Behavior Of Stacks Research Articles

Effects of Processing Parameters on the Forming Quality of C-Shaped Thermosetting Composite Laminates in Hot Diaphragm Forming Process

In this study, the effects of processing temperature and vacuum applying rate on the forming quality of C-shaped carbon fiber reinforced epoxy resin matrix composite laminates during hot diaphragm forming process were investigated. C-shaped prepreg preforms were produced using a home-made hot diaphragm forming equipment. The thickness variations of the preforms and the manufacturing defects after diaphragm forming process, including fiber wrinkling and voids, were evaluated to understand the forming mechanism. Furthermore, both interlaminar slipping friction and compaction behavior of the prepreg stacks were experimentally analyzed for showing the importance of the processing parameters. In addition, autoclave processing was used to cure the C-shaped preforms to investigate the changes of the defects before and after cure process. The results show that the C-shaped prepreg preforms with good forming quality can be achieved through increasing processing temperature and reducing vacuum applying rate, which obviously promote prepreg interlaminar slipping process. The process temperature and forming rate in hot diaphragm forming process strongly influence prepreg interply frictional force, and the maximum interlaminar frictional force can be taken as a key parameter for processing parameter optimization. Autoclave process is effective in eliminating voids in the preforms and can alleviate fiber wrinkles to a certain extent.

Analysis on Dynamic Piezoelectric Coupling Characteristics of PZT-4

Structural behavior of piezoelectric stacks has a great impact on electromechanical conversion efficiency of highway piezoelectric generators. In this paper, a single piezoelectric ceramic chip in piezoelectric stack structure was regarded as the object of study to construct the ANSYS model of a single piezoelectric ceramic entity. The selection of SOLID5 makes the piezoelectric ceramic chip into the grid cube and by the coupling field, maintains five aspects of static and dynamic piezoelectric coupling characteristic simulation analysis respectively, including static analysis, modal analysis, harmonic analysis, transient analysis and circuit simulation analysis. The results of all the analyses for the optimal structure design of highway piezoelectric generators are to provide a reference.

FEM Analysis on Warpage and Stress at the Micro Joint of Multiple Chip Stacking

Flip chip plastic ball grid array (FCPBGA) utilizing an interposer for multiple chip stacks, so called 2.5D or 3D package, is gaining prominence in order to achieve the next generation high performance computing. The multi-tier stacking of Si chip, Si interposer and organic substrate induces complicated warpage behavior and stress at the micro joints during the joining process. The authors studied warpage behavior and thermo-mechanical stress of multiple chip stacks on Si interposer package with different thickness of top chip, different thickness of Si interposer, different middle chip stacks and different metallurgy of micro joints after joining of stacked chips on the interposer using finite element method (FEM). Thicker stacked Si chips and thicker Si interposer showed higher von Mises stress at chip-to-interposer joint. Comparing SnAg joint and CuSn joint, CuSn joint always shows higher stress than SnAg joint because of its higher elastic modulus.

Tunable Bragg stacks from sol-gel derived Ta2O5 and MEL zeolite films

In this paper we investigated sol-gel derived Ta2O5 and nanosized MEL zeolite films obtained by spin coating of Tantalum sol and colloidal zeolite solution, respectively. Refractive index and thickness of the films were determined using non-linear curve fitting of measured reflectance spectra. The influence of the post deposition annealing on the optical properties and thickness of the films was studied. Besides tunable Bragg stacks were designed and prepared by layer-by-layer deposition of Ta2O5 and MEL suspensions with quarter-wave thicknesses. The influence of water, acetone and methanol on the optical behavior of Bragg stacks was discussed.

Interval Methods for Control-Oriented Modeling of the Thermal Behavior of High-Temperature Fuel Cell Stacks

Solid oxide fuel cells (SOFCs) can be used as decentralized energy supply devices for providing electricity and heat directly by converting chemical energy. In such applications of SOFCs, the electric power demand is commonly varying over time. Therefore, all processes in fuel cell systems are typically instationary. For example, the heating and cooling phases for starting up and shutting down the fuel cell system as well as the response to varying electrical load demands characterize the instationarity of the operating conditions for the thermal subprocess. In contrast to most existing control approaches, which only cover stationary operating strategies, our work aims at controlling SOFC systems in instationary operating regions. This means that a mathematical system model for these regions is necessary. Such control-oriented models for the temperature distribution in a fuel cell stack module can be obtained by the method of finite volume discretization. On the basis of the first law of thermodynamics, local energy balances are derived for each volume element, which leads to a system of coupled nonlinear ordinary differential equations. In this paper, parameter identification routines are compared which are based both on classical floating point techniques and on verified interval arithmetic approaches. In particular, interval techniques are employed to deal with imperfect system knowledge expressed by bounded parameter uncertainties and to search for globally instead of locally optimal system parameterizations.

Interface stability and microstructure of an ultrathin α-Ta/graded Ta(N)/TaN multilayer diffusion barrier

An ultrathin α-Ta (5 nm)/graded Ta(N) (1.5 nm)/TaN (2.5 nm) multilayer film coated with Cu film was deposited on the Si substrate using reactive magnetron sputtering in N2/Ar ambient. The film stacks of Cu/α-Ta/graded Ta(N)/TaN/Si were then annealed in a vacuum chamber at 400–700 °C for 1 h. X-ray diffraction (XRD), scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and energy-dispersive spectrometer (EDS) line scans were employed to investigate the microstructure evolution and the diffusion behavior of the film stacks, respectively. The results show that the α-Ta/graded Ta(N)/TaN multilayer film as a diffusion barrier had sufficient interface stability, which could be attributed to a relative stable amorphous layer forming at the interface of Cu and α-Ta layer, to prevent Cu atom diffusion at elevated temperatures up to 700 °C. The relationship between the interface stability and the microstructure of the multilayer barrier were also investigated.

Circular dichroism and circularly polarized luminescence triggered by self-assembly of tris(phenylisoxazolyl)benzenes possessing a perylenebisimide moiety

Chiral tris(phenylisoxazolyl)benzenes possessing a perylenebisimide moiety assembled to form helical stacks. The self-assembling behavior of the helical stacks responded to changes in solvent properties, temperature, and concentration. Strong circular dichroism (CD) and circularly polarized luminescence (CPL) of their assemblies were displayed, and were controlled by external stimuli.

Coordinate Transformation, Orthogonal Collocation, Model Reformulation and Simulation of Electrochemical-Thermal Behavior of Lithium-Ion Battery Stacks

In this paper, a simple transformation of coordinates is proposed that facilitates the efficient simulation of the non-isothermal lithium-ion pseudo 2-D battery model. The transformed model is then conveniently discretized using orthogonal collocation with the collocation points in the spatial direction. The resulting system of differential algebraic equations (DAEs) is solved using efficient adaptive solvers in time. A series of mathematical operations are performed to reformulate the model to enhance computational efficiency and programming convenience while maintaining accuracy even when non-linear or temperature dependent parameters are used. The transformed coordinate allows for efficient simulation and extension from cell sandwich to stack models. Furthermore, the transformation and reformulation techniques are used to simulate operation of an 8-cell battery stack subject to varying heat transfer coefficients as well as specified temperature boundary conditions.

Traps and trapping phenomena and their implications on electrical behavior of high-k capacitor stacks

The traps and trapping phenomena and their implications on leakage currents, conduction mechanisms, and stress-induced leakage current in high-k dielectrics have been investigated. Various dielectrics (mostly multicomponent materials) have been studied to demonstrate the large diversity of phenomena that govern the electrical behavior of the structures depending on dielectric material, trap and stack parameters, and measurement conditions. The effects common for the most of high-k dielectrics and those typical for each individual structure have been discussed. The singly positively charged oxygen vacancy has been elucidated as the main electron transport site in the high-k materials. The role of the pre-existing traps for the electrical degradation of high-k stacks has been evidenced.

3D Integrated Water Cooling of a Composite Multilayer Stack of Chips

New generation supercomputers with three dimensional stacked chip architectures pose a major challenge with respect to the removal of dissipated heat, which can reach currently as high as 250 W/cm2 in multilayer chip stacks of less than 0.3 cm3 volume. Interlayer integrated water cooling is a very promising approach for such high heat flux removal due to much larger thermal capacity and conductivity of water compared with air, the traditional cooling fluid. In the current work, a multiscale conjugate heat transfer model is developed for integrated water cooling of chip layers and validated with experimental measurements on an especially designed thermal test vehicle that simulates a four tier chip stack with a footprint of 1 cm2. The cooling heat transfer structure, which consists of microchannels with cylindrical pin-fins, is conceived in such a way that it can be directly integrated with the device layout in multilayer chips. Every composite layer is cooled by water flow in microchannels (height of 100 μm), which are arranged in two port water inlet-outlet configuration. The total power removed in the stack is 390 W at a temperature gradient budget of 60 K from liquid inlet to maximal junction temperature, corresponding to about 1.3 kW/cm3 volumetric heat flow. The computational cost and complexity of detailed computational fluid dynamics (CFD) modeling of heat transfer in stacked chips with integrated cooling can be prohibitive. Therefore, the heat transfer structure is modeled using a porous medium approach, where the model parameters of heat transfer and hydrodynamic resistance are derived from averaging the results of the detailed 3D-CFD simulations of a single streamwise row of fins. The modeling results indicate that an isotropic porous medium model does not accurately predict the measured temperature fields. The variation of material properties due to temperature gradients is found to be large; therefore, variable properties are used in the model. It is also shown that the modeling of the heat transfer in the cooling sublayers requires the implementation of a porous medium approach with a local thermal nonequilibrium, as well as orthotropic heat conduction and hydrodynamic resistance. The improved model reproduces the temperatures measured in the stack within 10%. The model is used to predict the behavior of multilayer stacks mimicking the change of heat fluxes resulting from variations in the computational load of the chips during their operation.

Characterization of epitaxial Pb(Zr,Ti)O3 thin films deposited by pulsed laser deposition on silicon cantilevers

This paper reports on the piezoelectric-microelectromechanical system micro-fabrication process and the behavior of piezoelectric stacks actuated silicon cantilevers. All oxide layers in the piezoelectric stacks, such as buffer-layer/bottom-electrode/film/top-electrode: YSZ/SrRuO3/Pb(Zr,Ti)3/SrRuO3, were grown epitaxially on the Si template of silicon-on-insulator substrates by pulsed laser deposition. By using an analytical model and finite element simulation, the initial bending of the cantilevers was calculated. These theoretical analyses are in good agreement with the experimental results which were determined using a white light interferometer. The dependences of the cantilever displacement, resonance frequency and quality factor on the cantilever geometry have been investigated using a laser-Doppler vibrometer. The tip displacement ranged from 0.03 to 0.42 µm V−1, whereas the resonance frequency and quality factor values changed from 1010 to 18.6 kHz and 614 to 174, respectively, for the cantilevers with lengths in the range of 100–800 µm. Furthermore, the effect of the conductive oxide electrodes on the stability of the piezoelectric displacement of the cantilevers has been studied.

Water diffusion and fracture behavior in nanoporous low-k dielectric film stacks

Among various low-dielectric constant (low-k) materials under development, organosilicate glasses (OSGs) containing nanometer-size pores are leading candidates for use as intrametal dielectrics in future microelectronics technologies. In this paper, we investigate the direct impact of water diffusion on the fracture behavior of film stacks that contain porous OSG coatings. We demonstrate that exposure of the film stacks to water causes significant degradation of the interfacial adhesion energy, but that it has negligible effect on the cohesive fracture energy of the nanoporous OSG layer. Isotope tracer diffusion experiments combined with dynamic secondary ion mass spectroscopy show that water diffuses predominantly along the interfaces, and not through the porous films. This unexpected result is attributed to the hydrophilic character of the interfaces.

Stacked Dual-Oxide MOS Energy Band Diagram Visual Representation Program (IRW Student Paper)

Energy band diagrams for MOS devices are essential for understanding device performance and reliability. Introduction of high-k gate stacks with a silicon dioxide (SiO2) interfacial layer requires an even greater understanding of the energy band behavior. A program that quickly determines the band diagrams based on a simple analytical model was created. It is used to explore the behavior of various oxide stacks with the ability to easily vary important parameters like oxide material, electron affinity, bandgap, dielectric constant, and thickness. The usefulness of this program to predict potential reliability issues is also demonstrated

Optical and In-Depth RBS Characterization of Porous Silicon Interference Filters

In the present work, Rutherford backscattering spectroscopy (RBS) measurements, scanning electron microscopy, and optical characterization are carried out on porous silicon (PS) multilayer interference filters in order to determine their compositional profile, homogeneity, and overall optical behavior. In addition, RBS measurements allow determination of the porosity profile of multilayer structures. Thus, both porosity and oxidation degree are determined for each individual layer forming the stack, giving a straightforward measure of the in-depth homogeneity of these structures. Finally, the aging effects on the optical behavior and compositional profile of the PS stacks are studied by comparing as-prepared and aged multilayers. The results reveal good in-depth homogeneity of the PS multilayers and an oxygen enrichment with ambient air exposure, which results in a lowering of the effective refractive index and a blueshift of the reflectance spectra of the filters.

Radio-frequency properties of stacked long Josephson junctions with nonuniform bias current distribution

We have numerically investigated the behavior of stacks of long Josephson junctions considering a nonuniform bias profile. In the presence of a microwave field the nonuniform bias, which favors the formation of fluxons, can give rise to a change of the sequence of radio-frequency induced steps. The amplitude of the steps is enhanced when the external frequency matches the fluxon shuttling regime.

Magnetic field behavior of vertical stacks of Josephson junctions with large idle regions

We report first measurements of the magnetic field dependence of the critical currents of vertical stacks of two Nb/Al-AlO/sub x//Nb Josephson junctions with large idle regions. We kept constant the width of the lateral idle region and changed the ratio of the longitudinal idle part to the junction length. To provide a close coupling of the two junctions of the stack, the thickness of the intermediate Nb electrode was smaller than the London penetration depth. The effect of the idle region on the magnetic behavior of the stack is discussed. In same experiments, a clear and nearly periodic current locking effect has been observed, that was independent of the dimensions of the idle region.

Power Plant Wind Design Problems and Practices

The wind influences the design, construction, and operation of a coal-fueled power plant in many respects. This presentation explores several design, fabrication, erection, and serviceability problems and practices. Specifically considered are shortcomings of standards with regard to wall panel and fastener design, concerns about fatigue failures at low stress levels and analytical needs for wind-sensitive structures such as steel stacks and transmission poles, requirements for quality control of materials and fabrication (including welding and galvanizing), and the need for attention to structural details for force transfer. Examples are also drawn from design of generation buildings, coal handling and substation structures; construction of silos, cooling towers, and pond liners; and behavior of chimneys and stacks in both along-wide and across-wind directions.