liquid crystal cell of the device described herein is produced by the method. Thus, features, embodiments, and advantages described in combination with the liquid crystal cell of the device also apply to the method and vice versa.
According to at least one embodiment of the method for producing a liquid crystal cell, the liquid crystal cell comprises a first substrate and a second substrate. The first substrate and the second substrate face each other. The liquid crystal cell further comprises a liquid crystal medium arranged between the first substrate and the second substrate. The liquid crystal medium has a phase transition temperature from a first, non-nematic phase to a second, nematic phase.
According to at least one embodiment of the method, the first substrate is provided. In particular, the first substrate comprises or consists of a glass. For example, the first substrate comprises at least one element selected from the group consisting of sealant rings, alignment layer, spacer beads, photo spacer, electrodes, thin film transistors (TFT), and combinations thereof In particular, these elements are arranged on and/or within the first substrate. The sealant rings are, for example, cured or uncured.
According to at least one embodiment of the method, the liquid crystal medium is deposited over the first substrate at a temperature below the phase transition temperature. In other words, the liquid crystal medium is deposited while being in the first, non-nematic phase. In particular, the liquid crystal medium is deposited while being in a crystalline phase. Advantageously, the liquid crystal medium in the first, non-nematic phase offers the possibility to deposit the liquid crystal medium as desired, for example evenly or unevenly, across the first substrate.
According to at least one embodiment of the method, the first substrate is heated to a temperature above the phase transition temperature to effectuate phase transition of the liquid crystal medium from the first, non-nematic phase to the second, nematic phase. In particular, the first substrate is heated after the liquid crystal medium is deposited over the first substrate.
According to at least one embodiment of the method, the second substrate is mounted onto the liquid crystal medium. In particular, the second substrate is mounted on the liquid crystal medium in the second, nematic phase.
According to at least one embodiment, the method for producing a liquid crystal cell comprising a first substrate and a second substrate facing each other, a liquid crystal medium with a phase transition temperature from a first, non-nematic phase to a second, nematic phase sandwiched between the first substrate and the second substrate, comprises at least the steps of providing the first substrate, depositing the liquid crystal medium over the first substrate at a temperature below the phase transition temperature, heating the first substrate to a temperature above the phase transition temperature to effectuate a phase transition of the liquid crystal medium to the second, nematic phase, and mounting the second substrate onto the liquid crystal medium. In particular, the steps are carried out in the order described.
During other methods for producing a liquid crystal cell, the liquid crystal medium is deposited over the first substrate in a liquid and/or nematic phase. In particular, in the other methods the liquid crystal medium is deposited using a one-drop-filling (ODF) process. For instance, the liquid crystal medium has to be in a liquid and/or nematic phase during the ODF process. Thus, all components and materials in contact with the liquid crystal medium have to be heated during the method for producing the liquid crystal cell.
Compared to the other methods, the method described herein has the advantage that heating is only necessary for final assembly of the liquid crystal cell, that is before and/or during the mounting of the second substrate onto the liquid crystal medium. Furthermore, the liquid crystal medium can be applied using simple and efficient techniques for deposition of solid materials.
According to at least one embodiment of the method, at least the step of mounting the second substrate onto the liquid crystal medium is performed under reduced pressure.
Additionally or alternatively, also the step of heating the first substrate is performed under reduced pressure. By "reduced pressure" is understood here and in the following, in particular, a pressure between and including 10 Pa and 1000 Pa, for example between and including 50 Pa and 250 Pa. Advantageously, due to the reduced pressure, the presence of gas bubbles inside the finished liquid crystal cell can be avoided or minimized.
According to at least one embodiment of the method, the second substrate is heated to a temperature above the phase transition temperature before or during mounting the second substrate onto the liquid crystal medium. In particular, by heating the second substrate it is insured that the liquid crystal medium remains in the second, nematic phase during mounting of the second substrate onto the liquid crystal medium.
According to at least one embodiment of the method, the first substrate and/or the second substrate are heated only once during the method for producing the liquid crystal cell. In particular, if the first substrate and/or the second substrate is heated only once, the first substrate and/or the second substrate is heated to such a temperature that the liquid crystal medium does not return to the first, non-nematic phase during mounting of the second substrate onto the liquid crystal medium.
According to at least one embodiment of the method, the first substrate and/or the second substrate are heated a plurality of times before and/or during mounting of the second substrate. In particular, using this heating procedure, the temperature of the liquid crystal medium does not fall below the phase transition temperature from the first, non-nematic phase to the second, nematic phase.
According to at least one embodiment of the method, the first substrate and/or the second substrate are heated constantly before and/or during mounting of the second substrate.
According to at least one embodiment of the method, the liquid crystal medium is in a solid state during depositing the liquid crystal over the first substrate. As the liquid crystal medium is solid, simple and efficient depositing methods can be used. Furthermore, if the liquid crystal cell has a high operating temperature, heating is advantageously just applied to the first substrate and the second substrate. Heating of other components and materials in contact with the liquid crystal medium is not necessary. Additionally, a solid liquid crystal medium allows for either an uneven or an even distribution of the liquid crystal medium over the first substrate.
According to at least one embodiment of the method, the liquid crystal medium is deposited using an electrostatic process. By using the electrostatic process, the liquid crystal medium can advantageously be selectively deposited over distinct regions of the first substrate.
According to at least one embodiment of the method, the liquid crystal medium is unevenly distributed over the first substrate. In other words, over some regions of the first substrate a higher amount of the liquid crystal medium is deposited compared to other regions of the first substrate. For example, a higher amount of the liquid crystal medium is applied in areas of thick metal electrode structures, spacers, or reservoir structures.
Furthermore, a method for operating a device is specified. In particular, the device described herein is operated. Thus, all features, embodiments, and advantages of the device and the method for producing a liquid crystal cell also apply to the method of operating the device and vice versa. In particular, the method for operating a device is a method for operating a liquid crystal cell.
According to at least one embodiment of the method for operating, the device comprises a liquid crystal cell. In particular, the liquid crystal cell comprises a first substrate and a second substrate facing each other, a liquid crystal medium arranged between the first substrate and the second substrate, wherein the liquid crystal medium has a phase transition temperature from a first, non-nematic phase to a second, nematic phase, and two electrodes for supplying an electric potential across the liquid crystal medium to drive the liquid crystal medium in a predetermined configuration, wherein the liquid crystal medium is in the first, non-nematic state when the liquid crystal medium has a temperature below a predetermined operating temperature of the device.
According to at least one embodiment of the method for operating, the liquid crystal cell is exposed to a radio-frequency signal to effectuate heating of the liquid crystal medium to a temperature above the phase transition temperature. In particular, a microwave signal is used to effectuate heating of the liquid crystal medium. The use of the radio-frequency radiation allows advantageously for an external heating of the liquid crystal cell.
Furthermore, if the liquid crystal cell is used in a microwave antenna, the microwaves impinging on the liquid crystal cell can be used to heat the liquid crystal medium. In particular, the liquid crystal medium is heated via dielectric loss.
Advantageous embodiments and developments of the device, the method for producing a liquid crystal cell, and the method for operating a liquid crystal cell will become apparent from the exemplary embodiments described below in conjunction with the figures.
In the figures: Figure 1 shows a schematic cross section of a device according to an exemplary embodiment.
Figure 2 shows a top view of a device according to an exemplary embodiment.
Figure 3 shows a schematic cross section of a device according to an exemplary embodiment.
Figures 4 to 6 show a schematic cross sections of a method for producing a liquid crystal cell according to an exemplary embodiment.
In the exemplary embodiments and figures, similar or similarly acting constituent parts are provided with the same reference signs. The elements illustrated in the figures and their size relationships among one another should not be regarded as true to scale. Rather, individual elements may be represented with an exaggerated size for the sake of better representability and/or for the sake of better understanding.
The exemplary embodiment of a device 1 shown in figure 1 comprises a liquid crystal cell 2 and heating means 3. Presently, the liquid crystal cell 2 is arranged above the heating means 3. The heating means 3 is a resistive heating means comprising a conductor selected from the group consisting of silver, chromium, copper, platinum, ITO, nickel, molybdenum, aluminum, cadmium, and an organic conductive material.
The liquid crystal cell 2 comprises a first substrate 4 and a second substrate 5. The first substrate 4 and the second substrate 5 face each other. A main extension plane of the first substrate 4 is parallel to a main extension plane of the second substrate 5. In other words, the first substrate 4 is arranged parallel to the second substrate 5. A liquid crystal -20 -medium 6 is arranged between the first substrate 4 and the second substrate 5. The liquid crystal medium 6 has a phase transition temperature from a first, non-nematic phase to a second, nematic phase. The liquid crystal medium 6 is in the first, non-nematic phase when the liquid crystal medium 6 has a temperature below a predetermined operating temperature of the device.
The heating means 3 are arranged and configured for maintaining the temperature of the liquid crystal medium 6 above the phase transition temperature from the first, non-nematic phase to the second, nematic phase during operation of the device 1. The first, non-nematic phase of the liquid crystal medium 6 is a crystalline phase, a smectic phase, or a glass. The liquid crystal medium 6 has a clearing temperature of about 160°C. The liquid crystal medium 6 comprises presently the following compound in at least 90 wt.% percent:
F F /=<1\
C511,1-44 NCS
F
The device 1 according to the present exemplary embodiment is configured to operate at a temperature of at least 60 K below the phase transition temperature of the liquid crystal medium from the second, nematic phase to a third, isotropic phase. Presently, an operating temperature of the device 1 is about 80°C.
The liquid crystal cell 2 shown in figure 1 comprises two electrodes 7. The electrodes 7 are configured and designed to drive the liquid crystal medium 6 in a predetermined configuration. The predetermined configuration of the liquid crystal medium 6 corresponds to a distinct orientation of the molecules in the liquid crystal medium 6. One electrode 7 is presently arranged between the first substrate 4 and the liquid crystal medium 6, whereas the other electrodes 7 is arranged between the second substrate 5 and a liquid crystal medium 6. Alternatively, it is possible that the electrodes 7 are part of the first substrate 4 and the second substrate 5.
Seen in top view, the liquid crystal cell 2 of the device 1 comprises a reservoir structure 8, as shown in figure 2. The reservoir structure 8 is presently a convexity which is at least partially filled with liquid crystal medium 6. The reservoir structure 8 is configured to adjust an amount of the liquid crystal medium 6 between the first substrate 4 and the second substrate 5 by having a cavity large enough to accommodate thermal expansion of the liquid crystal medium 6. It is also possible that upon cooling liquid crystal medium 6 is -21 -provided between the first substrate 4 and the second substrate 5 due to contraction of the liquid crystal medium 6. The reservoir structure 8 thus prevents damage of the liquid crystal cell 2 and therefore also of the device 1 due to temperature changes.
Compared to the device 1 shown in figure 1, in the device 1 according to figure 3, the heating means 3 is part of the first substrate 4. The heating means 3 comprises presently a mesh structure embedded in the first substrate 4. The heating means 3 is a resistive heating means comprising a conductor.
Also, in difference to the exemplary embodiment of the device 1 of figure 1, the liquid crystal cell 2 of figure 3 comprises an alignment layer 9 which is arranged in direct contact with the liquid crystal medium 6. The alignment layer 9 presently comprises an inorganic material such as SiO2 or A1203. Alternatively, a surfactant is used in the alignment layer 9. The alignment layer 9 induces a distinct orientation of the molecules in the liquid crystal medium 6. For example, the molecules align parallel or perpendicular to the main extension plane of the first substrate 4. A further alignment layer 9 may be arranged on the second substrate 5 (not shown). The further alignment layer 9 on the second substrate 5 induces a similar or different orientation of the molecules in the liquid crystal medium 6 compared to the alignment layer 9 on the first substrate 4.
The device 1 according to the exemplary embodiment of figure 3 further comprises a temperature sensor 10. The temperature sensor 10 is configured to measure the temperature of the liquid crystal medium 6. In combination with the heating means 3 it is possible to maintain a temperature of the liquid crystal medium 6 at a predefined temperature, for example the operating temperature of the device 1. In other words, the heating means 3 can be operated in dependence of the temperature of the liquid crystal medium 6 measured by the temperature sensor 10. In particular, the device 1 according to the present exemplary embodiment comprises a plurality of temperature sensors 10.
The devices 1 according to the exemplary embodiments of figures 1 and 3 are, for example, a liquid crystal based antenna element, a phase shifter, a tunable filter, a tunable metamaterial structure, a matching circuit, or a varactor.
In combination with figures 4 to 6, a method for producing a liquid crystal cell is described.
As shown in figure 4, a first substrate 4 with an electrode 7 is provided. The first substrate 4 can further comprise cured or uncured sealant rings, an alignment layer 9, spacer -22 -beads, photo spacer, additional electrodes for display or antenna functionality and/or thin film transistors (TFTs).
A liquid crystal medium 6 having a phase transition temperature from a first, non-nematic phase to a second, nematic phase is deposited over the first substrate 4 using an electrostatic process. This is done while the liquid crystal medium 6 is in the first, nonnematic phase which is a solid phase. The liquid crystal medium 6 is evenly distributed over the first substrate 4 which is presently a glass substrate. However, it is also possible that a higher or lower amount of the liquid crystal medium 6 is deposited over the first substrate 4 in some regions, for example in the regions of reservoir structures 9.
The substrate 4 with the liquid crystal medium 6 is heated such that a phase transition of the liquid crystal medium 6 occurs from the first, non-nematic phase to the second, nematic phase. This heating is performed under reduced pressure such that the liquid crystal medium 6 is degassed.
After the liquid crystal medium 6 is brought into the second, nematic phase, a second substrate 5 is mounted onto the liquid crystal medium 6. This step is shown in combination with figure 6. Thereby, the second substrate 5 is heated. During this assembly process, the temperature of the liquid crystal medium 6 is maintained such that the liquid crystal medium 6 remains in the second, nematic phase. This is achieved by either constant or repeated heating of the first substrate 4 with the liquid crystal medium 6 deposited over and of the second substrate 5. Alternatively, the first substrate 4 and the second substrate 5 are heated once to such a temperature that the temperature of the liquid crystal medium 6 does not fall below the phase transition temperature from the first, non-nematic phase to the second, nematic phase.
The assembly process is performed under reduced pressure to ensure that as little air as possible is sealed between the first substrate 4 and the second substrate 5.
The produced liquid crystal cell 2 comprises a first substrate 4 and a second substrate 5 which are arranged parallel to each other. Between the first substrate 4 and the second substrate 5, the liquid crystal medium 6 is arranged. Also, two electrodes 7 are arranged between the first substrate 4 and the second substrate 5. The electrodes 7 are configured and designed for supplying an electric potential across the liquid crystal medium 6 to drive the liquid crystal medium 6 in a predetermined configuration.
-23 -In a method for operating a device 1 according to an exemplary embodiment, the device comprises a liquid crystal cell 2. The liquid crystal cell 2 is, for example, produced by the method of figures 3 to 6. The liquid crystal medium 6 is in the first, non-nematic phase when the liquid crystal medium has a temperature below a predetermined operating temperature of the device 1.
During the method for operating the device 1 according to the exemplary embodiment, the liquid crystal cell 2 of the device 1 is exposed to a radio-frequency signal. Due to dielectric loss in the liquid crystal cell 2, in particular the liquid crystal medium 6, heating of the liquid crystal medium 6 is effectuated. In this way, the temperature of the liquid crystal medium 6 is raised to a temperature above the phase transition temperature from the first, non-nematic phase to the second, nematic phase.
The features and exemplary embodiments described in connection with the figures can be combined with each other according to further exemplary embodiments, even if not all combinations are explicitly described. Furthermore, the exemplary embodiments described in connection with the figures may have alternative or additional features as described in the general part.
The invention is not restricted to the exemplary embodiments by the description on the basis of the exemplary embodiments. Rather, the invention encompasses any new feature and also any combination of features, which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.
List of reference signs 1 device 2 liquid crystal cell 3 heating means 4 first substrate second substrate 6 liquid crystal medium 7 electrode 8 reservoir structure 9 alignment layer temperature sensor -24 -