Powder Metallurgy and Metal Ceramics, Vol. 42, Nos. 1-2, 2003

COMPUTERIZED SYSTEM FOR THE SINTERING OF NANOCERAMICS AT HIGH PRESSURES V. S. Urbanovich and G. G. Shkatulo UDC 621.315.612666.792.6 A computerized system of apparatuses for the sintering at high pressure of nanoceramics based on refractory compounds was developed. It allows for carrying out the sintering process according to a given program with rapid and accurate regulation of the P-T parameters. Information on the sintering process is stored in an internal data base with the possibility of export. The system can also be used as part of a shop local computing network. The properties of nanoceramics based on various refractory compounds produced with use of the system is given. Keywords: nanoceramics, high pressure apparatus, high pressure sintering, controller.

INTRODUCTION Sintering at high pressure is an effective method for the production of highly dense ceramics based on refractory compounds [1-4]. Using this method we have obtained practically pore-free titanium nitride-based nanoceramics with elevated microhardness [5, 6]. The structure and properties of nanomaterials sintered at high pressure are determined by the thermodynamic conditions of the process (pressure, temperature) as well as the kinetic parameters (time of action and the nature of structural change of the nanoceramics upon sintering). The thermodynamic parameters are determined by the nature of the high-pressure equipment, and the kinetic parameters by the ability of the system to control temperature and pressure in the sintering zone. Known systems for the control of temperature and pressure in the reaction chamber of high-pressure equipment do not allow for their variation according to a selected law [7-10]. In this work we present an automated system for the sintering of ceramic materials, including high-pressure equipment for hydraulic pressing units with forces 5-6.3 MN and a computerized system for the control and regulation of the thermobaric conditions of sintering.

EQUIPMENT FOR THE SINTERING OF CERAMICS AT HIGH PRESSURE High-Pressure and High-Temperature Apparatus

The high-pressure unit developed by us is an anvil-type with a recess for sintering the refractory compoundbased ceramics (Fig. 1). It is a modified version of the high-pressure equipment described in [6], and allows for the sintering of larger sized ceramic specimens (up to 12 mm diameter) at pressures up to 4 GPa and temperatures to 2200°C. The reaction chamber (Fig. 1) consists of two hard-alloy dies 41 mm in diameter and 20 mm high with central recesses of trapezoidal shape 25 mm in diameter and 5 mm deep on the opposing faces.The surfaces around the central recesses are furnished with a 5° angle of conicity. The dies are clamped with a three-layered band of steel supporting rings. The container is made of pressed lithographic limestone. Pressed graphitic heaters composed of two parts separated in a horizontal plane are placed in the axial opening of the container. Computer System for Sintering Control and Regulation

The equipment for automatic regulation of the temperature regime of sintering (synthesis) at high pressure [711] can be divided into: a unit consisting of an analogue driver and analogue regulator [7, 8]; a unit containing a digital Institute for the Physics of Solids and Semiconductors, National Academy of Sciences of Belarus, Minsk. Translated from Poroshkovaya Metallurgiya, Nos. 1-2(429). pp. 21-27, January-February, 2003. 1068-1302/03/0102-0019$25.00

2003

Plenum Publishing Corporation

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Fig. 1

Fig. 2

Fig. 1. High-pressure equipment for the sintering of nanoceramics. Fig. 2. Structural diagram of the heating circuits of the pressing unit. (1) High-pressure apparatus; (2) current gauge; (3) heating transformer; (4) thyristor voltage regulator; (5) electrical power unit of the pressing equipment; (6) sintering controller; (7) system unit; (8) mapping unit; (9) operator command input unit. instrument driver and analogue regulator [9]; a programmable units acting under the control of a microprocessor but containing an analogue regulator [10]. The units [7-9] in principle cannot solve the problem of sintering control, and the synthesis controller KS-1 [10], although it also contains a microprocessor, in view of its low efficiency and use of an analogue regulator also does not provide a basis for solution of the given problem. Fabrication of a controller with the use of units of the programmed logical controller S7-200 Simatic [11] is of definite interest. This controller is assembled from prepared modules, is quite easily programmed, and can be used for very different purposes. Shortcomings of controller S7-200 which hinder its use in solving the given problem are: insufficient processor efficiency (0.37 µsec at full boolian operation); lack of a mathematical coprocessor; low response speed of the available analogue signal input modules (≤250 µsec), making it necessary to use the original block of functional transformers of the acting current and voltage; absence of a unified housing. A different treatment of the computer system (controller) based on units and blocks of a PC-compatible commercial computer (work station), functioning in a regime of DDC (Direct Digital Control) appears to be optimal. This completely excludes the use of an analogue part in the regulation contour and substantially simplifies and decreases the cost of developing a system with high output parameters. Structural Scheme of the System. The sintering controller KS-3 is constructed on the basis of a small-scale PCcompatible work station (the use of a personal computer is also possible), equipped with a built-in LCD display and keyboard. The structural electrical circuit for connecting the controller to the electric power unit of the press is shown in Fig. 2. The given sintering temperature is attained by control of the voltage with a thyristor regulator connected to the primary coil of the heating transformer, the secondary coil of which is connected to the high-pressure equipment. Data on the voltage (U) on the heating element in the sintering zone is taken off directly from the high-pressure equipment, and data on the current (I) in the heater from a current transformer or low-ohm shunt which serve as an ammeter. These signals (U, I) and also the pressure signal (Pa) from the compression system of the press enter the controller. A number of other signals from the press also go there. After they are processed the controller takes the appropriate action on the thyristor regulator and gives a number of auxiliary signals in the electrical power unit of the press. The controller can be joined to a local computer network (LCN) with the aid of an RS-485 interface, and combining up to 25 controllers in one line is allowed. The developed system can control the pressing forces and cooling systems of different high-pressure units individually. System of Control and Regulation. The structure of the controller is shown in Fig. 3. The base of controller is a full-function board of the class “all in one Single Board Computer.” Processor and memory are preinstalled on the board. It has built-in interfaces for LCD/CRT display, RS-485, socket for DiskOnChip. Another important component

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P rogram to adjust the regulation param eters

Head program m odule

Database

A djusting file

P rogram to control the sintering

Fig. 3

Fig. 4

Fig. 3. Structural diagram of the sintering controller KS-3. (1) Display; (2) junction board; (3) fullfunction processor board; (4) keyboard; (5) multifunctional input-output board; (6) interface board. Fig. 4. General structure of the basic software.

Fig. 5. Dependence of the resistance of the reaction cell and electrical power on sintering time.

Fig. 6. Appearance of the monitor screen with given sintering parameters.

of the controller is a multifunctional board for the input and output of analogue and digital signals. The board includes a multichannel 12 bit A/D convertor with a conversion time of 8 µsec, D/A convertor, timer, and channels for digital input-output. A no less important role is played by the original interface board. It serves to normalize and galvanically isolate the input signals, synchronize the work of the controller with the thyristor unit and control it, coordinate the controller with the electrical power unit of the press. The rapid A/D-convertor together with suitable software make it possible to use the controller in the DDC regime, completely eliminate the use of analogue circuits in the regulation loop, which appreciably simplifies the controller and improves its operating parameters. Software and Technical Properties. The controller is equipped with original base software (SW), the general structure of which is shown in Fig. 4. The basic SW consists of: a head program module (with the aid of this the sintering program itself and a whole series of controller parameters are inputted, inspection of the data base, administrative, and others); sintering control program (with the aid of this the course of sintering is controlled and its interaction with the pressing unit in the operating regime of the press “cycle”); a program to adjust the regulation parameters. In addition, the basic SW includes the data base and adjustment file. Since the controller functions in a real time regime with strict requirements on fast action, the SW of the controller is written in the languages C++, Assembler with the use of a series of hardware controlled interruptions.

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Fig. 7. Fracture surface of nanocrystalline materials based on (a) TiN; (b) TiN/TiB2; (c) Al2O3 produced by sintering at high pressure. The basic technical parameters of the controller attained by close interaction of its programmatic and apparatus components are: Number of parts given by the operator for the sintering program, no more than ............................................. 24(100) Duration of each part of the sintering program, sec.....................................…................................…. 1-60 Maximum value of the regulated parameter: Power..................................................................... 10 kV⋅A with increments 0.01 kV⋅A Voltage................................................................... 10 V with increments 0.01 V Current................................................................... 5.0 kA with increments 0.01 A Basic regulation error, %, no more than..................… ±0.25 Increment of miscount and transmission of the regulating action to the regulating element, no more than.............................................................................................. 0.02 sec The controller provides: continuous digital regulation (according to the adaptive PID(PI) law); numerical indication of all given and controlled sintering parameters (P, U, I, R, current and real time); graphic mapping of the change of the regulated parameter and resistance of the reacting volume in the sintering process; storage of the detailed sintering and general information with the possibility of its autonomous review (the information can be recorded on a magnetic floppy disk and exported to specialized service programs) The controller is equipped with a RS-485 interface for operation in a local computer network, has protection from non-sanctified initiation of the SW. The fast action of the controller is illustrated by the dependence of the electric power of specimen heating and electrical resistance of the cell on time (Fig. 5). The abrupt decrease of electrical resistance after 6.8 sec produced only a puls increase in the electrical power, which demonstrates rapid action of the controller. In Fig. 6, as an example, the TABLE 1. Physico-Mechanical Properties of the Ceramics Material

Sintering temperature, °C

Density, g/cm3

Grain size, nm

Microhardness, GPa*

TiN TiN/TiB2 Al2O3

1200 1200 1000

5.10 4.72 3.87

~60 ~200 ~200

31.4 ± 1.0 34.2 ± 1.2 32.0 ± 2.4

*At a load of 0.5 N.

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appearance of the controller screen in the process of inputting the sintering parameters in the form of a complex relationship between electrical power and time is shown. The character of this relationship is simultaneously reflected in the lower part of the screen. The possibilities of the equipment are demonstrated by the results we have obtained: some of physical properties of ceramic specimens of different refractory compounds sintered at high pressures are superior to those of known bulk ceramic nanomaterials produced by other methods (Table 1); the specimen fracture surfaces exhibit a dense nanocrystalline structure (Fig. 7). The computerized system which was developed opens new broad possibilities for controlling the structure of nanoceramics in the process of sintering at high pressures, and obtaining new nanocrystalline materials with given properties, including also the technique of “rate-controlled sintering [12].”

CONCLUSIONS A computerized system of equipment for the sintering of nanoceramics based on refractory compounds at high pressure was developed. It permits the sintering process to be carried out in accordance with a given program with fast, highly accurate regulation of the PT-parameters. Information on the course of sintering is saved in an internal data base and can be exported. The developed system can also be used in the shop local computer network.

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