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            Causes of metal defects of platinum group elements in glass substrates for liquid crystal display

            From:CHANGZHOU ZHONGBO GLASS MACHINE AND EQUIPMENT CO.,LTD  Date:2019-3-25 14:09:36

            In the process of overflow pull-down, the main manufacturing method of glass substrates, in order to avoid the contact between glass melt and refractory material bringing in impurities and bubbles of refractory oxide stones, the glass melt formed by pre-melting in the pool furnace directly enters the pipes and containers made of high melting point refractory metal platinum and its alloys to clarify, homogenize, cool and transport the glass liquid. The forming overflow trough entering the muffle furnace is pulled down to make the required thickness glass substrate.

            Chemical erosion in the upper space of the free glass surface of the platinum-rhodium alloy channel refers to the space formed between the upper surface line of the clarification section and the tube wall of the platinum-rhodium alloy material, and the relative closed space formed between the liquid surface lines of the agitation tank and the upper cover of the feeding tank respectively (a, b, C shown in Fig. 1). At high temperatures, the following reversible oxidation reactions occur between platinum or rhodium in platinum-rhodium alloys and oxygen in space:

              Pt(s)+O2←→Pt O2


            The experimental results show that the degree of oxidation reaction is proportional to the partial pressure of oxygen in atmosphere, and increases exponentially with the space temperature and the velocity of gas flow in atmosphere. In addition, it should be pointed out that the liquid level line is at the three-phase coexistence interface, where the temperature is very high and the oxidation reaction is intense. Therefore, the following measures can be taken to reduce the oxidation corrosion defects of platinum and rhodium metals: (1) To reduce the temperature of glass liquid (space) in low channel under the conditions of bubble clarification and full uniformity of glass liquid. (2) Optimize the temperature control mode and heat preservation mode of each section of platinum-rhodium metal channel, especially the clarification section, to reduce the spatial temperature gradient and temperature fluctuation range to a great extent. (3) To minimize the fluctuation of liquid level in process operation. (4) Insulation and sealing facilities are added above the liquid level line and at the junction with the external environment to reduce the heat convection and gas convection between the space gas and the external environment. (5) If conditions permit, nitrogen or Sui gas, which does not react with platinum-rhodium alloys or other materials, may be introduced into the above-mentioned free space.

            Physical erosion of high temperature viscous glass liquid on the inner wall of stirring tank and stirring blade at high temperature. It was found in production practice that the content of Pt-Rh metallic inclusions brought into glass by the rotation of stirring rod in high temperature viscous glass liquid increased with the increase of rotational speed (stirring ability). The direct reason is that the shear stress acting on the stirring blade and the inner wall of stirring tank when the stirring rod rotates in high temperature viscous glass liquid causes physical erosion and peeling of platinum-rhodium alloy on the surface. In order to compensate for the degradation of fringe grade caused by the reduction of stirring efficiency due to the reduction of rotational speed, the uniformity of glass liquid must be improved from the source, which is seldom used.

            Contamination and erosion of impurities in glass raw materials mainly include two types: reducing impurities such as iron powder mixed in batch and glass crushing process; and crystallization defects of platinum and rhodium directly introduced into recovered platinum and rhodium exceeding standard broken glass. This requires that the necessary iron removal and other metal and oxide devices should be installed in the preparation of batches, especially in the preparation of broken glass, so as to strengthen the process control and avoid the mixing of other reducing foreign bodies. Attention should be paid to the management of recycled broken glass, so as to avoid the re-use of broken glass exceeding the platinum and rhodium standard, and to eliminate metal defects such as platinum and rhodium from the source.

            In this paper, the causes of platinum and rhodium and other precious metal defects in overflow pull-down process using platinum and rhodium alloy channels (containers) to clarify, homogenize and transport glass liquor are summarized comprehensively from the aspects of process, equipment and raw materials, and the corresponding solutions are put forward. In daily process management and work, according to the ideas provided in this paper to find the root causes of platinum and rhodium defects, and take corresponding measures, the above defects can be controlled at a lower level, so that products can meet user needs. However, in practical work, we should flexibly use the countermeasures introduced in this paper, and take into account the effects of the above measures on other properties and defects of glass substrates (such as reducing platinum and rhodium defects while possibly causing such as the rise of bubble incidence and the deterioration of stripe grade), so as to improve the quality and grade of glass substrates in an all-round way.

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