What Is a “Massive” Concrete Structure at Early Ages? Some Dimensional Arguments

The risk of early-age concrete cracking depends on the capacity of hardening concrete to support the thermal stresses caused by the exothermic nature of the hydration process. This has been recognized for “massive” concrete structures. However, with the increasing use of high performance concretes, it is apparent that this problem also concerns traditionally “thin” structural members (columns, beams). The definition of a “massive” concrete structure, and how the structural dimension affects intensity and occurrence of chemically-induced strucctural degradation is the main focus of this paper. Based on dimensional analysis of the governing equations, a characteristic length scale, the hydration heat diffusion length, is derived; beyond this length the structure needs to be considered as “massive,” and latent hydration heat effects affect the long-term structural integrity. From experimental data of normal strength concrete and high performance concrete, it is shown that this hydration heat diffusion length of high performance concrete is of the order of ?h = 0.2 m, and lh = 0.3 m for normal strength concrete. Through a number of case studies, the relevant similarity parameters of the risk of early-age concrete cracking are identified, which allow's the monitoring of the structural performance of early-age concrete structures.     

n‐GaAs/InGaP/p‐GaAs Core‐Multishell Nanowire Diodes for Efficient Light‐to‐Current Conversion

Heterostructure n‐GaAs/InGaP/p‐GaAs core‐multishell nanowire diodes are synthesized by metal‐organic vapor‐phase epitaxy. This structure allows a reproducible, selective wet etching of the individual shells and therefore a simplified contacting of single nanowire p‐i‐n junctions. Nanowire diodes show leakage currents in a low pA range and at a high rectification ratio of 3500 (at ±1V). Pronounced electroluminescence at 1.4 eV is measured at room temperature and gives evidence of the device quality. Photocurrent generation is demonstrated at the complete area of the nanowire p‐i‐n junction by scanning photocurrent microscopy. A solar‐conversion efficiency of 4.7%, an open‐circuit voltage of 0.5 V and a fill factor of 52% are obtained under AM 1.5G conditions. These results will guide the development of nanowire‐based photonic and photovoltaic devices.     

A novel 3-D integrated HFET/RTD frequency multiplier

A merged heterostructure field-effect transistor/resonant tunneling diode (HFET/RTD) combination is proposed to act as a (sub)millimeter wave source via highly efficient frequency multiplication and its functionality is demonstrated at intermediate frequency. On a semi-insulating InP-substrate a HFET followed by a double barrier RTD-layer sequence is grown in a single molecular beam epitaxy (MBE)-run. A novel monolithic frequency multiplier circuitry is developed where the RTD is used as load in contrast to other concepts where the RTD is inserted in the input of a three-terminal device. The resulting output voltage is rectangular type and rich in higher odd harmonics. This approach avoids classical disadvantages of the RTD because biasing in the negative-differential-regime is not required and bistability of the I-V-characteristic is used rather than the negative differential resistance. The small signal parameters of the single devices realized with optical lithography, wet etching, and self-aligned process technology are used as input data for a microwave design software (MDS) and the frequency multiplication is modeled up to submillimeter wave frequencies     

Threshold logic circuit design of parallel adders using resonanttunneling devices

Resonant tunneling devices and circuit architectures based on monostable-bistable transition logic elements (MOBILEs) are promising candidates for future nanoscale integration. In this paper, the design of clocked MOBILE-type threshold logic gates and their application to arithmetic circuit components is investigated. The gates are composed of monolithically integrated resonant tunneling diodes and heterostructure field-effect transistors. Experimental results are presented for a programmable NAND/NOR gate. Design related aspects such as the impact of lateral device scaling on the circuit performance and a bit-level pipelined operation using a four phase clocking scheme are discussed. The increased computational functionality of threshold logic gates is exploited in two full adder designs having a minimal logic depth of two circuit stages. Due to the self-latching behavior the adder designs are ideally suited for an application in a bit-level pipelined ripple carry adder. To improve the speed a novel pipelined carry lookahead addition scheme for this logic family is proposed     

On-wafer microwave measurement setup for investigations on HEMTsand high-Tc superconductors at cryogenictemperatures down to 20 K

An on-wafer measurement setup for the microwave characterization of HEMTs and high-T_c superconductors at temperatures down to 20 K is presented. Both S-parameter and noise measurements can be performed in the frequency range from 45 MHz to 40 GHz and 2 GHz to 18 GHz, respectively, using standard calibration techniques and commercial microwave probe tips. Microwave measurements on a pseudomorphic FET and an AlGaAs-GaAs HEMT as well as investigations on a superconducting filter are presented to demonstrate the efficiency of the developed system     

Untersuchungen zur Langkettenverzweigung in Polyäthylenen

Polyethylene samples of various densities and melt flow indices resulting from different polymerization processes have been investigated with respect to long chain branching (LKV). For that purpose several polymer fractions have been characterized by measurement of weight average molecular weights M_w and intrinsic viscosities [η], the latter ranging from 0,2 to 3,2 with high pressure samples and from 0,2 to 10 with low pressure material. The intrinsic viscosity difference of branched (high pressure) polyethylene compared to linear (low pressure) polyethylene is used as a measure of LKV. With high pressure polyethylene LKV increases with decreasing density. This dependence is strongest within the medium molecular weight range. Samples with varying LKV but constant density can be obtained by appropriate change of polymerization conditions. No LKV has been observed with low pressure polyethylene. This means a marked difference compared to high pressure material of equal density. Branching with low pressure polymers can therefore be ascribed to the short chain type only, which in particular results from copolymerization. Several mathematical approaches have been checked whether or not they can yield suitable information about n, the number of long chain branches per molecule. The best fit with our experimental data is obtained using the expression [η]v/[η]₁ = g^1,3 (n = f(g)) and assuming, that the average concentration of long chain branch points does not depend on molecular weight for fractions of the same sample (n/M = const.). If LKV ist taken into consideration, logarithmic normal molecular weight distributions are obtained for many high pressure polyethylenes (similar to low pressure material). Data are reported in support of the view, that performance characteristics are dependent on LKV. There is some evidence, that melt flow properties of polyethylene are improved with increasing LKV.     

InAlAs/InGaAs/InP HFET with suppressed impact ionization usingdual-gate cascode-devices

InAlAs/InGaAs dual-gate-HFETs (DGHFETs) and single-gate-HFETs (SGHFETs) have been fabricated and characterized with special emphasis on reducing the impact ionization. For the first time it is shown that in the case of the DGHFET, due to the second gate (V_G2S=0 V), impact ionization can be totally prevented in the channel underneath the RF-driven gate without reduction of the RF-relevant parameters such as transconductance, output resistance and voltage gain. The electric field and the potential distribution in the channel are discussed using a nomogram and confirmed by 2-D simulation. According to V_G2S=0 V, a new cascode design is presented by directly connecting the second gate to the source (ground)     

Scratch test model for the determination of fracture toughness

We revisit the scratch test within the framework of linear elastic fracture mechanics. In the analysis, we employ an Airy stress function approach to determine stresses and displacement in the vicinity of the scratch-blade–material interface, which serve as input for the evaluation of the energy release rate by means of the J-Integral. In contrast to previous models, the energy release rate thus found scales with the sum of the applied forces squared. This entails a linear relation between the applied forces and , where w is the scratch width and d the scratch depth. This analytical scaling is validated using experimental scratch data on cement paste and sandstone, which shows that the proposed approach provides a convenient way to determine the fracture toughness from scratch tests carried out with different scratch widths and depths.