Coprecipitation PZT piezoelectric ceramic powders Notes
Concentration of 1 precipitation solution: will affect the particle size, crystal, yield, purity and surface properties of precipitation. Typically, a relatively dilute solution of the precipitate, due to lower the nucleation rate, the particle size is easy to obtain a larger, more complete crystalline, high purity crystalline properties and surface precipitation, but the yield is lower, which is adapted to the chemical purity of the pure pursuit of particular products; Conversely, if the nucleation rate is too low, then the number of generated particles is small, the particle size of the individual particles becomes large, and for the preparation of a fine powder material which is disadvantageous, and therefore, the actual production according to the different performance requirements of the product, the appropriate precipitating solution to control the concentration of a certain extent control the nucleation rate and growth rate.
2 synthesis temperature: synthesis temperature precipitation also affect the particle size, crystal, yield, purity and surface properties of precipitation. In the hot solution, precipitation generally greater solubility, supersaturation is relatively low, so that the precipitate nucleation slowed conducive nuclei grow, precipitation was relatively close to facilitate settlement and washing; precipitation adsorption in the hot solution to be smaller, in favor of improved purity.
In the preparation of different sediment, due to the pursuit of different physical and chemical properties, depending on the specific use of the temperature test result should be. For example: If the temperature is too high, the product may decompose during synthesis and only give a black copper oxide; easily decomposed when using volatile precipitant, the temperature is too high will increase the loss of raw materials.
Join manner and speed 3 precipitating agent: Join manner and speed equally affects various physical and chemical properties of precipitation. If dispersing agent is added to the precipitate, and the slow rate of addition, while stirring, the solution to avoid formation of large number of nuclei locally too thick, the preparation of high purity in favor of the crystal forms of large particles to precipitate.
For example: Preparation of a white, amorphous precipitate of aluminum hydroxide powder, and the raw materials used for NaAlO2 ammonium, alkali metal and its main impurity, at the beginning of the line speed will be slower NH4HCO3 NaAlO2 was added to the hot solution until precipitation when most of the precipitation, the precipitation agent is then added at a rate to speed up until the end of the reaction. Thus obtained Al (OH) 3 particles greater need only washed several times, the product of alkali metal impurities to pass.
As will increase the concentration of the precipitating agent, the feeding speed, the reaction temperature is low, the thus obtained is Al (OH) 3 gel precipitation, even dozens of washing, the alkali metal content of the product is not easy to pass. Of course, this is only from the perspective of chemical purity to consider, or to produce special properties of Al (OH) 3 product, adding speed mode and precipitating agent should be based on the specific requirements.
4 feeding order: feeding methods points are added, the anti-Canada, and add three. Production of "positive plus" refers to the metal salts to put in a reactor, then add the precipitating agent; contrary to "add-back"; while the sediment-containing anions and cations in solution while proportionally added to the reactor The method, called "and add."
The order of addition of the precipitate, and the uniformity of the adsorbed impurities, which precipitate is closely related. "Is added to" main mode precipitated salt of anionic impurities adsorbed material; neutralization and precipitation at the time of, first, the produced precipitate, their different environments in which the PH value of the resulting difference in the uniformity of the precipitation product.
"Anti-plus" approach precipitated major anion adsorption of impurities; when and if the filling precipitation throughout the precipitation process of divination PH value changes little better product uniformity. "And add" excellent way to avoid a partial solution works too thick, the precipitation process is more stable, and the adsorption of impurities is small, and thus obtain a better physical and chemical properties of the product. In actual production depends on the specific requirements of the product should be.
Select 5 precipitant precipitant should consider the quality of products, processes, yield, and cost sources of raw materials, environmental pollution and safety issues. Under the process allows, with the option should be greater solubility, high selectivity, less by-products affect the precipitating agent, it is also easy to remove the excess of the precipitating agent, adsorption and reduce the incidence of side reactions.
The precipitate of carbonate in the production of product, the choice of precipitating agent with Na2CO3, NaHCO3 NH4HCO3 and a variety of other soluble carbonates, but generally NH4HCO3 as well, because of its solubility, easy to wash, volatile byproduct contamination also smaller, and extensive source of raw materials, the price is low. Use precipitating agent should generally be excessive in order to obtain high yields, reduce pollution metal ions; but not too much volume, otherwise they will be complex effects and salt effects, such as reduced yield. Typically 20% -50% excess will be able to meet the requirements.
6 precipitation aging aging precipitation process can be released into the most impurities. In the aging process, due to the precipitation of small particles of high specific surface area, surface energy is large; same specific surface area of particle deposition amount is small, the surface energy is small, the system changes from a high energy to low energy spontaneous tendency Therefore, precipitation will gradually dissolve small particles, large particles precipitate can slowly grow up again.
Secondly, from the view of the solubility of the precipitate, when the large system, when the small particles coexist, if the solution with respect to precipitation of large particles is saturated, then the precipitation of small particles is not saturated, so the precipitation of small particles dissolve and large particles of precipitate will be grow, so precipitation particle surface integrity, reduce moisture and caking, improve storage and use of performance precipitation. Aging process due to the dissolution of small particles, adsorption of impurities to reduce entrainment and parcels, play the role of a so-called partial recrystallization, the product can improve the purity of the precipitate.
When the aging conditions, such as time and temperature also affect the performance of the precipitation, and therefore, should be determined according to the specific requirements of the product. In actual production, it must be noted that the aging time exceeds a certain range may cause precipitation, but to make the purity of the product decreases.
