Recent progress in materials最新文献

筛选
英文 中文
Effect of Heat Treatment on Sliding Wear Resistance of Hybrid Aluminum Matrix Composite 热处理对杂化铝基复合材料滑动磨损性能的影响
Recent progress in materials Pub Date : 2023-04-12 DOI: 10.21926/rpm.2302015
{"title":"Effect of Heat Treatment on Sliding Wear Resistance of Hybrid Aluminum Matrix Composite","authors":"","doi":"10.21926/rpm.2302015","DOIUrl":"https://doi.org/10.21926/rpm.2302015","url":null,"abstract":"Composite materials with aluminum as matrix material have a wider amplitude of large-scale applications in engineering. Some salient features of aluminum matrix composites are, low density, low thermal coefficient of performance and low weight and high strength. Among the various series of aluminum alloy, Al6061 have been widely used by researchers due to its outstanding properties particularly as they are heat treatable. Aluminum 6061 alloys have been reinforced with various particulate reinforcements such as silicon carbide, and graphite to study their friction and wear resistance properties. Adding silicon carbide particulate reinforcement improves the sliding wear resistance of composite material. However, it makes the material brittle and hard resulting in machining difficulties and rough surface finish. On the other hand, it has been found from the literature survey that the addition of graphite particulate reinforcement increases ductility and sliding wear resistance. In this context, the present article focuses on developing hybrid aluminum matrix composites by incorporating both graphite and silicon carbide. Heat treatment has been carried out to further enhance the wear resistance and strength of the composites. Vortex-stir casting was successfully utilized to fabricate Al6061-SiC-Gr hybrid composites. There was excellent bonding between the matrix and reinforcement materials as revealed by the microstructure study. The sliding wear resistance of the Al6061-SiC composite was higher than the base matrix material. Heat treatment increases the sliding wear resistance of the composite. Ice quenching results in maximum improvement. Increased content of graphite increases the sliding wear resistance of Al6061-SiC composite. Further heat treatment increases the sliding wear resistance of the hybrid composites with ice quenching resulting in maximum improvement.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-04-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42356681","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
7Li Diffusion in Thin Disks of Single-Crystal Garnet LLZO-Ta Studied by PFG-NMR Spectroscopy 用PFG-NMR研究7Li在单晶石榴石LLZO-Ta薄片中的扩散
Recent progress in materials Pub Date : 2023-04-03 DOI: 10.21926/rpm.2302014
K. Hayamizu, T. Haishi, Y. Terada, K. Kataoka, J. Akimoto
{"title":"7Li Diffusion in Thin Disks of Single-Crystal Garnet LLZO-Ta Studied by PFG-NMR Spectroscopy","authors":"K. Hayamizu, T. Haishi, Y. Terada, K. Kataoka, J. Akimoto","doi":"10.21926/rpm.2302014","DOIUrl":"https://doi.org/10.21926/rpm.2302014","url":null,"abstract":"Pulsed field-gradient (PFG) NMR spectroscopy was applied to thin disk samples (0.5, 1 and 2 mm thick, 4 mm diameter) of LLZO-Ta (Li<sub>6</sub>La<sub>3</sub>Zr<sub>1.5</sub>Ta<sub>0.5</sub>O<sub>12</sub>) single crystal. We have measured the diffusion of <sup>7</sup>Li in thin pellets (0.5 to 3 mm thick) of NASICON-type LAGP powder and found that the Li<sup>+</sup> diffuses preferentially in the thicker direction. In this study, we placed 1 mm thick, 4 mm diameter disk-shaped single crystals vertically, i.e., parallel to the PFG direction. We found that <sup>7</sup>Li ions diffuse similarly as in rod-shaped single crystals. The <sup>7</sup>Li diffusion is slow (diffusion coefficient; ~2 × 10<sup>-13</sup> m<sup>2</sup>s<sup>-1</sup>) at Δ ≥ 100 milliseconds and gradually becomes faster to ~1 × 10<sup>-11</sup> m<sup>2</sup>s<sup>-1</sup> at Δ ≤ 10 milliseconds at 28°C. A diffraction pattern was observed in the echo decay plots, which was not observed in well-prepared single crystal rod samples. The diffraction patterns are often observed in powder inorganic electrolytes (LLZOs, LLTO, LAGP, and (Li<sub>2</sub>S)<sub>x</sub>(P<sub>2</sub>S<sub>5</sub>)<sub>y</sub>). In this study, edge effects are assumed for the diffraction pattern of the vertically placed thin disk of the single crystal. For horizontally placed disks, at long observation times (Δ ≥ 100 milliseconds), the diffusion coefficients of <sup>7</sup>Li agreed with those of the rod-shaped samples. However, as Δ became shorter, contrary to the vertically placed disk, the diffusion of <sup>7</sup>Li decreased and the plots became less linear. Surprisingly, as Δ became shorter (Δ ≤ 15 milliseconds), sinusoidal patterns were observed in the real and imaginary elements of the echo attenuation plots,. The pattern is reproducible and the first example of sinusoidal real and imaginary components of the echo attenuation plot was observed, but an adequate explanation has not been obtained.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41350273","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Non Solved Contemporary Scientific Problems of Non-Conventional Bio-Surfaces Lubrication 非常规生物表面润滑尚未解决的当代科学问题
Recent progress in materials Pub Date : 2023-03-27 DOI: 10.21926/rpm.2301013
K. Wierzcholski, J. Gospodarczyk
{"title":"Non Solved Contemporary Scientific Problems of Non-Conventional Bio-Surfaces Lubrication","authors":"K. Wierzcholski, J. Gospodarczyk","doi":"10.21926/rpm.2301013","DOIUrl":"https://doi.org/10.21926/rpm.2301013","url":null,"abstract":"This paper presents new non solved, scientific research directions and their applications in contemporary non-conventional lubrication problems of the bio-surfaces domain. Here are described new research scientific trends and research problem solutions of hydrodynamic lubrication for living tissue, human skin and human cartilage joints surfaces. At first in introduction is described the state of the art, a new purpose of anticipate research and the bio-tribology connections with other scientific domains are documented. The bio-tribology is a new scientific domain where the bio-properties of living, mutually cooperating bio-materials have an important influence on joint human functioning and on the consequences of obtained bio-tribology effects. Using experimental and numerical methods, the main result is focused on the influence of the bio-sweat dynamic viscosity variations across the film thickness on the load carrying capacity of the lubricated human skin and limbs.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-03-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46691261","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 1
Mechanics of ABS Polymer under Low & Intermediate Strain Rates 中低应变速率下ABS聚合物的力学特性
Recent progress in materials Pub Date : 2023-03-16 DOI: 10.21926/rpm.2301012
Suhail Hyder Vattathurvalappil, S. Hassan, M. Haq
{"title":"Mechanics of ABS Polymer under Low & Intermediate Strain Rates","authors":"Suhail Hyder Vattathurvalappil, S. Hassan, M. Haq","doi":"10.21926/rpm.2301012","DOIUrl":"https://doi.org/10.21926/rpm.2301012","url":null,"abstract":"Thermoplastics polymers like Acrylonitrile Butadiene Styrene (ABS) are often reinforced with nano/micro reinforcements to enhance their mechanical, thermal and electrical properties. However, the viscoelastic nature of these polymers results in their strong dependence on the applied strain rate and temperature sensitivity, leading to their characterization complexity. Hence it is paramount to study the strain rate-dependent mechanics of neat ABS. In this study, the effect of strain rate and temperature on Young’s modulus of ABS polymer was characterized using a dynamic mechanical analyzer (DMA). Storage modulus curves at various temperatures and frequencies were transformed into a representative master curve at a specific temperature using the time-temperature superposition (TTS) principle. Based on this curve‘s storage modulus and frequency relation, an empirical fit function was developed and the strain rate values were extrapolated. Using integral relations of viscoelasticity, the results were further transformed to a time domain relaxation function to extract the strain rate-sensitive Young’s modulus at different loading rates. This method was validated by comparing the data with tensile tests conducted on ABS coupons as per ASTM D638-14. The results were acceptable over a wide range of strain rates and indicated a clear sensitivity of ABS to strain rate and temperature. The strategy used in this work can be employed to study the effect of reinforcement morphology in ABS thermoplastics using DMA.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":"19 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-03-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41294941","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Retraction: Roy S. Performance of Grouted Coupler Embedded in Reinforced Concrete ABC Bridge Pier at Vehicle Impact. Recent Progress in Materials 2023; 5(1): 001 钢筋混凝土ABC桥墩内嵌灌浆扣件在车辆冲击下的性能。材料科学进展;2023;5 (1): 001
Recent progress in materials Pub Date : 2023-02-28 DOI: 10.21926/rpm.2301011
rpm rpm
{"title":"Retraction: Roy S. Performance of Grouted Coupler Embedded in Reinforced Concrete ABC Bridge Pier at Vehicle Impact. Recent Progress in Materials 2023; 5(1): 001","authors":"rpm rpm","doi":"10.21926/rpm.2301011","DOIUrl":"https://doi.org/10.21926/rpm.2301011","url":null,"abstract":"Retraction<strong>Retraction: Roy S. Performance of Grouted Coupler Embedded in Reinforced Concrete ABC Bridge Pier at Vehicle Impact. <em>Recent Progress in Materials</em> 2023; 5(1): 001</strong><em>Recent Progress in Materials</em> Editorial OfficeLIDSEN Publishing Inc., 2000 Auburn Drive, One Chagrin Highlands, Suite 200, Beachwood, OH, USA; E-Mail: <a href=\"mailto:rpm@lidsen.com\">rpm@lidsen.com</a><strong> </strong><em>Recent Progress in Materials</em>2023, volume 5, issue 1doi:10.21926/rpm.2301011<strong>Received:</strong> February 28, 2023<strong>Accepted:</strong> February 28, 2023<strong>Published:</strong> February 28, 2023","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-02-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48331324","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
An Interview with Assoc. Prof. Carlo Santulli Carlo Santulli副教授访谈录
Recent progress in materials Pub Date : 2023-02-27 DOI: 10.21926/rpm.2301010
Recent Progress in Nutrition Editorial Office
{"title":"An Interview with Assoc. Prof. Carlo Santulli","authors":"Recent Progress in Nutrition Editorial Office","doi":"10.21926/rpm.2301010","DOIUrl":"https://doi.org/10.21926/rpm.2301010","url":null,"abstract":"Interview<strong>An Interview with Assoc. Prof. Carlo Santulli</strong><em>Recent Progress in Materials</em> Editorial OfficeLIDSEN Publishing Inc., 2000 Auburn Drive, One Chagrin Highlands, Suite 200, Beachwood, OH, USA; E-Mail: <a href=\"mailto:rpm@lidsen.com\">rpm@lidsen.com</a><strong> </strong><em>Recent Progress in Materials</em>2023, volume 5, issue 1doi:10.21926/rpm.2301010<strong>Received:</strong> February 26, 2023<strong>Accepted:</strong> February 26, 2023<strong>Published:</strong> February 27, 2023","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-02-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43548858","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Accounting for the Influence of Heat-Moisture Treatment Mode When Designing Concrete Composition 在设计混凝土成分时考虑热湿处理方式的影响
Recent progress in materials Pub Date : 2023-02-24 DOI: 10.21926/rpm.2301009
L. Dvorkin, V. Zhitkovsky, Ruslan Makarenko
{"title":"Accounting for the Influence of Heat-Moisture Treatment Mode When Designing Concrete Composition","authors":"L. Dvorkin, V. Zhitkovsky, Ruslan Makarenko","doi":"10.21926/rpm.2301009","DOIUrl":"https://doi.org/10.21926/rpm.2301009","url":null,"abstract":"The article proposes a method that considers the parameters of heat and moisture treatment of concrete when designing its composition. Based on the analysis of experimental data, formulas are proposed that allow taking into account the type and characteristics of cement, the characteristics of aggregates, the effect of temperature and duration of steaming, and the water-cement ratio. Graphs and nomograms have been obtained, which makes it possible to simplify the calculation procedure according to the proposed method. Typical numerical examples of the application of this technique are given.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":"55 5","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-02-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41311332","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Advances in Quantum Properties of Graphene and Derivatives Applied to Functional Nanomaterials and Metamaterials 石墨烯及其衍生物在功能纳米材料和超材料中的量子特性研究进展
Recent progress in materials Pub Date : 2023-02-13 DOI: 10.21926/rpm.2301008
A. Bracamonte
{"title":"Advances in Quantum Properties of Graphene and Derivatives Applied to Functional Nanomaterials and Metamaterials","authors":"A. Bracamonte","doi":"10.21926/rpm.2301008","DOIUrl":"https://doi.org/10.21926/rpm.2301008","url":null,"abstract":"In the present work we describe the most important quantum properties of graphene and derivatives recently published. We discuss how these properties were incorporated into varied hybrid materials such as substrates for specific tuning of matter to track varied quantum signals. Their potential applications were analyzed from nanomaterial and nanotechnology. In particular graphene, its derivatives and other carbon-based allotropes were chosen due to their special chemical structure and properties from the nanoscale to larger lengths, according to specific applications. As expected, these carbon-based and related materials’ highly ordered and condensed electronic configuration showed particular electronic properties below the nanoscale. Thus, we discussed the generation of pseudo-electromagnetic fields and conduction bands. This particular property could also interact with different quantized energy levels and quantum properties, such as those focused on: i) Dirac electron interaction and conduction, ii) anomalous quantized hall effects, iii) magnetic effects, iv) excitons, v) polaron generations, d vi) Fermi and Landau levels. These different phenomena were discussed about the particular topological states of graphene by tuning their 3D chemical structures. Therefore, quantum phenomena and their possible modifications such as quantum interference, potential improvements and encrypted signal transduction were considered for applications.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-02-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42105653","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 2
Study of Recycled Spent Coffee Grounds as Aggregates in Cementitious Materials 回收废咖啡渣作为胶结材料骨料的研究
Recent progress in materials Pub Date : 2023-01-16 DOI: 10.21926/rpm.2301007
Judong Lee, Jinho Kim, Seungjoon Lee
{"title":"Study of Recycled Spent Coffee Grounds as Aggregates in Cementitious Materials","authors":"Judong Lee, Jinho Kim, Seungjoon Lee","doi":"10.21926/rpm.2301007","DOIUrl":"https://doi.org/10.21926/rpm.2301007","url":null,"abstract":"Most spent coffee grounds (SCGs), a byproduct of brewing coffee, are buried in landfill sites, and their decomposition produces significant greenhouse gases. As the recent warming of the Earth’s climate has made it imperative that industries reduce greenhouse gas emissions, the present study investigates the viability of recycling SCGs for use as a partial replacement of aggregates in concrete materials. Cement mortar samples with a fixed cement-to-water ratio and varying amounts of SCGs were fabricated and tested. Mechanical strength tests revealed that an appropriate amount of SCGs can improve compressive strength. However, since strength deterioration was also observed in samples with too much or too little SCG content, finding the optimal amount is necessary for implementation. The samples’ thermal conductivity decreased as the amount of SCGs increased, capturing the effective insulating substance of air within the SCGs’ porous structure. The increased insulating capacity of concrete materials resulting from the addition of SCGs could be beneficial in terms of a building’s lifecycle cost and carbon emissions. Thus, the SCGs once disposed of in landfills to emit greenhouse gases can be transformed into an appealing and eco-friendly building material if the proper concrete mix ratio is used.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-01-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44246837","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 2
Reuse of Sawdust in Developing Countries in the Light of Sustainable Development Goals 从可持续发展目标看发展中国家木屑的再利用
Recent progress in materials Pub Date : 2023-01-12 DOI: 10.21926/rpm.2301006
U. Udokpoh, C. Nnaji
{"title":"Reuse of Sawdust in Developing Countries in the Light of Sustainable Development Goals","authors":"U. Udokpoh, C. Nnaji","doi":"10.21926/rpm.2301006","DOIUrl":"https://doi.org/10.21926/rpm.2301006","url":null,"abstract":"The several furniture-making phases, from cutting to processing to polishing, produce a significant volume of sawdust of differing grades. The extraction and disposal of such waste in developing countries with no established or sustainable methods is an increasing problem, frequently posing environmental challenges. Sawdust is one of the most underused wood waste portions, with mounds dispersed around the region, detracting from its visual appeal and causing other ecological concerns. This study investigated potential applications of sawdust as it concerns sustainable development. It sought to employ an extensive systematic literature review to establish that sawdust, which is often a nuisance at sawmills, may be put to beneficial uses. Four cardinal focus areas of this study are (i) energy generation, (ii) environmental protection/pollution containment, (iii) building construction, and (iv) water treatment/supply. Specifically, the study dealt with the feasibility of energy generation from sawdust through sawdust cook stoves, briquetting, and pyrolysis; pollution containment through sawdust-derived adsorbent; affordable building construction through sawdust-based ceiling boards and particles boards; and (iv) water treatment using point of use (PoU) sawdust-clay water filter. Based on over two hundred (200) published papers consulted in this study, it was strongly established that sawdust had found successful wide application in all four thematic areas with strategic connection to the objectives of SDGs 3.0, 6.0, 7.0, 9.0, 11.0 and 12.0. This research believes that emphasis on sustainable sawdust management can help transform the over 1.5 million tonnes of sawdust waste burned or dumped indiscriminately into value-added products each year.","PeriodicalId":87352,"journal":{"name":"Recent progress in materials","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49353732","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 2
0
×
引用
GB/T 7714-2015
复制
MLA
复制
APA
复制
导出至
BibTeX EndNote RefMan NoteFirst NoteExpress
×
提示
您的信息不完整,为了账户安全,请先补充。
现在去补充
×
提示
您因"违规操作"
具体请查看互助需知
我知道了
×
提示
确定
请完成安全验证×
相关产品
×
本文献相关产品
联系我们:info@booksci.cn Book学术提供免费学术资源搜索服务,方便国内外学者检索中英文文献。致力于提供最便捷和优质的服务体验。 Copyright © 2023 布克学术 All rights reserved.
京ICP备2023020795号-1
ghs 京公网安备 11010802042870号
Book学术文献互助
Book学术文献互助群
群 号:481959085
Book学术官方微信