Nanometric Dry Powder Coatings Using a Novel Processt

A wide range of advanced technology for existing and emerging products based on high temperature metal-ceramic composites used in aircrafts, cutting tools, lithium-ion based rechargeable batteries, superconductors, field emission based flat-panel displays, etc. employ micron to submicron sized (0.1-10 microns) particulate precursors in their manufacturing process. Although there has been a significant emphasis given to control of the particle characteristics (shape, size, surface chemistry, adsorption, etc.), relatively little or no attention has been paid to concomitant designing desirable surface and bulk properties at the particulate level, which can ultimately lead to enhanced properties of the product. By attaching atomic to nano-sized inorganic, multi-elernental clusters either in discrete or continuous from onto the surface of the core particles, i.e nano-functionalization of the particulate surface, materials and products with significantly enhanced properties can be obtained. In this paper, we demonstrate for the first time the synthesis of artificially structured, nano-functionalized particulate materials with unique optical, cathodoluminescent, superconducting and electrical properties. In this paper, we show the feasibility of the pulsed laser ablation technique to make very thin, uniformly distributed and discrete coatings in particulate systems so that the properties of the core particles can be suitably modified. Experiments were conducted for laser deposition of Ag nano particles on Al20 3 and Si02 core particles by pulsed excimer laser (wavelength =248 nm and pulse duration=25 nanosecond) irradiation of a Ag, Y203:Eu3+, and TaSi2 sputtering targets. Analytical techniques using scanning electron microscopy (SEM), wavelength dispersive x-ray mapping (WDX), transmission electron microscopy (TEM), scanning transmission electron microscopy with z contrast (STEMZ), and photoluminesence (PL) were utilized to examine the structural, chemical, and morphological characteristics of the nanometric coatings. Qualitative surface uniformity measurements by WDX mapping techniques showed a high degree of coating uniformity on the core particulate. Structural TEM and STEMZ imaging showed both continuous and discrete polycrystalline, multiply twinned nano particle coatings ranging from 5-40 nm in thickness.


Introduction
Sub-micron to micron sized metallic and ceramic particles (100 nm to 10 microns) act at principal precursor materials for a wide range of existing and emerging products involving advanced ceramics, metals, composites which spans several industries such as aerospace, automobile, machining, vacuum electronics, batteries, data storage, catalysis, etc [1][2]. Particulate materials, as core technologies impact • Code 6372, Washington, District of Columbia 20375, USA •• Gainesville, Florida 32611, USA over 1 trillion dollar yearly on a worldwide basis [3]. To achieve desirable properties in the final product, typically the properties of the particles such as shape, size, composition, surface charge, flowability, etc., have been controlled. These characteristics play an important role in determining the final microstructure, and thus the product's properties. However with the rapid advancements in non-particulate technologies such as computers, telecommunications and electronics, there is a strong need to need to develop novel particulate systems which can results in valueadded products with enhanced properties [4].
Increasing interest in the recent years has been focused on a wide variety of nanostructured materials, which possess grain or phase structures modulated on a length scale of less than 100 nm, because it is anticipated that their properties will be different from and often superior to conventional materials that have phase or grain boundaries over a coarser size scale [5][6]. Using artificial engineered nanostructured materials, it may also be possible to engineer the properties by controlling the size of the constituent domains and the manner in which they are assembled. Some of the recent efforts have been focused on synthesizing atom clusters, zero dimensionality quantum well structures, one dimensional modulated multilayered materials, and three dimensional modulated nanophase materials and nanocomposite materials [3-7]. These artificially engineered nanostructured materials may possess novel properties, however typically they cannot be used along with the submicron to micron sized (100 nm to 10 micron) sized materials which constitutes the bulk of the existing commercial technologies.
By synthesizing nano-functionalized particulates, or artificially engineered materials formed by attachment of nanosized particles of complex stoichiometries onto core particles in different architectural forms (porous to compact, discrete to continuous), it is possible to produce particulate materials which concomitantly exhibit, distinct, disparate and unique properties. A schematic diagram of two types of nano-functionalized particulate materials is shown in Fig. 1. This figure shows both discrete and continuous nanofunctionalized layers onto core particles. Table I shows some of the unique characteristics of nano-functional-ized particulate materials. For example, thin, conducting continuous clusters ( <2 nm) are required for field emission display flat panel powders, so that the surface electrical charge can be dissipated, and electron beam induced surface oxidation can be prevented for long-life cathodoluminescent properties [8][9]. Similarly continuous nanoparticle intermetallics (such as TiA1) clusters of size <2 nm may be ideal for we core particles, so that defect-free, high-toughness, high-temperature strength composite materials may be synthesized for next generation high speed (>1500 m/min) cutting tool applications [10]. In contrast, thick (20-30 (lm) discontinuous CoO clusters may be ideal for Ni(OH)2/Co0 composites, in which the surface exposure of the core material is essential of efficient charging and discharging kinetics for high capacity rechargeable nickel metal hydride battery applications. It should be noted that in all these applications, it is imperative that the functionalized surface nano particles core particle Fig. 1 Schematic diagram of a nanofunctional particulate con· sisting of a core particle coated with nano particles. Fig. 1 (a) illustrates a discrete or porous coating while (b) shows a continuous nanoparticle coating. Although the synthesis of nanofunctionalized particulates is highly desirable from both fundamental and technological viewpoint, significant progress has not been made in this field due to several complicating factors [13][14][15]. Some of the cluster materials are in the form of multicomponent stochiometric materials such as high Tc superconductors (Y-Ba-Cu-0), colossal magnetoresistance (CMR) materials (such as La-Ca-Mn-0), phosphorescent materials (e.g, Eu doped yttrium oxide, sulphide). This complicates the uniformly deposition of these species onto the core particle [11][12]. In addition, the functionalized surface should typically have nanometric dimensionality control for optimum interaction between bulk and surface properties. Techniques presently used in the literature such as fluidized bed coating, powder blending, mechanofusion processing, chemical precipitation and chemical vapor phase condensation are not capable of overcoming the above mentioned barriers [ [13][14][15]. Thus, the state of the art methods cannot be utilized to synthesize nano-functionalized particulates with uniformly distributed multicomponent coatings in nanometric dimensions.

Experimental
The use of atomic to nano-sized particle fluxes to coat particulate materials has several advantages. Typically nano particles  nm) are very reactive and when brought into close contact with each other, aggregate to form larger particles, thus substantially decreasing the observed surface area. By adhering the nano particles to larger particles, the aggregation of the nano particles is prevented, thus significantly increasing their utilization. It should also be noted that the formation of a composite particle with atomic or nano particles will lead to improved adherence of the coating due to its size and reactivity of the nano particle. Another advantage of using nano particles to synthesize a composite particle is the small amount of material required in the vapor form. For example if 1% surface coverage is required, 1 gm of nano particles (<10 nm) can coat more than 100 kg of 10 Jlm core particles, if 100% efficiency of the coating process is assumed. Thus, the formation of nano thin coatings KONA No.17 (1999) on particulate materials not only provides unique properties but can be manufactured in large quantities. The pulsed laser deposition (PLD) technique has emerged as one of the most popular methods to deposit complex oxide thin films and deposition of compositionally varying multilayer systems [16][17][18][19]. Presently, most of the thin film deposition has been conducted on flat substrates which are normally kept parallel to the substrate at a distance of 3-10 em from the target. The flux generated by the laser interaction with the target and the ablated material, is composed of active atomic and molecular species. The background gas has been found to play a significant role in the film formation process. If the background gas chamber pressure consists of reactive gases such as oxygen, ammonia, nitrous oxide etc., a higher concentration of molecular species is observed in the plasma. At high background pressures (>200 mTorr), nano particles, (>3 nm) size in the plasma have been reported [20][21][22]. The formation of these clusters is due to increased collisions of the ablated species in the gas phase. Also, if the background gas is reactive (e.g. oxygen etc.), the ablated species may react to form new compound species [22]. Composite particulates which have been surface modified with a thin continuous or discontinuous layer have a large number of applications in existing and emerging technologies [23][24]. An example of a composite particulate material is shown in Fig. 1. This figure shows that the surface of the core particle is modified by the attachment of the secondary particles.
In this paper, we show the feasibility of the pulsed laser ablation technique to form discrete Ag nano particle coatings on core particles of Al 2 0 3 and Si0 2 • Fig. 2 shows a schematic diagram of the experimental setup to fabricate the particulate coatings. Except for the core particle suspension system, this deposition system resembles a standard PLD thin film deposition system. An excimer laser irradiates the target material through the ultraviolet transparent quartz window. Typical energy densities employed in the experiments were approximately 2-3 J/cm 2 . The laser plume is directed perpendicular to the target material to an agitated bed to core particles. The core particles are suspended in the system by a mechanical agitation method. The thickness and surface coverage of the coating is controlled primarily by the repetition rate of the laser and the residence time of the suspension. By controlling the energy as well as the background pressure in the system, the composition and size of the nano particles can be controlled. Earlier work has shown a correlation between the cluster Schematic diagram of the system employed to synthesize nano-cluster coatings. Fig. 2 (a) is a schematic of the overall processing system, including the pulsed excimer laser heat source, (b) shows an enlarged schematic of the nano particle growth process.
size and the background gas pressure. When the background gas pressure is increased, the cluster size changes from a few atoms to nanometer dimensions. The experiments have been conducted using A}z03 and Si0 2 core particles with a high purity Ag, Y 2 03:Eu 3 +, and TaSiz sputtering targets as sources. Figure 3 shows in-situ CCD images obtained during processing coating of 500 j.lm A}z03 particles.

Results and Discussion
As shown in Fig. 4. (a), the size of the ablated nano particles is below the resolution of the SEM, therefore the presence of the coating on the surface was determined by chemical analysis . Figures 4 (b), (c), respectively show SEM and Ag chemical images of approximately 18 j.lm faceted alumina particles, however as in (a), it does not reveal the presence of a nano particle film on the surface. From the WDX Ag chemical map shown in figure 4 (c), it appears that the Ag is distributed uniformly on the surface of the AI 2 03 particles. To determine the structure as well as the spatial distribution of the nano-functionalized layer, high resolution transmission electron microscopy and scanning transmission electron microscopy (HRTEM) with dark field contrast (STEM-Z) were conducted on these samples. The contrast in STEM-Z occurs due to incoherent Rutherford scattering, thus the intensity is proportional to the atomic number of the element [26][27]. Figure 4 (d) shows the STEM-Z micrograph of the surface morphology of the Ag/ AI 2 0 3 nano-functionalized particulate system. This figure shows that the surface of the core particle is partially covered with a dis-continuous layer of silver nanoparticles. To Fig. 4 (a) SEM of an Ah0 3 particle coated with Ag nano particles, (b) SEM of Ah0 3 particles coated with Ag nano particles and (c) corresponding WDX chemical map for Ag on (b) showing the 1 : 1 correlation of the Ag coating, (d) represents a dark field image of Ag on the Ab0 3 particle surface, at higher magnification, (e), (f) present dark field and bright field images of the Ag nano particle-Ah03 interface respectively, (g) shows a multiply twinned Ag nano particle on the Al203 particle substrate, (h), (i) show STEM structure reconstruction of a (110) twinned Ag nano particle on AbOJ.
investigate the interface further, the sample was tilted to look at the interface between the Ag nano particle and the A}z0 3 core particle_ Fig. 4. (e), (f) show dark and bright field STEM-Z images of the interface respec-tively_ From Fig. 4. (f), the brighter area corresponds to Ag and the darker Al 2 0 3 _ It appears that there is little if any intermixing between the Ag nano particles and the core Al 2 0 3 particle surface_ Fig. 4. (g) shows a dark field image of a multiply twinned Ag nano particle on the surface of the core Al 2 0 3 particle. Fig. 4. (h) and (i) show dark field images of a Ag nano particle on Al 2 0 3 , subsequent structure reconstruction showed prevalent (110) twinning behavior.
KONA No. 17 (1999) X-ray photo electron spectroscopy (XPS) was used to examine the relative surface coverage of Ag nano particles on Alz03 core particles as a function of background gas molecular weight and pressure. Figure 5 (a) shows the results of quantitative analysis of Ag surface coverage. Both the molecular weight and pressure of the backfilled gas appear to modify the deposition characteristics with respect to surface coverage in this system_ Mean Ag nano particle size distributions, shown in Fig. 5 (b) were gathered by collecting nanoparticles on TEM grids. Subsequent CCD camera time-gated Ag plume imaging at long times after the initial laser pulse are shown in  Figure 6 shows the plume evolution of the plume in the presence of Helium at 200 mTorr. Optical emission due to collisional heating and electron-ion recombination lasts for =70 J.lSec, in comparison to sustained emission lifetimes of =300 J.lSec in the presence of Krypton. Further analysis of the CCD imaging have shown that the molecular weight of the backfill gas controls both the nanoparticle formation and deposition characteristics by the formation of a semi stationary confined plasma (SSCP) [28].
Nano-functional particulate materials can be designed to achieve properties which cannot be obtained with existing particulate materials. An example of this is shown for particulate requirements for field emission based flat panel display applications [9]. This emerging technology has several advantages in terms of low power consumption, high brightness, low power consumption, large viewing angle and is expected to replace liquid based crystal technology in specific applications [9]. In this application, the multiple field em1sswn electron beam sources directly strike a phosphor particulate based screens possessing red blue or green activators to produce light which is emitted out of the surface. The phosphor screens are typically made from powder materials which exhibit a multitude of shapes and sizes distributions, thus causing non-uniformity in the thick film microstructure. Additionally, upon continuous irradiation with intense electron beams, the brightness of the screens degrade thus limiting its usefulness and have become a major impediment for commercialization [9]. Ftg. 8 (a) shows the SEM image of a continuous nano-functionalized Y 2 0 3 :Eu layer on 1 f.!m mono-sized silica particles. These nano-functionalized particulates were heat treated at 700°C for 1 hr in air to activate the phosphor layer. The silica particles, prepared by sol-gel techniques are typically spherical in shape, thus leading to the spherical morphology of the nano-functionalized particulates. The TEM micrograph in Fig. 8 (b) and its corresponding diffraction pattern, (8c), shows two important microstructural characteristics of the nano-functionalized Y 2 0 3 :Eu layer, namely (i) the layer forms a continuous film on the surface and (ii) a single crystal like electron diffraction pattern is obtained from nano-functionalized surface layer. The single crystal nature of the surface layer on a curvilinear substrate possibly suggests new mechanisms for stabilization of single crystal growth on curved surfaces. More studies are presently being conducted to understand the nature of layer growth on curved interfaces. Fig. 8 (d) shows the typical photoluminesence spectrum obtained from Y203:Eu 3 +/silica nano-functionalized particles. The figure shows that the particles yield a dominant red light emission peak at 611 nm due to 5 D 0 JF 2 transition. Due to shielding effect of 4f6 electrons by 5s 2 and 5p 6 electrons in outer shells of Eu ion, one expects a sharp emission within Eu ions. The photoluminesence brightness was significantly higher when the samples were annealed in air at 700°C for 1 hour. Another potential application of nano-functionalized particulates is shown in Ftg. 9. This example is related to reduction in the degradation in the brightness of the flat-panel phosphors due to aging effects. Coulomb Load (C/cm 2 ) Fig. 9 This figure shows that during cathodoluminescent degradation, the uncoated phosphor material (Y 2 0 2 S:Eu 3 +) degraded to approximately 50% of its original brightness if the total dose exceeds 15 C/ cm 2 (standard industrial conditions for a phosphor lifetime). However, the application of very thin nano-functionalized tantalum di silicide (TaSiz) and Ag layers by the laser ablation process reduces the degradation of this phosphor material by a factor of 5 and 2 respectively. Results of wet coatings with Si0 2 and phosphate materials are shown for comparison.
ure shows the change in brightness of an yttrium ox:ysulphide phosphor powder as a function of cumulative electron dose. This figure shows that the phosphor degraded to approximately 50% of its original brightness ifthe total dose exceeds 15 C/cm 2 . However, the application of a very thin nano-functionalized tantalum di silicide (TaSi 2 ) layer reduces the degradation of this phosphor material by a factor of 5. This reduction in degradation has been attributed to the reduced oxidation of the surface layer in presence of the protective nano-thin layer. It should be noted that nanometric dimensions of the layer is essential to ensure electron transparency of the surface.

Conclusions
A novel technique coupling pulsed laser ablation to synthesize engineered particulate materials has been presented. TEM observations have shown that the backfill gas pressure and molecular weight have significant effects on Ag nano particle formation during ablation. Ag nano particles formed during laser ablation have a multiply twinned, high defect structure. STEM investigations of the interface between the Ah0 3 core and the Ag nano particles contains very little intermixing with adhesion mechanisms most probably due to long range Van de Waals bonding. Further STEM characterization of the interfaces is presently 180 underway. In short, it is felt that PLD method represents a viable method to surface modify particulate systems, which are required for a wide variety of current and future applications.