Random Defect Model Research Articles

Delamination analysis of high-temperature superconducting tapes based on a random defect model

Random microscopic defects have been found in the fabrication of rare-earth-barium-copper-oxide (REBCO) high-temperature superconducting (HTS) tapes. Interlaminar failure of the tape is more likely when the interfacial bonding strength is reduced as a result of defects, which seriously jeopardizes the dependability of large magnet systems. In this paper, a three-dimensional finite element model (FEM) of the tape with random defects is developed using the Weibull distribution function to evaluate the effect of defects on the delamination of the tape. The delamination behavior under two loading scenarios were conducted (Case-I: out-of-plane tensile and Case-II: axial compression). The result show that the interfacial bonding strength is significantly affected by the defect fraction. The random characteristics of defects and their impact levels can be traced through different parameters. For case-I, the theoretical results of the effective interfacial bonding strength are in good agreement with the numerical results. During the delamination process, the occurrence of defects leads to stress oscillations in the constituent layers. Under axial compression loading (Case-II), the defect dramatically reduces the critical capacity of the tape. With increasing defect fraction, the rate of delamination propagation grows noticeably.

Oxygen Non-Stoichiometry, Defect Equilibrium and Electrical Properties of La0.8Sr0.2Sc0.2Mn0.8O3±δ

Non-stoichiometric variation of oxygen content in La0.8Sr0.2Sc0.2Mn0.8O3±δ were determined by means of coulometric titration as a function of oxygen partial pressure p(O2) and temperature. La0.8Sr0.2Sc0.2Mn0.8O3±δ oxides shows oxygen excess and oxygen deficient composition depending on p(O2). Random defect model was constructed based on the experimental oxygen non-stoichiometry. The non-stoichiometric variation of oxygen content of La0.8Sr0.2Sc0.2Mn0.8O3±δ can be explained by the defect equilibrium model. Thermodynamic quantities such as partial molar enthalpy and partial molar entropy were calculated using Gibbs-Hemholtz equation. Electrical conductivity measurements were performed as a function of oxygen partial pressure and temperature, which shows a typical p-type behavior.

Oxidation–reduction behavior of La0.8Sr0.2ScyMn1 − yO3 ± δ (y = 0.2, 0.3, 0.4): Defect structure, thermodynamic and electrical properties

Oxygen non-stoichiometry of La0.8Sr0.2ScyMn1−yO3±δ (y=0.2, 0.3, 0.4) oxide was studied by coulometric titration as a function of oxygen partial pressure, p(O2), and temperature in a range of 923–1023K. Depending on the Sc doping amount, p(O2), and temperature, oxygen non-stoichiometry varies significantly. Under a reducing condition, La0.8Sr0.2ScyMn1−yO3±δ shows both oxygen excess and oxygen deficient compositions. At the higher p(O2) region, the oxygen excess composition is due to metal ion vacancies, whereas in the lower p(O2) region, the oxygen deficient composition is due to the formation of oxygen vacancies. The experimental data were analyzed by a random defect model. Partial molar enthalpy and partial molar entropy of oxygen vacancy formation are calculated using the Gibbs–Helmholtz equation from the non-stoichiometric data. The electrical conductivity was measured as the function of the oxygen partial pressure and temperature. In the lower p(O2) region, electrical conductivity strongly depends on the oxygen non-stoichiometry.

An Efficient Framework for Scalable Defect Isolation in Large Scale Networks of DNA Self-Assembly

This paper proposes and evaluates an approach for defect isolation of DNA self-assembled networks made of a large number of processing nodes. The complexity of DNA self-assembled networks makes impractical to add a large amount of redundancy and employ inefficient and unscalable defect tolerant schemes. A previous framework based on a broadcast algorithm isolates defective nodes without incorporating redundancy for nodes. However, its disadvantage is the limited scalability, thus making it unsuitable for extremely large scale networks built by DNA self-assembly. The proposed framework improves upon the previous framework by involving three algorithmic tiers; namely, 1-hop wave expansion, efficient via placement, and unsafe node detection. The performance of the proposed framework is evaluated and compared with the original framework by considering large scale networks (up to 2,000 × 2,000 nodes), and a novel gross defect model (as well as a conventional random defect model as assumed in previous works). Simulation results indicate that the proposed framework outperforms the original framework in broadcast latency and efficiency and shows excellent scalability for DNA self-assembled nano-scale networks.

Defect chemistry and electrical properties of La1−xSrxCoO3−δ IV. Electrical properties

This paper considers electrical properties of La1−xSrxCoO3−δ in terms of defect models, such as random defect model and the cluster model. It is shown that the experimental data of the electrical conductivity may be explained in terms of the random defect model rather than the cluster model.

Defect chemistry and electrical properties of La1−xSrxCoO3−δ III. Oxygen nonstoichiometry

This paper concerns verification of the defect chemistry models of La1−xSrxCoO3−δ (LSC), involving the random defect model and the cluster defect model, against empirical data of oxygen nonstoichiometry. It appears that the experimental data may well be explained within the random defect model.

Defect chemistry and electrical properties of La1-xSrxCoO3-δ

This paper reports defect diagrams of Sr-doped lanthanum cobaltate, La1−xSrxCoO3−δ (LSC), involving both random defect model and the cluster defect model in the temperature range between 873 K and 1173 K. The diagrams are derived in the form of defects concentrations as a function of oxygen partial pressure using the nonstoichiometry data reported in the literature. The effect of the Sr content on defect structure of LSC is discussed in terms of both defect models.

Experimental determination of oxygen permeation flux through bulk and grain boundary of La 0.7Ca 0.3CrO 3

Oxygen permeation flux through La 0.7Ca 0.3CrO 3 was determined by an electrochemical method. Yttria-stabilized zirconia (YSZ) was used as an oxygen pump/sensor to apply an oxygen potential gradient over the sample, and the steady state current was recorded. The oxygen vacancy diffusion coefficient, D v , was determined to be 1 × 10 −5 cm 2 s −1 assuming a random defect model. The tracer exchange experiment was performed at 1273 K in 18O enriched O 2 (0.2 bar), and the sample was analyzed by a depth profiling and imaging technique using a secondary ion mass spectrometer (SIMS). The existence of fast diffusion paths along grain boundaries was confirmed. The bulk tracer diffusion coefficient, D ∗ , was calculated to be 2 × 10 −12 cm 2 s −1 in air, which was about 6 times higher than that expected from the electrochemical measurement.

Combined weakest link and random defect model for describing strength variability in fibres

A mathematical model which describes the strength variability along the length of a fibre was developed. The model is a combination of the modified weakest link and random defect models. This combined model describes very well the strength variability data of aramid fibres.

Antiferromagnetic exchange in manganese(II) monophenylphosphinate and the effects of cadmium doping

Mn[H(C6H5)PO2]2, its cadmium analogue, and mixed metal materials of composition Mn1−xCdx[H(C6H5)PO2]2 (x = 0.005, 0.01, 0.09, 0.27, and 0.47) have been synthesized and characterized by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry and low-temperature (4.2 to 80 K) magnetic susceptibility studies. The materials are shown to be isomorphous and are considered to have polymeric structures in which chains of metal atoms are linked by bridging phosphinate groups. The pure manganese compound is antiferromagnetic (maximum in xm at ~35 K) and the magnetic data for the compound have been analyzed according to two theoretical models for linear chains of antiferromagnetically coupled manganese(II) (d5, spin-free) ions. The Wagner and Friedberg model gives J = −3.00 cm−1 and the Weng model gives J = −2.78 cm−1. The effects of replacing Mn2+ by Cd2+ ions in the polymer is to increase the magnetic susceptibility (per mol of Mn) at all temperatures. Analysis of the data as a function of Cd doping indicates the incorporation of a paramagnetic component to the susceptibility which increases with increasing Cd content. In addition, the absolute value of the exchange coupling constant appears to decrease as the Cd content increases. These effects are considered in terms of a random defect model in which the replacement of Mn ions in the polymer by Cd ions results in the formation of Cd ion separated finite magnetic chain fragments. Keywords: manganese(II) monophenylphosphinate, magnetic properties, coordination polymer, cadmium doped antiferromagnet.

Specific heat and susceptibility of the 1-dimensional [formula omitted] Heisenberg antiferromagnet Cu(Pyrazine) (NO 3) 2. evidence for random exchange effects at low temperatures

Magnetic susceptibility and specific heat data are presented on the S = 1 2 antiferromagnetic Heisenberg chain Cu(pyrazine) (NO 3) 2 in the range 0.05 K < T < 18 K. The intrachain exchange is obtained as J/ k B = -5.20(5) K by a theoretical fit to data. No evidence for a transition to a 3-d ordered state could be detected down to the lowest temperatures (corresponding to k B T/| J| ≈ 0.01). The specific heat data provide a beautiful demonstration of the predicted linear chain low-temperature behaviour, c/ R = k B T/3| J|, over one decade in temperature. Superimposed upon the behaviour for the uniform antiferromagnetic chain, the data show contributions that can be interpreted in terms of a random exchange model. It is argued that these arise from random lattice imperfections (defects) which apparently break up the chains into weakly coupled finite segments with an average length of about 200 lattice sites. The segments with an odd number of spins act as a system of highly diluted, weakly interacting, spins 1 2 . The contributions due to defects may predominate the behaviour of the susceptibility and (to a lesser extent) the specific heat at very low temperatures, in case the interchain interactions are sufficiently small. In particular, the random defects will suppress the occurence of 3-d ordering between the chains. Furthermore, they lead to random exchange phenomena similar as observed in other quasi 1-d antiferromagnetic systems. It is suggested that many of the known examples of random exchange antiferromagnets can be explained in terms of the random defect model.