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在水热反应釜中加入乙二醇和蒸馏水的混合溶液,将适量铁、钴、镍、铜金属盐加入其中,于60 ℃水浴加热溶解,再依次加入淀粉、葡萄糖、碱和尿素,搅拌均匀,最后加入PVP并密封,放入180 ℃的烘箱中保温一段时间,取出反应釜,自然冷却后打开反应釜进行样品处理,用蒸馏水洗涤5~6次,离心过滤,将产物在40 ℃真空烘箱中进行干燥,将干燥后的产物放在管式炉中,在氮气保护条件下950 ℃焙烧2 h,得到最终样品。
因为碳多孔隙,假密度低,留空时间长,碳的密度为1.8 g/cm3,故复合材料的密度在2.5~4 g/cm3,比单独的铁氧体密度要轻[15]。
通过改变反应时间、淀粉和葡萄糖的配比等因素,试验得样品见表1。
表 1 样品试验条件
Table 1. Sample test conditions
Sample number Time/h Starch: Glucose/g The first group 1 16 8:8 2 16 6:10 The second group 3 18 6:6 4 18 9:3 5 18 6:10 The third group 6 20 8:8 7 20 9:3 The fourth group 8 22 8:8 9 22 6:6 -
红外干扰材料在空中分散形成“烟幕”,它对红外辐射的衰减机理与大气层中烟幕一样,主要是由烟幕中的颗粒对红外辐射的散射与吸收来实现。
颗粒对电磁波的散射理论主要有瑞利散射和Mie散射两种。图3为颗粒尺寸与散射强度关系示意图。
从图中可以看出,在起始阶段(X轴数值低于0.5),小粒子的散射远小于大粒子的(X轴数值大于1)散射;粒子尺度与波长相近时发生Mie散射,粒子同时具有吸收和散射作用会获得较大的消光。消光截面为:
$$ Q={Q}_{a}+{Q}_{s} $$ (1) 式中:
$ {Q}_{a} $ 、$ {Q}_{s} $ 分别为吸收截面和散射截面。通常烟幕对红外电磁波的散射主要以Mie散射为主。质量消光系数是单位质量烟幕的遮蔽面积,表征烟幕对电磁辐射衰减的能力大小,它的数值越大,说明衰减效果越好[16]。因此,质量消光系数α(也称为衰减系数β)是表征烟幕材料干扰效果好坏的重要参数,其计算公式为:
$$ \alpha ={Q}_{\rm e}G/\left(\rho V\right) $$ (2) 式中:
$ {Q}_{\rm e} $ 为消光效率;$ G $ 为粒子的几何截面积;$ \rho $ 为粒子的密度;$ V $ 为粒子的体积。提高消光效率或增大几何截面积与质量的比值都可使消光系数提高,这意味着在几何截面积相同时质轻的材料具有更大的消光系数。
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目前主要用傅里叶变换红外光谱仪、红外辐射计和红外热像仪3种技术测试烟幕材料对红外光的衰减性能[17]。
文中采用傅里叶变换红外光谱仪测试技术,将烟幕材料与KBr按一定浓度混合后压片,测试其红外透过率,根据朗伯-比尔计算公式定量计算质量消光系数[18-19]。计算式为:
$$T = \frac{{{I_t}\left( \lambda \right)}}{{{I_0}\left( \lambda \right)}} = {\rm{exp}}\left( { - \alpha CL} \right)$$ (3) $$\alpha = \frac{1}{{CL}}{\rm{ln}}\frac{{{I_0}\left( \lambda \right)}}{{{I_t}\left( \lambda \right)}} = \frac{1}{{CL}}{\rm{ln}}\frac{1}{T}$$ (4) 式中:
$ {I}_{0}\left(\lambda \right) $ 、$ {I}_{t}\left(\lambda \right) $ 分别为入射光通过烟幕前后的光强;$ \alpha $ 为质量消光系数;$ C $ 为KBr压片中烟幕材料的浓度;$ L $ 为光程,即KBr压片的厚度。 -
通过对样品进行红外分析,得到样品对不同波段波的吸收透过率,进一步通过计算分析得到样品对不同波段波的单位质量吸收情况,该数值为样品的质量消光系数,如图4所示,图4(a)为消光性能较好的7个样品,图4(b)为消光系数较低的2个样品。
由图4可见,在4~10 μm波段内,最好的是7号样品,消光系数最高达到0.37 m2/g,比传统的活性炭和炭黑的红外消光系数要高[10]。其次,样品5消光系数也大于0.3 m2/g,较好的样品还有8、6、4和2号,消光系数处于0.20~0.25 m2/g之间。样品1和9的消光系数低于0.20 m2/g。比较样品的制备条件及SEM分析,可以发现,反应时间为18 h和20 h,淀粉与葡萄糖配比为6:10和9:3的条件下制备的样品,焙烧后样品球形形貌保持较好,样品的消光系数也比较高,其他条件下碳球形貌相对较差,说明样品碳球的生长状态和形貌对样品的红外性能影响较大,同时碳球的形貌受淀粉和葡萄糖的配比影响。分析消光性能的影响因素:(1)反应时间。反应时间长,因为奥氏熟化,容易产生空心或空穴,消光面积增大,但反应时间过长,反而结块,焙烧后导致颗粒增大,降低消光性能。(2)淀粉和葡萄糖的配比。淀粉多容易出现孔洞,但也容易坍塌造成小颗粒,颗粒太小红外消光性能不好。根据XRD分析结果,4~9号样品产物成分一样,9号样品碳源较少,且颗粒细小,主要为瑞利散射和吸收,同时样品中有很多非球形粒子,存在各向异性等,导致消光较小。因此,采用适宜的反应时间和淀粉/葡萄糖配比才能得到空心且形貌好的球形颗粒,提高消光性能。
Preparation of carbon coated ferromagnetic composite materials by one-pot and IR extinction performance(Invited)
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摘要: 通过一锅水热法制备了炭包铁氧体前驱体,并在950 ℃氮气保护条件下焙烧得到了炭包铁磁体复合材料。通过XRD、FT-IR、SEM等方法,分析了复合材料的形貌、成分,研究了反应时间、淀粉和葡萄糖的配比等因素对复合材料形貌及红外消光性能的影响。采用傅里叶红外光谱仪的KBr压片法测试并计算了各材料在2.5~25 μm区间的红外消光系数。研究结果表明:反应时间为20 h和18 h,淀粉与葡萄糖的配比为9:3和6:10时,焙烧后样品的球形形貌较好,5号和7号样品在4~10 μm波段范围内消光性能较好,消光系数均大于0.3 m2/g,最高可达到0.37 m2/g。Abstract: Carbon coated ferrite precursor was prepared by one-pot hydrothermal method, which was calcined at 950 ℃ with N2 protection to obtain carbon coated ferromagnet composite materials. The morphology and composition of the materials were analyzed by XRD, FT-IR and SEM, and the effects of reaction time, the ratio of starch and glucose on the morphology and IR extinction of the composite were studied. The IR extinction coefficients of materials in the range of 2.5-25 μm were measured and calculated by KBr method of FT-IR. The results show that the samples have good morphology and extinction performance, that were prepared under the conditions of reaction time of 20 h and 18 h and the ratio of starch and glucose of 9:3 and 6:10. In the range of 4-10 μm, the extinction coefficient of sample 5 and 7 is greater than 0.3 m2/g and the maximum is 0.37 m2/g.
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Key words:
- electro-optical countermeasure /
- infrared smoke /
- carbon /
- ferromagnet /
- mass extinction coefficient
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表 1 样品试验条件
Table 1. Sample test conditions
Sample number Time/h Starch: Glucose/g The first group 1 16 8:8 2 16 6:10 The second group 3 18 6:6 4 18 9:3 5 18 6:10 The third group 6 20 8:8 7 20 9:3 The fourth group 8 22 8:8 9 22 6:6 -
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