Calibration bodies and methods for their manufacture

Abstract

The invention relates to the fields of materials science and measurement technology and concerns calibration objects, such as those that can be used in analytical spectroscopy. The object of the present invention is therefore to produce electrically conductive calibration blocks, in particular for the calibration of hydrogen measuring instruments. The calibration blocks can be provided in large quantities, simply and cost-effectively. The problem is solved by calibration bodies, in particular for the calibration of hydrogen measuring instruments, consisting at least of an electrically conductive, porous sintered body with a substantially homogeneous distribution of pores and / or defects, such as dislocations, stacking faults and / or twins, in the sintered body, in which an adjustable hydrogen concentration is present depending on the number and size of the pores and / or defects in the sintered body, wherein the hydrogen is physically and / or chemically bound in and / or on the surface of the pores and / or defects, and wherein the set hydrogen concentration in the calibration body decreases in a specified amount over time.

Description

[0001] The invention relates to the fields of materials science and metrology and concerns calibration objects, such as those that can be used in analytical spectroscopy, and a method for producing such calibration objects.

[0002] Calibration is used to check measuring instruments against their legal requirements.

[0003] Internal calibration is a calibration in which all measuring instruments are standardized to a company-internal or organization-internal standard.

[0004] A traceable calibration is a calibration whose reference is a national or international standard; consequently, the measurement results obtained are traceable to this standard.

[0005] Traceable measuring instruments are often also referred to as reference measuring instruments.

[0006] The advantage of measuring instruments that are traceable to national and international standards is the worldwide comparability of the measurement results.

[0007] Calibration in metrology is a measurement process for determining and documenting the deviation of a measuring instrument or standard from another instrument or standard, which in this case is referred to as the standard. Subsequently, taking the determined deviation into account, the readings are corrected during the subsequent use of the measuring instrument (Wikipedia, keyword calibration).

[0008] With regard to quality control, the materials used in lightweight construction, electromobility, power generation and power transmission must be checked for their homogeneity and chemical composition.

[0009] The measuring instruments required for this must be calibrated regularly to ensure the quality of the materials, for which so-called calibration blocks are used.

[0010] It can be assumed that the demand for calibration bodies will increase in the future for logistical and technological reasons, as well as due to the occurrence of damage in materials.

[0011] Calibrating measuring instruments for the analysis of light elements, especially hydrogen, presents a particular challenge, particularly when analyzing non-metallic alloying and interfering elements in iron, copper, or aluminum materials. Due to undesirable effects in these materials, such as increased susceptibility to cracking or reduced damage tolerance caused by the inclusion of atomic hydrogen, hydrogen analysis is of paramount importance, as is verifying the measurement accuracy of the necessary analytical instruments.

[0012] Analytical instruments that, for example, determine the hydrogen content of materials using hot gas extraction, are currently being calibrated with gaseous hydrogen.

[0013] Many measurement methods, such as optical emission spectrometry, require calibration with materials of known properties, such as element concentration, density, sputtering rate, etc., due to their functional requirements.

[0014] The intensity of spectral lines of elements with many electron transitions, such as Fe, is orders of magnitude lower than the intensity of the strongest lines of an element with few electron transitions, such as Cu. Therefore, the intensity of the emitted lines does not directly indicate the concentration of the elements. Calibration of the measurement setups is therefore particularly important.

[0015] Other measurement methods, in principle, allow direct inferences between the measurement signal and the sample properties, and calibration serves to improve the quality of this relationship. For example, there is a direct correlation between the number of ions counted in mass spectrometry and the quantity of the corresponding element, and the concentration can be directly inferred from the ratio of the count rates. However, more accurate results are obtained through calibration, since the sensitivity of the mass spectrometer is element-dependent, for example, due to different ionization energies and the mass-dependent sensitivity of the mass spectrometer.

[0016] If the relationship between the sample properties and the measurement signal changes due to drift of the measuring instrument, recalibration must also be performed. This is also possible with samples of unknown but constant properties.

[0017] Likewise, the investigation of different matrix materials for calibration blocks is important, as the independence of the calibration from the respective matrix material must be investigated and proven, or, if necessary, a matrix-specific calibration must be implemented.

[0018] The availability of massive calibration samples has been a problem for various elements in different matrix materials for many years. Critical elements in this context include not only the lighter elements H, O, and N, but also, for example, Na and Se.

[0019] For hydrogen calibration targets, it is also important that both large and small quantities of hydrogen can be reliably detected using the targets, which the known calibration targets cannot adequately achieve. Furthermore, the known calibration targets for determining the concentration of the element hydrogen are consumed quickly, making their use costly.

[0020] Hydrogen-containing layers on titanium sheets are also used as calibration blocks. However, these layers, often referred to as TiH2, are not homogeneous in depth (i.e., not actually TiH2) and their hydrogen content changes over time. Therefore, they cannot serve as calibration blocks, as the results of measurements calibrated in this way are not comparable.

[0021] According to DE 2 160 202 A, a process for manufacturing hydrogen storage electrodes is known, in which the electrode bodies consist of a base material made of elements from the 3rd to 5th group of transition elements and of an active material that promotes the absorption of hydrogen, wherein the base and active materials are intimately mixed in finely dispersed form and pressed into a shaped body and subsequently sintered, then the sintered material is again crushed, mixed, pressed into a shaped body and sintered again.

[0022] Furthermore, according to DE 10 2011 084 775.8 A1, massive hydrogen-containing vacuum-tight samples and a method for their production and use are known.

[0023] The massive hydrogen-containing vacuum-tight samples consist of a conductive matrix material with homogeneously distributed TiH2 and / or Pd2H embedded within it, wherein the density is at least 95% of the crystallographic density and the hydrogen content directly corresponds to the amount of TiH2 and / or Pd2H in the sample.

[0024] These samples are produced by mixing a powdered conductive matrix material with TiH₂ powder and / or Pd₂H powder, homogenizing, compressing, and subsequently sintering. Compression and sintering are carried out in an inert gas atmosphere or vacuum at elevated pressure and temperatures between 90 °C and the decomposition temperature of TiH₂ and / or Pd₂H. This is performed in two stages to a pressure of at least 0.1 GPa, with temperature and pressure maintained at each stage and maximum compression pressure for at least 10 minutes. The sample is then cooled to at least 90 °C at a rate of no more than 30 °C / min. The amount of TiH₂ and Pd₂H powder in the powder mixture is determined by the desired hydrogen content in the sample.

[0025] According to DE 10 2013 225 940 A1, a method for producing solid calibration samples for analytical spectrometry is known, in which sinterable powder made of technically pure or pure material of at least one element to be calibrated and a sinterable matrix material are mixed and homogenized, or a sinterable powder made of an alloy and / or a compound of at least one element to be calibrated is used, wherein, in the case of the use of alloys and / or compounds of the elements to be calibrated, the further alloy or compound components are a conductive sinterable matrix material, or a powdered conductive sinterable matrix material is added, mixed and homogenized, and subsequently the powder mixture is pressed and sintered into a solid sample.The pressing and sintering are carried out in an inert gas atmosphere at a pressure higher than normal pressure or in a vacuum at temperatures of 60-80% of the melting temperatures of the material with the lowest melting temperature, whereby the sinterable powders of the starting materials remain essentially unchanged in terms of their composition and structure during pressing and sintering.

[0026] A disadvantage of prior art solutions is that electrically conductive calibration blocks for light elements, and especially hydrogen, are not available in sufficient quantities, and in particular, calibration blocks for hydrogen with a defined and time-limited hydrogen content cannot be provided. Furthermore, no simple and cost-effective methods for producing such calibration blocks are known.

[0027] The object of the present invention is therefore to produce electrically conductive calibration blocks, in particular for the calibration of hydrogen measuring instruments. The calibration blocks can be provided in large quantities, simply and cost-effectively.

[0028] The problem is solved by the invention specified in the claims. Advantageous embodiments are the subject of the dependent claims, and the invention also includes combinations of the individual claims in the sense of an AND conjunction, as long as they are not mutually exclusive.

[0029] The calibration body according to the invention, in particular for the calibration of hydrogen measuring instruments, consists of at least one electrically conductive, porous sintered body with a substantially homogeneous distribution of pores and / or defects, such as dislocations, stacking faults and / or twins, in the sintered body, in which an adjustable hydrogen concentration is present depending on the number and size of the pores and / or defects in the sintered body, wherein the hydrogen is physically and / or chemically bound in and / or on the surface of the pores and / or defects, and wherein the set hydrogen concentration in the calibration body decreases in a specified amount over time.

[0030] Advantageously, the electrically conductive, porous sintered body consists of Fe, Cu or Al or of a base alloy of one or more of these elements.

[0031] Furthermore, the electrically conductive, porous sintered body advantageously contains ceramic particles.

[0032] The electrically conductive, porous sintered body also advantageously contains oxide particles.

[0033] And also advantageously, the electrically conductive, porous sintered body has been subjected to plastic deformation after sintering.

[0034] It is also advantageous if the electrically conductive, porous sintered body has a substantially homogeneous distribution of pores and / or defects, such as dislocations, stacking faults, or twins, within the sintered body.

[0035] In the inventive method for producing calibration bodies, in particular for the calibration of hydrogen measuring instruments, a defined concentration of atomic hydrogen is set in an electrically conductive, porous sintered body by means of an electrochemical process and a time-defined decrease of the hydrogen concentration in the sintered body is further realized.

[0036] Advantageously, the electrically conductive, porous sintered body is mixed from powdered starting materials and processed into a shaped body and subsequently sintered, whereby the sintering conditions are adjusted depending on the starting materials and the desired porosity of the sintered body.

[0037] It is also advantageous to set a density of > 90% of the relative density of the respective sintered body in the electrically conductive, porous sintered body.

[0038] It is also advantageous if at least electrically conductive materials are used as powdered starting materials for the sintered body, preferably Fe, Cu or Al or a base alloy of one or more of these elements, or even more advantageously steels.

[0039] It is also advantageous if, in addition to predominantly electrically conductive materials, ceramic materials are also used as powdered starting materials for the sintered body.

[0040] It is also advantageous if, in addition to predominantly electrically conductive materials, pre-oxidized materials or materials are used as powdered starting materials for the sintered body.

[0041] It is also advantageous if the electrically conductive, porous sintered body is subjected to plastic deformation after its sintering.

[0042] It is also advantageous if the electrically conductive, porous sintered body is subjected to electrochemical treatment several times, preferably 2 to 10 times.

[0043] It is also advantageous to set a hydrogen concentration of 0.25 ppm to 2000 ppm in the electrically conductive, porous sintered body.

[0044] The solution according to the invention makes it possible for the first time to produce electrically conductive calibration blocks, particularly for the calibration of hydrogen measuring instruments. The calibration blocks can be produced in large quantities, simply and cost-effectively.

[0045] The calibration body according to the invention, in particular for the calibration of hydrogen measuring instruments, consists at least of an electrically conductive, porous sintered body with a substantially homogeneous distribution of pores and / or defects, such as dislocations, stacking faults and / or twins, in the sintered body, in which an adjustable hydrogen concentration is present depending on the number and size of the pores or defects in the sintered body.

[0046] Advantageously, the electrically conductive, porous sintered body consists of Fe, Cu or Al or a base alloy of one or more of these elements.

[0047] Advantageously, the electrically conductive, porous sintered body can also contain ceramic particles.

[0048] Alternatively, the electrically conductive, porous sintered body can advantageously contain oxide particles.

[0049] It is also advantageous if the electrically conductive, porous sintered body has been subjected to plastic deformation after sintering.

[0050] It is also advantageous if the electrically conductive, porous sintered body has several or all of the advantageous features simultaneously.

[0051] Furthermore, according to the invention, it is important that the electrically conductive, porous sintered body has a substantially homogeneous distribution of pores and / or defects, such as dislocations or stacking faults or twins, in the sintered body.

[0052] The electrically conductive, porous sintered body according to the invention can advantageously have a density of > 90% of the relative density of the respective sintered body.

[0053] Within the scope of the present invention, the porosity of the sintered body is specified as density in relation to the relative density of the respective sintered bodies. High porosity means low density, and low porosity means high density.

[0054] It is also important according to the invention that an adjustable hydrogen concentration is present depending on the number and size of the pores and / or defects in the sintered body. The hydrogen is physically and / or chemically bound in and / or on the surface of the pores and / or defects. Pores and / or defects, or other local areas with higher energy in the sintered body, which are intended to absorb or physically and / or chemically bind the hydrogen in the sintered body, can also be referred to as hydrogen traps.

[0055] Furthermore, according to the invention, it is also important that the set hydrogen concentration in the calibration body decreases in the specified amount over time in the electrically conductive, porous sintered body.

[0056] This allows comparable measured values ​​to be achieved by the calibrated measuring device even when the hydrogen concentration in the calibration body according to the invention decreases, based on the hydrogen concentration known at the respective time.

[0057] The object of the invention is further solved by a method for producing calibration bodies, in particular for the calibration of hydrogen measuring devices, in which a defined concentration of atomic hydrogen is introduced into an electrically conductive, porous sintered body by means of an electrochemical process and a time-defined decrease of the hydrogen concentration in the sintered body is further realized.

[0058] The electrically conductive, porous sintered bodies according to the invention are advantageously produced from powdered starting materials by mixing them and processing them into a shaped body and subsequently sintering them, whereby the sintering conditions are adjusted depending on the starting materials and the desired porosity of the sintered body.

[0059] Different sintering conditions can be set, for example, a sintering atmosphere in vacuum or under pressure, or the application of mechanical pressure to the sintered body during sintering.

[0060] This means that higher pressure during sintering generally results in lower porosity in the sintered body. Similarly, it is advantageous to increase sintering times and / or temperatures to achieve lower porosity. The particle sizes of the starting powders can also be adjusted to produce the most porous sintered body possible.

[0061] Advantageously, a density of > 90% of the relative density of the respective sintered body is set in the electrically conductive, porous sintered body.

[0062] The starting powders for the production of the electrically conductive, porous sintered body according to the invention must in any case be selected in such a way that an electrical conductivity of the sintered body is achieved.

[0063] Advantageously, powdered starting materials for the sintered body can be Fe, Cu or Al or a base alloy of one or more of these elements, or even more advantageously steels such as 42CrMo4 or X5CrNi18-10.

[0064] In addition to electrically conductive starting powders, powdered ceramic materials such as Al₂O₃, oxide materials, or pre-oxidized metal powders can also be advantageously used as powdered starting materials for the sintered body. The resulting sintered body then contains homogeneously distributed oxide particles, which can also serve as hydrogen traps.

[0065] Oxide materials may have been added to the starting materials in an upstream mixing or grinding process.

[0066] For the purposes of this invention, pre-oxidized powder shall be understood to mean powders which have an increased oxygen content before the sintering process or to which an increased oxygen content has been added.

[0067] In various sintering processes, the use of such pre-oxidized powders is advantageous because the sintering is carried out under vacuum or protective gas atmosphere and therefore the powders do not oxidize during sintering.

[0068] All starting powders, such as ceramic and / or pre-oxidized powders, can be used simultaneously for the production of the sintered body according to the invention, whereby in each case it must be ensured that the sintered body has sufficient electrical conductivity for the subsequent electrochemical treatment.

[0069] The sintering of the starting materials can be achieved using any known sintering process. Advantageous methods include hot pressing, hot isostatic pressing, field- or current-assisted sintering processes such as current-assisted short-term sintering (FAST / SPS, Field Assisted Sintering Technique / Spark Plasma Sintering), or combined sintering processes such as hybrid FAST / SPS. In the hybrid FAST / SPS sintering process, the samples are heated by induction in a coil and also by resistance heating of the electrically conductive sintering material / sintering tool.

[0070] Furthermore, it is advantageous if the electrically conductive, porous sintered body is subjected to plastic deformation after sintering.

[0071] Plastic deformation of the sintered body particularly increases the number and density of defect sites, such as dislocations, stacking faults, and / or twins, in the sintered body, which are advantageous for the absorption and physical and / or chemical bonding of hydrogen to pores and / or defect sites.

[0072] According to the invention, the electrically conductive, porous sintered body is subjected to an electrochemical treatment in alkaline electrolyte and with the application of a direct current in the range of 1 mA / cm². 2 up to 2 mA / cm 2 and subjected to a DC voltage in the range of 300 mV to 800 mV relative to the resting potential.

[0073] The electrochemical process can be implemented in an arrangement consisting of at least two half-cells, each with three electrodes, and a liquid electrolyte. According to the invention, electrodes and electrolytes are used that facilitate the electrolysis of water. Advantageously, the electrochemical process can be carried out in a Devanathan-Stachurski cell (DS cell). The principle is based on the electrochemical generation of hydrogen on one side of a sample (by electrolysis of water) and the subsequent diffusion of hydrogen atoms adsorbed at the surface. Before loading, the sample is activated by grinding the front and back surfaces and removing the passive oxide.

[0074] Advantageously, the electrically conductive, porous sintered body is subjected to electrochemical treatment several times, preferably 2 to 10 times.

[0075] Through repeated electrochemical treatment, the hydrogen concentration in the sintered body can be increased, optionally enriched to the maximum possible concentration, and determined by recording the permeation or diffusion characteristics. Advantageously, hydrogen concentrations of 0.25 ppm to 2000 ppm are achieved in the electrically conductive, porous sintered body.

[0076] For example, increasing the temperature can increase the diffusion coefficient, or adding recombination poisons during electrochemical hydrogen charging can reduce the duration of hydrogen charging.

[0077] Of particular importance according to the invention is that the decrease in hydrogen concentration over time in the electrically conductive, porous sintered body can be specified.

[0078] This can be determined or calculated in the form of a concentration-time curve for a known sintered body according to the invention and electrochemical loading conditions and made available to the user.

[0079] In contrast to the prior art, the solution according to the invention does not produce any or predominantly no metal hydrides in the sintered body, which are known to be unstable at high sintering temperatures and thus cause deviations in the hydrogen concentration in the calibration body, which make the measured values ​​of the calibrated measuring instruments no longer comparable.

[0080] In addition, the inventive method allows both high and low hydrogen concentrations to be set in the calibration body and made available to the user as desired.

[0081] By selecting the starting materials and the manufacturing process of the electrically conductive, porous sintered body, the lattice type, the cold working state, the chemical composition and the homogeneity of the sintered body can be selected depending on the desired application, and the absorption and loading of hydrogen can be controlled by the conditions of the electrochemical treatment.

[0082] With the calibration body according to the invention and the manufacturing process according to the invention for these calibration bodies, compact bodies with a selected hydrogen concentration can be specified.

[0083] The combination of electrically conductive, porous sintered bodies, which are advantageously produced by means of powder metallurgy processes, and which are provided with a specifically adjustable hydrogen concentration by means of electrochemical processes, and which are then used as calibration bodies with the desired hydrogen concentration and with a predetermined decrease in the hydrogen concentration over time, is not known from the prior art.

[0084] The calibration bodies according to the invention can be used for the calibration of various analytical measuring instruments, such as in glow discharge spectroscopy or hot gas extraction.

[0085] The invention will now be explained in more detail using an exemplary embodiment. Example 1

[0086] 980 g of an argon-atomized powder made of 42CrMo4 steel with a mean particle size d 50A material with a porosity of < 32 µm is sintered into a cylindrical sample or compact body with dimensions of 80 mm diameter and 25 mm height using the combined sintering process Hybrid-FAST / SPS. The sintering process is carried out under a vacuum of approximately 500 Pa with a heating rate of 100 K / min up to a temperature of 1050 °C. During heating, the pressure is increased in parallel with the temperature from 4 MPa to a pressure of 50 MPa at the maximum temperature. Temperature and pressure are maintained at their maximum values ​​for a holding time of 5 minutes. Following this holding period, the sintering tool is cooled. The resulting electrically conductive sintered body according to the invention has a homogeneously distributed porosity and a relative density of 99.5%.

[0087] Samples measuring 55 x 55 x 1 mm are cut from the sintered starting material by wire EDM for electrochemical hydrogen loading, and the surface is subsequently finished by grinding. Electrochemical hydrogen loading is then carried out using a Devanathan-Stachurski cell (DS cell). For this purpose, the sintered body is placed centrally in the DS cell, and both half-cells are filled with a 0.1 M NaOH solution. The high pH of the solution is intended to prevent electrochemical corrosion of the sintered body.

[0088] The compact surface of the oxidation cell (anode side) is then anodically polarized by applying a constant positive overvoltage into the passive region, and the current-time profile is recorded and monitored for at least 24 hours. During this time, the current-time profile transitions into saturation, and the current value in the anodic region lies between 0.5 and 10 µA.

[0089] If this condition is met after at least 24 hours, the reduction cell (cathode side) is switched on and a constant negative polarization current of -10 mA is set. This reduction current initiates hydrogen evolution on this surface of the compact body.

[0090] The current-time profile of the oxidation cell continues to be recorded, and the point at which the reduction cell is switched on is marked in the curve. This point in time marks the beginning of hydrogen adsorption and diffusion in the compact. The current-time profile indicates the diffusion of hydrogen by the increase in current after a certain time.

[0091] When the current rises to saturation, the first charging process is complete and the reduction cell is switched off, preventing the formation of new hydrogen. The current-time curve then shows a discharge and a drop in the curve, indicating the efflux of hydrogen.

[0092] When this current reaches saturation again, the reduction cell is switched on a second time with the same parameters and the second hydrogen charging process is started.

[0093] Similar to the first charging process, the current-time profile is recorded again until the current increase reaches saturation.

[0094] The sintered body is now completely loaded with hydrogen.

[0095] The hydrogen concentration was calculated by data fitting through extraction of the diffusion coefficient, the base flow rate, and the saturation flow rate using the method according to Frappart et al. (DOI: 10.1016 / j.jpcs.2010.07.017). The hydrogen concentration of the calibration body according to the invention was 0.31 ppm. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 2 160 202 A

[0021] DE 10 2011 084 775.8 A1

[0022] DE 10 2013 225 940 A1

[0025] Cited non-patent literature

[0000] Frappart et al. (DOI: 10.1016 / j.jpcs.2010.07.017

[0095]

Claims

Calibration body, in particular for the calibration of hydrogen measuring instruments, comprising at least an electrically conductive, porous sintered body with a substantially homogeneous distribution of pores and / or defects, such as dislocations, stacking faults and / or twins, in the sintered body, in which an adjustable hydrogen concentration is present depending on the number and size of the pores and / or defects in the sintered body, wherein the hydrogen is physically and / or chemically bound in and / or on the surface of the pores and / or defects, and wherein the set hydrogen concentration in the calibration body decreases in a specified amount over time. Calibration body according to claim 1, wherein the electrically conductive, porous sintered body consists of Fe, Cu or Al or of a base alloy of one or more of these elements. Calibration body according to claim 1, wherein the electrically conductive, porous sintered body comprises ceramic particles. Calibration body according to claim 1, wherein the electrically conductive, porous sintered body comprises oxide particles. Calibration body according to claim 1, wherein the electrically conductive, porous sintered body has been subjected to plastic deformation after sintering. Calibration body according to claim 1, wherein the electrically conductive, porous sintered body has a substantially homogeneous distribution of pores and / or defects, such as dislocations or stacking faults or twins, in the sintered body. Method for the production of calibration bodies, in particular for the calibration of hydrogen measuring instruments, in which a defined concentration of atomic hydrogen is set in an electrically conductive, porous sintered body by means of an electrochemical process and a time-defined decrease of the hydrogen concentration in the sintered body is further realized. The method according to claim 7, wherein the electrically conductive, porous sintered body is mixed from powdered starting materials and processed into a shaped body and subsequently sintered, wherein the sintering conditions are adjusted depending on the starting materials and the desired porosity of the sintered body. Method according to claim 7, wherein a density of > 90% of the relative density of the respective sintered body is set in the electrically conductive, porous sintered body. The method according to claim 7, wherein at least electrically conductive materials are used as powdered starting materials for the sintered body, advantageously Fe, Cu or Al or a base alloy of one or more of these elements, or even more advantageously steels. Method according to claim 7, wherein, in addition to predominantly electrically conductive materials, ceramic materials are also used as powdered starting materials for the sintered body. Method according to claim 7, wherein pre-oxidized materials or materials are used as powdered starting materials for the sintered body in addition to predominantly electrically conductive materials. Method according to claim 7, wherein the electrically conductive, porous sintered body is subjected to plastic deformation after its sintering. Method according to claim 7, wherein the electrically conductive, porous sintered body is subjected to electrochemical treatment several times, advantageously 2 to 10 times. Method according to claim 7, wherein a hydrogen concentration of 0.25 ppm to 2000 ppm is set in the electrically conductive, porous sintered body.