DE3005725A1 - Porous metal battery electrode - with active material impregnated in the pores - Google Patents

Abstract

Battery electrode has a plaque of sponge-like porous metal matrix (1) containing a multiplicity of cells (2) connected with each other three-dimensionally. The sectional area of the gratings forming the plaque is reduced continuously in the thickness of the plaque from the surface to the centre thereof and an active material is impregnated in the plaque. Sectional area of the grating near the surface is pref. between 1 and 5 times the area of the grating at the central part. Provides a battery electrode with a high density of active material which is produced at low cost.

Description

Battery electrode

The invention relates to a battery electrode with a plate made of a sponge-like porous metal matrix in which a large number of cells are arranged that are three-dimensionally connected to each other.

Common electrodes for primary batteries currently on the market are manufactured using a process in which one is mainly composed of one active material impregnated powder mixture directly into the battery case is or with a carrier, such as. B a grid, a sieve, a punched one Metal or a stretched metal, which is arranged on the middle part of the mixture This manufacturing process is simple and has the advantage that a number of active materials can be used for impregnation, so the process is suitable for the # primary battery # which does not require very high strength. Representative Secondary batteries or accumulators close the lead-sulfuric acid accumulator and the nickel-cadmium accumulator. The electrodes for the lead accumulator are made in a process in which a paste, which is mainly composed of consists of at least one active material, with a lead grid or an expanded metal is coated as a support, or a method using a metal cylinder with a multitude of small pores is impregnated with an active material. These methods result in a simple manufacturing process because the major ones work only in the coating of an active material or in the impregnation this material directly into the cylinder exist. The electrodes of the nickel-gadmium accumulator, on the other hand, are manufactured by means of a process in which an active material is poured into a sintered plate made of nickel powder is used, or a method is used in which the active material is directly in a metal bag is filled, which has a plurality of small pores. The former method was developed and required after the last world war a rather complicated process of dipping the electrodes in a salt of the active material to convert the salt into an active material. on the other hand the electrode produced by this process dominates that of others Process manufactured electrodes in terms of strength and electrical Operational characteristics. The latter method is used to make a pocket plate electrode used and is simple in view of the fact that the active material powder directly is filled, although on the other hand the electrical operating characteristics of the electrode manufactured in this way are inferior to those of the electrode below Using a sintered plate.

The usual methods of making a battery, as well as their Electrodes have been outlined briefly above, with those used below The term "battery" is intended to include both primary cells and accumulators. Considerable effort is currently being made, a secondary battery or a Accumulator or especially an alkaline accumulator (mainly a To develop a nickel-cadmium accumulator, which has a non-sintered electrode, which is not of the pocket plate type and which has electrical properties having, which are essentially identical to the sintered ones which are better in terms of their strength Electrode, it is desired that this electrode in a simple manufacturing process can be impregnated with a high density active material. Such procedures are for example in U.S. Patents 2,474,502 and 2,694,743 for a sponge-like porous metal matrix, while in U.S. Patents 3,287,164 and 3,597,829 an electrode using such a sponge-like porous one Metal matrix is described. This sponge-like porous metal matrix is made by the following procedure was prepared.

In one method, a sponge is made of resin material with a metal coated and after removing the resin, the metal is annealed. According to a second method, a metal powder is poured into a resin sponge and the entire structure is sintered. According to a further method, a mixture sintered or melted from a # pore .forming material and a metal powder and the remainder to form pores. Any material that remains is through Etching or the like removed. Another method is to turn a gas into a molten metal is blown in and the metal is cooled, with the bubbles held therein, whereupon high pressure is applied to the films between to remove the bubbles. The process by which the resin is coated with metal, is already used industrially. In any case, the sponge-like has porous Metal matrix on pores, the mean diameter of which is generally larger than that Diameter of the pores a sintered plate made of a metal powder (about 1 / im). This mean pore diameter is for the grain size of the active Material and makes a direct filling of the metal matrix possible. That Sintered substrate, on the other hand, has a smaller mean pore diameter so that it is almost impossible to fill the active material powder directly. Instead, it is after immersing the plate in a saline solution of the active Material required, the salt impregnated in the plate into an active material to convert, or the plate must be cathodically polarized in a saline solution, to achieve the active material. This shows that in the case of a spongy porous metal matrix the active material using essentially the same simple manufacturing process can be filled as it is for a non-sintered Electrode is used. Another feature of the electrode using a Plate made of a sponge-like porous metal matrix is that due to the fact that the active material is completely covered by a metal grid is, a long service life and a high electron conductivity of the electrode is achieved so that it is suitable for high current discharge.

Furthermore, the porosity can easily exceed the value of the sintered Plate (about 80%) can be enlarged so that the density of the filled active Material is enlarged. In the case of a sponge-like porous metal matrix, the for example made by coating with a metal. is a porosity possible up to a maximum of about 98%. In continuous production however, in industrial applications a porosity is about 96% or less desirable, to maintain the strength of a plate with a thickness of 1 to 2 mm. The sponge-like porous metal matrix, the porosity of which is about 20% higher than that of of the sintered plate (about 80%) can be with a proportionally larger Amount of active material to be filled.

The electrode with a sponge-like porous metal matrix existing plate has advantages in terms of various electrical properties as a battery electrode and it can be used in a simple process with a large Volume of active material to be impregnated. Therefore it is for primary batteries, which should have high current discharge properties, as well as good for accumulators suitable. This electrode can also be used as a spiral electrode.

When using an electrode with a plate of the sponge-like porous metal matrix with the above-mentioned characteristics for a nickel electrode The following advantages: By coating a resin sponge with nickel, a sponge-like porous metal matrix with substantially uniform lattice diameters (mean porosity of 95% and about 21.5 cells per cm). This metal matrix was used as a plate filled with a paste of a mixed powder which consisted mainly of hydroxide nickel powder (86% by weight), whereby results in a filling density of 300 to 320 mAh / cm3. I4erm a pressure of 400 kg / cm2 is exercised, the density is increased to 430 to 450 mAh / cm3. This value is already greater than the density of 350 to 400 mAh / cm3 for a common sintered Plate.

The invention is based on the object of providing a battery electrode To create the type mentioned at the outset, which is an essential at low manufacturing costs has increased storage density and strength.

This task is carried out by the characterizing part of the claim 1 specified invention solved.

Advantageous refinements and developments of the invention result from the subclaims.

It was found that when the cross-sectional area of the grating near the surface of the sponge-like porous metal matrix plate is increased, while the cross-sectional area of the grid at the central part of the plate is reduced in size, a larger amount of active material is impregnated into the plate can be, regardless of the same porosity of the overall structure, this high density is maintained after the print processing. The cross-sectional area the grid can be changed by changing the conditions when covering the sponge with the Metal can be changed, for example by changing the current density or by Moving the source material. A sectional view of such an embodiment of the sponge-like porous metal matrix is shown schematically in FIG.

According to one embodiment, a foamed resin becomes uniform inside coated with soot and on both Pages in a watt solution without agitation at a current density of 3.0 A / cm for a period of approximately 5 minutes plated with nickel. The resulting structure is washed in water and that Resin is removed by baking, followed by annealing in a hydrogen atmosphere takes place at 800 ° C for 30 minutes, so that a sponge-like, consisting of nickel, porous metal matrix is produced with an average porosity of 96%, the a cross-sectional area of the grid near the surface of about 2800 e 2, while the cross-sectional area of the grid in the central part is approximately 700 / um. The number of cells is approximately 21.6 cells per cm. These porous web is compressed between rollers so that the surface is flattened and the porosity is brought to a controlled value of 95%. This Material is then used as a plate covered with a paste of a powder mixture is filled, the 86 wt.% Of hydroxide nickel with an average grain size of 25 to 150 / µm. This arrangement is then dried and placed between levels Plates exposed to a pressure of 400 kg / cm2. This gives a density of 500 to 520 mÄh / cm3 of the active material in the electrode was achieved.

In a further experiment a similarly foamed Resin on both sides with nickel at a current density of 1.0 A / cm2 and with sufficient Stir the bath and the resulting sponge-like porous nickel-metal matrix is coated with uniform cross-sectional area of the grid was used as the plate, the was filled with a similar paste, essentially a hydroxide-nickel powder contained. the The plate thus formed was subjected to a pressure of 400 kg / cm2 exposed, and there was a density of the active filled in the electrode Materials from 430 to 450 mAh / cm3.

In the latter case, the density before the press processing was 300 to 320 mAh / cm3, which is compared to the density of 350 to 380 mAh / cm3 in the former case is much less. This shows that a greater amount of active material is involved the plate according to the first-mentioned embodiment can be used as in the latter attempt. It was also found that the cross-sectional area of the Lattice in the middle part of the resin sponge compared to the cross-sectional area the grid near the surface of this sponge, at a magnification of the Current density or with a decrease in the movement of the bath when coating with Metal or when the thickness of the resin sponge increases.

This reducing the cross-sectional area of the grids inside the Sponge or the then formed sponge-like porous metal plate allows the absorption of a larger amount of active material without loss of strength the impregnated plate.

It is also advantageous if the cells in the spongy porous metal matrix having a spindle-shaped shape running in one direction have longer and shorter dimensions because this is particularly a winding of the plate-shaped electrode in the direction along the shorter diameter of these cells.

Embodiments of the invention are described below with reference to Drawings explained in more detail.

The drawings show: FIG. 1 a photomicrograph of an embodiment a sponge-like, porous metal matrix consisting of nickel, in which the nickel Lattice (with a cross-sectional area of approximately 2000 / µm2) forms the spherical Surrounding cells, Fig. 2 is a sectional view schematically showing the reduction of Cross-sectional area of the lattice of the sponge-like porous metal matrix along the Thickness of the metal matrix, starting from the surface to the central part, shows 1 denotes the metal grids and 2 denotes the cells, FIG. 3 is a graphical representation Illustration of the relationship between the ratio of the cross-sectional area of the grid near the surface and the cross-sectional area of the grid near the middle part as well as the density of the active material of hydroxide-nickel, the is made from hydroxide nickel powder (with the composition of the below described first embodiment), this plate after filling with a pressure of 400 kg / cm2 was applied, Fig. 4 is a photomicrograph showing an embodiment a nickel plate made of a sponge-like porous metal matrix with essentially spindle-shaped cells showing the largest Part run in one direction and are delimited by nodding-itter, which the plate form, where the abscissa and ordinate represent the short and long sides, respectively, Fig. 5 is a graph showing the relationship between the ratio of the longer to the shorter diameter and the number of bending cycles that go along a break cause of the shorter diameter of the electrode, which a plate according to Fig. 4, which is impregnated with the active material, with the electrode between Rolls with a diameter of 100 mm and alternating around 900 in two directions, Fig. 6 is a graph showing the relationship between the number of #ade discharge cycles and the discharge capacity of a half cell shows using an electrode according to the first embodiment of the invention; where (a) the case of using an electrode having dimensions of 60 x 50 x 1 mm and a counter electrode made of a nickel screen and an electrolyte in the form a 30% aqueous KOH solution, while (b) shows the case of using a sponge-like, porous metal matrix made of nickel with the same cross-sectional area the grid on the middle part and near the Surface shows and wherein (c) the use of a conventional sintered nickel positive electrode Fig. 7 is a graph showing the relationship between voltage and the discharge capacity, the discharge curve a showing a discharge at 1650 mA of a cylindrical enclosed nickel-cadmium storage battery (C-2yp) shows an aqueous KOH solution with 30 wt. In an amount of 6.5 cm3, a separator and a common cadmium electrode similar to the third one described below or fourth embodiment, while the discharge curve b is the case of use a spiral-shaped nickel positive electrode according to the first embodiment shows in which the application of pressure to achieve a lower density of 430 mAh / cm3 has been decreased, and where the discharge curve c is the case of an ordinary battery represents.

In Fig. 1 is a photomicrograph of a spongy porous Metal matrix shown with spherical cells with a diameter of 450 to 550 / µm, which are bordered by grids. The graph of Fig. 3 shows the Relationship between the change in the ratio of the cross-sectional area of the grid in the middle part to the cross-sectional area of the grid near the surface along the strength of a spongy, porous matrix made of nickel a middle one Porosity of 95 96 versus the density of a Powder mixture consisting mainly of hydroxide-nickel powder (86% by weight) and which was impregnated into the electrode and subjected to a pressure of 400 kg / cm2. The graph of Fig. 3 shows that the density of the impregnated material with increasing value of the ratio of the cross-sectional area of the grating in the Proximity of the surface multiplied by the cross-sectional area of the grid in the central part increases with 100. If the value of this ratio multiplied by 100 is the However, if the value exceeds 500, the strength of the grid becomes in the middle part too small, so that there is a risk that the plate will fall apart into upper and lower parts, so that the strength of the electrode is reduced to an insufficiently low level will. This problem exists with an average porosity, the greater than about 93%, while below this value the severity of the decreased Strength does not occur, but in this case the density of the impregnated active material with the porosity is reduced, so that the advantage of high There is no density of the impregnated active material. That is why this is above mentioned the ratio multiplied by 100 preferably 500 °, 6 or less. at A common method is electrodes with plates of different porosity or the like are superposed and the active material becomes high density impregnated into the panels. In contrast to this, according to the invention, the electrode designed so that the diameter of the grids forming the plate is continuous changes so that an active material is impregnated into this one-piece plate that will have higher porosity in their Has interior. this results in a long service life with an increased number of charge and discharge cycles on the one hand and the high strength of the grid along the longer side on the other hand it contributes to the fact that the grid is better able to contain the active material to hold on.

In the usual case a spiral electrode with a plate This spongy, nickel-made porous metal matrix is formed Break or crack in the spiral winding even with one electrode a thickness of 1 mm, if a mixture of the in the manner described above composite active materials with a high density of 520 mAh / cm2 will. In an extreme case, the plate is broken, so that the electrical operating properties of the electrode will be greatly deteriorated. Therefore, when an electrode in more effective Way to be used in this form, must be the density of the impregnated active material to a value of less than 430 mAh / cm3 in the case of an electrode be reduced with a thickness of 1 mm. This leads to the disadvantage that it it is not possible to fill the active material with high density. It was, however found that essentially no jump occurs when the plurality of cells d. H.

the spherical spaces, which according to FIG. 1 continuously in the sponge-like, porous metal matrix consisting of nickel are overlapped, are deformed in such a way that that most of the cells have a spindle-shaped shape with a special orientation assumes, whereupon an active material with a density of up to 520 mAh / cm3 in the electrode is filled in and the electrode spirals along the shorter side of the spindle-shaped cells is wound up. When the electrode is wound spirally along the longer side, there is a risk of Formation of a large number of cracks, especially in the case of deformed cells, the danger here is greater than with spherical cells. About the probability of the occurrence of cracks in spiral bending became a Test carried out in which an electrode with a width of 100 mm, a Length of 200mm and a thickness of 1mm with nickel hydroxide with the one mentioned above Composition filled to a density of 520 mAh / cm3 and between two cylinders was held with a diameter of 100 mm. The electrode was repeated bent back and forth in the longitudinal direction at an angle of 900. In Fig. 5 is the Relationship between the mean of the ratio of the longer diameter of the essentially spindle-shaped cells in the plate to the shorter diameter, multiplied with 100, shown opposite the number of bending periods (each of the bending of the electrode in both directions at an angle of 900) carried out before breakage occurred. The result of this experiment shows that a break is harder to achieve when the ratio between the longer diameter and the shorter diameter is enlarged. The sponge-like porous metal matrix with the essentially spindle-shaped cells was produced by the fact that a Resin sponge was coated with metal while stretched or in one direction was pulled apart. For the middle value of 150% or more of the ratio the longer diameter to the shorter diameter, multiplied by 100, arises a crack or break in the layer of soot, which is not then at the broken point coated with metal, and in some cases the grids broke when burned out of the resin to give a weak plate. A photomicrograph one sponge-like, made of nickel porous metal matrix with essentially spindle-shaped cells is shown in FIG. In this way an electrode using a plate made of a sponge-like porous nickel made of nickel Metal matrix with essentially spindle-shaped cells wound up in a spiral when an active material has been impregnated at a high density so that the achievement of a high capacity in a storage battery with spiral electrodes is possible. This is also true in the case of primary batteries having a spiral shape enlarged electrode area to achieve a high current discharge and others has improved electrical properties. The embodiment described is of course with the same effect on a spiral electrode a lead-sulfuric acid storage battery.

This sponge-like porous metal matrix with essentially spindle-shaped Cells can easily be made continuously. In particular, with regard to insist that the only requirement is to keep the resin floating as it is coated with metal to pull apart a long strip of this plate in a simple one Process are produced. In this case, the long strip should be preferred be pulled apart and stretched in the longitudinal direction.

The reason for the increased density of the active material contained in the sponge-like porous metal matrix of the electrode is impregnated in which the cross-sectional area of the grids towards the central part for enlargement the reduced porosity appears to be that the greater porosity in the plate it allows a larger amount of active material to be more easily can be impregnated, while at the same time there is no risk that the impregnated active material falls out again. The reason why there are no cracks when the active material spirals along the shorter diameter of the substantially spindle-shaped cells are wound up in the sponge-like porous metal matrix, seems to consist in the fact that the grids forming the plate still form one Have expansion area under the force in the direction of the outer circumference is exerted in spiral winding. (Of course, a Compression force exerted in the area of the inner circumference, but represents the expansion force in the area of the outer circumference represents the main external force in the case when a sufficient amount of the active material or the like is in the porous Metal is impregnated.) If the active material, on the other hand, spirals along of the longer diameter is wound, there is a greater risk of occurrence of cracks than when spherical cells are formed. In the case of a spongy porous metal matrix with spherical cells (95% porosity and 21.6 cells per cm) a break is hardly achievable given the density of the impregnated active material is reduced from 520 mAh / cm3 to 460 mAh / cm3. This is also not due to failure when the due to spongy porous nickel material, as was temporarily accepted because it was found that the conditions remain unchanged even if the annealing temperature is increased from 80,000 to 100,000 or if the glow time has been extended from 30 minutes to 1 hour.

Examples of electrodes are explained in more detail below.

First embodiment: a foamed urethane resin sheet (with a Thickness of 1.8 mm and 21.6 cells per cm) was in a colloidal liquid dispersion immersed in carbon black to coat the surface of the resin with the carbon black. the The resulting structure is from both sides in a standard Watt's solution coated with nickel for 5 minutes at a current density of 3 A / cm2 without stirring. The structure coated in this way was washed in water and in air at 50000 heated for 30 minutes. After removing the resin in this way the structure became in a hydrogen atmosphere when heated to 8000C annealed for 30 minutes, so that a sponge-like porous nickel plate with a mean porosity of 96 0.6, with 21.6 cells per cm, with a cross-sectional area 2 surface of the grids with a surface of 2800 cm and a cross-sectional area of Grating in the middle range of 700 / Im2 was made. This nickel plate was between rollers subjected to pressure to produce a plate with a thickness of 1.45 mm and a middle one To achieve a porosity of 95 96.

This plate is made with a paste-like mixture of 87% by weight hydroxide-nickel powder (most of which had a mean grain size of 25 to 150 µm) from 10% by weight nickel powder (with an average grain size of 2 to 6 / lm), cobalt powder (Average grain size 2 'to 6 and 0.3% by weight of an aqueous solution of carboxymethyl cellulose filled. The resulting structure was dried in a fluororesin suspension (with 1 wt, resin) immersed, dried again and one between two flat plates Subjected to a pressure of 400 kg / cm so that an electrode was formed.

Second embodiment: This embodiment corresponds to the first Embodiment except that a desired position of the spongy porous nickel plate a pressure of 500 kg / cm2 before impregnation with a has been exposed to active material. An electrode lead was then crimped onto the Welded position.

Third embodiment: This embodiment corresponds to the first Embodiment, however, with the exception that a foamed urethane resin under Applying a tensile force of 2 kg / cm2 in one direction was plated with nickel.

Fourth Embodiment: The electrode according to the third embodiment becomes substantially spindle-shaped in the direction of the shorter diameter Cells of the plate with a standard cadmium electrode and a non-woven polyamide separator wound spirally so that spiral electrodes result. On this Follow the usual steps for making a cylindrical wrapped step Nickel-Oadmium Storage Battery. Although the inventive steps in this Embodiment were only carried out for the nickel electrode, they can be carried out in the same way with the cadmium electrode, so that the same beneficial effect in a battery both positive and is achieved at the negative electrode. In the embodiment under consideration here the sponge-like porous metal matrix is used for the nickel electrode. if the metal of the sponge-like porous metal matrix is changed is the present one Invention applicable to electrodes of secondary or primary batteries in the same way, the good operating properties in terms of a high discharge rate should have, regardless of whether these electrodes are plate-shaped or are spiral.

In Fig. 6 is the result of an examination of the operating life cycles the nickel electrode according to the invention in a storage battery or an accumulator shown using a half-wave. The electrode had dimensions of 50 x 60 x 1 mm and it corresponds to the first embodiment. A nickel screen became was used as the opposite electrode and an aqueous KOH solution was used as the electrolyte used at 25% by weight. The charging process was at 200 mA for 10 hours at 200C performed while discharging at the same temperature and at 1000 mA was carried out, up to a voltage drop of 150 mV compared to Hg / Hgo of 25 Ges.0,6 of KOH, and the result is shown in curve (a) of FIG. For comparison, curve (b) shows the charge-discharge properties of an electrode of the same size made of spongy porous nickel metal with the grids in the have substantially the same cross-sectional area, the electrode being similar processed as in the first embodiment. For further comparison are the properties of a common sintered positive electrode of the same size represented by curve (c). In all curves (a), (b) and (c) there is a mean value shown for five half-cells. This graph shows that the invention Nickel electrode has a discharge capacity that is 20 to 25 ° h greater than the usual sintered nickel electrode (c) is during the service life is the same for both electrodes. On the other hand, the electrode according to the invention has has a discharge capacity which is approximately 15% higher than the electrode according to (b), it also has a longer service life. This is also true in the case of the electrode of the third embodiment.

The curve (a) of Fig. 7 shows the discharge characteristics of a Electrode of the third embodiment, in which the time that the resin floated was coated with nickel, was shortened, so that a spongy porous Nickel matrix with a thickness of 2 mm and a porosity of 97.5% resulted in and the plate obtained in this way between rollers to a thickness of approximately 1 mm was compressed. This particular electrode was made with a cylindrical Enclosed C-type nickel-cadmium storage battery used after the same method as in the fourth embodiment and they became after charging with 200 mA for 20 hours at 20 ° C with a current of 1650 mA discharged. The nickel positive electrode used had dimensions of 3ES x 210 x 0.68 mm, while the negative cadmium electrode has dimensions of 39 x 260 x 0.55 mm. During a test production of this storage battery, the Nickel existing positive electrode is not charged while the nega.tive # oadmium electrode was charged at approximately 500 mAh, so that the remaining theoretical capacity was about -mAh.

For comparison, curve (b) represents the electrical operating properties a battery with a nickel positive electrode of the same size as in Trap (a) and with a density of the impregnated material of 430 mAh / cm3 according to of the first embodiment. Furthermore, the curve (c) shows the operational characteristics a battery using a conventional sintered nickel positive electrode of the same size. It can be seen that the nickel electrode according to the invention has a larger capacity than the usual sintered nickel positive electrode, while the voltages are essentially the same.

Claims (7)

Claims jl: battery electrode with a plate of a sponge-like porous metal matrix with a multitude of cells inside, which are three-dimensionally connected to each other, thus g e k e n n -z e i c h n e t that the cross-sectional area of the forming the sponge-like porous metal plate Grid (1) continuous along the thickness of the plate, starting from the surface the plate towards its central part, is reduced and that an active material is impregnated into the porous plate. 2. Battery electrode according to claim 1, characterized in that g e -k e n n z e i c that is, the cross-sectional area of the grating is close to the surface of the sponge-like porous metal matrix more than 100% of the mean value of the cross-sectional area of the latticework in the middle Deil and a maximum of 500% of this. 3. Battery electrode according to Ar.spruch 1, thereby g e it is not noted that the sponge-like porous metal matrix has the shape of a long strip and that the porous metal matrix with an active material is impregnated, whereupon the impregnated metal matrix is formed under pressure and on cut to the desired size. 4. Battery electrode according to one of claims 1 to 3, characterized g e it does not indicate that most of the plurality of cells (2) in the sponge-like porous metal matrix a unidirectional spindle shape with longer and having shorter diameters. 5. Battery electrode according to claim 4, characterized in that g e -k e n n z e i c Note that the longer diameter does not exceed 150% of the mean of the shorter diameter. 6. Battery electrode according to claim 4 or 5, characterized in that g e k e n n z e i c h n e t that the sponge-like porous metal matrix has the shape of a long Has strip and is impregnated with an active material and that the impregnated active material molded under pressure and cut to the desired size is. 7. Battery electrode according to one of claims 4 to 6, characterized g e do not indicate that the electrode is in the direction along the shorter Diameter of the substantially spindle-shaped cells in the porous plate are wound spirally is.