This application is a continuation under 35 U.S.C. § 120 of International application PCT/EP2012/002250 filed on May 25, 2012, designating the U.S. and claiming priority to European application 11004375.9 filed on May 27, 2011 in Europe. The content of each prior application is hereby incorporated by reference in its entirety.
The disclosure is related to a contact material and particularly a contact material having Cu/Cr composite materials that include the element Si in a small quantity <1 wt. % in order to achieve a high short-circuit current interruption performance of the vacuum interrupter.
The predominant contact materials for vacuum interrupters can include Cu/Cr composite materials consisting of 50-75 wt. % copper and 25-60 wt. %, for example, chromium, known powder metallurgy techniques are mainly used for the production of these materials. It is known that the chemical composition of the contact material can be used for the application in vacuum interrupters. Finterstitial gas contents as well as copper-oxides and chromium-oxides should be kept as low as possible. Alumino- or silicothermic produced Cr powders are widely used as raw powder sources for the production of Cu/Cr materials. The alumino- or silicothermic method (also known as Goldschmidt-Process) is a comparable cheap technique used in the chromium metal industry. In this process the metals Al or Si or a mixture thereof is used to reduce Cr2O3 to Cr via the following reactions:
Cr2O3+2 Al→2 Cr (I)+Al2O3 (1)
2 Cr2O3+3 Si→4 Cr (I)+3 SiO2 (2)
As a consequence of the reducing agents (Al or Si), Cr powder products contain residual contaminations of Si or/and Al in elemental and oxide form. The Cr is produced in batches in very big quantities (e.g., several tons each batch). In this technique a precise control of the Si contaminants can be difficult and in some cases impossible to achieve. As a consequence, there can be rather high variations in local Si concentration within a production batch and of course from batch to batch. This result can present severe problems for the vacuum interrupter application, where a very precise control of the Si content at ppm-level is specified. It was found experimentally that variations in Si content of the contact material, which are related to different chromium batches, can lead to a very poor and random current interruption performance.
An exemplary contact material for a vacuum interrupter is disclosed, comprising: copper Cu and chromium Cr, wherein a content of the chromium is above 10 wt. % and the material is doped with silicon below 0.2 wt. % (2000 ppm Si) and the remainder is copper Cu.
An exemplary method for making a contact material is disclosed, the contact material including copper Cu and chromium Cr, wherein a content of the chromium is above 10 wt. % and the material is doped with silicon below 0.2 wt. % (2000 ppm Si) and the remainder is copper Cu, and the method comprising: coating chromium (Cr) particles with a silicon-precursor.
Exemplary embodiments of the nozzle ring according to the disclosure are described in the following with reference to the drawings, in which:
a illustrates a polymer structure following NH3 curing in accordance with an exemplary embodiment of the present disclosure;
b illustrates a cross-linking reaction of the precursor layer in accordance with an exemplary embodiment of the present disclosure;
a and 4b illustrate SEM micrographs of the uncoated and coated Cr powder in accordance with an exemplary embodiment of the present disclosure;
a and 7b illustrate fractographs of the two materials of
Exemplary embodiments of the present disclosure enable precise control of the Si concentration of Cu/Cr contact materials. Furthermore, the Si resulting from the exemplary techniques described herein can be homogeneously distributed within the contact material in order to generate the maximum doping effect. This distribution can result in a reliable performance of vacuum interrupters at very high level, which is independent of quality variations between different Cr raw powder batches.
In accordance with the exemplary embodiments disclosed herein the chromium content can be above 10 wt. % and that the material is doped with silicon below 0.2 wt. % (2000 ppm Si) and the remainder is copper Cu. In another exemplary embodiment, the microstructure can include chromium (Cr) particles which are covered by a thin layer of silicon (Si) or Si-based material (e.g. SiOx).
Further advantages provided by the disclosed exemplary embodiment can include the silicon (Si) or Si-based material (e.g. SiOx) being located at phase boundaries between chromium (Cr) and copper (Cu) and are therefore homogeneously distributed within the microstructure. In detail, the unnotched impact bending strength of the material can be higher than 30 J/cm2. Furthermore the electrical conductivity of the material can be in the range of 30-35 MS/m, for example.
According to an exemplary embodiment of the present disclosure, the chromium (Cr) particles are coated with a silicon-precursor, in order to bring in the silicon. To introduce the silicon in a very effective way, the silicon-precursor can be a polysilazane or similar Si-containing polymer.
Exemplary embodiments disclosed herein provide advantages in using mostly advantageous, a powder metallurgical process for making the contact material in the manner discussed above, that the coated Cr particles are further mixed with copper, pressed into contact shape, and finally sintered. The contact material resulting from the exemplary process can be used for contacts or contact surface coverage material for low, medium or high voltage switchgears. Furthermore, the contact material can be used for shielding, or shielding surface coverage material for medium or high voltage switchgears, such as in vacuum interrupters for medium voltage.
According to an exemplary embodiment of the present disclosure, the resulting material includes chromium (Cr) particles either dispersed or arranged in a network within a continuous copper (Cu) matrix phase. In another exemplary embodiment chromium (Cr) and copper (Cu) can also form an interpenetrating network of phases, depending on the content of chromium (Cr) and copper (Cu). The Cr particles or Cr phases can be covered by a silicon (Si) based coating.
According to an exemplary embodiment of the present disclosure, issues concerning distribution and precise control of Si dopant in Cu/Cr contact materials can be addressed by coating the Cr particles with a Si-precursor in a very simple wet-chemical process. In an exemplary embodiment, a polysilazane of the type PHPS (=perhydropolysilazane) can be used as Si-precursor. It should be understood that any type of polysilazane or other similar Si-containing precursor can be used to achieve the exemplary Cu/Cr microstructure of the disclosed contact material. The coating causes a very homogeneous distribution of Si in the final Cu/Cr material, which can generate a maximum doping effect. Moreover by adjusting the Si-precursor concentration, a precise control of Si content of the final Cu/Cr material can be achieved. This guarantees a reliable performance of vacuum interrupters and a stable contact material production independent of raw powder variations.
a illustrates a polymer structure following NH3 curing in accordance with an exemplary embodiment of the present disclosure. In accordance with an exemplary embodiment of PHPS precursor is purely inorganic. As shown in
For the coating of Cr powders diluted precursor solutions can be made. Dibutylether can be used as a solvent. The PHPS concentration in the precursor solution can be in the range of 0.5-1.5 wt. %, for example. The PHPS concentration can be adjusted to the target Si value of the final Cu/Cr material. As a result, a precise control of final Si-concentration in the ppm-range can be achieved. For the coating process, the Cr powder can be immersed in the precursor solution. The precursor reacts immediately with the particle surface forming strong chemical (covalent) Si—O—Cr bonds. After a few minutes of mixing, the dibutylether is removed by evaporation. Most of the solvent can be recovered in a condensation gap and can be reused. The resulting dry Cr powder particles are covered with a thin layer (few nm thick) of Si-precursor. Due to the fact that the drying step is performed in air, the Si-precursor layer on top of the Cr particle surface undergoes a slow cross-linking reaction which starts when the dried Cr powder comes in contact with air.
a illustrates SEM micrographs of the uncoated and coated Cr powder in accordance with an exemplary embodiment of the present disclosure. The coated powder exhibits a very homogeneous Si distribution covering all surface area of the Cr particles. The colour code red expresses Si in the EDX mapping. XPS measurements revealed that the PHPS precursor was transformed during a full cross-linking reaction to a dense SiOx layer, with x ranging from −0.9 to −1.1, for example, on the outmost surface region (4-5 nm). After the full powder metallurgy processing to a sintered Cu/Cr contact material the Si concentration was measured by ICP-OES. The measured concentration of 280 ppm Si matched the target value of 290 ppm very well. (
In the following, the main advantages of exemplary embodiments of the present disclosure are summarized briefly.
In order to demonstrate the fundamental different behaviors of undoped and Si-doped Cu/Cr contact materials, two examples for both types of material are described in the following. Both materials have been processed using the same raw powder source and almost identical processing steps. The only difference in processing was in the doping with Si. One was undoped (not coated with the Si precursor) and the other was Si-doped (coated with the Si precursor). In order to evaluate the current interruption performance of contact materials, the materials were installed into commercial vacuum interrupters of the same design and tested under the same conditions. A standard three-phase electrical test procedure was performed to determine the limit in short-circuit current interruption ability.
As
According to an exemplary embodiment of the present disclosure a further improvement of Cu/Cr contact materials can be achieved with respect to their mechanical performance. Contacts for vacuum interrupters should withstand comparable high mechanical impact loads, because of the fast opening and closing speeds at which the interrupter can be operated in service. It was found that Cu/Cr materials with Si-doping exhibit higher impact strengths compared with undoped materials.
This difference in mechanical performance can be explained by the accompanying fracture surfaces.
a shows the fracture surface of undoped Cu/Cr contact material shows large gaps between Cr particles and Cu matrix phase. The bonding between Cr and Cu phase is comparably poor. In
Another important material property for the application in vacuum interrupters is the electrical conductivity of the contacts. In their major applications vacuum interrupters are operated in closed position most of the lifetime. A high electrical conductivity is of significant advantage in order to generate minimum losses under nominal currents. It is to be noted, that Si-doped Cu/Cr materials offer a comparable high electrical conductivity. This result was surprising, as usually almost all additives to copper based conductor materials lead basically to a decrease in electrical conductivity. However, the electrical conductivity of Si-doped Cu/Cr material (containing 280 ppm Si) is even higher than of the same undoped Cu/Cr material. The electrical conductivities of both materials, which have been processed are almost identical. The only difference was in the doping with Si.
A chromium raw powder batch with a measured (by ICP) Si content of 88 ppm was used as starting material. The target value of Si concentration in the final Cu/Cr contact material was set to be 290 ppm. A concentrated solution of 20 wt. % PHPS precursor was further diluted to 1.00 wt. % PHPS by addition of dibutylether. 1000 g of chromium powder was added to 128.0 g of the diluted precursor solution and mixed for a short period (<30 min). After this the dispersion is dried by removal of the dibutylether solvent by rotational evaporation at a pressure of 40 mbar and a temperature of 60° C. for approximately 1 hour. After this treatment the dry Cr powder was mixed with Cu powder in the ratio 25 wt. % Cr to 75 wt. % Cu. After pressing the Cu/Cr powder mixture and final sintering to a dense contact material the Si content was determined (by ICP) to be 280 ppm. The Si is homogeneously distributed within the sintered microstructure. The Si can be located at the phase boundary between Cr and Cu.
A chromium raw powder batch with a measured (by ICP) Si content of 52 ppm was used as starting material. The target value of Si concentration in the final Cu/Cr contact material was set to be 600 ppm. A concentrated solution of 20 wt. % PHPS precursor was further diluted to 1.40 wt. % PHPS by addition of dibutylether. 1000 g of chromium powder was added to 232.0 g of the diluted precursor solution and mixed for a short period (<30 min). After this step, the dispersion is dried by removal of the dibutylether solvent by rotational evaporation at a pressure of 40 mbar and a temperature of 60° C. for approximately 1.5 hours. After this treatment, the dry Cr powder was mixed with Cu powder in an exemplary ratio of 25 wt. % Cr to 75 wt. % Cu, for example. After pressing the Cu/Cr powder mixture and final sintering to a dense contact material, the Si content was determined (by ICP) to be 589 ppm. The Si is homogeneously distributed within the sintered microstructure. The Si can be located at the phase boundary between Cr and Cu.
Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
Number | Date | Country | Kind |
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11004375.9 | May 2011 | EP | regional |
Number | Date | Country | |
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Parent | PCT/EP2012/002250 | May 2012 | US |
Child | 14092037 | US |