Doping-induced coloration in titania

Thermal decomposition of Ti₃GeC₂ MAX phase at 1773 K yields an orange-colored titania powder. Micro-XRD of the powder under oscillation mode reveals a pure rutile phase (space group P42/mnm). X-ray photoelectron spectroscopy confirmed substitutional doping of Ge in the titania lattice. The presence of Ti-O-Ge bond was observed in O 1s spectrum and confirmed by the shift in binding energy in Ti 2p_3/2 and Ge 3d peaks. The UV-visible Diffuse Reflectance Spectrum studies on the Ge-doped titania powder show wide absorption in the visible region (380 to 650 nm) yielding a bandgap of 2.83 eV, which is desirable for photocatalytic applications. Defect states formed due to Ge doping led to lowering of the titania conduction band inducing an orange coloration in the powder.     

Sample period effects in optimally designed digital control systems

A systematic procedure for obtaining an optimal sampled data control system which treats the sample period as a free design parameter is developed. The sensitivity of the closed-loop performance with respect to the sample period is examined for various choices of the penalty weighting factors and the compensator structure. An optimal nonzero sample period may exist, depending on the penalty weighting assigned to the system states even when a boundary condition is imposed that requires the optimal sampled data control system to converge to an optimal continuous control system as the sample period approaches zero.     

A novel structure of real-time expert control system for process industry

A new structure of on-line real-time expert control system for process industry is proposed in this paper. The overall system can be hierarchically visualized as three levels. The highest level performs the supervisory functions, such as overall system performance monitoring, possible system components failure detections and diagnoses; and commands generations regarding to the necessary controller parameter tunings or structure variations. It is in this level that the expert control system stores the knowledge and makes the decisions. The second level utilizes some sophisticated signal processing algorithms, such as adaptive filtering, system identification, state estimation, to pre-process the raw measurement data coming from the sensors. So that the procedure of decision-making and structure of the knowledge base become simpler and more concise. It is this level that makes the present structure unique in comparison with the other existing expert control systems. The lowest level consists of the conventional control hardwares which execute the control commands. A prototype system is under construction on an IBM PC-AT microprocessor using PROLOG for knowledge manipulation, decision-making and FORTRAN for numerical computations.     

Analysis of axially dispersed systems with general boundary conditions—II: Solution for dispersion in the appended sections

The axial dispersion problem formulated in Part I[1] of this series is solved here for “case 1” which corresponds to the situation of finite, non-zero dispersion in both semi-infinite appended sections. The solutions are obtained in terms of the spectrum of the operator consisting of a continuous spectrum and frequently a finite set of discrete eigenvalues. Transient solutions are presented for a first order reaction in the axially dispersed reactor for a variety of initial conditions demonstrating where and where not the Danckwerts boundary conditions may be valid. It is shown that “controlling” eigenvalues may be exploited from persisting tails of transient concentration measurements on tracer experiments to obtain dispersion parameters.     

Stirred pots,tubular reactors,and self-adjoint operators

Boundary value problems involving continuous flow reactors have been considered in which tubular and well-stirred tank reactors have been considered together with an axial dispersion model for the tubular reactor. This formulation does away with the customary but non-physical discontinuity in the state of the feed stream at the inlet to the stirred tank reactor. The problems, restricted to isothermal reactors entertaining first order reaction systems, have been solved by means of an elegant formalism in tune with the general theory of self-adjoint operators in abstract Hilber space and consistent with the elementary treatment.     

CFD simulation of bubble columns incorporating population balance modeling

A computational fluid dynamics (CFD)-code has been developed using finite volume method in Eulerian framework for the simulation of axisymmetric steady state flows in bubble columns. The population balance equation for bubble number density has been included in the CFD code. The fixed pivot method of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51, 1311-1332] has been used to discretize the population balance equation. The turbulence in the liquid phase has been modeled by a k-ε model. The novel feature of the framework is that it includes the size-specific bubble velocities obtained by assuming mechanical equilibrium for each bubble and hence it is a generalized multi-fluid model. With appropriate closures for the drag and lift forces, it allows for different velocities for bubbles of different sizes and hence the proper spatial distributions of bubbles are predicted. Accordingly the proper distributions of gas hold-up, liquid circulation velocities and turbulence intensities in the column are predicted. A survey of the literature shows that the algebraic manipulations of either bubble coalescence or break-up rate were mainly guided by the need to obtain the equilibrium bubble size distributions in the column. The model of Prince and Blanch [1990. Bubble coalescence and break-up in air-sparged bubble columns. A.I.Ch.E. Journal 36, 1485-1499] is known to overpredict the bubble collision frequencies in bubble columns. It has been modified to incorporate the effect of gas phase dispersion number. The predictions of the model are in good agreement with the experimental data of Bhole et al. [2006. Laser Doppler anemometer measurements in bubble column: effect of sparger. Industrial & Engineering Chemistry Research 45, 9201-9207] obtained using Laser Doppler anemometry. Comparison of simulation results with the experimental measurements of Sanyal et al. [1999. Numerical simulation of gas-liquid dynamics in cylindrical bubble column reactors. Chemical Engineering Science 54, 5071-5083] and Olmos et al. [2001. Numerical simulation of multiphase flow in bubble column reactors: influence of bubble coalescence and breakup. Chemical Engineering Science 56, 6359-6365] also show a good agreement for liquid velocity and gas hold-up profiles.     

UV nanoimprint lithography of sub-100 nm nanostructures using a novel UV curable epoxy siloxane polymer

We reported the replication of sub-100 nm nanostructures by an ultraviolet (UV) nanoimprint lithography (NIL) technique. We used a novel UV curable epoxy siloxane polymer as the NIL resist to achieve features as small as 50 nm. The polymeric soft molds for the NIL were fabricated by casting toluene diluted poly(dimethyl-siloxane) (PDMS) on the hydrogensilsesquioxane (HSQ) hard mold. The NIL results were characterized by using a scanning electron microscope and an atomic force microscope. Our results illustrate that, with the epoxy siloxane resist, the 50 nm HSQ features on the hard mold can be successfully replicated using PDMS soft molds.