氧弹法测量硼粉燃烧热值的方法学研究

Methodology of Measuring Heat of Combustion of Boron Powder by Oxygen Bomb Calorimetry

关键词:  硼粉  燃烧热  酸碱滴定  硼酸层  氧弹法

Key words:  Boron powder  Heat of combustion  Acid-base titration  Boric acid layer  Oxygen bomb calorimetry

摘要:

硼是一种高能固体燃料，其燃烧热值的准确测定十分重要。用甲醇-水混合溶剂洗脱去除硼粉表面的氧化物，然后用氢氧化钠溶液滴定洗脱液中硼酸含量，计算得到500nm原始硼粉表面氧化层平均厚度为1.8~5.5nm，40nm原始硼粉表面氧化层平均厚度0.5~1.6nm，采用氧弹法测定了500nm和40nm原始硼粉及其去除氧化物后的燃烧热值，通过滴定法分析燃烧产物中硼酸含量，换算得到实际燃烧的单质硼质量，从而计算出单质硼燃烧热值。实验结果表明，500nm原始硼粉及其去除氧化物后燃烧得到的单质硼平均燃烧热值分别为55.2±0.8kJ·g^-1和55.4±1.2kJ·g^-1，两者非常接近，且落在文献值范围内；40nm原始硼粉及其去除氧化物后燃烧得到的单质硼平均燃烧热值分别为53.3±0.9kJ·g^-1和51.6±0.9kJ·g^-1，略低于文献值。为了准确测量单质硼的燃烧热值，宜选择颗粒尺寸较大（如500nm）的硼粉样品。在上述厚度范围内，氧化层对500nm原始硼粉或40nm原始硼粉的燃烧效率均无影响。当硼颗粒的尺寸为40~500nm时，燃烧效率也与硼颗粒的粒径大小无关。

Abstract:

Boron is a well-known solid high energy fuel. It is important to measure accurately the heat of combustion of boron. The oxides or boric acid on the surface of boron powder was eluted with a methanol-water mixed solvent， and the amount of boric acid in the eluent was analyzed by titration of sodium hydroxide solution. The average thickness of the surface oxide layer on the as-received boron powder was 1.8~5.5nm for 500nm powder and 0.5~1.6nm for 40nm powder. The heat of combustion of as-received boron powder with sizes of 500nm and 40nm as well as the oxide-free ones were measured by oxygen bomb calorimetry. The amount of boric acid in the combustion product was analyzed by titration method， which was used to calculate the actual mass of boron burned and thus the heat of combustion of boron. Results show that the average heat of combustion of boron is 55.2±0.8kJ·g^-1 obtained from as-received 500nm boron powder and 55.4±1.2kJ·g^-1 obtained from the washed one. These values fall in the range of reported values in the literature. The average heat of combustion of boron is 53.3±0.9kJ·g^-1 obtained from as-received 40nm boron powder and 51.6±0.9kJ·g^-1 obtained from the washed one. These values are slightly lower than those reported in the literature. Boron powder with larger particle size （such as 500nm） is recommended for measuring accurately the heat of combustion of boron. The oxide layer in the above-mentioned thickness range on the boron surface has little effect on the combustion efficiency for either 40nm or 500nm as-received boron powders. The combustion efficiency is also independent on the size of boron powder in the range from 40 to 500nm.