Sunday, November 22, 2009

DuPont Photoimageable Silver and CNT Paste to Challenge Indium Tin Oxide in FED Applications

DuPont scientists have created novel photoimageable silver and carbon nanotube (CNT) paste compositions and processing methods, which can be co-processed, for improving the manufacture of cathode electrodes and emitters of field emission devices. The compositions and method are useful in field emission display (FED) television and backlighting applications, according to U.S. Patent Application 20090284122.

The DuPont CNT process and paste compositions are useful in flat panel computer, television and other types of displays, vacuum electronic devices, emission gate amplifiers, klystrons and in lighting devices. The process is especially advantageous for producing large area electron field emitters for flat panel displays, i.e., for displays greater than 30 inches (76 cm) in size. The flat panel displays can be planar or curved. The photoimageable CNT composition is comprised of CNT, acrylate monomer/oligomer blend, Trimethylolpropane ethoxy triacrylate, 2-methyl-1-[-(methylthio)phenyl]-2-(4-morphol inyl)-1-propanone and 2,4-diethyl-9H-thioxanthen-9-one.

The application discloses a novel co-processing method for formation of FED cathode electrode and emitter, and a co-processable photoimageable silver composition and a CNT composition for the processing method. The manufacturing steps for an electrode and an emitter in a field emission device, include: (a) printing a photoimageable conductive composition on a substrate, (b) printing a photoimageable carbon nanotube (CNT) composition on the conductive composition, (c) exposing the printed substrate to UV light, (d) developing the exposed substrate, and (e) firing the developed substrate.

The CNT composition is used as a screen printable paste containing solids comprising carbon nanotubes. The carbon nanotubes are 0.01 to 9 wt % of the total weight of solids in the paste depending on the embodiment. The silver metal powder are from 10-80 wt % of the total weight of solids in the paste. 2,4-diethyl-9H-thioxanthen-9-one is 1 to 7 wt % of the CNT paste, the 2-methyl-1-[-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone is 2 to 14 wt % of the CNT paste. The silver conductor paste and CNT emitter paste may be prepared by three-roll milling a mixture of silver or acicular carbon, organic medium, surfactant, a solvent and either low softening point glass frit, metallic powder or metallic paint or a mixture thereof.

There is a continuing need for improved technology enabling the use of acicular carbon (e.g.carbon nanotubes) in electron field emitters. In the current industrial process, conductor electrodes and emitters are formed by a separate imaging, development and firing steps. If the electrode is made of ITO, ITO is sputtered via a mask pattern in vacuum. If the electrode is made of silver, a photoimageable silver thick film paste, such as Fodel® from DuPont, it is first screen-printed, dried, photo-imaged, developed, and fired. Then a CNT emitter paste is printed on top of conductor electrodes, dried, imaged, developed and fired. There is a need to minimize the number of process steps and improve the process. The DuPont’s invention addresses this need, according to inventors inventors Haixin Yang, Lap-Tak Andrew Cheng, Terry Roland Suess, Chien and Lung Weng.

Tables 1, 2 and 3 reveal material compositions used in the photoimageable silver and carbon nanotube (CNT) paste as well as weight percentages. 



FIG. 5 is an optical micrograph of the CNT dashes over silver line after air firing.
FIG. 6 is an image of phosphor illumination by electrons emitted by the separate-image, co-develop, and air co-fire field emission cathode part.

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