AC Values Research Articles

Defect detection of composite adhesive joints using electrical resistance method

Mechanical joint and adhesive bonding are two typical joining methods for composite materials. The bolt fastening method is reliable but increases the weight of the structure and cuts the composite fiber, which may cause stress concentration and strength degradation. The adhesive bonding method has a higher bonding strength than the mechanical fastening, but the bonding strength can deteriorate depending on the various parameters that influence the environmental conditions and manufacturing process. In addition, bonding defects due to the presence of foreign substances on the bonding surface or immature surface treatment is called a Kissing Bond, and can significantly reduce the bonding strength. The electrical resistance method is a very promising technique for detecting kissing bond defects by measuring the electrical characteristics after the dispersion of CNTs in the adhesive.In this study, composite-to-composite single-lap joint specimens with CNTs were fabricated and the defect detection capability and the static strength of composite joints were evaluated using the electrical resistance method. A carbon/epoxy composite was used as an adherend and the electrical resistance of the adhesive joint was measured by varying the content of CNTs to 0.5 wt%, 1.0 wt%, and 2.0 wt%. The AC and DC resistance values of composite adhesive joints with artificial defects were measured using an LCR meter and a high-resistance meter, and their lap shear strengths were evaluated.

EFFECT OF TOROIDAL MAGNETIC FIELD RIPPLE VALUE ON THE l=1, m=1 YAMATOR MAGNETIC SURFACES

To form a ripple toroidal magnetic field, the calculated l=1, m=1 Yamator model is equipped with N=16 toroidal magnetic field coils of ac radius. In order to study the effect of toroidal magnetic field ripple value δ on the magnetic surface parameters the calculation was performed for different values of radius ac. In the paper the range of ac radius values is identified within the scope of which the magnetic surface parameters are close to that of the l=1, m=1 Yamator with ideal axially symmetric toroidal magnetic field (δ=0).

The role of the calcium concentration effect on the structural and dielectric properties of mixed Ni–Zn ferrites

The calcium substituted nickel-zinc ferrites with the formula of CaxNi0.75−xZn0.25Fe2O4 (x = 0, 0.25, 0.5 and 0.75) have been prepared by using the chemical co-precipitation method. The X-ray diffraction (XRD) analyses reveal the results that all the samples crystallize in cubic spinel structure and the lattice constants of the samples for x = 0, 0.25, 0.50 and 0.75 are found to be 8.334, 8.348, 8.380 and 8.538 A, respectively. The crystallite size of the samples, obtained from Debye Scherrer’s equation, varies between 12 nm and 27 nm. The scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses have been conducted to reveal and determine the morphology and stoichiometry of all the prepared CaxNi0.75−xZn0.25Fe2O4 ferrite samples. The SEM images show that the partical sizes for all the samples are at nano size in accordance with the XRD results and EDX results confirm the contents of the produced samples. The dielectric and impedance properties of the prepared ferrite samples have been investigated in the frequency range from 20 Hz to 10 MHz and in the temperature range from 350 to 700 K. The real and imaginary parts of dielectric constant, tan θ, AC and DC conductivity values decrease with increasing calcium content (except x = 0). Contrary to this behavior, real and imaginary parts of impedance increase with increasing calcium content. The general AC conductivity behavior of all samples is like semiconductor behavior. The conductivity mechanism of the sample with x = 0 is explained by the mechanism of correlated barrier hopping (CBH), while it has turned into overlapping large polaron tunneling (OLPT) mechanism for all other samples. From the relaxation time graphs obtained from the impedance data, activation energies of the grain and grain boundaries are obtained. The Nyquist plots are also presented in the temperature range of 350–700 K to determine the conductivity mechanism of the prepared samples and all the plots show only one semi-circle, which means that the dominant transmission comes from the grain boundaries.

Understanding aerosol–cloud interactions through modeling the development of orographic cumulus congestus during IPHEx

Abstract. A new cloud parcel model (CPM) including activation, condensation, collision–coalescence, and lateral entrainment processes is used to investigate aerosol–cloud interactions (ACIs) in cumulus development prior to rainfall onset. The CPM was applied with surface aerosol measurements to predict the vertical structure of cloud development at early stages, and the model results were evaluated against airborne observations of cloud microphysics and thermodynamic conditions collected during the Integrated Precipitation and Hydrology Experiment (IPHEx) in the inner region of the southern Appalachian Mountains (SAM). Sensitivity analysis was conducted to examine the model response to variations in key ACI physiochemical parameters and initial conditions. The CPM sensitivities mirror those found in parcel models without entrainment and collision–coalescence, except for the evolution of the droplet spectrum and liquid water content with height. Simulated cloud droplet number concentrations (CDNCs) exhibit high sensitivity to variations in the initial aerosol concentration at cloud base, but weak sensitivity to bulk aerosol hygroscopicity. The condensation coefficient ac plays a governing role in determining the evolution of CDNC, liquid water content (LWC), and cloud droplet spectra (CDS) in time and with height. Lower values of ac lead to higher CDNCs and broader CDS above cloud base, and higher maximum supersaturation near cloud base. Analysis of model simulations reveals that competitive interference among turbulent dispersion, activation, and droplet growth processes modulates spectral width and explains the emergence of bimodal CDS and CDNC heterogeneity in aircraft measurements from different cloud regions and at different heights. Parameterization of nonlinear interactions among entrainment, condensational growth, and collision–coalescence processes is therefore necessary to simulate the vertical structures of CDNCs and CDSs in convective clouds. Comparisons of model predictions with data suggest that the representation of lateral entrainment remains challenging due to the spatial heterogeneity of the convective boundary layer and the intricate 3-D circulations in mountainous regions.

Pseudo-resonance and energy band gaps in plasmonic crystals

In this paper, using the generalized coupled pseudoforce model with driving elements, we develop a method to study the plasmon excitations and energy band structure in a plasmonic crystal. It is shown that the presence of the periodic ion core potential leads to a pseudo-resonance condition in the plasmon wavefunction and electrostatic potential profiles, quite analogous to the frequency resonance, leading to the gap formation in the energy dispersion profiles. It is found that the dual length scale character of plasmon excitations leads to the occurrence of a critical value of ac = 2πλp for the lattice constant (λp being the plasmon wavelength) above and below which the energy band structure of plasmonic crystals becomes substantially different. It is also found that energy bandgap positions of the parabolic free electron energy dispersion relation are slightly higher compared to those of the plasmon excitations. The wavefunction and electrostatic potential solution for a one dimensional plasmonic lattice with a generalized periodic potential are also derived in this research. Current development helps us to illuminate the basic properties of the collective quantum phenomenon in many physical contexts by appropriately incorporating the collective effect in the root level and can have a wide range of applications in developments of the rapidly growing fields of nano-technology, nano-electronics, and plasmonics.

Built Environment, Psychosocial Factors and Active Commuting to School in Adolescents: Clustering a Self-Organizing Map Analysis.

Although the built environment and certain psychosocial factors are related to adolescents’ active commuting to and from school (ACS), their interrelationships have not been explored in depth. This study describes these interrelationships and behavioral profiles via a self-organizing map (SOM) analysis. The sample comprised 465 adolescents from the IPEN (International Physical Activity and the Environment Network) Adolescent study in Valencia, Spain. ACS, barriers to ACS, physical self-efficacy, social support and sociodemographics were measured by questionnaire. Street-network distance to school, net residential density and street intersection density were calculated from the Geographic Information System. The clustering of the SOM outcomes resulted in eight areas or clusters. The clusters which correspond to the lowest and highest ACS levels were then explored in depth. The lowest ACS levels presented interactions between the less supportive built environments (i.e., low levels of residential density and street connectivity in the neighborhood and greater distances to school) and unfavorable psychosocial variables (i.e., low values of physical self-efficacy and medium social support for ACS) and good access to private motorized transport at home. The adolescents with the lowest ACS values exhibited high ACS environment/safety and planning/psychosocial barrier values. Future interventions should be designed to encourage ACS and change multiple levels of influence, such as individual, psychosocial and environmental factors.

Assessment of co-seismic landslide hazard using the Newmark model and statistical analyses: a case study of the 2013 Lushan, China, Mw6.6 earthquake

The April 20, 2013 Mw6.6 earthquake of Lushan County, Sichuan Province, China, has triggered 4540 landslides (> 1000 m2). Exploring a more effective method to assess landslide hazard in the affected area of this event is of great significance for disaster prevention and mitigation. By applying the Newmark model and two statistical analysis models (logic regression and support vector machine, LR and SVM), this study addressed this issue. In the Newmark model, we used the landslide point density, the average gradient (mean slope) and the mean peak ground acceleration to group the lithology and created a critical acceleration (ac) map. The Newmark displacements and the probability of the slope instability are mapped by combining the ac map and PGA map. In the statistical analysis models of LR and SVM, 7040 samples (4540 landslide sites and 2500 random non-landslide sites) were randomly divided into the training set (5000 samples) and validation set (2040 samples). Based on the relationship between landslide distribution and influence factors, we selected the critical acceleration (ac) value, topographic relief, PGA, and distance to rivers as the independent variables for LR and SVM. Finally, the ROC curves for three landslide hazard models were drawn and the AUC values were calculated. The landslide hazard maps produced by LR are similar to those by applying SVM. The AUC values indicate that these two models combined with ac data perform better than the simplified Newmark model. In this study, a new method of integrating statistical analysis models (LR and SVM) with critical acceleration (ac) for earthquake landslide hazard assessment is presented, which can be used to carry out seismic landslide hazard assessment more effectively than the simplified Newmark model.

Critical loads of acidity and exceedances for 1138 lakes and ponds in the Canadian Arctic

Sulphur emissions associated with increased anthropogenic activity, such as resource extraction and marine shipping, may lead to the acidification of aquatic freshwater systems in the Arctic. In the current study, acid sensitivity (based on a critical load (CL(Ac)) approach) of 1138 lakes and ponds in the Canadian Arctic was quantified using the Steady-State Water Chemistry (SSWC) model. Ecosystem protection was based on an Acid Neutralising Capacity limit (ANClimit) for Arctic Char. Acidification risk was estimated under modelled sulphur deposition for the year 2010 (two scenarios). Overall, surface water CL(Ac) values for the Canadian Arctic were low (median = 35.8 meq·m−2·yr−1, mean = 96.3 meq·m−2·yr−1), with approximately 40% (n = 455) of sites estimated to be sensitive to acidification (CL(Ac) < 20 meq·m−2·yr−1). Higher CL(Ac) values were found in Yukon (434.5 meq·m−2·yr−1, n = 40), Ellesmere Is. (262.2 meq·m−2·yr−1, n = 143), and Southampton Is. (251.5 meq·m−2·yr−1, n = 35), while lower CL(Ac) values were found in Melville Is. (5.5 meq·m−2·yr−1, n = 48), Banks Is. (18.4 meq·m−2·yr−1, n = 45), and Bylot Is. (20.4 meq·m−2·yr−1, n = 36). Under modelled deposition for 2010, 12–12.5% (n = 136–142) of all sites were exceeded. The highest proportion of exceeded sites were observed in Ellef Ringnes Is. (48–60%; n = 12–15), Melville Is. (38–44%; n = 18–21), and Northwest Territories (24–26%; n = 23–26).

The Effect of Optical Crosstalk on Accuracy of Reflectance-Type Pulse Oximeter for Mobile Healthcare.

According to the theoretical equation of the pulse oximeter expressed by the ratio of amplitude (AC) and baseline (DC) obtained from the photoplethysmographic signal of two wavelengths, the difference of the amount of light absorbed depending on the melanin indicating the skin color is canceled by normalizing the AC value to the DC value of each wavelength. Therefore, theoretically, skin color does not affect the accuracy of oxygen saturation measurement. However, if there is a direct path for the light emitting unit to the light receiving unit instead of passing through the human body, the amount of light reflected by the surface of the skin changes depending on the color of the skin. As a result, the amount of crosstalk that varies depending on the skin color affects the ratio of AC to DC, resulting in errors in the calculation of the oxygen saturation value. We made crosstalk sensors and crosstalk-free sensors and performed desaturation experiments with respiratory gas control on subjects with various skin colors to perform oxygen saturation measurements ranging from 60 to 100%. Experimental results showed that there was no difference in the measurement error of oxygen saturation according to skin color in the case of the sensor which prevented crosstalk (−0.8824 ± 2.2859 for Asian subjects, 0.6741 ± 3.2822 for Caucasian subjects, and 0.9669 ± 2.2268 for African American subjects). However, a sensor that did not prevent crosstalk showed a large error in dark skin subjects (0.8258 ± 2.1603 for Asian subjects, 0.8733 ± 1.9716 for Caucasian subjects, and −3.0591 ± 3.9925 for African Americans). Based on these results, we reiterate the importance of sensor design in the development of pulse oximeters using reflectance-type sensors.

Amorphous silica and its effects on shale reservoir: A case study about Yanchang formation lacustrine shale, Ordos Basin

ABSTRACTThe content of amorphous silica in Yanchang formation shale ranges from 1.48% to 19.89%, with an average of 11.77%. The Chang 7–3 submember (the submember 3 of the seventh member of Upper Triassic Yanchang formation) shale contains much larger content of amorphous silica. Amorphous silica appears as opal rim closely adjacent to detrital quartz, cement among clay minerals, and cement aside detrital minerals. Silica originates from clay mineral transformation and pressure solution of detrital quartz. Mineral composition, growth space, and overpressure affect the content of amorphous silica. Amorphous silica reduces the brittleness of shale reservoir, decreases the density of Chang 7–3 submember shale, and causes an increase in AC values for Chang 7–3 submember shale.

Bamboo Fiber and Sugarcane Skin as a Bio-Briquette Fuel

The present study deals with the issue of bio-briquette fuel produced from specific agriculture residues, namely bamboo fiber (BF) and sugarcane skin (SCS). Both materials originated from Thừa Thiên Huế province in central Vietnam and were subjected to analysis of their suitability for such a purpose. A densification process using a high-pressure briquetting press proved its practicability for producing bio-briquette fuel. Analysis of fuel parameters exhibited a satisfactory level of all measured quality indicators: ash content Ac (BF—1.16%, SCS—8.62%) and net calorific value NCV (BF—16.92 MJ∙kg−1, SCS—17.23 MJ∙kg−1). Equally, mechanical quality indicators also proved satisfactory; bio-briquette samples’ mechanical durability DU occurred at an extremely high level (BF—97.80%, SCS—97.70%), as did their bulk density ρ (BF—986.37 kg·m−3, SCS—1067.08 kg·m−3). Overall evaluation of all observed results and factors influencing the investigated issue proved that both waste biomass materials, bamboo fiber and sugarcane skin, represent suitable feedstock materials for bio-briquette fuel production, and produced bio-briquette samples can be used as high-quality fuels.

Vector semirational rogue waves for a coupled nonlinear Schrödinger system in a birefringent fiber

Under investigation in this paper is a coupled nonlinear Schrödinger system with the four-wave mixing term, which describes the propagation of optical waves in a birefringent fiber. Via the Darboux dressing transformation, the semirational solutions which give rise to the vector rogue waves and breathers are obtained. We display the vector rogue waves and the interaction between the rogue waves and bright–dark solitons. During the interaction, breather-like structures arise because of the interference between the dark and bright components of the soliton. Besides, it can be observed that the rogue wave and soliton merge together. Interactions between the breathers and bright–dark solitons are shown graphically. Keeping |α1|2a+|α2|2c+bα1α2∗+b∗α1∗α2 invariant, we find that the smaller value of ac−|b|2 yields the more obvious breather-like structure, with a and c representing the self- and cross-phase modulations, respectively, b representing the four-wave mixing effect, α1 and α2 being two constants. Similarly, keeping ac−|b|2 invariant, we find that the smaller value of |α1|2a+|α2|2c+bα1α2∗+b∗α1∗α2 yields the more obvious breather-like structure. Bound state forming between the Kuznetsov-Ma soliton and breather-like structure is illustrated.

Lymphoma images analysis using morphological and non-morphological descriptors for classification

Mantle cell lymphoma, follicular lymphoma and chronic lymphocytic leukemia are the principle subtypes of the non-Hodgkin lymphomas. The diversity of clinical presentations and the histopathological features have made diagnosis of such lymphomas a complex task for specialists. Computer aided diagnosis systems employ computational vision and image processing techniques, which contribute toward the detection, diagnosis and prognosis of digitised images of histological samples. Studies aimed at improving the understanding of morphological and non-morphological features for classification of lymphoma remain a challenge in this area. This work presents a new approach for the classification of information extracted from morphological and non-morphological features of nuclei of lymphoma images. We extracted data of the RGB model of the image and employed contrast limited adaptive histogram equalisation and 2D order-statistics filter methods to enhance the contrast and remove noise. The regions of interest were identified by the global thresholding method. The flood-fill and watershed techniques were used to remove the small false positive regions. The area, extent, perimeter, convex area, solidity, eccentricity, equivalent diameter, minor axis and major axis measurements were computed for the regions detected in the nuclei. In the feature selection stage, we applied the ANOVA, Ansari-Bradley and Wilcoxon rank sum methods. Finally, we employed the polynomial, support vector machine, random forest and decision tree classifiers in order to evaluate the performance of the proposed approach. The non-morphological features achieved higher AUC and AC values for all cases: the obtained rates were between 95% and 100%. These results are relevant, especially when we consider the difficulties of clinical practice in distinguishing the studied groups. The proposed approach is useful as an automated protocol for the diagnosis of lymphoma histological tissue.

Open-label clinical trial of bezafibrate treatment in patients with fatty acid oxidation disorders in Japan

IntroductionFatty acid oxidation disorders (FAODs) are rare diseases caused by defects in mitochondrial fatty acid oxidation (FAO) enzymes. While the efficacy of bezafibrate, a peroxisome proliferator-activated receptor agonist, on the in vitro FAO capacity has been reported, the in vivo efficacy remains controversial. Therefore, we conducted a clinical trial of bezafibrate in Japanese patients with FAODs. Materials and methodsThis trial was an open-label, non-randomized, and multicenter study of bezafibrate treatment in 6 patients with very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency and 2 patients with carnitine palmitoyltransferase-II (CPT-2) deficiency (median age, 8.2 years; ranging from 5.8 to 26.4 years). Bezafibrate was administered for 6 months following a 6-month observation period. The primary endpoint was the frequency of myopathic attacks, and the secondary endpoints were serum acylcarnitines (ACs, C14:1 or C16 + C18:1), creatine kinase (CK) levels, degree of muscle pain (VAS; visual analog scale) during myopathic attacks, and quality of life (QOL; evaluated using validated questionnaires). ResultsThe frequency of myopathic attacks after bezafibrate administration decreased in 3 patients, increased in 3, and did not change in 2. The CK, AC, and VAS values during attacks could be estimated in only three or four patients, but a half of the patients did not experience attacks before or after treatment. Changes in CK, AC, and VAS values varied across individuals. In contrast, three components of QOL, namely, physical functioning, role limitation due to physical problems (role physical), and social functioning, were significantly elevated. No adverse drug reactions were observed. ConclusionIn this study, the frequency of myopathic attacks and CK, AC, and VAS values during the attacks could not be evaluated due to several limitations, such as a small trial population. Our findings indicate that bezafibrate improves the QOL of patients with FAODs, but its efficacy must be examined in future investigations.

Fatigue strength improvement of an aluminum alloy with a crack-like surface defect using shot peening and cavitation peening

The effects of shot peening (SP) and cavitation peening (CP) on the bending fatigue strength of the 7075 aluminum alloy with a crack-like surface defect were investigated. Test specimens containing a small semicircular slit were subjected to SP and CP. Bending fatigue tests were conducted on the specimens to determine the fatigue strength and critical depth (ac) of the semicircular slit that can be rendered harmless by peening. The results indicated that fatigue strengths increased by peening and that semicircular slits less than 0.1 and 0.2 mm in depth were rendered harmless by SP and CP, respectively. The ac values estimated based on fracture mechanics were similar to experimental values, indicating that ac was affected by the distribution of the compressive residual stress induced by peening.

The change of optical vortex on the paraxiality of fully coherent and partially coherent Airy beams

The analytical expressions for the degree of paraxiality for fully coherent and partially coherent Airy vortex beams are obtained, respectively, and used to investigate the change of optical vortex on the degree of paraxiality. The results show that the paraxiality decreases when the optical vortex is nested in the Airy beam, while the noncanonical strength of optical vortex contributes nothing to the change of the paraxiality. A larger truncation factor leads to a higher paraxiality, and the effect of topological charge on the paraxiality gradually emerges with the increasing of the truncation factor. For a partially coherent Airy vortex beam, the paraxiality increases with an increase of transverse scale or truncation factor. While it is also shown that there exists critical value of w0c or ac, where a lower coherence length has an advantage in acquiring a higher paraxiality for w0 < w0c or a < ac. Whereas for w0 > w0c and a > ac, a higher coherence length contributes the partially coherent Airy vortex beam to be more paraxial. In addition, the changes between the paraxiality of the partially coherent Airy beams and partially coherent Airy vortex beams become small as the increasing of transverse scale or truncation factor.

Use of cover crop fields by migratory and resident birds

Cover crops, established between the growing seasons of primary crops to improve soil and water quality, have become increasingly popular in the Midwest region of the United States; however, the impact on migratory and resident birds is largely unknown. We conducted avian surveys on four field types in east central Illinois in the spring of 2015 and 2016: maize (Zea mays L.) stubble with cover crop, maize stubble only, soybean [Glycine max (L.) Merr.] stubble with cover crop, and soybean stubble only. For each field type, we calculated relative bird abundance and the Avian Conservation Significance value (ACS). Relative bird abundance was greater in cover crop fields than non-cover crop fields, with the maize fields planted with cover crops providing the greatest value (nearly twice the number of individuals and twice the species compared with non-cover crop soybean fields). The most common species were Red-winged Blackbird (Agelaius phoeniceus), Common Grackle (Quiscalus quiscula), and American Robin (Turdus migratorius). ACS values were most influenced by the Eastern Meadowlark (Sturnella magna), a species of high conservation concern. Many agricultural landscapes lack habitat in the spring and cover crop fields may be important areas to provide shelter and forage for birds. While we documented greater use of these fields more research is needed to understand why birds use these fields; more explicitly are birds finding cover, foraging, or attempting to breed in cover crop fields? As the amount of cover crop area increases, the value of these fields for migratory and resident bird use may increase. While habitat for wildlife is a secondary consideration when planting cover crops, my research suggests the use of cereal rye and later termination of cover crops benefits birds. Cover crops will not replace natural habitats for birds, but the widespread use of cover crops may benefit some bird populations.

Impact of CuS on the crystallization kinetics of Na2S-P2S5 glasses

The xCuS-45P2S5-(55−x)Na2S (mol% 2.5≤x≤10) glasses are synthesized through melt-quench method. Compositional dependent local structure, thermal stability, crystallization kinetics and electrical conductivity are investigated through various techniques. X-ray diffraction showed that a crystalline peak of Cu1.8S is observed in all the glass samples. The intensity of Cu1.8S peak increases with the addition of CuS in the glass samples. The de-polymerization of phosphate structural units from Q2→Q1→Q0 is observed by increasing CuS content, as evidenced from FT-IR and Raman studies. The glass transition temperature (Tg) and crystallization temperature (Tc) peaks shift towards higher temperature with increased CuS content. This phenomenon indicates that the thermal stability of the glass samples enhanced with increase in CuS content. The crystallization kinetics is investigated using Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods. It is observed that the trend of activation energies remains identical in both crystallization zones, as calculated from both methods. The mechanism involved during the crystallization of glass samples is proposed by evaluating Local Avrami exponent (LAEX). The room temperature electrical conductivities were measured and the maximum value of ac (2.86×10−4Scm−1) and dc (3.85×10−5Scm−1) conductivities were obtained for 10CuS-45P2S5-45Na2S glass sample. The dielectric response of the sample was also investigated and the dielectric constant was found maximum for 10CuS-45P2S5-45Na2S glass sample.

Experimental study on breakdown voltage of high pressure and high temperature helium gas between parallel electrodes

The DC and 50Hz AC breakdown voltage of helium between two parallel copper electrodes were measured to obtain the base data for insulation design of the electrical equipment working in the high pressure and high temperature helium gas in High Temperature Reactor (HTR). The measurements were performed in the distance of 0.25–0.53mm, the temperature of 15–180°C, and the pressure of 0.05–7MPa. It has been proved that, for both DC and 50Hz AC, the breakdown voltage increases with the increasing of the pressure, with the increasing of the distance, and with the decreasing of the temperature. The DC breakdown voltage is about 1.4 times of the value of 50Hz AC or, in other words, is similar to the peak value of 50Hz AC. The Paschen law, describing the relationship between the breakdown voltage of helium (Ub) and the product of gas density and electrode distance (ρd), was presented for DC and 50Hz AC respectively, based on the experimental data at high pressure and high temperature. Good linearity is shown in the data. Furthermore, the linear fitting equation was also presented. The slope of Paschen curve about Ub-ρd at the range of large ρd is influenced by the collision cross section and the ionization energy of helium atom, and not influenced by the temperature, for DC. Both for DC and 50Hz AC, the slope of Paschen curve about Ub-ρd is about 6×105 times of the slope of Paschen curve about Ub-Pd, where Pd is the product of gas pressure and electrode distance. Both for DC and 50Hz AC, the breakdown voltage of helium is less than one fifth of the value of air, at 0.1MPa and room temperature. However, the breakdown voltage of helium at 180°C and 7MPa is larger than the value of air at 0.1MPa and room temperature.

Integrated core-log facies analysis and depositional model of the gas hydrate-bearing sediments in the northeastern continental slope, South China Sea

Host sediments may exert a significant influence on the formation of gas hydrate reservoirs. However, this issue has been largely neglected in the literature. In this study, we investigated the types, characteristics and the depositional model of the fine-grained gas hydrate-bearing sediments in the northeastern margin of the South China Sea by integrating core visual observations and logging-while-drilling downhole logs. The gas hydrate-bearing sediments consist dominantly of muddy sediments formed in the inter-canyon ridges of the upper continental slope, including hemipelagites, debrites (mud with breccia) and fine-grained turbidites. Cold-seep carbonates and associated slumping talus, muddy breccia debrites, as well as coarse-grained turbidites, may locally occur. Four classes and six sub-classes of log facies were defined by cluster analysis. Core-log correlation indicates that gas hydrates are majorly distributed in fine-grained sediments with high resistivity and low acoustic transit time (AC) log responses, which are easily differentiated from the fine-grained background sediments of high gamma-ray (GR), high AC, and low resistivity log values, and the seep carbonates characterized by low GR, high resistivity, high density, low AC and low porosity log values. The primary host sediments consist of fine-grained hemipelagic sediments formed by deposition from the nepheloid layers of river material and from the microfossils in seawater column. Most of the hemipelagic sediments, however, might have been extensively modified by slumping and associated gravity flow processes and were re-deposited in the forms of debrites and turbidites. Locally developed seep carbonates associated with gas hydrate dissociation and leakage provided additional sources for the gravity flow sediments.