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Wind power generation yields significant economic and social benefits, especially for developing countries. However, large wind power plants (WPPs) are usually constructed in remote areas, leading to a weak grid. And, serious challenges, such as the power blocking, threaten the stable operation of the power system and results in a great deal of wind energy wasted. For this issue, this article presents a comprehensive study for doubly fed induction generators (DFIGs) to analyze and enhance its active power transfer capability in weak grid. Firstly, a steady model for DFIGs is established, which emphasizes the influence of reactive power and power angle features. Based on it, the active power transfer capability under common reactive power control (RPC) modes is analyzed in detail. The analysis results indicate that both the rotor current limit and the power angle will affect the active power transfer capability, and the constant voltage RPC mode is preferred in the very weak grid. Further, combined with the dq-axis current priority, a grid-side converter (GSC)-dominated RPC method is proposed to fully utilize the GSC capacity and substantially improve the active power transfer capability. Finally, the analysis and proposed method are validated by experiments.

Compared with the gas metal arc welding (GMAW), the cold metal transfer technology can significantly reduce the welding heat input owing to obtain the output current of almost zero in the droplet detaching, which has a greater advantage in additive manufacturing. In this work, the cold metal transfer was employed to study the law of appearance and mechanical behavior of additively manufacturing of Ti–6Al–4V alloy. The results showed that the continuous and uniform weld appearance could be obtained when the current of 120 A–130 A was used. In the top layer, middle layer and bottom layer of thin-walled components, the same microstructure was obtained with the different parameters, which were mainly consisted of the columnar original β grains and acicular martensite α’. The max-hardness was obtained in the bottom zone (BZ) of thin-walled component. With the increase of welding current, the hardness values in BZ were decreased. With the increases of current, the tensile strength was reduced, which caused by the grain growth.

This paper presents a comparative study on the laser-induced ignition and combustion characteristics between nanometer-sized aluminum (nano-Al) and micron-sized aluminum (micro-Al) powder stacks. S...

Carbon nanotubes (CNTs) were in-situ grown on TiB2 substrates via CVD using Ni/Y2O3 as catalysts. The effects of methane (CH4), acetylene (C2H2), and carbon monoxide (CO) gas as the carbon source on the quality and quantity of the carbon products was studied and compared using XRD, SEM, TEM, and Raman characterization techniques. Using C2H2 showed the highest carbon yield, which was due to the increasing amount of impurities. Combined with the results of TEM and Raman spectroscopy, the CNTs formed in C2H2 or CO gas exhibit more defects. Although the amount of products formed in CH4 gas is smaller, the purity and graphitization of CNTs are much higher than that formed in C2H2 or CO gas. The difference can be attributed to the different thermal stabilities of CH4, C2H2, and CO. Since CH4 possesses a basic structure and exhibits higher chemical stability at high-temperature, high-purity and high-quality CNTs are more likely to grow in CH4 gas rather than C2H2 and CO. Due to the comparative instability of C2H2 and CO, they have been found to be more reactive, which leads to increased decomposition and the formation of impurities and defects, in addition to the CNTs.

Chemical vapor deposition was employed for the growth of carbon nanotubes (CNTs) on titanium boride (TiB2), with nickel (Ni) as the catalyst and methane (CH4) as the carbon precursor. The effects of the Ni catalyst and the CH4 supply on the growth of CNTs were evaluated. The crystallinity and microstructure of the CNTs were characterized by XRD, SEM, EDS, TEM, and Raman spectroscopy. Analysis showed that the dispersion and morphology of the carbon products depended on the distribution of the Ni catalyst, and the quantity and length of the grown CNTs were affected by the CH4 gas flow rate. The CNTs grown on the TiB2 substrate with a small amount of impurities and high productivity were well distributed under the following conditions: Ni concentration, 10 wt%; CH4 gas flow rate, 100 mL min−1; growth temperature, 850 °C; and duration, 150 min.

7A52 (Al-Zn-Mg-Cu) alloy is a high-strength aluminum alloy, its welded joints are often accompanied by defects such as poor wear resistance and low fatigue strength. Herein, we try to optimize the welded joint of 7A52 aluminum alloy by using ultrasonic impact treatment (UIT). Generally, the mechanical properties such as microhardness and fatigue strength of the welded joint after UIT will be improved. 7A52 aluminum alloy tandem metal inert gas (MIG) welded joints with UIT time per unit area of 2.5 min, 5 min, 10 min, 15 min, 30 min, and 75 min were studied. Through the surface topography, microstructure observation, and mechanical properties test, the time parameters of excessive treatment, lack of treatment, and proper treatment were selected, and the effects of UIT, excessive treatment, lack of treatment, and proper treatment on fatigue strength were analyzed. Test results show that, the mechanical properties of welded joints after UIT are improved. The proper treatment time is 15min and its fatigue strength is 37.86MPa, respectively under the stress ratio of 0.1. Compared to the original welding condition with a fatigue strength of 28.61MPa, the fatigue strength of the welded joints of 7A52 aluminum alloy increased by 32.33%. The largest percentage of grain size reduction occurs when the UIT is 15 min. Moreover, excessive treatment and lack of treatment will not further refine the grains and optimize the mechanical properties.

The grid codes have strict rules for reactive power response time of wind turbines (WTs) during low voltage ride through (LVRT). For the type-3 WT, the response time is mainly depended on the attenuation rate of transient stator flux, which is closely related to the rotor-side control modes (i.e., LVRT solutions). Unfortunately, the characteristic of stator flux attenuation time-scale is hardly considered in the existing work on transient behavior analysis. For this issue, this paper firstly calculates the attenuation time constant of the stator flux under various LVRT control modes. Then, their flux attenuation time-scales are analyzed and compared using a typical MW-level WT. The analysis helps to assess the feasibility for satisfying LVRT requirement of reactive response time, besides that it contributes to the optimization of the reactive compensation configuration in wind farms. The analysis is validated by the simulation on a 1.5-MW WT.

Carbon nanotubes (CNTs) are successfully grown on the surface of TiB2 matrix by chemical vapor deposition with Fe as catalyst. However, CNT growth has to be improved before conducting integration procedures. The effects of synthesis conditions on the structure of CNT(Fe)–TiB2 are evaluated by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Results indicate that with an increase in synthesis temperature, carbon yield increases because of the improvement in the initial growth rate of CNTs. The morphology of as-received carbon changes due to changes in surface roughness of TiB2. With an increase in Fe catalyst, the density and diameter of carbon particles increase as well because of the increased number of activity sites and agglomeration of Fe particles. As the gas flow rate of methane supply rises, the density and length of CNTs initially increase and then decrease. This behavior is attributed to the increase in carbon diffusion rate and decrease in contact time between the catalyst and the carbon precursor. As growth duration is extended, CNT growth initially improves because of the sufficient reaction time between the carbon precursor and the catalyst. However, the quality of CNTs subsequently decreases, and a large amount of amorphous carbon is produced owing to the aggregation of the carbon precursor, which leads to excessive concentration of Fe.

Carbon nanotubes (CNTs) were synthesized in situ over TiB2 powders by using a Fe catalyst at temperatures ranging from 800 °C to 1000 °C. The CNT(Fe)–TiB2 composites were then successfully consolidated by spark plasma sintering. Results indicated that CNT(Fe)–TiB2 composites with 20 wt% Fe sintered at 1400 °C for 5 min provided the optimal combination of dense microstructures and excellent properties. These properties included the following: relative density, 97.3%; flexural strength, 985 ± 48 MPa; and fracture toughness, 11.8 ± 0.4 MPa m1/2. Retention of the perfect structure and homogenous dispersion at the molecular level of CNTs in TiB2 matrix were identified as the main factors that contribute to the marked improvement of the mechanical properties of the composites.

Carbon nanotubes (CNTs) were grown on TiB2 powder by chemical vapor deposition using a Co catalyst. Then, the bulk CNTs(Co)-TiB2 composite was consolidated from these in-situ synthesized powder by spark plasma sintering. Homogeneously-dispersed CNTs with herringbone-like structure composed of well-crystallized graphite were observed. The flexural strength and fracture toughness of the in-situ synthesized CNTs(Co)-TiB2 composite were determined to be 987 ± 42 MPa and 12.4 ± 0.5 MPa m1/2 respectively, which were much higher than those of monolithic TiB2 and TiB2-CNTs composites synthesized by traditional ball-milling and sintering. The improvement of the composite performance was ascribed to the hybrid effect of the fully dense structure of the composite, the appropriate interfacial strength between the CNTs and the matrix, and the higher strengthening ability of well-dispersed CNTs.

CNTs were successfully synthesized on refractory metal diborides (HfB2, ZrB2, and TiB2) via CVD, with Ni as the catalyst at about 850–1200 °C. The obtained powders were densified via SPS at 1400–1700 °C to obtain a composite. The microstructures of the grown CNTs were characterized. The mechanical performances of the as-received materials were tested to evaluate the enhancing impact of CNTs grown in situ. Phase analysis of the final materials was conducted by XRD to eliminate the effects of impurities. The optimal strength and toughness, respectively, were 1013 ± 48 MPa and 10.7 ± 0.5 MPa·m1/2 for TiB2-Ni-CNT composites; 988 ± 47 MPa and 9.4 ± 0.4 MPa·m1/2 for ZrB2-Ni-CNT composites; and 912 ± 43 MPa and 8.5 ± 0.3 MPa·m1/2 for HfB2-Ni-CNT composites. The CNTs grown in the boride matrix exhibited uniform dispersion, resulting in further enhancement.

This paper deals with active power limit and the effect of reactive power compensation under different short circuit ratios (SCRs) of doubly fed induction generation (DFIG)-based wind turbine. The steady-state equations of a DFIG connected to weak grid are established to depict the relation between the external and internal variables of DFIG. Based on the equations, critical factors including system power angle and rotor current limit are further analyzed to quantify the power transmission capability of a DFIG plugged in weak grid. Moreover, the influence of reactive compensation on the active power limit is also demonstrated. Simulation results are finally presented to validate the correctness of analysis result.

α-synuclein is a predominantly expressing neuronal protein for understanding the neurodegenerative disorders. A diagnosing system with aggregated α-synuclein encoded by SNCA gene is necessary to make the precautionary treatment against Parkinson’s disease (PD). Herein, gold-nanourchin conjugated anti-α-synuclein antibody was desired as the probe and seeded on single-walled carbon nanotube (SWCN) integrated interdigitated electrode (IDE). The surface morphology of SWCN-modified IDE and gold urchin-antibody conjugates were observed under FESEM, FETEM and AFM, the existing elements were confirmed. Voltammetry analysis revealed that the limit of fibril-formed α-synuclein detection was improved by 1000 folds (1 fM) with gold-nanourchin-antibody modified surface, compared to the surface with only antibody (1 pM). Validating the interaction of α-synuclein by Enzyme-linked Immunosorbent Assay was displayed the detection limit as 10 pM. IDE has a good reproducibility and a higher selectivity on α-synuclein as evidenced by the interactive analysis with the control proteins, PARK1 and DJ-1.

Carbon nanotubes (CNTs) were synthesized on zirconium diboride (ZrB2) through CVD using a Ni catalyst. The influences of the synthesis temperature, time and catalyst amount on the quality and quantity of grown CNTs were studied. The catalyst content determined the temperature at which the CNTs began to grow. When the Ni catalyst content was low, the temperature at which the CNTs began to grow should be higher, and was 1050 °C for 5 wt% Ni content, but was 950 °C for 15 wt% Ni content. Meanwhile, agglomeration of metal atoms was observed if the synthesis temperature was increased until reaching the upper limit temperature for CNT synthesis, which was 1100 °C for 5 wt% and 10 wt% Ni content, but was 1000 °C for 15 wt% Ni content. Increased Ni catalyst content also led to higher catalytic activity as long as the Ni catalyst content did not exceed 15 wt%. Meanwhile, prolonged synthesis time extended the reaction time for the decomposition of C2H2, but resulted in the agglomeration of carbon atoms when the synthesis time was 150 min. The results showed that the uniform distribution of CNTs can be obtained in ZrB2 powder and there was no reaction between ZrB2 and Ni catalyst. The suitable synthesis conditions for obtained uniform size of CNTs grown with a higher degree of graphitization on ZrB2 that identified in this work were: Ni catalyst content of 10 wt%, synthesis temperature of 1050 °C, and synthesis time of 120 min.

TiB 2 -based composites containing 15 vol% multi-walled carbon nanotubes were synthetized at the temperature of 1600 °C to 1800 °C for 10 min under a uniaxial load of 20 MPa by spark plasma sintering. Microstructure, mechanical properties and densification behavior of the composites were studied. It was shown that mechanical properties were increased by introduction of carbon nanotubes, especially for the fracture toughness. The fracture toughness was ∼9.1 MPa m 1/2 , which was much higher than that of monolithic TiB 2 ceramics. The dense microstructure was the primary factor to improve the mechanical properties. The toughening mechanisms were crack deflection and carbon nanotubes pull-out. The results presented here provide a potential method for improving the fracture toughness of TiB 2 ceramics.

TiB2-based ceramics reinforced with SiC particles and carbon nanotubes were densified using spark plasma sintering. The introduction of SiC and CNTs improved the densification and mechanical properties of the TiB2-based ceramics. XRD spectra data indicated that no reactions were found during the sintering process. The fracture toughness of the TiB2–SiC–CNTs composites was increased to 10.2±0.9 MPa m1/2, which was significantly higher than the monolithic TiB2. Crack deflection, crack bridging, and nanotube pull-out were the main toughening mechanisms used.

Owing to its excellent properties, titanium boride (TiB2) has drawn considerable attention. This study focuses on in-situ synthesis of carbon nanotubes (CNTs) on the surfaces of TiB2 particles by using Co as a catalyst in catalytic chemical vapor deposition to improve the toughness of TiB2 through the toughening effect of CNTs. Analyses of crystal structure, phase and microstructure confirm the successful grafting of CNTs on TiB2 matrix. Carbon yield is also calculated to estimate the effects of carbon deposition. Raman spectroscopy indicates that the CNTs grafted on TiB2 at 850 °C for 150 min exhibit low ID/IG, suggesting satisfactory quantity and quality of CNTs. An appropriate increase in synthesis temperature can promote the rapid growth of CNTs at the beginning, extending the growth time of CNTs. Meanwhile, sufficient growth time would allow the process of carbon precursor conversion to CNTs, enhancing the purity and graphitization of CNTs.

This paper proposes a three-layer ultra-wideband metamaterial absorber, which consists of layered ring resonators and four rectangular patch resonators on the top. The structure is rotationally symmetrical and hence becomes polarization-insensitive. This absorber realizes an absorption bandwidth of 22.39 GHz within 6.37–28.76 GHz for absorptivity level above 90%. The overall thickness of this absorber is 4.9mm, which is only 0.10X and 0.21X at the lowest and highest frequencies, respectively. The proposed structure exhibits wider absorption bandwidth in comparison with the state of arts.

We propose a spin field effect transistor using a graphene nanoribbon as the channel. The control and manipulation of the electron spin in the channel modulate the spin-polarized current. The modulation is carried out by the magnetic exchange field which arises from the electron exchange interaction with ferromagnetic gate and quantum confinement effect. Numerical estimation indicates that a substantial magnetic exchange field can generate a phase difference on the order of π within a timeframe far below the spin lifetime and timescale between successive collisions, which ensures both the spin coherence and ballistic conduction during the electron transport. A graphene ribbon with armchair boundaries has the desired Dirac point and metallic character. This Dirac-like dispersion relation, together with negligible spin–orbit coupling, makes large on-off ratio feasible even in the presence of thermal dispersion.