Nanomaterials dry lyophilized
    Freeze- drying is a drying method for a direct , direct drying method is the liquid phase precipitate obtained directly ( or simply filtered after washing ) using conventional drying or high temperature calcination product obtained nanomaterials method of operation .
   Freeze-drying method is to prepare - given concentration aqueous salt solution , so that the spray nozzle into tiny droplets , highly dispersed into the cooling agent ( such as liquid nitrogen ), and is rapidly frozen solid particles were filtered and then frozen , the rapid pre- cooling with liquid nitrogen into a vacuum freeze dryer , then heated to sublimation of ice , the combustion of a salt or oxide nanoparticles . Such methods include spray drying and vacuum freeze- drying two Lo process , the spray freezing process thing to be careful not to make ice salt separation and vacuum drying process must be done can not make the ice melt, but should make it sublime . For freeze- drying the gel , the gel is generally frozen into the ice water , and then under vacuum sublimation of ice into the water vapor is removed.
    When the water is frozen into ice , its volume expansion becomes large. The phase change of water in the process of expansion that is close to the original properly separated gel particles , while the formation of the solid state , to prevent re- aggregation of the gel ; other hand, since the vapor - liquid interface into the lower energy vapor - solid interface , therefore , in theory , can greatly ease the freeze-drying process drying effect caused by interfacial tension reunion .
    Nanomaterials vacuum freeze-drying has the following characteristics .
    1 can be a soluble salt to prepare a homogeneous solution consisting of powder material complex .
    2 by the rapid freezing of metal ions in the solution can be kept in a uniformly mixed state.
    3 simply by freeze-drying can be prepared without salt . Hydrated salt melt without touch , generally in the ratio of non- contact temperature melting salt occurs under conditions much lower , and thus , to avoid the molten salt mixture composed of separation occurs .
    4 Preparation of freeze-dried products , gas permeability is good.
    5 a long drying time . Because the nanoparticles freeze drying process , leaving small pores ice sublimation , the channel narrow , discontinuous , sublimation of water vapor formed by diffusion or permeation can elect ; sublimation of ice at low temperatures is a slow pass transfer process , the entire operation time longer. If the sample having a thickness of 20mm, the drying time required to 54h; thickness of 15mm is 45h; thickness is reduced to 12mm, the drying time is still required 32h.
    6 large temperature difference within the material . As the drying process , the dry layer is very low thermal conductivity , convection , conduction difficult under vacuum sublimation efficiently transmitted to the interface , so the use of radiation heating ( eg laboratory using infrared lamps, etc. ) is commonly used to sublimation of ice a means , most of the radiant heat absorbed by the material used for the sublimation of the ice , and the rest temperature heat to dry layer . Material by liquid nitrogen to freeze dryer lower layer maintained at a temperature of below the melting point , which makes the material there is a large temperature difference between the upper and lower (up to above 30 ℃ ) , causing the drying temperature is difficult to control .
    7 easy to produce " disintegration " phenomenon. As mentioned above , the drying process of sublimation vapor passage narrow and discontinuous porosity , water vapor to exit by way of diffusion or permeation , low temperature ice mass sublimation is a slow process , so that the drying rate is slow , if the control temperature a little attention, excessive temperature rise , the right is easy to produce " disintegration " phenomenon ( ie, dry ice sublimation layer has been formed after the skeleton is very dirty, weak , when it reaches a certain temperature , it will cause the skeleton stiffness decreased, so that water vapor channel narrowing or blockage , slow down or even prevent the sublimation of water vapor , resulting in collapse of the dry layer , melt the end of the dry layer of a phenomenon ) . With the disintegration phenomenon, dry layer of the powder has been the impact of flying pressure difference , which not only affects the yield of the product , more so the drying process can not proceed.