Simple preparation of crystalline NaCl ultrafine particles at atmospheric pressure

Abstract

In recent years ultrafine particles below 0.1/xm, in addition to the common use for aerosols, have become more important in nanometre-scale particle technology for electronic devices for advanced materials such as nanocomposites. In these fields NaC1 crystalline ultrafine particles can be used as tracers or markers, since they show distinct crystal habits different from the other materials. However, technical problems arising in the preparation of these particles have kept them from more extensive application. In most previous papers on the production of crystalline ultrafine particles, a reduced pressure has been applied for the preparation [1-3]. However, this condition causes many difficulties in the manipulation subsequent to the generation of ultrafine particles. For instance, the longer mean free path due to the reduced pressure causes a large amount of the particles produced to adhere to the walls, and mechanical devices such as vacuum vessels and pumps make it difficult to treat Particles separately. On the other hand, NaC1 ultrafine particles produced using a furnace operated at atmospheric pressure have often been used for studies in the aerosol science field because of the convenience and simplicity [4, 5]. However, the shape of most particles produced by the above method are round or indistinct, which implies they are amorphous. Consequently, long-term observation to obtain moreprecise information by using transmission electron microscopy (TEM) cannot be carried out, since the electron beam vaporizes the particles within a few seconds [4]. We also confirmed this fact. This tendency becomes more remarkable as the size of the particles decreases. A method of using atomization of diluted NaC1 solution and subsequent drying of the resultant small droplets has often been carried out as another method for producing cubic NaC1 crystal aerosol at atmospheric pressure [6]. The method is good for the preparation of relatively large particles, but the generation of ultrafine particles below 0.1/zm (and particularly below 0.05/xm) seems to be difficult, probably because of the excessive dilution of NaC1 solution to be atomized. Also, it is difficult to attain higher concentrations of particles, since the large amount of dry gas necessary for vaporizing solvent (water) inevitably results in a lowering of the concentration. We were unable to prepare NaC1 ultrafine particles below 0.05/xm with the desired concentration by the above method.