• 作者：Martin Gangl ; Miroslav Kocifaj ; Gorden Videen ; Helmuth Horvath
• 关键词：Nano-sized aerosols ; Absorption ; Coated carbonaceous particles ; Discrete dipole approximation
• 刊名：Atmospheric Environment
• 出版年：2008
• 期刊代码：11_13522310
• 类别：env
• 出版时间：April 2008
• 卷：42
• 期：11
• 页码：2571-2581
• 文件大小：681 K

In order to obtain dry powder aerosols from the same carbonaceous material at two different particle sizes, several problems have to be solved:

(1) The first problem is to find a method to mill or to disperse the original material into very fine particles. As a rule, the force needed to separate fine particles increases inversely with the diameter wanted,

(2) For the interpretation of the biological results of the experiments it is highly desirable to ensure that the two different particle size aerosols have small size ranges, i.e. have virtually non-overlapping particle size distributions,

(3) Since there are many ways for particles to join or clog in air, it is expected that most if not all dry powder aerosols will consist of aggregates of small particles. Therefore, if carbonaceous aerosols consisting of single particles are required, clogging should be prevented.

Finally, the total volume flow for each aerosol should be about 8 L/min.

In theory, these criteria could be met as follows:

* generation of ultra fine carbonaceous particles (diameter about 50 nm).

An aerosol of ultra fine particles may be generated from the gaseous phase of the original material, using a high voltage electrical discharge between carbon electrodes. Inside the discharge the temperature will rise very quickly above 1200 °C, resulting in a burst of gaseous particles. The gap between the electrodes will then be swept and cooled with an inert gas like helium or argon at a low temperature. Nitrogen is not suitable because of the chemical reactions between C and N at high temperatures. The strong flow of cold inert gas will cool the gaseous particles into ultra fine single solid particles.

The temperature zone around the electrical discharge will, however, give rise to thermal diffusion and singlets will aggregate to clusters. Simultaneously, the particles will be transported and diluted by the flow of gas and as the distance between the particles grows and the temperature drops, formation of aggregates and conglomerates will be impeded. In the next step, the primary aerosol flow has to be discharged by a radioactive Krypton source after which the aerosol will be diluted to its final concentration with oxygen-enriched air to compensate for the presence of the inert gas.

* generation of fine carbonaceous particles (diameter about 500 nm).

This larger sized aerosol will be generated from conventional pure acetylene carbon black. There are several methods available of which the simplest may seem a fluidized bed. The expected massive formation of aggregates inside the generator, resulting in a poor efficiency and little control, should be inhibited to some extend by superimposing ultrasonic waves on the gas stream through the generator. Isolation of the ultrasound noise may then be needed in order not to disturb the experimental animals.

A second method consists of dispersing the carbon black material in a high speed air jet mill. Although the effectiveness of milling of this size of particles is rather poor, some abrasion and segregation will occur which may interfere with the required size distribution width. Moreover, in these types of mills, the total amount of air flow is rather large, requiring removal of superfluous aerosol.

It will be discussed which generation methods finally were chosen and which measures had to be taken to avoid the formation of conglomerates and to overcome electrical charges.