Adhesion of silicate-settled phosphor screens

When a phosphor screen is laid by sedimentation in a solution of weak potassium silicate mixed with a salt additive, gelatinous silica is deposited at the powder‐glass interface and the adhesion depends on this rather than on the electrostatic forces. In order to measure this adhesive force and its increase with time before decanting off the supernatant liquid an underwater jet test has been devised. The method for calibration of the jet in absolute units is described and the influences of various mechanical factors are reviewed. With this test the course of setting in typical phosphor screens has been quantitatively followed, and it has been shown that the important solution factors are the ratio of the concentration of cation to the SiO 2 content of the solution, and the cationic charge. More powerful jets have been used to study the kinetics with screens of greater adhesion. In an attempt to explain the anomalous adhesion experienced with phosphors of different particle size, a study has been made of the variation of the adhesion index with particle size by using specially prepared homodisperse fractions. A brief study has been made of the factors concerned with adhesion after drying in order to throw further light on the gelation reaction. Finally a critical discussion and review of the literature is given to compare the interfacial gelation reaction with the much studied bulk reaction. It is concluded that owing to the very high silica concentration in the interface the reaction is probably limited by a diffusion mechanism.