Preparation, properties and application of highly hazy and transparent cellulose films for solar cells
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摘要: 将可持续的纤维素材料与电子器件结合是当今学术界的研究热点。高雾度透明纤维素薄膜是一种具有特殊光学性能的纸张。它除了具有普通纸张的优点(可降解、成本低、柔性、质轻等)外,还呈现出高的透光率和优异的光散射性能,可作为绿色光学透明材料应用于太阳能电池,提升电池的光电转化效率。本文首先简要介绍了高雾度透明纤维素薄膜的发展历程;接着,详细总结了高雾度透明纤维素薄膜的制备方法及其性能(如光学、力学、热稳定性、耐水等);然后论述了现阶段这类薄膜在太阳能电池中的应用进展;最后,总结了高雾度透明纤维素薄膜存在的科学技术问题,并对其今后的研究方向以及应用前景进行了展望。Abstract: Integrating sustainable cellulose materials into electronic devices is a hot research topic in academic communities. Highly transparent cellulose film with high transmission haze is a kind of paper with special optical properties. In addition to the advantages (degradability, low cost, flexibility, light weight, etc.) of ordinary paper, it also presents high transparency and strong light scattering behavior (high transmission haze), and has the potential to use in solar cells as a green optical transparent material to improve the power conversion efficiency. In this review, the development process of highly hazy and transparent cellulose film was first introduced. Then, the preparation and properties (such as optical properties, mechanical properties, thermal stability and water resistance) of highly transparent and hazy cellulose films were summarized in detail. After that, the progress in the use of transparent and hazy cellulose film in solar cells was discussed. Finally, scientific and technical problems of highly transparent and hazy cellulose films for solar cells were summarized, and their challenges and future research direction were provided as well.
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Keywords:
- high transmission haze /
- high transparency /
- cellulose film /
- light scattering /
- solar cells
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工程用水泥基复合材料(Engineered cementitious composites,ECC)是基于微观力学和断裂力学系统设计的高性能纤维增强材料,具有超高韧性、良好的应变硬化和裂缝控制能力,已广泛应用于混凝土结构加固[1-3]。然而,作为水泥基材料本身,其抗拉强度有限,需要结合相应的增强材料以充分发挥其材料特性。由于高强钢绞线(High-strength steel wire rope,HSSWR)抗拉强度高、耐腐蚀,且极限拉应变与聚乙烯醇(PVA)-ECC接近,故课题组将高强钢绞线网嵌入ECC并开展相关研究。结果表明:钢绞线网与ECC有良好的粘结性能;钢绞线网增强ECC(High strength steel wire mesh reinforced ECC,HSSWM-ECC)用于加固钢筋混凝土(Reinforced concrete,RC)梁的抗弯承载力、抗裂性、弯曲韧性等受力性能均得到了显著提升[4-7]。
然而,大多数HSSWM-ECC加固RC梁试件出现了跨中或端部的局部剥离,不利于HSSWM-ECC加固效果的充分发挥。由于材料间的界面粘结性能是实现加固效果的关键[8-10],有必要对HSSWM-ECC与混凝土的粘结性能展开研究。Mansour等[11]基于斜剪、劈裂拉伸和直剪试验探究表面处理方法对超高性能纤维增强混凝土-混凝土和混凝土-混凝土界面粘结性能的影响,包括钢丝刷、钻孔、刻槽、涂环氧树脂和胶体胶。结果表明:钻孔和刻槽能明显改善界面粘结性能和失效模式。张阳等[12]基于抗剪推出试验研究不同界面处理下的超高性能混凝土与混凝土的抗剪粘结性能,结果表明:凿毛和刻槽界面的试件抗剪承载力最高,而植筋和刻槽界面的试件破坏时延性较好。Al-Madani等[13]研究采用光滑、钻孔和喷砂3种处理方式的超高性能混凝土-混凝土在斜剪、双剪、劈裂抗拉和三点弯曲试验下的粘结性能,结果表明:除了双剪试验,喷砂处理试件的粘结强度高于钻孔试件。上述研究表明,通过对粘结界面采取不同处理方式,改变界面粗糙程度,界面粘结性能均可得到不同程度的改善。但是,上述结论不是在统一控制指标下获得的,且凿毛、喷砂等处理方式的粘结作用机制难以量化;而机械刻槽只由槽深、槽宽和槽间距3个参数确定,能够定量研究各参数的粘结机制 [14-15],此外,课题组前期试验结果表明,采用刻槽方法处理的HSSWM-ECC与混凝土试件的粘结性能明显优于凿毛组[16]。然而,目前国内外研究主要集中于刻槽构造的作用效果,鲜有对其粘结机制进行研究,尤其是对于刻槽连接HSSWM-ECC与混凝土。
基于此,本文考虑刻槽数量、刻槽深度、钢绞线直径、纵向钢绞线配筋率及ECC强度等刻槽参数与加固基体性能的影响,通过HSSWM-ECC与混凝土梁式试验,分析试件破坏形态、荷载-端部滑移曲线、界面应变分布及各影响因素作用机制等,并建立多参数影响的刻槽构造钢绞线网增强ECC与混凝土界面剥离承载力计算模型。
1. 试验概况
1.1 试件设计
与单剪、双剪试验相比,梁式试验可以更真实地反映加固梁界面受力状态,故采用梁式试件对刻槽处理的HSSWM-ECC与混凝土界面粘结性能进行试验研究,其试件设计如图1所示。为了探究刻槽处理方式对HSSWM-ECC与混凝土界面粘结性能的影响,主要考虑了键槽特征(键槽数量(3、4和5道)、槽深(3 mm、5 mm和7 mm))、钢绞线直径(2.4 mm、3.2 mm和4.5 mm)、纵向钢绞线配筋率(0.376%、0.627%和0.877%)和ECC强度等参数,设计制作了12组36个梁式试件,其设计参数见表1所示。表1中试件以“变量-变量参数”命名,如N3试件,N表示键槽数量变化,3为键槽数量;其他组变量基于试件N4确定,如H3,H表示变化量为槽深,3为槽深3 mm;D3.2表示变化量为钢绞线直径,其值为3.2 mm;R表示纵向配筋率变化,T为ECC配方不同。为保证试验结果的有效性,每个编号试件均浇筑3个相同试件。
表 1 HSSWM-ECC-混凝土试件设计参数Table 1. Design parameters of HSSWM-ECC-concrete specimensGroup number Specimen number Groove number Groove height/mm HSSWR diameter/mm HSSWR ratio/% ECC type A A0 0 0 2.4 0.627 Type 1 N N3 3 5 2.4 0.627 Type 1 N4 4 5 2.4 0.627 Type 1 N5 5 5 2.4 0.627 Type 1 H H3 4 3 2.4 0.627 Type 1 H7 4 7 2.4 0.627 Type 1 D D3.2 4 5 3.2 0.878 Type 1 D4.5 4 5 4.5 0.855 Type 1 R R3 4 5 2.4 0.376 Type 1 R7 4 5 2.4 0.877 Type 1 T T2 4 5 2.4 0.627 Type 2 T3 4 5 2.4 0.627 Type 3 Notes: A—Control group; N—Groove number changed group; H—Groove height changed group; D—Strand diameter changed group; R—Longitudinal strand ratio changed group; T—ECC tensile strength changed group. 1.2 材料性能
试验用ECC由粉煤灰、微硅粉、水泥、石英砂、聚乙烯醇(PVA)纤维、水、减水剂和增稠剂拌合形成,其配合比如表2所示。混凝土采用设计强度等级为C40的混凝土,原材料水泥、水、砂、石的比例为1∶0.53∶1.92∶3.41。钢绞线选用规格为2.4 mm、3.2 mm及4.5 mm的小直径钢绞线。为测量各材料力学参数,不同配合比的ECC和不同直径的钢绞线分别预留3个试件,由于混凝土分3批浇筑,每批试件也预留3个伴随试件在同等条件下养护。
表 2 ECC配合比Table 2. Mix proportion of ECCwt% ECC Cement Sand Fly ash Micro silica Water Water reducer Thickener Polyvinyl alcohol (PVA) fiber Type 1 1 0.4 3 0.073 1.02 0.0407 0 0.072 Type 2 1 0.4 3 0.073 1.02 0.0407 0.00182 0.072 Type 3 1 0.4 3 0.073 1.15 0.0407 0 0.072 试验加载前,对预留混凝土(受压:150 mm×150 mm×150 mm)、ECC (受压:70.7 mm×70.7 mm×70.7 mm;受拉:270 mm×30 mm×13 mm)伴随试块和钢绞线进行材料性能测试[17-18]。其中,混凝土立方体抗压强度平均值为45.72 MPa,标准差为2.11 MPa,ECC和钢绞线具体参数分别列于表3和表4,图2为ECC受拉应力-应变试验曲线。
表 3 ECC力学性能结果Table 3. Mechanical properties results of ECCECC Cracking strength/MPa Cracking strain/% Tensile strength/MPa Ultimate tensile strain/% Compressive strength/MPa Type 1 1.842 0.031 2.516 3.587 31.2 Type 2 1.606 0.055 2.107 3.512 26.5 Type 3 2.315 0.043 3.282 3.724 25.4 表 4 HSSWR力学性能结果Table 4. Mechanical properties results of HSSWRDiameter/mm Measured area/mm2 Elastic modulus/GPa Ultimate tensile strength/MPa Ultimate tensile strain/% 2.4 2.82 110.348 1566.53 3.195 3.2 4.95 103.805 1581.30 3.787 4.5 9.64 97.782 1564.82 4.108 1.3 加载及量测方案
试验加载装置如图3所示。采用100 kN液压千斤顶以10 N/s的速度加载,直至试件破坏。竖向荷载通过压力传感器自动采集。为量测加载过程中界面应变发展,三排应变片平行粘贴于ECC侧面粘结边缘、非粘结边缘及混凝土底部。考虑课题组前期相关试验中在凹槽附近观测到存在应变集中区,故本次试验在每道凹槽两侧各粘贴一个应变片,以进一步明确该处拉压应变状态变化过程。另外,在自由端和加载端各粘贴一对应变片。ECC侧面测点布置如图3所示。鉴于在自由端HSSWM-ECC层与混凝土相对滑移较小,位移计仅布置于加载端,测量的差值即为界面相对滑移。
梁式试件受力模型如图4所示,界面剪力(V)等于HSSWM-ECC层跨中拉力(F),可表示为
V=F=PL1−L22L3 (1) 式中:P为竖向荷载;L1、L2和L3分别为支座中心、加载点和纵向钢绞线形心至钢铰转动中心的距离,分别取为345 mm、75 mm 和98.6 mm。
由此,纵向钢绞线名义拉应力σs和界面名义剪应力τa分别为
σs=FnAs (2) τa=Vlb (3) 式中:n为纵向钢绞线根数;As为单根钢绞线实测截面面积;l和b分别为粘结长度和宽度。
2. 试验结果与分析
2.1 HSSWM-ECC与混凝土粘结试件破坏形态
HSSWM-ECC与混凝土界面粘结性能试验试件的破坏形态如图5所示。根据破坏特征和破坏面位置,其破坏模式可归纳为2类:界面剥离破坏(粘结层剥离破坏、混凝土层内剥离破坏、HSSWM-ECC层内剥离破坏)和钢绞线断裂破坏。
(1) 粘结层剥离破坏(A类):此模式只发生于非刻槽处理的试件。由于界面是整个加固体系最薄弱的区域,破坏时,HSSWM-ECC层与混凝土均未出现材料层面的损坏,剥离界面较光滑平整,如图5(a)所示。
(2) 混凝土层内剥离破坏(B类):界面处理或HSSWM-ECC基体性能的提升使界面粘结强度大幅增加,甚至超过混凝土的抗剪强度。随着刻槽处ECC 被剪断,界面薄弱位置即混凝土层内部,发生剥离破坏。因此,该破坏主要出现于界面粘结性能良好的N组、H组及D3.2、R7、T3试件,是试验理想的破坏类型。典型破坏形态如图5(b)所示,刻槽位置处剪切面平整,局部表面附着有1~2 mm厚的混凝土砂浆,且集中分布于第一道槽至第四道槽范围内,表明HSSWM-ECC对混凝土的销钉作用主要集中于前四道槽,第五道槽对刻槽咬合力的贡献较少。值得注意的是,对于H3试件(图5(c)),刻槽处可观察到部分的ECC 剪切残留,说明槽深较浅(小于槽宽)时,刻槽咬合处的ECC会出现从刻槽中“拔出”的现象,界面的锚固效果有所降低。
(3) HSSWM-ECC层内剥离破坏(C类):破坏时有部分ECC粘附在混凝土表面,此类破坏主要发生于加固层基体性能较差的试件。对于D4.5组,为保证相同的纵向配筋率,其钢绞线直径由2.4 mm增大至4.5 mm,相应的钢绞线根数由7根减至2根,这使纵向钢绞线间距由10 mm变为30 mm,从而削弱了钢绞线网对ECC的约束作用。在荷载作用下,纵向钢绞线间距较远的ECC易出现拉剪破坏。而对于T2组,由于Type 2的ECC材料抗拉性能低于Type 1和Type 3,随着荷载增加,ECC砂浆内部发生断裂,并逐渐传递至自由端。
(4) 钢绞线断裂破坏(D类):只发生于钢绞线数量较少的R3组,在加载过程中,界面保持可靠粘结,但钢绞线内正应力逐渐超过其极限抗拉强度,发生钢绞线受拉断裂。
由上述分析可知,刻槽数量、刻槽深度和ECC抗拉强度对HSSWM-ECC与混凝土界面破坏形态影响较显著。此外,对于所有发生界面剥离的试件,在破坏之前并无明显的预兆,属于脆性破坏。
2.2 HSSWM-ECC与混凝土粘结试验结果
将各试件粘结性能的力学指标及破坏模式列于表5。其中,Fu及τa,p为界面峰值荷载及其对应的峰值粘结应力;σs,p为钢绞线峰值名义拉应力;su为界面最大滑移量。可知:刻槽数量对HSSWM-ECC与混凝土界面粘结性能有较显著的影响,N4试件的τa,p较N3提高了约31%,而N5与N4基本相同,这说明刻槽数量在一定范围内对粘结性能有利。与此相同,在一定范围内增加刻槽深度(3 mm到5 mm),τa,p先增加后趋于稳定。在相同纵向钢绞线配筋率下,增大钢绞线直径,界面粘结强度呈下降趋势。τa,p随纵向钢绞线配筋率的增加而增大,但单根钢绞线名义拉应力σs,p有所下降,表明增加纵向钢绞线配筋率对界面粘结性能的提升还有较大潜力。ECC材料力学性能对粘结界面性能有一定影响,结合表3和表5结果,在本试验条件下,τa,p随ECC 抗拉强度的增大而增大。
表 5 HSSWM-ECC与混凝土粘结试件试验结果Table 5. Test results of HSSWM-ECC-concrete specimensSpecimen number Fu/kN τa,p/MPa σs,p/MPa su/mm Failure mode A0-1 4.24 0.39 300.71 0.0645 A A0-2 4.42 0.41 313.48 0.0620 A A0-3 3.68 0.34 260.99 0.0558 A N3-1 10.21 0.95 724.11 0.1365 B N3-2 11.64 1.08 825.40 0.1442 B N3-3 11.96 1.11 848.55 0.1576 B N4-1 15.18 1.41 1076.60 0.1190 B N4-2 14.27 1.32 1012.06 0.0830 B N4-3 14.85 1.38 1053.19 0.1070 B N5-1 15.69 1.45 1112.77 0.0970 B N5-2 – – – – B N5-3 14.94 1.38 1059.57 0.0850 B H3-1 10.63 0.98 754.07 0.1726 B H3-2 10.66 0.99 756.03 0.1651 B H3-3 9.72 0.90 689.36 0.1789 B H7-1 15.31 1.42 1085.82 0.0920 B H7-2 14.90 1.38 1056.74 0.1043 B H7-3 13.86 1.28 982.98 0.1279 B D3.2-1 14.71 1.36 743.16 0.1093 B D3.2-2 16.52 1.53 834.34 0.1155 B D3.2-3 16.98 1.57 857.59 0.1263 B D4.5-1 14.49 1.34 751.56 0.1171 C D4.5-2 14.73 1.36 763.78 0.1252 C D4.5-3 15.32 1.42 794.61 0.1535 C R3-1 11.44 1.06 1352.01 0.1213 D R3-2 12.23 1.13 1445.21 0.1281 D R3-3 10.75 1.00 1271.05 0.1451 D R7-1 16.63 1.54 842.55 0.0863 B R7-2 17.71 1.64 897.34 0.0825 B R7-3 16.47 1.53 834.57 0.0950 B T2-1 12.41 1.15 880.40 0.1117 C T2-2 13.54 1.25 960.51 0.1240 C T2-3 13.16 1.22 933.56 0.1290 C T3-1 16.51 1.53 1170.59 0.0882 B T3-2 16.20 1.50 1148.97 0.0844 B T3-3 15.68 1.45 1111.80 0.0914 B Notes: Fu—Ultimate load; τa,p—Interface peak bond stress; σs,p—Peak nominal tension of longitudinal HSSWR; su—Maximum slip; A—Peeling failure in interface; B—Peeling failure in concrete layer; C—Peeling failure in HSSWM-ECC layer; D—Strand fracture damage. 2.3 HSSWM-ECC与混凝土粘结试件典型试验曲线
2.3.1 荷载-滑移曲线
由试验结果可知,各试件均发生界面剥离及钢绞线断裂的脆性破坏,故在试验过程中试件达到峰值荷载后迅速破坏,未能获得曲线下降段。各工况典型的荷载-滑移曲线,如图6所示。可以看出,刻槽界面处理试件的界面承载力、粘结刚度及破坏时的滑移量较未处理界面试件均有较大增幅。这表明刻槽处理是提高界面粘结性能的有效方式。
从图6可以看出,刻槽试件的荷载-滑移曲线呈现出两个阶段的特点:微滑移段和滑移段。
(1) 微滑移段OA:加载初期,荷载随加载端滑移线性增长, HSSWM-ECC层与混凝土协同工作,界面粘结范围内无微裂缝产生。随着荷载继续增加,混凝土层内出现微裂缝,并不断延伸。由于刻槽对混凝土内裂缝发展的约束作用,粘结刚度虽然有所降低,但总体仍处于较高水平。
(2) 滑移段AB:随着荷载继续增加至极限荷载的80%左右时,部分微裂缝贯通键槽底部,粘结刚度出现明显退化。HSSWM-ECC层与混凝土相对滑移明显增大,直至槽被完全剪断或拔出,试件破坏。
2.3.2 应变分布曲线
图7为刻槽处理试件在粘结长度范围内各应变测点应变分布图。如图7(a)所示,在有效粘结长度范围内,界面未处理试件(A0)整体上应变由加载端向自由端递减,此结果与文献[19-20]结论一致。加载初期,应变增长速率较慢,仅加载端附近测点处应变值有较大增长;之后,伴随着荷载的增加,自由端附近测点处应变值亦开始加速增长直至试件破坏。
由图7(b)可知,刻槽试件应变也是从加载端向自由端逐渐减小,但不同于A0试件,其在粘结长度内均产生应变,且刻槽两侧拉压应变交替,出现应力集中现象。分析原因:刻槽处理改变了界面粘结力组成,使机械咬合力占比明显增加,应变分布主要体现出键槽咬合力的作用,即加载时咬合齿共同抵抗界面剪切荷载。
3. HSSWM-ECC与混凝土界面粘结性能影响因素
3.1 刻槽数量
将不同刻槽数量试件(N3、N4和N5)的荷载-滑移曲线绘于图8。可以看出,随着刻槽数量的增加,界面承载力先增加后保持稳定;而对应的滑移值则随刻槽数量的增加不断减少。此外,随刻槽数量的增加,图中曲线的初始斜率逐渐增大。分析原因:刻槽位置由自由端至加载端依次确定,故刻槽数量最多的N5试件加载端与最近槽的距离最短,凹槽最先产生“锚固”作用,因此其界面粘结刚度最大。
为了进一步分析刻槽数量对界面粘结性能的影响规律,将相关试件的界面剥离荷载及其对应的滑移量与刻槽数量的关系绘于图9。可以看出,相较于非刻槽处理的试件,仅N3试件的界面剥离荷载就显著提高了174%;且随刻槽数量的增加仍能获得较大提高。在界面粘结长度相同时(本试验为120 mm),N4和N5试件的剥离荷载基本相当,约为15 kN。对于非刻槽处理试件,由于其破坏时荷载较小,故其最大滑移量较刻槽试件偏小。但相同荷载下的滑移量随刻槽数量的增多而减小。上述分析结果表明:界面刻槽处理可提高钢绞线网/ECC与混凝土的界面承载能力,但刻槽数量在一定范围(刻槽参与总受力宽度小于20 mm)内效果明显;但对滑移却是负相关的关系。分析原因:刻槽是通过增加材料间的机械咬合力来提高界面承载力的,但受材料性能的限制,在加载过程中,能够发挥作用的刻槽宽度是有限的,超过有效宽度后,继续增加刻槽数量,界面承载力不再提高,类似于有效锚固长度的原理。而端部滑移由HSSWM-ECC层的受拉伸长与材料间的相对位移两部分组成,由于HSSWM-ECC层拉伸变形较小,端部滑移主要是由材料间的相对位移提供,在具有一定数量的刻槽后,继续增加刻槽,会加强材料间的“锚固”作用,约束了材料间的相对位移,使滑移量有所降低。
3.2 刻槽深度
图10为不同刻槽深度(3 mm、5 mm和7 mm)试件的荷载-滑移试验曲线。可以看出,N4(5 mm)和H7(7 mm)试验曲线变化趋势基本一致,界面剥离荷载约为15 kN。而H3(3 mm)试件的剥离荷载和界面粘结刚度明显较低,这说明刻槽深度对界面粘结性能影响显著。
图11为相关试件界面剥离荷载及其对应端部滑移量与刻槽深度的关系曲线。可以看出,刻槽深度由3 mm增至5 mm,其界面剥离荷载提高了约43%,而对应滑移量则降低了约40%。但随着刻槽深度继续增加,二者均无明显变化。分析原因:槽深较浅时,刻槽处ECC被“拔出”而破坏,无法充分发挥HSSWM-ECC层的力学性能,导致界面峰值荷载有明显降低,也使界面滑移明显增大。上述分析表明:刻槽深度在一定范围内(槽深5 mm内)增加,可提高界面承载力和粘结刚度,使机械咬合力充分发挥。
3.3 钢绞线直径
图12为相同配筋率下不同纵向钢绞线直径(2.4 mm、3.2 mm和4.5 mm)试件的荷载-滑移试验曲线。可以看出,加载初期,各试件的荷载-滑移曲线基本一致。这是由于此阶段钢绞线网还未发挥作用,界面荷载全部由ECC承担。随着ECC开裂,界面粘结刚度随钢绞线直径增大而降低。
图13为相关试件的界面剥离荷载及其对应端部滑移与钢绞线直径的关系曲线。可以看出,钢绞线直径与界面剥离荷载呈负相关;与滑移量呈正相关,且均近似为线性关系。其中,D4.5 (4.5 mm)试件的剥离荷载较R7 (2.4 mm)试件降低了约12%,而对应的滑移量增加了约50%。这表明,钢绞线直径的增大会降低界面粘结性能。分析原因:刻槽咬合处的ECC 受拉开裂后,荷载由钢绞线网承担;为保证相同的配筋率,随着钢绞线直径的增大,需求的钢绞线数量减少,从而使钢绞线的间距增大,对ECC约束作用削弱,进而钢绞线间ECC容易出现拉剪破坏,界面承载力有所降低。此外,钢绞线对ECC的约束作用的降低也导致材料间的锚固性能有所降低,使其在相同荷载作用下,界面滑移增大。
3.4 纵向钢绞线配筋率
图14为钢绞线直径为2.4 mm,纵向钢绞线配筋率不同试件(0.376%、0.627%和0.877%)的荷载-滑移试验曲线。可以看出,加载初期,配筋率较低的R3试件的滑移量增长速率较快;随着荷载增加,N4试件曲线斜率变化明显,与R3试件曲线斜率接近;当荷载约为12 kN时,R3试件达到峰值荷载前出现滑移大幅增长阶段。与之相比,试件R7整个加载过程中荷载-滑移曲线基本保持上升趋势。分析原因:R3试件配筋率较低(0.376%,3根),对ECC的约束较差,故在相同荷载下产生较大的滑移;且在界面剥离破坏前,钢绞线达到极限抗拉强度,发生钢绞线断裂破坏。而N4试件在ECC开裂失效后,界面粘结刚度减小至与R3试件相近,类似地,R7试件在突变后的粘结刚度接近N4试件,表明钢绞线网主要通过限制ECC裂缝的开展发挥作用。
图15为不同钢绞线配筋率与各试件界面峰值荷载及其对应端部滑移的关系曲线。可以看出,界面峰值荷载及其滑移与钢绞线配筋率基本呈线性关系。R7试件的峰值荷载较R3试件提高了约48%,而相应滑移量降低了约33%。这表明增大钢绞线配筋率有利于改善界面粘结性能。
3.5 ECC 强度
通过改变水胶比和增稠剂含量来制备不同强度的ECC,图16、图17分别为ECC抗拉强度不同时各试件的荷载-滑移试验曲线及ECC抗拉强度与界面剥离荷载及其滑移的关系曲线。可以看出,随着ECC抗拉强度的提高,试件的界面剥离荷载随之提高,而对应的滑移相应降低。其中,T3试件的剥离荷载较T2试件增大了约24%,而滑移则相应降低了约28%。这是由于ECC基体性能显著影响着化学胶结力,且较高的抗拉强度使其在凹槽处不易被剪断,从而提高了材料间的机械咬合力。上述结果表明,ECC抗拉强度的提高有利于界面粘结,故其也是影响界面粘结性能的重要参数。
4. HSSWM-ECC与混凝土粘结界面承载力预测模型
4.1 HSSWM-ECC与混凝土界面粘结机制
未经机械刻槽处理的试件,其界面粘结力主要由化学胶结力、摩阻力和机械咬合力三部分组成。其中,化学胶结力是由水化反应过程中骨料与水泥浆体的化学键结合而形成,其主要来源有:(1) ECC中的胶凝材料与基体混凝土中水化产物发生化学反应后产生的吸附作用;(2) ECC中的胶凝材料与基体混凝土孔隙中的骨料化学反应后产生的吸附作用。化学胶结力的大小受ECC 及基体混凝土水泥的力学性能及粘结面积影响较大。摩阻力与机械咬合力则主要受界面粗糙程度影响,浇筑后的混凝土表面不可避免的会产生“凹凸”缺陷,进一步浇筑ECC后,这些局部“凹凸”会形成咬合齿。在界面荷载的作用下,发挥机械咬合作用与摩阻作用。上述三部分作用中,化学胶结力是界面粘结力的主要来源;而摩阻力与机械咬合力由于其存在较强的不确定性和不稳定性,且部分依赖于化学胶结力而存在,因而属于次要部分。该类界面的受力模型如图18(a)所示。
界面经机械刻槽处理后,其界面粘结力仍为化学胶结力、摩阻力和机械咬合力。其中,化学胶结力和摩阻力的形成机制未发生改变,其所能提供的界面粘结力也基本保持稳定。而刻槽的设置使材料间的“咬合”作用更加显著,其成为此类界面粘结力的主要来源。此类界面的受力模型如图18(b)所示。
4.2 参数分析
由上述界面粘结作用机制分析可知,未经机械刻槽处理的界面,其界面粘结力主要以化学胶结力为主。因此,此类界面的粘结力主要与界面粘结长度l、宽度b和材料强度有关。其中,材料强度主要与ECC 抗拉强度有关。原因在于:在既有混凝土表面浇筑ECC,其界面化学胶结力主要由ECC 水泥浆体的凝结、硬化形成,且粘结界面主要受剪切荷载作用。因此取ECC 抗拉强度fet为强度特征值,以FB=g(b, l, fet)为目标函数,对A0组数据进行拟合。得到FB的计算公式如下:
FB=0.15blfet (4) 刻槽界面的粘结力由化学胶结力、摩阻力和机械咬合力组成,故该类界面承载力Fu可在未刻槽界面的基础上,考虑键槽形成的“咬合力”,即下式所示:
Fu=FB+FG (5) 式中:FB为未刻槽界面的承载力,采用式(4)计算;FG为刻槽提供的机械咬合力,主要与刻槽特征和材料性能有关。其中,刻槽数量N与宽度wG是通过改变刻槽参与受力总宽度(wGZ=NwG)来影响界面承载力的。由试验结果分析可知,界面参与受力的总宽度存在一个限值,称为有效受力宽度wGE。钢绞线直径、纵向钢绞线配筋率及ECC强度均是通过改变HSSWM-ECC的材料性能来影响界面承载力。因此建立FG的计算模型:
FG=KGbwGEβwβhfse (6) 式中:KG为综合调整系数;b为刻槽长度,同界面粘结宽度;wGE为界面刻槽沿受力方向上的有效宽度,由于N4与N5试件界面峰值荷载接近,且如图5所示,第5道槽对刻槽咬合力的贡献较少,本文取wGE=20 mm(4道槽,单槽宽5 mm)进行计算;βw为界面刻槽参与受力宽度的影响系数;βh为界面刻槽深度的影响系数;fse为HSSWM-ECC层的抗拉强度特征值。
由试验结果分析可知,界面粘结性能与ECC抗拉强度、钢绞线直径及钢绞线配筋率均有关。而HSSWM-ECC 作为一种复合材料,可认为上述因素是其性能的综合体现。因此,为了综合考虑上述因素对界面承载力的影响,采用HSSWM-ECC层的抗拉强度特征值fse来进行表征,其计算模型如下式所示:
fse=kf[nAsbtσstβlg+fet] (7) 式中:kf 为强度调整系数;As为纵向单根钢绞线截面积;n为纵向钢绞线根数;b和t分别为加固层的宽度和厚度;σst为钢绞线极限拉伸强度;βlg为钢绞线间距的影响系数;fet为 ECC 极限拉伸强度。
由于式(7)中计算变量较多,可分步处理以确定式中参数。首先,以相同钢绞线直径、间距试件的试验数据,拟合出kf,其值为0.82;同理,确定βlg 的计算公式,如下式所示:
βlg=0.545(l2.4lg)+0.455[0,1] (8) 式中:lg为纵向钢绞线间距;l2.4为基准间距,本文为15 mm。
进而对槽宽、槽深分别影响FG进行拟合计算,得到界面刻槽受力宽度、深度影响系数的计算公式,如下式所示:
βw=wGZwGE (9) βh=√hGwG (10) 式中, hG和wG分别表示为刻槽的深度和宽度。
以N3、N4、H3、D3.2、R3、R7、T3试件为拟合组,对其数据进行线性回归分析,得到拟合参数KG=0.545。拟合结果如图19 所示,回归直线相关系数COD=0.989。
通过上述计算分析,HSSWM-ECC与混凝土刻槽处理的界面承载力预测模型如下式所示:
{Fu=FB+FGFB=0.15blfetFG=0.545bwGEβwβhfse (11) 4.3 模型验证
为了验证本文所提界面承载力预测模型的精确性,以D4.5、T2、H7 和N5组试件的试验数据为验证组,其计算结果与试验结果如表6 所示。其中,试验值与预期界面承载力值之比的平均值为0.986,变异系数为0.069,预期值与试验值吻合良好,具有较高的精度。由于制作误差造成的各试件力学性能不尽相同,且试验误差无法完全消除,部分试件预测值与实测值存在差异。
综上所述,该模型对于高强钢绞线网增强ECC与混凝土刻槽界面剥离承载力预测具有较好的适用性。
表 6 HSSWM-ECC-混凝土粘结试件界面承载力预测模型验证结果Table 6. Validation results of bearing capacity prediction model of HSSWM-ECC-concrete specimensGroup number Specimen number Test data/kN Calculated value/kN Test data/Calculated value D4.5 D4.5-1 14.49 13.95 1.039 D4.5-2 14.73 13.95 1.056 D4.5-3 15.32 13.95 1.098 T2 T2-1 12.41 13.01 0.954 T2-2 13.54 13.01 1.041 T2-3 13.16 13.01 1.012 N5 N5-1 15.69 16.48 0.952 N5-2 — — — N5-3 14.94 16.48 0.907 H7 H7-1 15.31 15.81 0.968 H7-2 14.90 15.81 0.942 H7-3 13.86 15.81 0.876 5. 结 论
(1) 高强钢绞线网增强工程用水泥基复合材料(HSSWM-ECC)与混凝土界面粘结性能梁式试验中,各试件主要发生两类破坏模式:界面剥离破坏和钢绞线断裂破坏。其中,界面剥离损伤破坏面位置受刻槽数量、刻槽深度和工程用水泥基复合材料(ECC)抗拉强度影响。
(2) 刻槽界面沿粘结范围内的应变分布整体呈现自加载端向自由削弱传递的现象,且刻槽附近出现应力集中。
(3) 在一定范围内(刻槽参与受力总宽度20 mm及槽深5 mm范围内),界面峰值荷载随刻槽数量和刻槽深度的增加而增大,破坏时的最大滑移量则与之相反。这表明键槽特征(键槽数量、槽深)对界面粘结性能影响较大。
(4) 界面峰值荷载与钢绞线配筋率和ECC抗拉强度呈线性正相关,而与其对应的滑移量呈线性负相关。这表明HSSWM-ECC层的设计能改善其与混凝土界面粘结性能。
(5) 基于刻槽界面粘结机制分析,建立了考虑界面键槽特征(刻槽数量、槽深)及HSSWM-ECC层强度特征(钢绞线配筋率、钢绞线直径、ECC抗拉强度等)的界面抗剪承载力预测模型。通过验证组对比分析,所提预测模型与试验结果吻合良好。
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图 4 高雾度透明纤维素薄膜的制备方法:(a)真空抽滤法[9,35];(b)铸涂法[39];(c)涂布法[12];(d)浸渍法[43];(e)纤维表面选择性溶解法[31,45];(f)“自上而下”法[32]
Figure 4. Preparation methods of highly hazy and transparent cellulose films: (a) Vacuum filtration[9,35]; (b) Casting[39]; (c) Coating[12]; (d) Impregnation[43]; (e) Surface selective dissolution[31,45]; (f) Top-down[32]
TEMPO—2,2,6,6-Tetramethylpiperidine-1-oxyl radical; TOCN—2,2,6,6-Tetramethylpiperidine-1-oxyl radical oxidized cellulose nanofibrils; TOWFs—TEMPO-oxidized wood fibers
图 5 一束光分别通过普通纸(a)、超清晰膜(b)、高雾度透明薄膜(c)所发生的反射、折射及散射现象[43];(d)高雾度透明纤维素薄膜的照片[31];高雾度透明纤维素薄膜(Hazy paper)与聚对苯二甲酸乙二酯(PET)、柔性玻璃(Flexible glass)的透光率(T) (e)及雾度(H)对比(f)[31]
Figure 5. Reflection, refraction, and scattering of a beam of light through common paper (a), ultra clear film(b), and highly hazy and transparent film[43]; (d) Photograph of highly hazy and transparent cellulose film[31]; Comparison of transmittance (T) (e) and haze (H) (f) of highly hazy and transparent cellulose film (Hazy paper), poly (ethylene terephthalate) (PET) and flexible glass[31]
图 6 (a)普通纸、羧甲基纤维素(CMC)膜以及高雾度透明复合膜的应力-应变曲线[43];(b)各向异性高雾度透明薄膜和各向同性纳米纸的应力-应变曲线[33];(c)普通纸、TEMPO-氧化纸、CMC膜以及高雾度透明复合膜的耐折度[43];(d)高雾度透明复合膜和TEMPO-氧化纸的耐折性能比较[43]
Figure 6. (a) Stress-strain curves of common paper, carboxymethyl cellulose (CMC) film and highly hazy and transparent composite film[43]; (b) Stress-strain curves of anisotropic highly hazy and transparent film and isotropic nanopaper[33]; (c) Folding endurance of common paper, TEMPO-oxidized paper, CMC film, and highly hazy and transparent composite film[43]; (d) Folding measurement of highly hazy and transparent composite film and TEMPO-oxidized paper[43]
图 7 高雾度透明纤维薄膜的热稳定性:由木质纤维和CMC组成的高雾度透明薄膜、纯CMC薄膜、普通纸以及由TEMPO氧化木浆组成的高雾度透明薄膜的热重分析(a)和极限氧指数分析(LOI) (b);(c)由木质纤维和CMC组成的高雾度透明薄膜(A)、纯CMC膜(B)、普通纸(C)由TEMPO氧化木浆组成的高雾度透明薄膜(D)的垂直燃烧测试[43]
Figure 7. Thermal stability of highly hazy and transparent cellulose film: Thermogravimetric (a) and Limiting oxygen index (LOI) (b) analysis of highly hazy and transparent composite film made by wood fibers and CMC, CMC film, common paper and TEMPO-oxidized paper; (c) Vertical flame testing of highly hazy and transparent composite film (A), CMC film (B), common paper (C) and TEMPO oxidized paper (D)[43]
图 8 高雾度透明纤维复合薄膜的耐水性能:(a)高雾度透明纤维素薄膜吸水率和厚度随时间的变化以及吸水率的拟合曲线;(b)高雾度透明纤维素薄膜长度和宽度随浸泡时间的变化;(c)普通纸和高雾度透明纤维素薄膜的初始水接触角(WCA);(d)普通纸和高雾度透明纤维素薄膜在水中浸泡2小时后的照片;(e)普通纸、高雾度透明纤维素薄膜以及再生纤维素薄膜(RCF)的湿强度[34]
Figure 8. Water resistance of highly hazy and transparent cellulose composite film: (a) Water absorption and thickness change of composite film as a function of immersion time and corresponding fitting curves of time-dependent water absorption based on Box Lucas1 model; (b) Changes in length and width of composite film with increasing immersion time; (c) Original water contact angles (WCA) of paper and composite film; (d) Digital images of paper and composite film after immersing into water for two hours; (e) Wet strength of paper, composite film and RCF[34]
图 9 (a)贴有高雾度透明纤维素薄膜的有机太阳能电池的结构[9];贴附高雾度透明纤维素薄膜前后有机太阳能电池(b)和砷化镓太阳能电池(c)的电流密度-电压曲线(W和W/O分别代表未贴膜有和贴薄[9],插图为贴附高雾度透明纤维素薄膜的砷化镓太阳电池);(d)贴附高雾度透明纤维素薄膜前后砷化镓太阳电池在全可见光波段、不同入射角条件下的光反射率;(e)太阳能电池上入射光分布的示意图[32];(f)贴附高雾度透明纤维素薄膜前后砷化镓太阳能电池的电流密度-电压曲线[32]
Figure 9. (a) Structure of organic solar cells coated with highly hazy and transparent cellulose films[9]; Current densities-voltage curves of organic solar cells (b) and gallium arsenide (GaAS) solar cells (c) with (W) and without (W/O) highly hazy and transparent cellulose film[9]; (d) Optical reflectance of GaAs solar cells at different incidence angles and at all visible wavelengths before and after attaching highly hazy and transparent cellulose film[32]; (e) Schematic diagram of incident light distribution on a solar cell[32]; (f) Current densities-voltage curves of GaAs solar cells before and after attaching highly hazy and transparent cellulose film[32]
表 1 高雾度透明纤维素薄膜的制备与性能
Table 1 Preparation and properties of highly hazy and transparent cellulose films
Material Film
namePretreat-
mentPreparation
methodPreparation
timeT and H Mechanical
propertyThermo-
stabilityWater
resistanceRef. Northern wood pulp and CMC All-cellulose composite films — Impregnation >5 h T: 90%;
H: 82%Tensile stress:
140 MPa;
Toughness:
8.5 MJ·m−3;
Folding times:
3342Td: 254℃; LOI: 30% CA: 43° [43] Northern wood pulp and CMC Transparent and hazy paper — Impregnation + protonation >10 h T: 91%;
H: 84%Tensile stress:
108 MPa;
Folding times: 994— CA: 72°; Saturated water Absorptivity: 60%; Change in thickness: 25% [67] Bleached softwood kraft pulp Hazy TOCN films TEMPO oxidation + homogeni-
zationCasting + coating >24 h T: 85%;
H: 62%— — — [12] Bleached softwood kraft pulp Highly translucent and light-diffusive film TEMPO oxidation + homogeni-
zationCasting >12 h T: 90%;
H: 78%— — — [25] Bleached softwood kraft pulp Ag NW paper TEMPO oxidation + homogeni-
zationVacuum filtration — T: 91%;
H: 65%— — — [29] Bleached softwood pulp Highly transparent and hazy paper — Surface selective dissolution >8 h T: 90%;
H: 91%— — — [31] Bleached softwood kraft pulp Nanostruc-
tured paperTEMPO oxidation Vacuum filtration >10 h T: 96%;
H: 60%Tensile stress:
105 MPa— — [9] Basswood Anisotropic transparent paper — “Top-down” >8 h T: 90%;
H: 90%Tensile stress:
350 MPa; Toughness:
7.38 MJ·m−3— — [32] Basswood Anisotropic wood film — “Top-down” >8 h T: 90%;
H: 80%— — — [33] Northern wood pulp Highly transparent paper TEMPO oxidation Vacuum filtration >5 h T: 90%;
H: 84%Tensile stress:
89.2 MPa; Bursting strength: 85 MPa; Young's modulus: 7.73 GPa— — [39] Northern wood pulp Highly transparent paper TEMPO oxidation Casting 41-53 h T: 88%;
H: 72%Tensile stress:
85.3 MPa; Bursting strength: 124 MPa; Young's modulus: 12.89 GPa— — [39] Bleached softwood kraft pulp Highly hazy and transparent cellulose film Carboxy-
methylationVacuum filtration >2.5 h T: 89%;
H: 85%Tensile stress:
138 MPaTd: 179℃ — [40] Pine dissolving pulp Highly hazy transparent cellulose film TEMPO oxidation + ultrasoni-
cationCasting — T: 90%;
H: 76%Tensile stress:
22 MPaTd:
295-305℃;
CTE: 8.5-10.6 ppm/K— [78] Cellulose pulp Hazy transparent cellulose nanopaper Alkali treatment + homogeni-
zationVacuum filtration >10 h T: 90%;
H: 90%— — — [38] Bleached softwood pulp and CNF Bilayer hybrid paper TEMPO oxidation + homogeni-
zationVacuum filtration — T: 92%;
H: 70%— — — [23] Bleached softwood kraft pulp Nanostruc-
tured paperMicrofibrilla-tion Vacuum filtration >2.3 h T: 83%;
H: 89%Tensile stress:
18.5 MPa;
Young's modulus:
3.04 GPa— — [79] Bleached softwood kraft pulp Transparent and hazy all-cellulose composite films Impregnation >12 h T: 90%;
H: 95%Tensile stress: 37.03 MPa; Young's modulus: 1.90 GPa; Toughness:
2.78 MJ·m−3— CA: 76°; Saturated water Absorptivity: 59%; Change in thickness: 30% [34] Notes: CNF—Cellulose nanofibrils; Ag NW—Silver nanowire; Td—Thermal decomposition temperature; CTE—Coefficient of thermal expansion; CA—Contact angle. -
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