The field degradation of carbon nanotube field emitters in diode emission at constant current was demonstrated to be highly dependent upon the presence of water at partial pressures as low as 10−9Torr. The anode surface was also seen to significantly impact the degradation rate, with metallic Al films yielding the worst degradation rates. Coating the anode surface with a carbon or polymer film lowered the degradation rate. It is suggested that a majority of the degradation seen in nanotube field emission devices is due to ionization of water adsorbed at the anode surface.

The effect of carbon containing gasses on the field emission degradation rate of carbon nanotube field emission devices has been measured. Long chain hydrocarbons were seen to form a carbonaceous deposit on the anode surface which effectively lowered the degradation rate. Simple hydrocarbons such as methane, ethylene, and acetylene reversed degradation by continually enhancing emission. This continuous enhancement was repeatable and continued over 500h. Carbon dioxide exposure at low partial pressures resulted in an increased field emission degradation rate similar to oxygen and water exposure as reported earlier.

A screen printed. under-gate triode with carbon nanotube field emitters is demonstrated. The geometry of the device allows it to be matrix addressable. Fabrication methods including CVD deposition of dielectric and/or nanotubes, wet etching, and photolithography are eliminated by the use of screen printing, effectively lowering the cost of manufacture of the device. Through the use of a charge dissipation layer the emission current is greatly enhanced, and instabilities which plagued previous under-gate devices are eliminated. When a square Pulse of 30 mu s is applied at a rate of 120 Hz emission currents of >4 mu A/cm(2) are achieved for gate voltages of < 300 V without any ungated emission. This structure is a prime candidate to be used in a field emission backlight unit (BLU) for an LCD due to its reduced cost of manufacture, stable operation, and local delivery Of Pulsed light. (C) 2008 Elsevier B.V. All rights reserved.

A system of an insulating Bi2O3‐based frit and a conductive SnO2:Sb has been developed for printing and firing on glass substrates. Resistivity can be progressively decreased from >200 GΩ to the megaohm region by adding micron size SnO2:Sb in the thick film formulation. More SnO2:Sb is required for a given resistivity when alumina is added as a filler, and resistivity is dependent on alumina particle size. Controllably reducing resistivity of a dielectric is useful in field emission display applications to eliminate charging between emitter lines and to function as a ballast resistor layer.

A new picture of environmentally-enhanced fracture in highly brittle solids is presented. It is asserted that the fundamental relations for crack growth are uniquely expressible in terms of the surface force functions that govern the interactions between separating walls in an intrusive medium. These functions are the same, in principle, as those measured directly in the newest submolecular-precision microbalance devices. A fracture mechanics model, based on a modification of the Barenblatt cohesive zone concept, provides the necessary framework for formalizing this link between crack relations and surface force functions. The essence of the modification is the incorporation of an element of discreteness into the surface force function, to allow for geometrical constraints associated with the accommodation of intruding molecules at the crack walls. The model accounts naturally for the existence of zero-velocity thresholds; further, it explains observed shifts in these thresholds in cyclic load-unload-reload experiments, specifically the reduction in applied loading needed to propagate cracks through healed as compared to virgin interfaces. The threshold configurations emerge as thermodynamic equilibrium states, definable in terms of interfacial surface energies. Crack velocity data for cyclic loading in mica, fused silica and sapphire are presented in support of the model. Detailed considerations of the theoretical crack profiles in these three materials, with particular attention to the atomic structure of the “lattice” (elastic sphere approximation) at the interfaces, shows that intruding molecules must encounter significant diffusion barriers as they penetrate toward the tip region. It is concluded that such diffusion barriers control the fracture kinetics at low driving forces. At threshold the barriers become so large that the molecules can no longer penetrate to the tip region. This leads to a crucial prediction of our thesis, that the cohesive zone consists of two distinct parts: a “protected” primary zone adjacent to the tip, where intrinsic binding forces operate without influence from environmental influences; and a “reactive” secondary zone more remote from the tip, where extrinsic interactions with intruding chemical species are confined. The prevailing view of chemically enhanced brittle fracture, that crack velocity relations are determined by a concerted reaction with reactive species at a single line of crack-tip bonds, is seen as a limiting case of our model, operative at driving forces well above the threshold level. The new description offers the potential for using brittle fracture as a tool for investigating surface forces themselves.

The effect of lateral cracks on strength controlling contact flaws in brittle materials is examined. Inert strength studies using controlled indentation flaws on a range of ceramic, glass, and single crystal materials reveal significant increases in strength at large contact loads, above the predicted load dependence extrapolated from strength measurements at low indentation loads. The increases are explained by the growth of lateral cracks decohesing the plastic deformation zone associated with the contact from the elastically restraining matrix, thereby reducing the residual stress field driving the strength controlling radial cracks. A strength formulation is developed from indentation fracture mechanics which permits inert strengths to be described over the full range of contact loads. The formulation takes account of the decreased constraint of the plastic deformation zone by lateral crack growth as well as post-contact nonequilibrium growth of the radial cracks. Simple extensions permit the strengths of specimens controlled by impact flaws to be described, as well as those failing under nonequilibrium (fatigue) conditions. The implications for materials evaluation using indentation techniques are discussed and the dangers of unqualified use of strength measurements at large indentation loads pointed out. The work reinforces the conclusion that a full understanding of the residual stress field at dominant contact flaws is necessary to describe the strength of brittle materials.