高雾度透明纤维素薄膜制备、性能及其太阳能电池应用

侯高远; 李冠辉; 胡招湘; 李玉洁; 张德健; 崔锦怡; 方志强

doi:10.13801/j.cnki.fhclxb.20210609.002

高雾度透明纤维素薄膜制备、性能及其太阳能电池应用

华南理工大学　制浆造纸工程国家重点实验室，广州 510640

基金项目: 国家重点研发计划(2018YFD0400701)；国家自然科学基金(21978103；31700508)；广东省自然科学基金(2020B1515020021)；广州市珠江科技新星专项(201806010141)；中央高校基本科研业务费(2019MS083)

详细信息

作者简介:
方志强，工学博士，副研究员。从事透明纤维素薄膜的设计、制备、性能调控及其在柔性电子、能源、环境领域的应用研究。主持国家自然科学基金、广东省杰出青年基金、广州市珠江科技新星专项等多个纵向项目课题。已在EES，Matter，ACS Nano, Nano Letters等国内外期刊上发表SCI文章80余篇，被引5 400余次（Google citation），授权15项中国发明专利，2项美国发明专利，参编英文专著2部，获广东省技术发明奖一等奖（12/15），在国内外会议上做口头报告20余次

通讯作者:
方志强，博士，副研究员，硕士生导师，研究方向为纤维素基材料及其高值化利用　E-mail: mszhqfang@scut.edu.cn

中图分类号: TB383.2; TM914.4
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出版历程
- 收稿日期: 2021-04-07
- 修回日期: 2021-05-19
- 录用日期: 2021-06-02
- 网络出版日期: 2021-06-08
- 刊出日期: 2022-03-22

Preparation, properties and application of highly hazy and transparent cellulose films for solar cells

State Key Lab of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China

摘要

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。
- 高雾度 /
- 高透光率 /
- 纤维素薄膜 /
- 光散射 /
- 太阳能电池
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.
- high transmission haze /
- high transparency /
- cellulose film /
- light scattering /
- solar cells

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复合材料由于具有高比强度、高比刚度、抗疲劳、抗腐蚀及可设计性等优点，被广泛地运用在航空航天、船舶、汽车、能源等领域^[1-5]。随着使用范围的逐渐扩大和应用场景的不断丰富，明确不同环境下复合材料的力学性能至关重要^[6]。水分是复合材料面临的一个主要环境因素，当复合材料长期暴露在水分中时，由于吸湿作用，复合材料基体、纤维和基体界面及复合材料层间会出现不同程度的损伤，导致复合材料的力学性能退化^[7]。实际服役过程中，复合材料受到水分影响的同时还面临不同载荷工况的作用，外载荷可能会在复合材料中诱发微裂纹和界面脱粘，从而影响复合材料的吸湿行为，进而导致复合材料的力学行为和失效机制更加复杂^[8]。因此，研究长期水分-载荷作用下复合材料的力学行为对复合材料的安全评估具有重要意义。

复合材料的基体为高分子材料，对水分较敏感，研究者最先关注的是复合材料的吸湿性能。Pradip等^[9]研究了不同纤维类型及混杂纤维复合材料的吸湿性能，同时应用ANSYS仿真模拟了水分扩散过程。Rocha等^[10]选取纤维随机分布材料的代表性体积单元研究了界面对水分扩散过程的影响。国内学者同样通过试验研究了不同型号复合材料在各种湿热环境下的吸湿性能，并应用有限元分析、理论计算等方法对复合材料吸湿性能进行了模拟预测^[11-12]。吴以婷等^[13]研究了碳纤维增强环氧树脂基复合材料层合板的吸湿特性，发现材料在脱湿和吸湿过程中均呈现出两段式的特点。此外，材料吸湿后的力学性能退化也备受关注。Shetty等^[14]围绕吸湿后复合材料性能退化的机制进行研究，发现经过湿热老化后碳纤维/环氧树脂复合材料层合板的层间剪切强度降低近1/3。张裕恒等^[15]测试了碳纤维增强乙烯基树脂复合材料在水箱中老化90天后的压缩强度和层间剪切强度，二者分别下降了7.6%和12.3%。对于酚醛树脂复合材料和环氧树脂复合材料，吸湿后性能同样会发生退化^[16-17]。

上述的研究仅考虑了水分单独作用的情况，未考虑吸湿过程中载荷的影响。研究表明拉伸预应力对单向带复合材料的吸湿特性影响明显，能加速水分的扩散。Humeau等^[18]在吸湿过程中给±45°铺层的层合板施加了静态拉伸载荷，发现拉伸载荷对试样的吸湿特性影响明显。李彪等^[19]还研究了海水浸泡与拉伸应力耦合下碳纤维增强复合材料单向层合板的剩余强度，指出预应力的存在会造成复合材料的抗拉强度和弹性模量的进一步退化，且应力水平越高，这种退化现象越明显。当复合材料层合板受到弯曲载荷-水分作用时，其吸湿特性和剩余强度也会受到影响，当弯曲应力为60%弯曲强度时，吸湿量上升最快，材料强度下降最严重，达 21.15%^[20]。

相较于单向复合材料，编织复合材料中编织物增强体的纤维相互缠结，显示出较强的整体性，使其具有更好的抗冲击性和损伤容限等优点，常作为主传力或次传力结构参与结构的载荷传递过程^[21]。在承受载荷的同时，编织复合材料同样受到水分等环境因素的影响。而关于水分-载荷耦合作用下编织复合材料吸湿行为和性能退化机制的研究较匮乏。因此，本文针对碳纤维增强环氧树脂(T300/H69)二维(2D)编织复合材料在长期水分-载荷作用后的力学行为展开研究，设计了水分-拉伸载荷耦合作用下的加载装置，开展了不同拉伸预应力下的吸湿试验和强度性能退化试验，研究拉伸预应力对其吸湿特性的影响，分析不同水分-载荷作用下T300/H69编织复合材料弹性模量及失效强度的退化规律，并结合宏微观形貌分析，揭示长期水分-载荷耦合作用下T300/H69编织复合材料力学性能的退化机制，最后建立水分-载荷作用下复合材料的剩余强度预测方法。

1. 试验方法

1.1 原材料与制备

选取平纹编织的碳纤维增强环氧树脂基复合材料(T300/H69)进行研究。其中每10 mm×10 mm的面积内，经纱和纬纱各铺设5条，每条纱束中包含碳纤维单丝约3000根。通过真空袋成型工艺制备T300/H69编织复合材料，固化压力为0.16 MPa，材料的铺层方式为[(±45°)₁₀]，单层厚度为0.18 mm。T300平纹编织复合材料的基本力学性能参数如表1所示。

表 1 碳纤维增强环氧树脂(T300/H69)复合材料的基本力学性能参数

Table 1. Basic mechanical properties of carbon fiber reinforced epoxy resin (T300/H69) composites

Engineering constant	Value	Engineering constant	Value
E₁/GPa	55.90	X_T/MPa	602
E₂/GPa	54.60	X_C/MPa	431
E₃/GPa	7.220	Y_T/MPa	597
G₁₂/GPa	3.812	Y_C/MPa	422
G₁₃/GPa	3.122	Z_T/MPa	60
G₂₃/GPa	3.122	Z_C/MPa	152
$\nu _{12}$	0.068	S₁₂/MPa	83
$\nu _{13}$	0.310	S₁₃/MPa	49
$\nu_{23}$	0.310	S₂₃/MPa	49
Notes: E₁, E₂ and E₃—Elastic modulus in different directions; G₁₂, G₁₃ and G₂₃—Shear modulus in different directions; $\nu_{12}$ , $\nu_{13}$ and $\nu_{23}$ —Poisson's ratio of the material; X_T, X_C, Y_T, Y_C, Z_T, Z_C—Axial tension, axial compression, transverse tension, transverse compression, normal tension, normal tension; S₁₂, S₁₃, S₂₃—In-plane shear strength.

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固化完成后，参考GB/T 1447—2005^[22]切割成试样，切割完成的T300/H69编织复合材料尺寸如图1(b)所示。

图 1 拉伸预应力施加系统

Figure 1. Tension prestressing application system

R—Radius

下载: 全尺寸图片幻灯片

1.2 水分-载荷环境试验

为了实现水分-载荷耦合环境的作用，设计了如图1所示的夹具及载荷施加装置。整套装置主要包含穿心千斤顶(配有液压泵及泵表)、应力支撑型材架、传力杆、夹板。其中穿心千斤顶用于施加拉伸载荷，通过观察液压泵表的数值控制手动泵的行程，可以实现不同大小拉伸载荷的施加，利用传力杆将载荷传递到试样上。试样两端通过不锈钢夹板进行夹持，并且通过多个夹板实现试样串联，通过螺丝固定在应力支撑型材架上。参考标准HB 7401—2020^[23]，通过浸泡法进行吸湿试验，吸湿量通过下式进行计算：

$M_t=\left|\frac{W_t-W_0}{W_0}\right|\times100\%$

(1)

式中：W₀为干燥试样的质量；W_t为t时间材料试样的质量；M_t为t时刻试样的吸湿量。

将夹持好的试样放置于亚克力水槽中进行浸泡，水温为室温。定期将试样取出，用吸水纸擦干水分后记录试样质量，称重完放回水槽继续浸泡，每组试样包含4件重复试验件。通过静力试验获得材料的屈服强度σ_s和弹性模量E。以屈服强度为参考，分别设置低应力状态80%σ_s、标准应力状态100%σ_s、高应力状态120%σ_s、140%σ_s这4种预应力水平来分析T300/H69编织复合材料力学性能的退化行为。

1.3 拉伸失效试验

对干燥试样和环境处理后的试样进行拉伸失效测试，以研究水分单独和水分-载荷耦合作用下复合材料的强度变化。考虑到不同时间下材料的吸湿量不同，对材料性能的影响也不同。因此选取144 h的时间间隔，分别以0 h、144 h、288 h、432 h为试验工况进行吸湿后的拉伸性能测试，当浸泡时间达到144 h、288 h、432 h后将试样取出称重，之后立即开展静力试验。同时以屈服应力为界，设置仅有预应力作用的对照组。试验矩阵如表2所示。

表 2 试验矩阵

Table 2. Test matrix

Pretreated	Specimen group	Test time/h
Untreated	Dry	0
Moisture	Wet	144, 288, 432
Load	D-80%σ_s	288
	D-110%σ_s	288
	D-120%σ_s	288
Moisture-load	W-80%σ_s	144, 288, 432
	W-100%σ_s	144, 288, 432
	W-120%σ_s	144, 288, 432
	W-140%σ_s	144, 288, 432
Notes: D and W—Dry state and moisture absorption state of the material; σ_s—Yield stress.

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采用深圳万测试验设备有限公司制造的TSE 504C电子万能试验机进行拉伸试验，试验中通过引伸计测试试件的变形，按1 mm/min的加载速度进行试验，每类工况重复测试4件试样。试验均在室温下进行。

1.4 微观形貌表征

从受水分-载荷耦合作用后的复材试样的破坏区域切割出长宽各10 mm的小方块型试样，通过ZEISS公司制造的SUPRA 55场发射扫描电子显微镜 (SEM) 观察试样拉伸断口处纤维和基体的损伤形貌。观察前对试样进行喷金处理。

2. 结果与讨论

2.1 T300/H69编织复合材料的吸湿特性分析

通过浸泡吸湿试验获得了T300/H69编织复合材料的吸湿量与吸湿时间的关系，如图2中的散点所示。

图 2 不同预应力下T300/H69编织复合材料的吸湿量-时间变化曲线

Figure 2. Moisture absorption-time curves of T300/H69 braided composites under different prestresses

下载: 全尺寸图片幻灯片

可以看出在不同的拉伸预应力下，T300/H69编织复合材料的吸湿规律基本一致。在吸湿前期，吸湿量随着吸湿时间迅速增大，随着吸湿时间不断增加，吸湿量的增加速率有所减缓。此外，拉伸预应力对吸湿量有明显影响。比如吸湿432 h后，5种拉伸预应力下材料的吸湿量分别为0.404%、0.543%、0.656%、0.881%、1.240%，说明在相同的浸泡时间下，预应力越大，编织复合材料的吸湿量越大，相较于无应力状态的吸湿量，140%σ_s状态下的吸湿量增加了207%。

进一步分析拉伸预应力对材料吸湿过程的影响，选取144 h、288 h、432 h这3个典型时刻的吸湿量，得到吸湿量随拉伸预应力的变化曲线如图3所示。可知，在水分-载荷耦合环境下，T300/H69编织复合材料的吸湿量与拉伸预应力总体上呈现正相关。而且以屈服应力为界限，当拉伸预应力小于材料的屈服应力时，吸湿量增加相对平缓，越靠近屈服应力，吸湿量的增加幅度越明显，一旦拉伸预应力超过材料的屈服应力，T300/H69编织复合材料的吸湿量显著提高。

图 3 不同预应力下T300/H69编织复合材料的吸湿量

Figure 3. Moisture absorption of T300/H69 braided composites under different prestresses

下载: 全尺寸图片幻灯片

Fick模型^[24-25]常被用于表征水分在材料中的扩散过程，如下式所示：

$\frac{M_t}{M_{\infty}}=\left\{1-\frac{8}{\text{π}^2}\sum\limits_{j=0}^{\infty}\frac{\exp\left[-\left(2j+1\right)^2\text{π}^2\left(Dt/h^2\right)\right]}{\left(2j+1\right)^2}\right\}$

(2)

在实际工作中，常常对Fick模型进行数学简化，简化结果如下式所示：

$\frac{{{M_t}}}{{{M_\infty }}} = 1 - \exp \left[ { - 7.3{{\left( {\frac{{Dt}}{{{h^2}}}} \right)}^{0.75}}} \right]$

(3)

式中：h为材料的厚度；t为试样的吸水时间；D为试样的水分扩散系数；M_∞为平衡吸湿量。

针对图2中的试验点，将水分扩散系数D和平衡吸湿量M_∞看作未知参数，采用式(3)对其进行拟合，得到的水分扩散系数和平衡吸湿量汇总于表3。此外Fick模型的拟合曲线如图2中实线所示，可以看出Fick模型可以很好地描述水分-载荷作用下编织复合材料的吸湿行为。

表 3 Fick吸湿模型的相关参数

Table 3. Relevant parameters of Fick moisture absorption

Tensile prestress	Diffusion coefficient D/(mm²·h⁻¹)	Balanced moisture content M_∞/%
0	0.00175	0.464
80%σ_s	0.00192	0.612
100%σ_s	0.00213	0.706
120%σ_s	0.00229	0.917
140%σ_s	0.00257	1.297

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由表3可知，对于T300/H69编织复合材料而言，在432 h试验测量的吸湿量均小于Fick模型预测的结果，表明试样未到达吸湿平衡。从表中可以看出吸湿后的T300/H69编织复合材料的吸湿扩散系数和平衡吸湿量均与拉伸预应力正相关，随拉伸预应力的增大逐渐增大，表现出明显的非线性关系。这是由于拉伸预应力会使材料产生一定程度的微裂纹等缺陷，导致水分进入材料的速率变快，T300/H69编织复合材料对水分的吸收和容纳能力变强。

2.2 T300/H69编织复合材料的剩余力学性能

2.2.1 载荷-位移分析

对于施加相同预应力的试样，其在不同时间环境处理后的载荷-位移曲线变化规律一致，因此选取作用288 h的试样进行研究。分别给出干态、仅预应力、仅吸湿、水分-载荷耦合作用下的载荷-位移曲线进行分析，如图4所示。

对于本文研究的 [(±45°)₁₀]铺层的T300/H69编织复合材料，从图4(a)和图4(e)可看出，干态和水分单独作用后其载荷-位移曲线具有相同的变化趋势，均表现明显的双线性特征；从图4(b)~图4(d)可以看出，仅预应力也会对材料的拉伸性能产生劣化影响，当预应力超过屈服应力时，双线性特征不再明显。对于水分-载荷耦合作用的试样，从图4(f)~图4(i)可以看出，引入水分作用后，材料性能劣化程度会进一步加剧，当应力水平小于100%σ_s时，其载荷-位移曲线也具有明显的双线性特征，但当拉伸预应力超过100%σ_s时，这种双线性特征受到的影响更加显著，在120%σ_s时两阶段曲线的界限已不再分明，当预应力为140%σ_s时，载荷-位移曲线已无双线性变化规律。对于144 h和432 h的工况均存在同样的现象。

图 4 不同工况下作用288 h后T300/H69编织复合材料的载荷-位移曲线

Figure 4. Load-displacement curves of T300/H69 braided composites after 288 h under different working conditions

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上述结果表明，相对于水分单独作用和预应力单独作用，水分-载荷耦合对T300/H69编织复合材料拉伸变形造成的影响更明显。

2.2.2 模量及强度分析

进一步分析水分-载荷耦合作用对T300/H69编织复合材料失效强度(σ_f)和弹性模量(E)的影响，结果如表4和图5所示。

表 4 T300/H69编织复合材料的弹性模量及强度

Table 4. Elastic modulus and strength of T300/H69 braided composites

Category		144 h		288 h		432 h
Category		E/GPa	σ_f/GPa	E/GPa	σ_f/GPa	E/GPa	σ_f/GPa
Moisture	Average	13.721	0.195	13.627	0.193	13.586	0.192
Moisture	CV	2.13%	2.31%	4.61%	2.01%	1.09%	1.82%
D-80%σ_s	Average	—	—	13.204	0.191	—	—
D-80%σ_s	CV	—	—	2.61%	2.30%	—	—
D-100%σ_s	Average	—	—	12.715	0.187	—	—
D-100%σ_s	CV	—	—	1.89%	1.6%	—	—
D-120%σ_s	Average	—	—	10.643	0.180	—	—
D-120%σ_s	CV	—	—	3.64%	1.78%	—	—
W-80%σ_s	Average	12.873	0.190	12.682	0.184	12.535	0.181
W-80%σ_s	CV	2.05%	0.74%	3.01%	1.36%	2.43%	1.88%
W-100%σ_s	Average	11.947	0.182	11.516	0.174	11.419	0.169
W-100%σ_s	CV	4.19%	2.58%	3.32%	2.82%	5.08%	3.02%
W-120%σ_s	Average	10.404	0.162	9.506	0.151	8.975	0.143
W-120%σ_s	CV	3.27%	3.33%	4.12%	3.88%	5.39%	2.66%
W-140%σ_s	Average	9.232	0.151	7.521	0.138	6.136	0.128
W-140%σ_s	CV	4.78%	2.72%	5.66%	3.38%	7.12%	4.11%
Notes: E—Elastic modulus of the material; σ_f—Failure strength of the material; CV—Coefficient of variation.

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图 5 不同环境中T300/H69编织复合材料的弹性模量(a)及失效强度(b)

Figure 5. Elastic modulus (a) and failure strength (b) of T300/H69 braided composites in different environments

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从表4和图5中可以看出，对于所有的工况，弹性模量及失效强度的变异系数(CV)均不超过10%，表明试验数据具有较好的重复性，且失效强度和弹性模量随着吸湿时间的增加均逐渐下降。水分单独作用下，材料的弹性模量及强度下降幅度较低，此时两者最大分别仅下降了2.34%和3.03%。预应力单独作用时，模量和强度均会随应力水平的增加而降低。水分-载荷耦合作用下，材料的弹性模量及强度的下降幅度更大，例如在288 h，D-120%σ_s工况下的弹性模量和失效强度分别下降了23%和9.1%，W-120%σ_s工况下弹性模量和失效强度分别下降了31.7%和 23.7%。这说明水分-载荷耦合效应会导致材料内部损伤加剧。

进一步分析失效强度与预应力的变化关系，如图6所示。可知，不论是干态还是水分-应力耦合作用，只要预应力超过屈服应力，材料的强度均会有明显下降。下降幅度与拉伸预应力相关，应力水平越高，弹性模量及强度越低，两者最大下降达55.9%和35.4%。同时，以屈服应力为界限，当预应力小于材料的屈服应力时，材料强度的下降幅度较平缓，一旦预应力超过屈服应力，材料强度的下降趋势加剧。这表明超过屈服应力的预加载应力会使材料的潜在损伤更加剧烈。

图 6 T300/H69编织复合材料的预应力-失效强度曲线

Figure 6. Prestress-failure strength curves of T300/H69 braided composites

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在长期的水分-载荷环境作用中，吸湿量是能够直接检测的参数，也是反映材料受到环境影响的重要指标。因此，图7进一步分析不同工况下T300/H69编织复合材料弹性模量及失效强度随吸湿量的演变规律。

可以看出，在任意应力水平下，T300/H69编织复合材料的弹性模量和强度均随着吸湿量的增大而减小，而且表现出明显线性关系。随着拉伸预应力的增大，拟合曲线的斜率增加，表明材料的弹性模量和强度劣化更严峻。此外，对比图7(a)和图7(b)还可以发现，T300/H69编织复合材料弹性模量的下降程度明显大于失效强度，该规律不受预应力水平的影响。

2.3 T300/H69编织复合材料的失效机制

为了揭示水分-载荷作用后编织复合材料力学性能的退化机制，分别对破坏后的宏观失效模式和微观形貌进行分析。

2.3.1 宏观失效模式分析

不同工况下编织复合材料的宏观失效模式基本一致，选取432 h的失效照片和干燥情况对比，如图8所示，可以看出，断裂位置全部位于试验件的中央标距段内，断口均呈现出V字型，断口较齐整，左右两侧断口区域能完全重叠，并且绕断裂中心处旋转对称，断裂面沿着纱束的走向进行扩展，并可见一定数量的断裂纤维。由断口形状可知，水分-载荷耦合作用对T300/H69编织复合材料的宏观破坏模式影响较小。

图 7 T300/H69编织复合材料的弹性模量(a)及失效强度(b)随吸湿量的变化

Figure 7. Elastic modulus (a) and failure strength (b) of T300/H69 braided composites change with moisture absorption

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图 8 T300/H69编织复合材料宏观断口形貌

Figure 8. Macroscopic fracture surface of T300/H69 braided composites

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2.3.2 微观形貌分析

图9为干燥和吸湿432 h试样的SEM结果对比。可以看出：(1)干燥状态下试样断口的树脂较光滑，树脂内裂纹较少，如图9(a)所示；(2)当水分单独作用时，图9(b)显示树脂基体表面较粗糙，出现大量的“褶皱”条纹，表明在长期的水分作用下树脂基体发生了塑化，此外，还能观察到少量的基体微裂纹；(3)对于水分-载荷耦合作用的试样也出现了吸湿塑化，从图9(c)~9(f)中同样能观察到粗糙的基体断面。与此同时，受预应力作用的基体微裂纹数量明显增多。此外，当拉伸预应力为100%σ_s时，观察到纤维-基体界面脱粘，当拉伸预应力为120%σ_s和140%σ_s，纤维-基体界面脱粘的区域显著增加。故而基体微裂纹和纤维-基体界面脱粘程度的加剧是水分-载荷作用后编织复合材料剩余强度进一步劣化的主要诱因。

图 9 T300/H69编织复合材料试样拉伸断口的SEM图像

Figure 9. SEM images of tensile fracture of T300/H69 braided composites specimen

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2.4 T300/H69编织复合材料的剩余强度预测

虽然通过试验能获得T300/H69编织复合材料在水分-载荷耦合作用下力学性能的变化规律，但试验成本较高，试验周期较长。因此，建立水分-载荷耦合作用下复合材料剩余强度的有效预测方法，对于复合材料的工程应用具有重要意义。

Shiva等^[26]认为复合材料的剩余强度与水分和外部载荷均呈现指数型下降规律，如下式所示：

$\frac{{{\sigma _t}}}{{{\sigma _{{\mathrm{dry}}}}}} = \exp \left[ { - {{\left( {\frac{D}{\phi }} \right)}^2}At} \right]\exp \left[ { - {{\left( {\frac{D}{\phi }} \right)}^2}B{\sigma _{{\mathrm{prestress}}}}t} \right]$

(4)

式中：σ_t为t小时后复合材料的抗拉强度；σ_dry为干燥条件下的抗拉强度；σ_prestress为拉伸预应力；t为水分-载荷耦合作用时间；A为水分老化常数，由吸湿老化后的试验结果获得；B为应力系数，可从材料在水分-载荷耦合作用后的强度退化结果获得； $\phi$ 为几何参数，取材料表面积之和与上下表面面积之和的比值。

Shiva模型中假设吸湿老化时间较长时，复合材料的剩余强度为0，这一假设与实际工程情况存在差异。在无穷时间吸湿后，材料往往还具有一定的剩余力学性能。因此，考虑到无穷老化时间后复合材料仍具有残余性能这一特性，本文对Shiva剩余强度模型做了修正，引入水分-载荷耦合作用无穷时间后的剩余强度σ_∞，得到水分-载荷耦合作用下T300/H69编织复合材料剩余强度预测模型，如下式所示：

${ {\sigma _t} = \left( {{\sigma _{{\mathrm{dry}}}} - {\sigma _\infty }} \right)\exp \left( { - {{\left( {\frac{D}{\phi }} \right)}^2}At} \right)\exp \left( { - {{\left( {\frac{D}{\phi }} \right)}^2}B{\sigma _{{\mathrm{prestress}}}}t} \right) + {\sigma _\infty } }$

(5)

从试验测量的吸湿量可以发现，试样并未达到吸湿平衡，但从图7可知材料的强度和吸湿量存在线性关系，因此结合图7中的线性关系和表3中的平衡吸湿量(1.297%)对σ_∞进行预测，得到σ_∞为119 MPa，基于该值对强度进行预测，结果如图10所示。

图 10 T300/H69编织复合材料不同模型下的拟合结果

Figure 10. Fitting results under different models of T300/H69 braided composites

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可以看出，对于受水分-载荷耦合作用的T300/H69编织复合材料而言，当拉伸预应力不超过屈服应力时，Shiva剩余强度模型的预测效果较好，而一旦拉伸预应力超过屈服应力，其预测精度明显较低，尤其是在高应力水平时，误差较大，对于W-140%σ_s 工况下的试样，在432 h的预测误差达7.5%。较Shiva剩余强度模型而言，本文建立的模型能够很好地预测不同应力水平下复合材料的剩余强度，最大相对误差仅约3.2%，预测精度高于Shiva模型。

为了进一步验证本文提出的理论模型，对文献[20]中30%弯曲抗拉强度、60%弯曲抗拉强度下受弯吸湿层合板的剩余弯曲强度数据进行了拟合，拟合结果如图11所示。

图 11 文献[20]数据拟合

Figure 11. Literature [20] data fitting

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可知，两种弯曲载荷下，预测值与试验数据的最大相对误差仅为1.85%。进一步验证了本文建立的剩余强度模型的有效性，可指导受水分-载荷作用后复合材料的安全评估。

3. 结论

本文通过试验和理论研究了二维(2D)编织复合材料在水分-载荷长期耦合作用下的力学性能退化行为。主要结论如下:

(1)碳纤维增强环氧树脂(T300/H69)编织复合材料的吸湿量随拉伸预应力的增加而增大，以屈服应力为界限，当拉伸预应力小于材料的屈服应力时，吸湿量增加相对平缓，一旦拉伸预应力超过材料的屈服应力，材料的吸湿量显著提高。Fick吸湿模型能够较好地描述材料在水分-载荷下的吸湿特性；

(2)长期水分-载荷环境作用对T300/H69编织复合材料力学性能影响显著，随着应力水平的增加，其弹性模量和强度的退化越明显，且弹性模量的下降程度大于失效强度，在140%σ_s (σ_s为屈服强度)预应力下作用432 h后下降分别达55.9%和35.4%。此外，T300/H69编织复合材料的弹性模量和强度退化比例与吸湿量存在线性关系；

(3)微观形貌分析表明吸湿会引起基体塑化，同时预应力导致基体微裂纹和纤维-基体界面脱粘程度加剧，揭示了水分-载荷作用后编织复合材料力学性能的下降机制；

(4)对Shiva剩余强度理论进行修正，建立了复合材料在水分-载荷耦合作用下的剩余强度预测模型，实现了良好预测。

图 1 光照射纤维素薄膜时的光线路径图

Figure 1. Diagram of light path when a light passes through the cellulose film

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图 2 利用紫外/可见/近红外分光光度计检测雾度的装置示意图^[29]

Figure 2. Experimental haze measurement setup with the UV/vis/NIR spectrophotometer^[29]

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图 3 高雾度透明纤维素薄膜的发展历程^{[9,10,28,30-32,34]}

Figure 3. Development process of highly hazy and transparent cellulose films^{[9,10,28,30-32,34]}

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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

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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]

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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]

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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]

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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]

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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]

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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 name	Pretreat- ment	Preparation method	Preparation time	T and H	Mechanical property	Thermo- stability	Water resistance	Ref.
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: 3342	T_d: 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- zation	Casting + coating	>24 h	T: 85%; H: 62%	—	—	—	[12]
Bleached softwood kraft pulp	Highly translucent and light-diffusive film	TEMPO oxidation + homogeni- zation	Casting	>12 h	T: 90%; H: 78%	—	—	—	[25]
Bleached softwood kraft pulp	Ag NW paper	TEMPO oxidation + homogeni- zation	Vacuum 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 paper	TEMPO 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- methylation	Vacuum filtration	>2.5 h	T: 89%; H: 85%	Tensile stress: 138 MPa	T_d: 179℃	—	[40]
Pine dissolving pulp	Highly hazy transparent cellulose film	TEMPO oxidation + ultrasoni- cation	Casting	—	T: 90%; H: 76%	Tensile stress: 22 MPa	T_d: 295-305℃; CTE: 8.5-10.6 ppm/K	—	[78]
Cellulose pulp	Hazy transparent cellulose nanopaper	Alkali treatment + homogeni- zation	Vacuum filtration	>10 h	T: 90%; H: 90%	—	—	—	[38]
Bleached softwood pulp and CNF	Bilayer hybrid paper	TEMPO oxidation + homogeni- zation	Vacuum filtration	—	T: 92%; H: 70%	—	—	—	[23]
Bleached softwood kraft pulp	Nanostruc- tured paper	Microfibrilla-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; T_d—Thermal decomposition temperature; CTE—Coefficient of thermal expansion; CA—Contact angle.

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