Sub-micrometer CoAl 2 O 4 particles are prepared by applying the polyol method. Colloidal particles 50 to 200 nm in size with spherical shape are formed. Their average size can be controlled by adjusting the concentration of the components (metal precursor, water). The resulting suspensions in diethylene glycol can contain up to 10 wt.% CoAl 2 O 4 . The colloidal particles are well stabilized against agglomeration even when mixed with water. Based on such colloidal CoAl 2 O 4 suspensions planar substrates (e.g. glass plates) as well as non-planar substrates (e.g. phosphor powder) can be covered homogeneously with thin pigment ®lms. In order to establish the characteristic deep blue body colour of CoAl 2 O 4 , additional brief heating (15 min, 600 ³C) is necessary. Further characterisation of CoAl 2 O 4 powders as well as suspensions was carried out by diffuse re¯ection measurements, X-ray powder diffraction, scanning electron microscopy, laser diffraction and atomic force microscopy.
Highly stable field emission current densities of more than 6A∕cm2 along with scalable total field emission currents of ∼300μA per 70μm diameter carbon nanotube (CNT)-covered electron emitter dot are reported. Microwave-plasma chemical vapor deposition, along with a novel catalyst sandwich structure and postdepositional radio-frequency (rf) oxygen plasma treatment lead to well-structured vertically aligned CNTs with excellent and scalable emission properties. Scanning electron and transmission electron microscope investigations reveal that postdepositional treatment reduces not only the number but modifies the structure of the CNTs. Well-structured microwave-plasma-grown nanotubes become amorphous during rf oxygen plasma treatment and the measured work functions of CNTs change from 4.6eVto4.0eV before and after treatment, respectively. Our experiments outline a novel fabrication route for structured CNT arrays with improved and scalable field emission characteristics.
Chemical and physical aspects of the adhesion of colloidal ZnO particles (d(50)=81 nm) on the surface of ZnS-type phosphors have been studied. Here, the green-emitting phosphor ZnS:Cu,Al,Cu (d(50)=5.0 µm) applied in TV screens was chosen as model compound. The ZnS material was pretreated in various ways (H(2)O, HCl, H(2)O(2)) and reacted thereafter with a suspension containing colloidal ZnO particles. Analytical investigations (SEM, ESCA) have shown that the adhesion of colloidal ZnO particles is strongly affected by the degree of hydrolysis of the ZnS surface. Electroacoustic investigations (ESA) prove that both types of surfaces, hydrolyzed ZnS as well as colloidal ZnO, are positively charged. Even so, adhesion of ZnO particles is encouraged very much under these conditions, indicating that secondary attractive forces (electrostatic interaction, chemical bonding) determine the amount of colloidal ZnO adhered on a ZnS-type phosphor. Copyright 2000 Academic Press.
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