碳纤维水泥基复合材料的正、负压敏性及其机理分析

对碳纤维水泥基复合材料的压敏特性进行了实验研究。结果发现, 在含水量不同的情况下, 碳纤维水泥基复合材料分别具有正、负两种压敏特性。含水量较高时, 碳纤维水泥基材料的电阻相对变化率随应力的增大而增大, 随应力的减小而减小, 表现出正压敏性。而含水量较低时, 电阻相对变化率随应力的增大而单调减小,随应力的减小而单调增大, 表现出负压敏性, 进而提出了一种双效应机理, 并对这一物理现象进行了合理解释。      

温度对CF/ABS 复合材料导电性的影响

研究了温度对ＣＦ／ＡＢＳ树脂复合材料导电性的影响。所制复合材料的电阻率随温度的升高按指数规律上升,而且,随着复合材料中炭纤维含量的增加,电阻率随温度的变化率逐渐降低。     

碳纤维增强水泥基复合材料的压阻效应

试验研究了短切碳纤维增强水泥基复合材料(CFRC)的压阻效应, 获得了正、负两种压阻效应相互转换的全过程。从隧道效应和孔隙的连通性角度对该现象的产生机理进行了探讨。结果表明, 在连续烘干和单向循环加载条件下, CFRC的压阻效应会随含水量变化而发生改变。多数情况下, CFRC的体积电阻率随压应变单调减少, 压阻效应为正。含水量越少, 正压阻效应越明显。当含水量减少到约3.19%~4.04%的范围时, CFRC的体积电阻率随压应变单调增加, 压阻效应为负。与正压阻效应相比, 负压阻效应表现更强。CFRC的正、负压阻效应及其相互转换是隧道效应和孔隙连通性两方面相互影响的必然结果。      

碳纤维水泥基复合材料的阻-温特性

研究了碳纤维水泥基复合材料的阻-温特性。发现当温度低于某一特定值时，电阻率随温度的升高而单调降低(NTC效应)。超过此值时，则表现为电阻率随温度升高而大幅度单调增加(PTC效应)。普通水泥基材料不具有相应的阻-温特性．     

硅微粉及热处理温度对铝矾土基喷涂料性能影响

为了研究硅微粉在铝矾土基喷涂料中的应用,以铝矾土为主要原料,铝酸钙水泥为结合剂,分别研究不同硅微粉含量对铝矾土基喷涂料性能的影响。试样自然干燥24h脱模后,再经110℃烘干24h,分别于1000、1300、1500℃热处理3h。检测各温度热处理后试样的线变化率、体积密度、抗折强度、抗压强度以及试样的热膨胀系数。结果表明:铝矾土基喷涂料经过110℃烘干,1000、1300℃热处理后,抗折强度和抗压强度随着硅微粉含量的增加而增加。铝矾土基喷涂料的热膨胀系数随着硅微粉含量的增加而减小。制备铝矾土基喷涂料的最佳硅微粉质量分数为5%。     

热处理温度对聚酰亚胺基炭纤维结构性能影响

以聚酰亚胺(PI)纤维为前驱体,经800~2800℃连续高温处理,制备出不同性能的聚酰亚胺基炭(石墨)纤维。采用元素分析、SEM、HR-TEM、Raman、纤维强力仪、电阻率仪等分析手段研究热处理温度对炭纤维(CF)元素含量、结构形貌、力学性能、传导性能等方面的影响。结果表明,随着热处理温度的升高,聚酰亚胺基炭纤维中碳含量从78.97%(800℃)提高到99.72%(2 800℃),非碳原子含量降低;聚酰亚胺基炭纤维表面缺陷数目增加且尺寸增大。同时,纤维的微观结构也从二维乱层石墨结构向有序的三维层状结构发展,表现为石墨化程度的提高及石墨微晶尺寸的增大;炭纤维拉伸强度先增加后降低,最大拉伸强度924.4 MPa,断裂伸长率降低,电阻率减小,热导率增加,2 800℃石墨化处理后纤维热导率为228.4 W·m-1·K-1,是800℃处理后的50.4倍。     

碳纤维水泥基材料的机敏特性研究

水泥基材料是一种高电阻率的惰性材料,通过掺入一定体积含量(0.2 vol%~1.2 vol%)的短切碳纤维,可显著提高其电导率。本文作者研究了碳纤维水泥基材料表观电导特性与其内部微观结构的关系,探讨了电阻率的变化与材料所受外部荷载的关联性。结果表明碳纤维水泥基材料具有实时诊断内部损伤的机敏性:当水泥基材料内部裂纹产生或扩展时,表现为材料电阻率上升;而当水泥基材料内部裂纹闭合时,其电阻率下降。碳纤维水泥基材料的电阻变化与所受荷载呈良好的线形关系;而不含碳纤维的普通水泥基材料在整个受荷过程中,其电导特性则无明显变化。碳纤维水泥基材料电导特性的变化反映了水泥基材料内部损伤状况丰富的信息,根据这一特性可以及时预报水泥基材料内部潜在的损伤状况,有效地防止灾难性的破坏。     

基于炭纤维水泥基材料电阻率变化的水化进程监测

研究了水化过程中空白水泥石与炭纤维水泥石的电阻特性,探讨了炭纤维水泥石的电阻率与其自身水化进程以及强度的关系,并对掺加早强剂和热养护炭纤维水泥石的电阻率与水化进程以及强度之间的关系进行了进一步的探讨。研究结果表明:水化过程中水泥石的极化作用较强,测试时不能获得稳定的电阻率,掺入质量分数0.30%以上的炭纤维后可以获得稳定的测试电阻。纤维掺量为0.35%的水泥石的电阻率与水化进程呈线性关系。掺加早强剂和热养护的炭纤维水泥石的电阻率与水化进程同样呈现线性关系。应用炭纤维水泥基材料电阻率对其自身水化进程的监测是可行的。     

炭纤维/ABS树脂复合材料的导电性研究

研究了炭纤维长度、填充量及表面处理条件下对炭纤维／ＡＢＳ树脂复合材料导电性的影响以及材料导电性同环境温度的相关性。结果表明复合材料的电阻率随着纤维填加量的增多而降低。纤维长度越长,复合材料导电性越好。纤维表面处理后,复合材料电阻率增大,相同纤维填充量下,表面处理时间越长,材料电阻率越大。温度对复合材料电阻率的影响随纤维填加量的增多而减少。而且,纤维表面状态的改变对复合材料的电阻率－温度关系有较大影响。     

乙炔炭黑水泥基复合材料的压敏性

为探讨乙炔炭黑水泥基复合材料的压敏性 , 验证了利用基于埋入式环状电极的四电极法测试复合材料电阻的可行性; 研究了一次加载至破坏时乙炔炭黑水泥基复合材料的电阻率变化规律 , 并通过在弹性阶段的加载考察了乙炔炭黑水泥基复合材料压敏性的重现性 ; 分析了测试电流和试件偏压对压敏性的影响。研究结果表明 ,基于埋入式环状电极的四电极法测试压敏水泥基材料的电阻误差小于 10 % , 用于复合材料电阻的测试是可行的。掺量 15 vol %的乙炔炭黑水泥基复合材料一次加载至破坏 , 电阻率变化率最大可达 55 % , 弹性阶段电阻率变化率可达 35 %以上 , 且其压敏性基本不受测试电流和试件偏压的影响。乙炔炭黑是制备具有高感知灵敏度的压敏水泥基材料的一种有效功能组分。     

Electrical resistivity studies on acid-treated E-glass fabric

E-glass fabrics were soaked in 0.1 to 2N hydrochloric acid for 0.5 to 3 h to remove the non-siliceous ions, and the volume resistivity of the treated fabrics was measured using d.c. voltages of 100, 250, 400 and 500. The electrical resistivity of the treated fabric increased sharply with increase in normality from 0.5N to 1.0N for a constant soaking time of 3 h; further increase was marginal at higher normalities (above 1.0N). A sharp increase in the electrical resistivity of the fabric was also found in the initial 1/2 h of soaking when fabrics were soaked in constant normality acid (2N) and resistivity was measured with time. The initial sharp increase in the electrical resistivity was attributed to the bulk removal of Na₂O + K₂O ions which are the main contributors to the conductivity. The removal of bivalent and trivalent ions did not contribute much to the increase in the electrical resistivity at this stage. The electrical resistivity of the treated fabrics increased with increase in the post heat-treatment temperature. During acid treatment, the non-siliceous ions are removed, which makes the fabric porous and susceptible to water absorption. Owing to absorption of moisture, the electrical resistivity of the fabric decreases with time and reaches a constant minimum value. The rate of decrease in electrical resistivity decreases with increase in the normality of the soaking acid. The constant minimum value of electrical resistivity increases with increase in post heat-treatment temperature for a fabric soaked in a constant normality acid. The mechanism of electrical conduction in the acid-treated fabrics after absorption of water is attributed to the proton movement which occurs by the polarization of the diffused water to the interior of the structure.     

Electrical characteristics of hierarchical conductive pathways in cementitious composites incorporating CNT and carbon fiber

Cementitious composites incorporating CNT and carbon fiber were produced in the present study, and the effect of the incorporation of carbon fiber on the electrical characteristics of the composites was investigated. Polycarboxylic acid based superplasticizer and silica fume were used to ensure the dispersion of the CNT and carbon fiber. Independent variables were the water to cement ratio, the amount of carbon fiber added to the composites. The test results showed that the cementitious composites incorporating CNT and carbon fiber had more stable electrical resistivity compared to those without carbon fiber. A microstructural analysis was conducted and the mechanism of the improved electrical characteristics of the cementitious composites incorporating CNT and carbon fiber was discussed.     

基于纳米碳材料的上光机干燥源制备与研究

目的 探寻合适的纳米碳材料取代传统上光机干燥热源,使之更节能,干燥效果更优化。研究不同制备条件对该材料发热板固化和导电性的影响及其电热规律,为上光机干燥源的关键技术提供依据。方法 采用丝网印刷技术制作纳米碳材料发热板,通过分析不同碳基材料发热板的物理性能以选择合适的材料,并进一步研究其在含碳量比、烧制温度、涂层厚度下的导电性能,以及进行通电试验研究其电热性能。结果 直径为150~200 nm纳米碳纤维更适用于本文制备方法,纯净水作为溶剂调制效果良好且获取方便。纳米碳纤维发热板导电性随烧结温度升高而降低,随厚度、含碳量增加而增强。发热板电阻越高、通电时间越长、电流越大,其温升越大、产生热量越高。结论 纳米碳纤维导电性和热效率高、绿色环保,适用于制作上光机干燥设备的干燥源,以提高干燥效率、降低能耗。     

Soaking–drying frequency effect on gypsum-rich roadbed sand

This article presents the California bearing ratio (CBR) and moisture content behaviour of a gypsum-rich roadbed sand with a gypsum content of about 39% subjected to cyclic soaking and drying. Frequencies of 8-, 14- and 60-day cycle lengths were adopted together with six cycles of soaking and drying for each frequency. Each cycle consisted of two equal time periods of soaking and drying. For each frequency, 13 pairs of CBR samples were prepared and subjected to cyclic soaking and drying in the laboratory under a surcharge load of 45 lb (200 N). The time variation of moisture content during cyclic soaking and drying at the top, middle and bottom of each CBR sample was studied as well. The article reveals that for each cycle, the CBR decreases during soaking and increases during drying for all frequencies.     

导电炭黑Super-P对混凝土性能的影响

采用低成本和高稳定性的纳米导电炭黑Super-P (CBSP)作为水泥混凝土的添加剂。通过设置不同的水灰比和不同的CBSP掺量,研究了CBSP的加入对混凝土各方面性能的影响(即坍落度、力学性能、抗渗性能、导电性能和温敏性能)。通过SEM对混凝土微观形貌进行分析。实验结果显示,掺入纳米材料CBSP使混凝土坍落度不断降低。随着CBSP的掺入量不断增大,混凝土的力学性能先提升后降低且各龄期变化趋势相似,当CBSP的掺入量为0.75wt%时力学性能达到最大。同时混凝土的力学性能随水灰比的增加而降低。混凝土的抗渗性能随着CBSP掺量的增加而先提高后降低,且当水灰比较大时抗渗性能有所降低。当CBSP掺入量为0.75wt%～2wt%时,混凝土电阻率迅速降低。标准养护条件比室内干燥养护的混凝土电阻率低。不同水灰比混凝土之间电阻率相差较小。SEM显示了CBSP的填充孔隙和隧道导电作用。通过实验证明CBSP的加入可以改善混凝土的各项性能。     

Utilization of polymer degradation to modify electrical properties of poly(l-lactide)/poly(methyl methacrylate)/carbon filler composites

This research attempts to utilize polymer degradability in modifying electrical properties of poly(l-lactide) (PLLA)/poly(methyl methacrylate) (PMMA)/carbon fillers composites. Three kinds of carbon particles, i.e. carbon black, vapor-grown carbon fiber, and carbon nanotube, were compounded with PLLA/PMMA blend, followed by hydrolytic degradation of the composites, resulted in degradation of PLLA molecular chain from the surface of samples, with PMMA and carbon particles remained undegraded. By controlling degradation rate, it was possible to prepare samples with low surface resistivity, yet at the same time exhibited high value of volume resistivity. It was also found that final electrical properties of degraded composites depend on the size and the shape of the fillers.     

Influence of aggregate fraction in the mix on the reliability of the rapid chloride permeability test

The present paper elucidates the influence of aggregate content of the mix on the reliability of rapid chloride permeability test (RCPT) results. For this purpose, test specimens prepared with mixes varying in total aggregate content were subjected to soaking test, RCPT and electrical resistivity measurements, and the results from these tests were compared and conclusions drawn. The RCPT results indicated the plain cement concrete to be relatively more resistant against chloride penetration than the plain cement mortar, whilst the opposite was true according to the 90-day soaking test results. The above trend did not change despite the addition of silica fume (SF) to the concrete and mortar mixes. The lower aggregate content or higher paste content of plain cement mortar and the mortar with SF is shown to mislead the RCPT results. The higher paste content in the above mix promotes the conduction of higher charge as a result of lower electrical resistivity. Thus the results derived from the present investigation emphasize the need to consider the volume fraction of aggregate in the mix with and without SF while interpreting the RCPT results. Furthermore, regardless of the total aggregate and SF content in the mix, the total charge passed (from the RCPT) through the mix decreased exponentially with increasing electrical resistivity. On the other hand, for those mixes containing either SF or a high volume fraction of aggregates the linear correlation between the total charge passed and chloride penetration coefficient (K) was poor. However, for the mix with relatively lower aggregate content and with no SF the charge passed was well correlated linearly with K.     

Fabrication of carbon papers using polyacrylonitrile fibers as a binder

Carbon papers (CPs) have been fabricated using wet-laying carbon fibers (CFs) and polyacrylonitrile (PAN) fibers. Scanning electron microscopy revealed that the PAN fibers tightly interconnected the CF junctions with the pores between the fibers. The tensile strength of the carbon webs (CWs) increased as the fraction of PAN fibers used as the binder increased. The CW fabricated with 0.15 wt% PAN fibers had a tensile strength six times greater than that of the CW without PAN fibers. Moreover, by mixing the CFs with PAN fibers in water, the CFs separated from each other in the webs due to the interruption of hydrophobicity between the CFs. After mixing with PAN fibers, the CWs were carbonized at 1200 °C in the presence of a phenolic resin. The PAN fibers maintained their morphology due to their high carbon content after carbonization. The electrical resistivity of the CPs with high PAN fiber content was significantly lower than that of a CP without PAN fibers due to the interconnection of the CFs by the carbonized PAN fibers.     

Effect of CNTs on morphology and electromagnetic properties of non-firing CNTs/silica composite ceramics

Carbon composite ceramics have much attention for industry because of their excellent properties such as strong toughness, high electrical conductivity as well as low percolation threshold. Therefore, carbon nanotubes (CNTs) were used to incorporate with silica ceramics in order to improve their electromagnetic properties. The amount of CNTs in CNTs/silica composite ceramics was varied in order to investigate its effect on morphologies and electromagnetic properties of those. The composites were successfully fabricated by non-firing process. The results revealed that the obtained CNTs/silica composite ceramic have an electrical resistivity of 66.6?Ω·cm with a bending strength of 13.8?MPa. At the same time, the electromagnetic wave absorption ability achieved 70% over a wild frequency. This indicates that the CNTs in CNTs/silica composite ceramics may be potentially applied for an electromagnetic wave reflective material.