| 1. |
- Ahmad, Muhammad Ashfaq, et al.
(författare)
-
Structural and electrical characterisation of nanostructure electrodes for SOFCs
- 2014
-
Ingår i: International journal of hydrogen energy. - : Elsevier BV. - 0360-3199 .- 1879-3487. ; 39:30, s. 17487-17491
-
Tidskriftsartikel (refereegranskat)abstract
- This paper reports the effects of sintering temperature on structure, particle size and conductivity of electrodes (Sn0.2Zn0.8Fe0.2O & Sn0.8Zn0.2Fe0.2O). The electrode material was prepared by the chemical method combining a solid state reaction. Structural analyses were performed using X-ray diffraction and scanning electron microscopy. The particle size of the material obtained using Scherrer's formula was 50-60 nm and the nanostructure's surface was studied using electrochemical characterisations tools. Electrical conductivity was determined using the 4-probe DC method, which was compared with the 4-probe AC method. These results suggest a promising substitute for the conventional electrodes of solid oxide fuel cells (SOFCs). It is known that a sintering temperature above 1000 degrees C causes an increase in density and a reduction of porosity. Therefore, we optimised the sintering temperature at 1000 degrees C and obtained electrical conductivity of about 5 S Thus, this electrode could play a vital role in the development of high performance SOFCs at intermediate temperatures.
|
|
| 2. |
- Akram, Muhammad Nadeem, et al.
(författare)
-
Improved wide-field emmetropic human eye model based on ocular wavefront measurements and geometry-independent gradient index lens
- 2018
-
Ingår i: Optical Society of America. Journal A. - : Optical Society of America. - 1084-7529 .- 1520-8532. ; 35:11, s. 1954-1967
-
Tidskriftsartikel (refereegranskat)abstract
- There is a need to better understand the peripheral optics of the human eye and their correction. Current eye models have some limitations to accurately predict the wavefront errors for the emmetropic eye over a wide field. The aim here was to develop an anatomically correct optical model of the human eye that closely reproduces the wavefront of an average Caucasian-only emmetropic eye across a wide visual field. Using an optical design program, a schematic eye was constructed based on ocular wavefront measurements of the right eyes of thirty healthy young emmetropic individuals over a wide visual field (from 40° nasal to 40° temporal and up to 20° inferior field). Anatomical parameters, asymmetries, and dispersion properties of the eye’s different optical components were taken into account. A geometry-independent gradient index model was employed to better represent the crystalline lens. The RMS wavefront error, wavefront shapes, dominant Zernike coefficients, nasal-temporal asymmetries, and dispersion properties of the developed schematic eye closely matched the corresponding measured values across the visual field. The developed model can help in the design of wide-field ophthalmic instruments and is useful in the study and simulations of the peripheral optics of the human eye.
|
|
| 3. |
- Akram, Muhammad Nadeem, et al.
(författare)
-
Influence of base-region thickness on the performance of Pnp transistor-VCSEL
- 2014
-
Ingår i: Optics Express. - 1094-4087. ; 22:22, s. 27398-27414
-
Tidskriftsartikel (refereegranskat)abstract
- We have recently reported a 980nm GaAs-based three terminal Pnp transistor-vertical-cavity surface-emitting laser (TVCSEL) operating at room temperature with optical power up to 1.8mW. However, the current gain beta = Delta I-c/Delta I-b was near zero just before lasing and became negative after the lasing threshold. The main cause of the negative current gain was found to be a gradual and position-dependent forward-biasing (saturation) of the base-collector junction with increasing bias even before lasing threshold. In this article, detailed multi-physics device simulations are performed to better understand the device physics, and find ways to avoid the premature saturation of the base-collector junction. We have optimized the thickness of the base region as well as its doping concentration and the location of the quantum wells to ensure that the T-VCSEL is in the active mode throughout its range of operation. That is, the emitter-base junction is forward biased and base-collector junction is reversed biased for sweeping the excess charges out of the base region.
|
|
| 4. |
- Nadeem Akram, Muhammad, et al.
(författare)
-
Accurate Wide-field Emmetropic Human Eye Model Based On Ocular Wavefront Measurements
- 2017
-
Konferensbidrag (refereegranskat)abstract
- Purpose: To develop accurate wide-field eye model that reproduces the wavefront aberration of emmetropic eye. To demonstrate the ability to develop individual eye models based on the newly developed generic eye model. Methods: A high resolution Shack-Hartmann wavefront sensor is employed to measure the wavefront error from 40 deg Nasal to 40deg Temporal field and up to 20deg Inferior field of view in steps of 10deg of thirty young emmetropic subjects. Zernike polynomials up to 6th order are fitted to the measured wavefront over a circular exit pupil diameter of 4 mm. A mean wavefront is constructed for each field angle from the measured data set. A lens design program is used to reconstruct an model-eye that reproduces same wavefront as the mean wavefront of the measured data set over each field angle. Anatomical parameters range limits as well as dispersion of the eye (cornea, aqueous, lens and vitreous regions) is incorporated during model development to give a realistic eye model. Our model is different from previously published models in that it does not use Gradient index lens (GRIN) as the lens material simplifying optical modeling and ray tracing. In addition, the cornea and lens components are allowed to be tilted, decentered and rotated with respect to the optical axis to provide a much better fit to the measured set of wavefronts. Results: The RMS values, the wavefront shapes, Zernike coefficients and chromatic performance as predicted by the developed model closely match the measured values over the field of view. Our model does re-reproduce the Nasal-Temporal asymmetries found in the eye performance. Personalized eye models developed using the generic eye model also give excellent fit to the measured wavefront over the field of view. Conclusion: We believe our wide-field emmetropic eye model provides better ability to model peripheral vision. It can also be used for the design of advanced ophthalmic instruments, designing lenses for myopia control and low vision optical aids.
|
|
| 5. |
- Yu, Xingang, et al.
(författare)
-
Room-temperature operation of transistor vertical-cavity surface-emitting laser
- 2013
-
Ingår i: Electronics Letters. - : Institution of Engineering and Technology (IET). - 0013-5194 .- 1350-911X. ; 49:3, s. 208-209
-
Tidskriftsartikel (refereegranskat)abstract
- The first room-temperature operation of a transistor vertical-cavity surface-emitting laser (T-VCSEL) is demonstrated. Fabricated using an epitaxial regrowth process, the T-VCSEL is electrically a pnp-type bipolar junction transistor and consists of an undoped AlGaAs/GaAs bottom DBR, an InGaAs triple-quantum-well active layer, an Si/SiO2 dielectric top DBR, and an intracavity contacting scheme with three electrical terminals. The output power is controlled by the base current in combination with the emitter-collector voltage, showing a voltage-controlled operation mode. A low threshold base-current of 0.8 mA and an output power of 1.8 mW have been obtained at room temperature. Continuous-wave operation was performed up to 50 degrees C.
|
|
