EP0914604A1

EP0914604A1 - Method and device for compacting and measuring the resistivity of powders

Info

Publication number: EP0914604A1
Application number: EP97935597A
Authority: EP
Inventors: François SEITE
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1996-07-23
Filing date: 1997-07-22
Publication date: 1999-05-12
Also published as: FR2751751A1; FR2751751B1; WO1998003856A1

Abstract

A device for complete compacted powder resistivity measurement during powder compacting is disclosed. The device mainly includes two pistons (10S, 10I) slidably moveable inside a compacting sleeve (1). The compacting surfaces (10C) of the pistons and the inner surface (2) of the compacting sleeve (1) each comprise four electrodes, i.e. two current electrodes (4, 14I) and two voltage measurement electrodes (4, 14U). The device is useful for measuring the resistivity of compacted powders.

Description

METHOD AND DEVICE FOR COMPACTING AND MEASURING THE RESISTINITY OF POWDERS

DESCRIPTION

Field of the invention

The invention relates to the measurement of the resistivity of powders during their compaction. These are of course powders containing at least one conductive material.

Prior art and problem posed

It is known to measure the resistivity of conductive materials in the form of powders by a method requiring a lot of technicality in order to obtain precise and reliable results. The compression of the powder adds to the difficulty due to the phenomena of friction, pressure, variation in density. Indeed, the appearance and the physical behavior of the compacted powder evolve according to the degrees of compaction and throughout this operation. The object of the invention is to solve the problems associated with the difficulties of measuring the resistivity of all the conductive powders during their compaction.

Summary of the invention

A first main object of the invention is a device for compacting and measuring the resistivity of powders during compaction comprising: - A compacting chamber inside which is enclosed the compacted powder to be measured; and

- electrodes emerging inside the compacting chamber to subject the compacted powder to an electric current and measure the potential difference which results therefrom.

According to the invention, the compacting chamber comprises: - a compacting jacket inside which move

- two movable pistons sliding along a common axis and each equipped with at least four aligned electrodes, including two electrodes for passing the electric current and two electrodes for measuring the potential difference, in order to carry out axial resistivity measurements.

In the preferred embodiment of the device according to the invention, the compacting jacket also has four electrodes opening onto the internal surface, oriented at 90 ° relative to each other around the axis of the mobile pistons, for carrying out resistivity measurements radial.

In this case, the electric current injection electrodes are placed next to each other, as are the voltage measurement electrodes.

Preferably, the compacting jacket is made of ceramic based on alumina. Still in the main embodiment, the compacting jacket is located in an external cylinder.

In this case, the compacting jacket preferably has vertical radially oriented and through slots on the central part of this jacket. compaction, positioned at 90 ° to each other to receive the four electrodes of the compaction jacket.

Preferably, the pistons have peripheral grooves to reduce the friction of the mobile pistons in the compacting jacket.

The second main object of the invention is a method of compacting and measuring the resistivity of the powders being compacted in a device comprising a compacting chamber inside which is enclosed the powder being compacted to be measured. and comprising a compacting jacket inside which two movable pistons slide along a common axis, each equipped with at least four aligned electrodes, each having two electrodes for passing the electric current through the compacted powder and two electrodes for measuring the potential difference, so as to carry out axial resistivity measurements. According to the invention, in a first implementation of the method, the measurement of the axial resistivity is made with the four electrodes of the same mobile piston.

In a second implementation of the method, the measurement of radial resistivity is carried out with the electrodes of the compacting jacket.

In a third implementation of the method, the measurement of the axial resistivity, in particular during the evolution of compaction, is carried out with two electrodes of a first piston and two electrodes of a second piston. List of Figures

The invention and its various technical characteristics will be better understood on reading the following description, illustrated by the figures representing respectively:

- Figure 1, in section, the device 1 according to the invention; Figure 2, in cross section, the section of the device according to the invention, along the line II-II of Figure 1;

- Figure 3, in simplified section, a first measurement mode according to the method according to the invention; FIG. A, in simplified section, a second measurement mode according to the method according to the invention; FIG. 5, in simplified section, a third measurement mode according to the method according to the invention; and FIG. 6, in simplified section, a fourth measurement mode according to the method according to the invention.

Detailed description of the invention

Referring to Figure 1 showing the main embodiment of the invention, the device constitutes a compacting chamber represented by the compacted powder and referenced 6. The main elements thereof are a compacting jacket 1 placed inside d an outer cylinder 3 and whose inner surface 2 constitutes the cylindrical surface of the compacting chamber. The device is completed by two mobile pistons, an upper mobile piston 10S and a lower mobile piston 10T. All these parts are preferably made of ceramic based on alumina.

The compacting liner 1 is in the form of a cylinder supporting radial electrodes 4. For this purpose, it has radial vertical slots opening both on the internal surface 2 and on the external surface in contact with the external cylinder 3, at the level of the compaction chamber. These slots make it possible to block the four radial electrodes 4.

With reference to FIG. 2, it can be seen that the arrangement of these radial electrodes 4 is an arrangement at 90 ° around the axis of the movable pistons 10S and 101. Returning to FIG. 1, each radial electrode 4 is connected to an electrical connection cable 5 emerging through the upper part of the compacting jacket 1. The latter is preferably made of alumina. The two movable pistons 10S and 101 have circular sections and are arranged opposite one another inside the compacting jacket 1. They are designed to slide inside thereof, one towards the 'other, under the effect of pressure, so as to compress the material to be tested. In addition to this mechanical role of piston compression, there is the role of measurement.

In fact, each compression face 10C of the mobile pistons 101 and 10S constitutes a probe equipped with four aligned electrodes. Among these four electrodes 141 and 14U, the two end electrodes 141 inject the electric current into the compressed powder test tube, while the two central electrodes 14U collect the voltage through the material to be measured. The rear ends 12 of the movable pistons 10S and 101 have a larger section intended to receive and transmit the mechanical pressure supplied by a jack. In addition, these rear portions 12 each have a blind hole 18 whose purpose is to allow each of the pistons to be easily extracted from the interior of the device, once the compression test has been carried out.

Each of the movable pistons 10S, 101 has two peripheral annular grooves 15 about 3 mm wide and formed on the outer surface of the cylindrical parts 11 of the pistons. The first is positioned approximately 3 mm from the compression surface 10C of each piston, the second being positioned 3 mm above. These grooves 15 have a depth of approximately 0.5 mm and have the role of reducing the surfaces in contact between the internal surface 2 of the compacting jacket 1 and the external surface of the cylindrical part 11 of the movable pistons 10S and 101. They can also serve as a receptacle in the event of the powder rising, during the compression of the sample. These grooves 15 then recover the excess material and limit the friction between the movable pistons 10S and 101 and the compacting jacket 1, in the area where the measurement is carried out.

Each movable piston 10S and 101 is preferably made in two parts 16 and 17, precisely adjusted and immobilizing the electrodes 14 which are shouldered in order to immobilize them.

It is specified that the measurement of the variation in height of the powder column is done by means of a non-contact proximity sensor, fixed on the actuator generating the compression force and not represented. It is equipped with a permanent magnet and thanks to the magnetic field, detects any movement of the movable pistons 10S and 101.

Inside each movable piston 10S and 101, there are four holes allowing the passage of the measurement wires 131 and 13U connected to the electrodes 141 and 14U. They are connected two by two, some to a stabilized direct current generator, not shown in FIG. 1, the others to a precision voltmeter, not also shown. These mobile pistons 10S and 101 are made of ceramic.

The fixed or static assembly of the system mainly comprises two concentric elements. The first is the outer cylinder 3 in the form of a cylinder with a diameter of about 60 mm and a height of about 40 mm. It has a hole of about 30 mm over its entire height.

Inside this is the compacting jacket 1 which carries the electrodes 41 and 4U and which fits perfectly inside the external cylinder 3. A cylindrical recess of internal surface 2 is provided, of a diameter of approximately 16 mm at its internal surface 2. At each end of this recess, there is a slight frustoconical chamfer 7 facilitating the reception and introduction of the two movable pistons 10S, 101. This compacting liner 1 receives four radial electrodes 4 arranged at 90 °, equidistant, around the main axis of the device which is that of the movable pistons 10S and 101. They are machined so as to open out inside the compacting chamber. These electrodes 4 are provided with a heel which is in contact with the internal surface of the external cylinder 3, so that these radial electrodes 4 are immobilized perfectly. In addition, they have a tongue 8 embedded in the ceramic of the shirt compaction 1 and intended to bring the current or take the voltage to be measured during the measurements. Each tab is connected to a conductive cable 5. FIG. 2 clearly shows the 90 ° arrangement of the four radial electrodes 4 inside the compacting jacket 1. The radial electrodes 4 are associated in adjacent pairs. The two intensity electrodes 41, intended to apply the current in the powder to be compacted, are adjacent and connected to a direct current generator 16. Likewise, the voltage electrodes 4U are also adjacent and connected to the precision voltmeter 19.

The compression of the powder does not present any particular difficulty. It is not the same for the measurement of the variations in resistivity during this compression operation. Indeed, this measurement depends on a number of parameters, such as density, pressure, duration of compression, friction, etc. In addition, certain powders, such as graphite, have a special feature which is the preferential orientation, generally perpendicular to the axis of application of the force, that is to say the axis of displacement of the pistons. Finally, the homogeneity of the compact sample is difficult to obtain. It is by taking into account the problems associated with this type of measurement that the apparatus, object of the invention, was designed. It is based on the principle of measuring resistivity by the method of four aligned or square points. This method of compacting and measuring the resistivity according to the invention leads to measuring the true resistivity of the material, which is impossible to achieve with a cell equipped with only two electrodes. In addition, the fact that the two movable pistons 10S and 101 are mobiles largely contributes to achieving this homogeneity.

With reference to FIG. 3, the first measurement case uses four tips aligned in the same plane, that is to say on the compacting surface of the upper movable piston 10S. The two end electrodes 141 pass the electric current through the compacted powder 6. The two central electrodes 14U are intended to capture the potential difference in the compacted powder 6. This type of measurement is perfectly suitable at the end of the compacting phase of the powder, when the sample densifies, while its thickness is reduced.

With reference to FIG. 4, the assembly used is exactly the same, except that it is applied to the lower movable piston 101.

By cons, the assembly of Figure 5 is a variant. In fact, the current electrodes 141 of the upper mobile piston 10S are used to inject the current into the compacted pouore 6. And in this case, it is the electrodes 14U of the lower mobile piston 101 which are used to sense the potential difference in compacted powder 6. This method makes it possible to control the homogeneity of the compacted product, as well as small variations in resistivity during a compression plateau.

Finally, the assembly of Figure 6 is slightly different. Indeed, it is the radial electrodes 4 of FIGS. 1 and 2 and referenced here 4U and 41 of the measurement jacket 1 which are stressed. It will be recalled that these four radial electrodes 4U and 41 are arranged in a square at 90 °, the current being injected on one side, the voltage being measured on the other. This measurement makes it possible to study the orientation effects observed with certain powders, such as graphite, while limiting the effects inherent in inclusions and inhomogeneities.

These three types of measurement, which can be performed alternately, allow permanent monitoring of the behavior of the powder during the compaction test. Operating on two perpendicular planes, the apparatus according to the invention can make it possible to analyze the variations in resistivity during compression, as during a plateau when the pressure is stabilized.

The theory of calculation allowing to exploit these measures of resistivity is based on the theorem of LAPLUME which treats the use of the electrodes with four points, aligned or in square. The basic formula is as follows:

p = K.U / I,

p being the resistivity, I the injected current and U the measured voltage, K the form coefficient (diameter and height of the powder column).

Claims

1. Method for compacting and measuring the resistivity of the powders being compacted in a compacting chamber inside which the powder to be compacted and to be measured is enclosed, by means of electrodes (41, 4U, 141, 14U ) opening into the compacting chamber in order to subject the powder being compacted to an electric current and to measure the potential difference which results therefrom, the compacting chamber comprising a compacting jacket (1) inside which move two movable pistons (10S, 101), along a common axis, each of the movable pistons (10S, 101) having at least four aligned electrodes (141, 14U), including two electrodes (141) for passing the electric current in the compacted powder, including two electrodes (14U) to measure the potential difference in the powder during compaction and to carry out axial resistivity measurements.

2. Method according to claim 1, characterized in that it consists in measuring the axial resistivity with the four electrodes (141, 14U) of the same movable piston (10S, 101).

3. Method according to claim 1, characterized in that it consists in measuring the radial resistivity with radial electrodes (4, 41, 4U) of the compacting jacket (1) oriented at 90 ° to the periphery of the compaction, to measure the radial resistivity.

4. Method according to claim 1, characterized in that it consists in measuring the axial resistivity, in particular during the evolution of compaction, with two electrodes (141) of a first mobile piston (10S) or (101) and with two electrodes (14U) of a second mobile piston (101 or 10S).

5. Device for compacting and measuring the resistivity of powders during compaction, comprising:

- a compaction chamber inside which the compacted powder to be measured is enclosed; and

- electrodes (141, 14U, 4, 41, 4U) opening into the compacting chamber to subject the compacted powder to an electric current and measure the resulting potential difference, characterized in that the compaction includes: a compaction mediation (1) inside which move:

- two mobile pistons (10S, 101) along a common axis, each mobile piston being fitted with at least four electrodes (141, 14U) including two electrodes

(141) for passing the electric current and two other electrodes (14U) for measuring the potential difference, in order to carry out axial resistivity measurements.

6. Device according to claim 5, characterized in that the compacting jacket (1) has four radial electrodes (4, 41, 4U) opening onto the internal surface (2) of the compacting jacket (1) and oriented at 90 ° relative to each other around the axis of the movable pistons (10S, 101) for carrying out radial resistivity measurements.

7. Device according to claim 5, characterized in that the compacting jacket (1 ⁾ is made of ceramic based on alumina.

8. Device according to claim 5, characterized in that the compacting jacket (1) is in an external cylinder (3) made of alumina-based ceramic.

9. Device according to claim 6, characterized in that the radial electrodes (4, 41, 4U) of the compacting jacket (1) are oriented radially at 90 ° from each other.

10. Device according to claim 9, characterized in that the compacting jacket (1) has slots oriented radially at 90 ° and passing through the entire thickness of the compacting jacket (1) to receive the electrodes (4, 41 , 4U).

11. Device according to claim 5, characterized in that the mobile pistons (10S, 101) have peripheral grooves (15).