State of the art fabrication of LED modules based on chip-on-board (COB) technology comprises some shortcomings
both with respect to the manufacturing process itself but also with regard to potential sources of failures and
manufacturing impreciseness. One promising alternative is additive manufacturing, a technology which has gained a lot
of attention during the last years due to its materials and cost saving capabilities. Especially direct-write technologies like
Aerosol jet printing have demonstrated advantages compared to other technological approaches when printing high
precision layers or high precision electronic circuits on substrates which, as an additional advantage, also can be flexible
and 3D shaped. Based on test samples and test structures manufactured by Aerosol jet printing technology, in this
context we discuss the potentials of additive manufacturing in various aspects of LED module fabrication, ranging from
the deposition of the die-attach material, wire bond replacement by printed electrical connects as well as aspects of high-precision
phosphor layer deposition for color conversion and white light generation.
The aim of the present work was to demonstrate the fabrication technique for semitransparent layers of nanoparticulated (~50 nm) LuPO4:15%Eu phosphors. Furthermore, to present their basic luminescent properties and provide results regarding their performance in a planar imaging system incorporating a CMOS photodetector. Parameters such as the Detective Quantum Efficiency (DQE), the Normalized Noise Power Spectrum (NNPS) and the Modulation Transfer Function (MTF), were investigated.
The NNPS was found to present significantly higher values near the zero frequency for the 67 μm, 100 μm films, pointing on their higher non uniformities compared to the 220 and 460 μm films For the two thickest films (460 μm and 220 μm) the MTF curves practically do not differ, while MTFs for the thinner layers of 100 μm and 67 μm are higher as the layer’s thickness decreases. The higher DQE values observed for the 220 μm and 460 μm films up to medium frequencies, while at high frequencies the DQE values are comparable. Although the MTF values of these films are much lower than the thinner screens, the capability of the higher x-ray absorption, in conjunction with the low noise properties, lead to higher DQE values.
The LuPO4:Eu semitransparent films seems to be a very promising scintillator for stationary x-ray imaging. The acquired data allow to predict that high-temperature sintering of our films under pressure may help to improve their imaging quality, since such a processing should increase the luminescence efficiency without significant growth of the grains, and thus without sacrificing their translucent character.
The purpose of the present study was to experimentally evaluate the imaging characteristics of the Lu2O3:Eu
nanophosphor thin screen coupled to a high resolution CMOS sensor under radiographic conditions. Parameters such as
the Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum (NNPS) and the Detective Quantum
Efficiency (DQE) were investigated at 70 kVp under three exposure levels (20 mAs, 63 mAs and 90 mAs). Since
Lu2O3:Eu emits light in the red wavelength range, the imaging characteristics of a 33.3 mg/cm2 Gd2O2S:Eu conventional
phosphor screen were also evaluated for comparison purposes.
The Lu2O3:Eu nanophosphor powder was produced by the combustion synthesis, using urea as fuel. A scintillating
screen of 30.2 mg/cm2 was prepared by sedimentation of the nanophosphor powder on a fused silica substrate. The
CMOS/Lu2O3:Eu detector`s imaging characteristics were evaluated using an experimental method proposed by the
International Electrotechnical Commission (IEC) guidelines.
It was found that the CMOS/Lu2O3:Eu nanophosphor system has higher MTF values compared to the CMOS/Gd2O2S:Eu
sensor/screen combination in the whole frequency range examined. For low frequencies (0 to 2 cycles/mm) NNPS values
of the CMOS/Gd2O2S:Eu system were found 90% higher compared to the NNPS values of the CMOS/Lu2O3:Eu
nanophosphor system, whereas from medium to high frequencies (2 to 13 cycles/mm) were found 40% higher. In
contrast with the CMOS/ Gd2O2S:Eu system, CMOS/Lu2O3:Eu nanophosphor system appears to retain high DQE values in the whole frequency range examined.
Our results indicate that Lu2O3:Eu nanophosphor is a promising scintillator for further research in digital X-ray
radiography.
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