WO2015193295A1

WO2015193295A1 - Process for the preparation of porous nitrided iron material

Info

Publication number: WO2015193295A1
Application number: PCT/EP2015/063451
Authority: WO
Inventors: Thomas L. CHRISTIANSEN; Marcel A. J. Somers; Rasmus Berg Frandsen
Original assignee: Danmarks Tekniske Universitet
Priority date: 2014-06-16
Filing date: 2015-06-16
Publication date: 2015-12-23

Abstract

The present invention relates to a novel two-step process for preparing porous nitride iron material. In the first step a porous iron material is provided, and in the second step said porous iron material is subjected to nitriding treatment at a low temperature. The present invention also relates to a method of preparing porous iron material.

Description

PROCESS FOR THE PREPARATION OF POROUS NITRIDED IRON MATERIAL

Field of invention

The present invention relates to a novel process for preparing a porous nitrided iron material and to a novel process for preparing a porous sponge-like iron material having a multimodal pore size distribution. The present invention also relates to products produced by the novel processes, i.e. to a porous nitrided iron material, a porous sponge-like iron material having a multimodal pore size distribution and a porous sponge-like nitrided iron material having a multimodal pore size distribution. The present invention further relates to any use of the products produced by the novel processes.

Background of invention

The iron nitride phase Fe16N2 (denoted a") was first discovered in 1951 by Jack [1 ] by ageing of Fe-N martensite. The crystallographic structure was described as eight (2x2x2) distorted and expanded body-centered tetragonal units of the original martensite. Nitrogen is ordered and occupies 2 of 48 octahedral interstices. Fe16N2 is found as an intermediate phase in the decomposition of Fe-N martensite to Fe4N (γ) nitride. The nitrogen content of the stoichiometric Fe16N2 phase corresponds to 3.04 wt%. Fe16N2 is observed at temperatures below approximately 200 °C or slightly lower. In 1972 Kim and Takahashi [2] claimed that Fe16N2 had special magnetic properties, i.e. that it had a giant magnetic moment. Several publications about this topic have appeared since, but it remains inconclusive whether the phase Fe16N2 has a giant magnetic moment or not. This uncertainty is largely attributed to the difficulties in synthesizing pure or single-phase Fe16N2. Furthermore it can be difficult to distinguish Fe16N2 from the non-ordered martensite structure with overall composition Fe8N (same overall composition as Fe16N2) which appears during ageing as a precursor for the ordered Fe16N2. Fe16N2 is considered to be a candidate as new magnetic material due to its - alleged - interesting magnetic properties, viz. giant magnetic moment.

Today several synthesis routes for preparing the Fe16N2 phase are known.

Ageing of Fe-N martensite: Quenching of Fe-N austenite from its stability region results in formation of nitrogen martensite (α') and retained austenite. The maximum solubility of nitrogen in austenite is 2.8 wt% at 650 °C which means that a full conversion into Fe16N2 is not possible. The method was applied by Jack in 1951 [1 ]. For nitrogen contents approaching the maximal 2.8 wt% the austenite does not transform upon quenching to room temperature due to the austenite stabilizing effect of nitrogen; partial transformation can be obtained by deep-cooling. Deep-cooling in liquid helium, liquid nitrogen and dry ice have been done in order to convert - or partially convert - austenite into martensite [3]. For low- and intermediate nitrogen contents

transformation to martensite occurs upon quenching to room temperature (with some retained austenite). When the nitrogen martensite is tempered or aged at temperatures below approximately 200 °C, redistribution of nitrogen takes place which results in a disordered structure with composition Fe8N (martensite) and ferrite without nitrogen. Eventually, ordering of nitrogen in Fe8N occurs and Fe16N2 forms. The highest fraction of Fe16N2 reported to be formed according to this procedure is 55% and the composition of the material is a mixture of austenite, ferrite and Fe16N2.

Ageing of quenched Fe(N) ferrite: Quenching of Fe-N ferrite from its stability region results in a supersaturated solid solution of nitrogen in ferrite at room temperature. Ageing of this supersaturated solution leads to precipitation of Fe16N2 [e.g. 4]. Due to the very limited solubility of nitrogen in ferrite at elevated temperature only small fractions of Fe16N2 are possible. This procedure (quench ageing) is also known from nitriding of steel (surface hardening) where the component is quenched from the nitriding temperature.

Ageing of guenched low nitrogen epsilon phase:

Quenching of Fe-N from the epsilon phase field with low nitrogen contents (above 650 'Ό, e.g. 4-5 wt% nitrogen) results in unstable low-nitrogen epsilon phase. Upon aging below 190°C this low nitrogen epsilon transforms into stable nitrogen rich epsilon and Fe16N2 (and Fe4N) [5]. Only relatively minor fractions of Fe16N2 are obtainable by this route.

Direct synthesis by nitriding of nano-oxide powder:

It has been shown that it is possible to produce Fe16N2 from iron nanosized oxide powder with sizes below, about 20-30 nm [6]. The nanosized iron oxides

(Fe203/Fe304) are reduced in an atmosphere of pure hydrogen at 500 'Ό for 8 hours and subsequently cooled to temperatures below 200 ^ and nitrided in pure NH3 for various durations. This results in formation of Fe16N2 (and ferrite). Fe16N2 fractions up to nearly 100% have been reported. The direct synthesis (nitriding) at temperatures below 200 °C is only possible because the required diffusion distance is very short. The process is very sensitive to oxidation due to the vast surface area of the reduced oxides. Furthermore the reactivity of the nanopowders poses a fire risk and the powder is difficult to handle.

"Energy-assisted" methods: Other methods to produce Fe16N2 include epitaxial growth by sputtering [e.g. 2], nitrogen ion implantation [e.g. 8] and molecular beam deposition [e.g. 9]. These methods only allow formation of very thin films on a substrate and offer no accurate thermodynamic control of the process unlike thermochemical processes.

Hence, all of these known methods suffer from different kinds of drawbacks as pointed out above. The inventors of the present patent application have developed a new process in which all of said drawbacks are overcome. This novel method can be described as a two-step process for preparing porous nitride iron material. In the first step a porous iron material is provided, and in the second step said porous iron material is subjected to nitriding treatment at a low temperature. As can be seen from the above, it has not previously been demonstrated that Fe16N2 can be synthesized directly from bringing an iron surface in contact with a gaseous atmosphere of high nitrogen activity containing NH3 at low temperature.

Porosity in iron and iron-based alloys is a well-known phenomenon which occurs during nitriding and nitrocarburising. The phenomenon is a consequence of the metastable nature of the iron-based nitrides with respect to N₂ (gas). Very high pressures of N₂ are required to establish equilibrium between the gas and the solid phase (iron nitrides). In nitriding, NH₃ is applied which provides a nitrogen activity in the solid state that corresponds to the nitrogen activity that an N₂ pressure of several thousands of bars would provide. At locations further away from those where the nitrogen activity is imposed to the system tends to create a gaseous atmosphere with a corresponding nitrogen activity, hence the development of N₂ filled porosities. The topic of N₂ gas development (porosity) in nitriding has so far almost exclusively been investigated phenomenologicaly for growing layers of iron (carbo)-nitrides during nitriding/nitrocarburizing [9]. Iron foils containing porosity have been used for thermodynamics investigations [10].

There exists a need for a process for preparing porous iron material, where the porosity is controlled so that the physical properties, such as for example material thickness, of the final product is very well characterized. The inventors of the present invention have solved this problem by developing a novel process for preparing a porous (sponge-like) iron material. Summary of invention

In a first aspect the present invention relates to a novel two-step process for preparing porous nitride iron material. In the first step a porous iron material is provided, and in the second step said porous iron material is subjected to nitriding treatment at a low temperature.

In a second aspect the present invention relates to a method of preparing porous iron material.

Thus the first aspect of the present invention relates to a process for preparing a porous nitrided iron material, said method comprising the steps of:

(i) providing a porous iron material, and

(ii) treating said porous iron material with a high nitrogen activity fluid at a temperature lying in the range of 80-200°C. The high nitrogen activity fluid may be a gas such as an ammonia containing gas.

The first aspect of the present invention also relates to a process for preparing a porous nitrided iron material, said method comprising the steps of:

(i) providing a porous iron material, obtained by the process according to the second aspect of the present invention, and

The second aspect of the present invention can be described as a process for providing porous iron material comprising the steps of: (a) providing an iron material,

(b) treating said iron material with a high nitrogen activity fluid at a temperature lying in the range of 450-800°C in order to obtain a nitrided porous iron material, and

(c) optionally further treating said nitrided porous iron material obtained in step (b) one or more times with a high nitrogen activity fluid at a temperature lower than the temperature in step (b), and for each further treatment step lowering the temperature compared to the previous treatment step, and

(d) optionally further treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c) in one or more times with a high nitrogen activity fluid at a temperature higher than the temperature in the step immediately preceding this treatment step, and

(e) treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c) and/or one or more optional steps (d), with a fluid, with a nitrogen activity that is lower than necessary to stabilise iron nitrides and at a temperature lower or higher than that of the final nitriding step in order to obtain a de-nitrided porous iron material.

A further aspect related to the second aspect of the present invention can be described as a process for providing porous iron material, comprising the steps of:

(a) providing an iron material,

(e) treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c) and/or one or more optional steps (d), with a fluid with a nitrogen activity that is lower than necessary to stabilise iron nitrides and at a temperature lower than that of the final nitriding step in order to obtain a de-nitrided porous iron material, wherein step (b) and optional steps (c) and/or (d) preferably are performed for a duration of time such that a steady-state is achieved.

In a preferred embodiment of the second aspect of the invention, at least one of steps (c) and/or (d) are mandatory.

In relation to the second aspect of the present invention, the preparation of a porous iron material, the fluid of steps (a) to (c) (and to step (f) of the methods hereunder) may be an ammonia containing gas and the fluid of step (g) may be a hydrogen containing gas. The fluid of steps (a) to (c) (and to step (f) of the methods hereunder) may also be called a first fluid or first gas and the fluid of step (g) may be called a second fluid or gas.

Another aspect of the present invention relates to the process for preparing a porous nitrided iron material, wherein the method comprises the steps of:

(i) providing a porous iron material prepared by the method of the second aspect of the present invention, and

Another aspect of the present invention relates to a porous iron material having a bimodal or a multimodal pore size distribution and produced according to the second aspect of the present invention.

Another aspect of the present invention relates to a porous nitrided iron material produced by the process according to the first aspect of the present invention.

A further aspect of the present invention relates to the use of the porous nitrided or de- nitrided iron material as obtained by either the first or the second aspect of the present invention, as a magnetic material, and/or as a starting material for a catalyst, and/or as a substrate material for deposition of other materials. Brief Description of Drawings

Figure 1 : Scanning electron microscopy. Multimodal porosity in nitrided 25 μηι iron foil. A) low magnification showing the entire cross-section of the through-nitrided foil. B) High magnification showing porosities (1 ) in iron material (2) in detail. The converted foil has a stack-like appearance with a remaining "wall-thickness" of iron of less than 100-200 nm.

Figure 2: Thermogravimetry: recorded mass change (2) and time-temperature profile (1 ). Steps I and II: porosity formation; step III: denitriding step; steps IV to VI: low temperature nitriding steps, formation of Fe16N2.

Figure 3: A) Thermogravimetry: recorded mass change (2) and time-temperature profile (1 ). Steps I to III: porosity formation steps; step IV: denitriding step; steps V and VI: low temperature nitriding steps, formation of Fe16N2. B) X-ray diffractogram. (1 ): Fe16N2 reflections; (2): body-centered cubic (BCC) iron reflections.

Figure 4: X-ray diffractogram. (1 ): Fe16N2 reflections; (2): body-centered cubic (BCC) iron reflections.

Figure 5: Thermogravimetry: recorded mass change (2) and time-temperature profile (1 ). Steps I to V: porosity formation steps; step VI: denitriding step; steps VII and VIII: low temperature nitriding steps, formation of Fe16N2 Detailed description of the invention

The present invention relates to a novel process for preparing a porous nitrided iron material. This novel process comprises at least two steps, namely:

(i) provision or preparation of the porous iron material, and

(ii) treating said porous iron material with an ammonia containing fluid at a temperature lying in the range of 80-200°C.

Specifically, the novel process comprises at least the two steps of:

(i) provision or preparation of a porous iron material, where the porous iron material is provided or prepared according to a further method of the present invention,

(ii) treating said porous iron material with a high nitrogen activity fluid at a temperature lying in the range of 80-200°C.

The process of the present invention differs from known processes in that the treatment of the porous iron material with the ammonia containing fluid (i.e. step (ii) also known as a nitriding step) proceeds at very low temperatures as compared with known methods. In the present invention the nitriding temperature lies in the range of 80- 200°C, such as for example in the range of 100-190°C, preferably in the range of 130- 180°C. By the term "nitrided iron material" material as used herein is meant an iron material, in which parts or all of the iron has been converted into the iron nitride phase Fe16N2 (denoted a"). The iron nitride phase can be written both with and without subscripts, i.e. as both Fe16N2 and Fe₁₆N₂. By the term "fluid" is understood a gas or a liquid or plasma. Furthermore, fluidized beds, powder packs and so forth involve gas and thus are of relevance as media to be fluidized for the present processes.

The high nitrogen activity fluid or ammonia gas used in step (ii) may in principle be any kind of fluid with high nitrogen activity. In some embodiments the fluid with high nitrogen activity is a gas. In specific embodiments, the gas is ammonia gas. Preferably the ammonia gas comprises 70-100% ammonia. In some embodiments the fluid with high nitrogen activity is a plasma or a liquid with salts comprising nitrogen. By the term "nitrogen activity" is meant the effective atomic fraction of nitrogen with respect to N₂ gas at 1 bar, as defined by chemical thermodynamics. For a gas, the activity is the effective partial pressure (or fugacity, for a non-ideal gas) relative to a reference pressure. By the term "iron material" is meant a material comprising iron, such as pure iron (Fe), iron alloys, including steel and stainless steel, or iron oxides.

The porous iron material that either is provided or prepared may in principle be any kind of iron material that is provided in a porous form or is prepared and thus provided prior to step (ii). Non-limiting examples of porous iron material are porous iron foil, metal foam and/or porous iron in any other geometry. Examples of iron material that is to be prepared or provided in a porous form include, but are not limited to, pure iron or iron alloys such as Fe-Ni and/or Fe-Co and/or Co-Fe-Ti and/or Fe-Ti. Other examples of iron material include, but are not limited to, iron oxides. An advantage of the present invention is that the porous iron material and the nitrided iron material can be easily fabricated and/or produced in any possible form, shape or geometry, and is thus not restricted to a specific size. In an embodiment of the invention, the porous iron material is provided or prepared according to a method of the present invention. In a further embodiment, the provided or prepared porous iron material may assume any shape, i.e. geometrical shape. By the term geometrical shape as used herein is meant the bulk geometry, and the bulk geometry is further porous. An example of a porous bulk geometry is a sponge block, where the block or box shape defines the bulk geometry.

In a preferred embodiment of the invention, the iron material provided is selected from the group consisting of iron foil, iron plate, iron sheet, iron powder, iron granulate, iron wire, iron slab, and iron rod.

In a further embodiment, the iron material provided is a powder or granulate, wherein the particles are in the micron size range, such as equal to or above 1 μηι, more preferably equal to and above 2 μηι or 5 μηι or 30 μηι, and most preferably between 3- 6 μηι.

The porous iron material that either is provided or prepared may in principle be fabricated by any methods. For example a metal foil may be fabricated using techniques such as metal rolling, calendering , metal deposition, or metal powder processing including wet shaping techniques such as extrusion, and consolidating by sintering. In another embodiment of the invention, the iron material provided has been fabricated from sintered powder comprising iron.

In a preferred embodiment of the invention, the iron material is iron or an iron alloy. The present invention also relates to a novel process for preparing porous iron material, which includes the steps of:

(a) providing an iron material,

(b) treating said iron material with a high nitrogen activity fluid at a temperature lying in the range of 450-800°C in order to obtain a nitrided porous iron material, and (c) optionally further treating said nitrided porous iron material obtained in step (b) one or more times with a high nitrogen activity fluid at a temperature lower than the temperature in step (b), and for each further treatment step lowering the temperature compared to the previous treatment step, and

(g) treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c), with a fluid with a nitrogen activity that is lower than necessary to stabilise iron nitrides and at a temperature lower than that of the final nitriding step in order to obtain a de-nitrided porous iron material. Optionally, a nitriding step is performed where the temperature is higher than the nitriding step preceding it.

The porous structure of the the prepared porous iron material is controlled by the nitriding and denitriding steps (b) and (c). The treatments as well as the number of steps will determine the resulting porosity, pore sizes, and microstructure.

In an embodiment of the invention, at least one of the steps (c) and/or (d) are mandatory. In a further embodiment, the step (b) and/or step (c) is carried out for each temperature for a period of below 33 hours, more preferably below 24 hours, and most preferably equal to or below 12 hours, such as 8, 6, 4 or 2 hours.

In an embodiment of the invention, the process for providing or preparing porous iron material, comprises the steps of:

(a) providing an iron material,

(d) optionally further treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c) in one or more times with a high nitrogen activity fluid at a temperature higher than the temperature in the step immediately preceding this treatment step, and (e) treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c) and/or one or more optional steps (d), with a fluid with a nitrogen activity that is lower than necessary to stabilise iron nitrides and at a temperature lower than that of the final nitriding step in order to obtain a de-nitrided porous iron material.

In an embodiment of the present invention, the de-nitriding step is performed at the same temperature as the nitriding step preceding it or at a temperature higher than the nitriding step preceding it. Thus in an embodiment of the present invention the process for preparing porous iron material includes the steps of:

(a) providing an iron material,

(g) treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c), with a fluid with a nitrogen activity that is lower than necessary to stabilise iron nitrides in order to obtain a de-nitrided porous iron material.

The process for preparing porous iron material can also be described by the following steps:

(a) providing an iron material, such as for example a material selected from the group comprising or consisting of iron foil, iron plate, iron sheet, iron powder, iron granulate, iron wire, iron slab, and/or iron rod.

(b) treating said iron material with a fluid with high nitrogen activity at a temperature lying in the range of 450-800°C in order to obtain a nitrided porous iron material, and

(c) optionally further treating said nitrided porous iron material obtained in step (b) with a fluid with high nitrogen activity relative to the porous iron material obtained in step b) at a temperature lying in the range of 520-700°C, and

(d) optionally further treating said nitrided porous iron material obtained in step (c) with a fluid with high nitrogen activity relative to the porous iron material obtained in step c) at a temperature lying in the range of 500-650°C, and (e) optionally further treating said nitrided porous iron material obtained in step (d) with a fluid with high nitrogen activity relative to the porous iron material obtained in step d) at a temperature lying in the range of 450-580°C, and

(f) optionally further treating said nitrided porous iron material obtained in step (e) with a fluid with high nitrogen activity relative to the porous iron material obtained in step e) at a temperature lying in the range of 400-500°C, and

(g) treating said nitrided porous iron material obtained in step (b) or optional steps (c), (d), (e) or (f) with a fluid having a nitrogen activity that is lower than that of the nitrided porous iron material obtained in either of steps (b), (c), (d), (e) or (f) at a temperature lower than that of the final nitriding step, and where the nitrogen activity is such that it allows de-nitridation of the material, in order to obtain a de-nitrided porous iron material.

The process for preparing porous iron material can also be described by the following steps, wherein steps (c), (d), (e) are replaced with the following steps:

(c) optionally further treating said nitrided porous iron material obtained in step (b) with a high nitrogen activity fluid at a temperature lying in the range of 520-700°C, and

(d) optionally further treating said nitrided porous iron material obtained in step (c) with a high nitrogen activity fluid at a temperature lying in the range of 500-650°C, and (e) optionally further treating said nitrided porous iron material obtained in step (d) with a high nitrogen activity fluid at a temperature lying in the range of 350-580°C, and

(f) optionally further treating said nitrided porous iron material obtained in step (e) with a high nitrogen activity fluid at a temperature lying in the range of 400-500°C, and

(g) treating said nitrided porous iron material obtained in step (b) or optional steps (c), (d), (e) or (f) with a fluid having a nitrogen activity that is lower than necessary to stabilise iron nitrides and at a temperature lower than that of the final nitriding step in order to obtain a de-nitrided porous iron material and wherein the temperature of each steps (c) to (g) is lower than that of the preceding step and/or wherein the step before step (g) is performed at a temperature higher than step (g), and wherein each of the optional steps (c), (d), (e), (f) and/or (g) preferably are performed for a duration of time such that a steady-state is achieved.

In one embodiment the method of preparing porous iron is conducted so the temperature of a step is lower than the temperature of the preceding step. In another embodiment, the temperature may increase or decrease between the steps. In a particular embodiment the fluid of step (g) is a gas, such as a hydrogen containing gas. In another embodiment the temperature of step (g) is in the range of 350°C to less than 800°C and lower than that of the final nitriding step.

Preferably the fluid is a gas. In preferred embodiments, the gas is an ammonia containing gas. Preferably the process of preparing a porous iron material can be described by the steps of:

(a) providing an iron material, such as for example a material selected from the group consisting of iron foil, iron plate, iron sheet, iron powder, iron granulate, iron wire, iron slab, and/or iron rod.

(b) treating said iron material with an ammonia containing gas at a temperature lying in the range of 450-800°C in order to obtain a nitrided porous iron material, and

(c) optionally further treating said nitrided porous iron material obtained in step (b) with an ammonia containing gas at a temperature lying in the range of 520-700°C, and

(d) optionally further treating said nitrided porous iron material obtained in step (c) with an ammonia containing gas at a temperature lying in the range of 500-650°C, and

(e) optionally further treating said nitrided porous iron material obtained in step (d) with an ammonia containing gas at a temperature lying in the range of 450-580°C, and (f) optionally further treating said nitrided porous iron material obtained in step (e) with an ammonia containing gas at a temperature lying in the range of 400-500°C, and (g) treating said nitrided porous iron material obtained in step (b) or optional steps (c), (d), (e) or (f) with a gas, preferably a hydrogen gas at a temperature lying in the range of 350-800°C and at least at a temperature lower than that of the final nitriding step, in order to obtain a de-nitrided porous iron material.

In an embodiment the temperature is decreased for each step. In another embodiment at least one step is conducted at a temperature higher than the preceding step. In an embodiment, the temperatures of steps (c) and (d) lie in the range of 350 to 800°C.

When applying any of the above processes for providing porous iron, pores are formed in the iron material during the nitriding step(s) when ammonia gas at temperatures higher than about 450°C is brought in contact with the starting iron material. When ammonia gas is brought in contact with the heated iron material during the nitriding process, ammonia molecules dissociate into nitrogen and hydrogen. The nitrogen then diffuses into the material thereby creating a nitrided zone. Pores develop in iron nitride at some distance from the surface. The longer the iron material is treated with ammonia gas, the thicker a nitrided zone will grow and the more porosity will develop. A skilled person will know that if it is aimed at converting 100% of the starting iron material into iron nitride, then a thin starting iron material would be a suitable starting material. The inventors of the present invention have surprisingly found that it is possible to tailor the pore size distribution by carefully controlling the combination of temperature and treatment time when using the above process, so that fast nitriding kinetics is obtained at low temperature. This is a major advantage of the process of the present invention when compared to known processes, where the pore size distribution is only very poorly controlled, and often not controlled at all.

More precisely, it has been found that the pore size distribution can be controlled by performing the nitriding process stepwisely. Hence, in step (b), the iron material is brought in contact with ammonia gas, preferably at a constant temperature, lying in the range of 450-800°C, and for a specific period of time, preferably until a steady state condition is reached. In this step one type of pores having a pore size characterised by the normal distribution is formed. Next, in step (c) the iron material is brought in contact with ammonia gas at a temperature, which preferably is lower than the temperature in step (b), and is in the range of 520-700°C, and in this step a second type of pores having a different size than the pores formed during step (b) will be formed. Hence, when an iron material is subjected to steps (b) and (c), an iron nitride material with bimodal pore size distribution is formed. If the iron material then also is subjected to step (d), where the temperature is lying in the range of 500-650°C, an iron nitride material with trimodal pore size distribution is formed, and so on. In principle, nitriding steps at several temperatures can be performed one after another, and thereby nitrided iron material with a multimodal pore size distribution can be obtained. In an

embodiment the temperature in the steps after at least a bimodal, such as a tri- or multimodal pore size distribution has been obtained, is increased to accelerate the process. By the term "multimodal pore size distribution" as used herein is meant any pore size distribution, where porosity is obtained by holding an iron nitride material in a fluid of high nitrogen activity, preferably containing high contents of ammonia, at several combinations of temperature and nitrogen activity to combine different driving forces and different nitrogen diffusion distances for pore formation. The pores thus formed have different sizes and different separations. A bimodal distribution is obtained by only employing two of the temperature steps e.g. steps (b) and (c) (or (b) and (d) etc.)

After one or more nitriding steps, the nitrided porous iron material is subjected to a de- nitriding step (step (g)), in which the nitrided porous iron material is brought in contact with a fluid of very low nitrogen activity. Preferably a gaseous atmosphere of very low nitrogen activity and containing high contents of hydrogen is used, to withdraw all nitrogen from the iron nitride material by forming ammonia. The de-nitriding step is carried out at a temperature lying in the range of 350-800°C and preferably at a temperature lower than the final nitriding step used. Thus the denitriding step can be carried out at a temperature lower than the final nitriding step and may, dependent on the temperature hereof, be in the range of 350-750°C, such as 350-700°C, such as 350-650°C such as 350-600°C such as 350-550°C such as 350-500°C, such as 350- 450°C, such as 380°C or 420°C.

Alternatively, the denitriding step is performed in an inert atmosphere, in which case the temperature does not need to be lower than the final nitriding step and may be in the range of 420-850°C. In this case, nitrogen gas is formed by warming up the inert atmosphere to high temperatures, leading to denitriding. If hydrogen is used in this step the dissolved nitrogen is retracted from the material by development of ammonia, which desorbs from the large internal surface area of the material and leaves an iron sponge in the depth region where the material was nitrided.

Alternatively the nitrided porous material is placed in a vacuum, i.e. at low pressure without a specific composition of the remaining gaseous atmosphere, for the denitriding process. When using vacuum for the de-nitriding process the temperature must be at least 400°C, such as 450°C , 500°C or higher. In an embodiment the denitriding step is performed by placing the nitrided porous iron material in a vacuum at a temperature of at least 400°C in order to obtain a de-nitrided porous iron material. In another embodiment the temperature is at least 450°C. In yet another embodiment the temperature is between 400°C and 800°C, such as between 450°C and 800°C.

In cases where the starting iron material for the process of providing porous iron is a very thin sheet or plate, such as for example from about 10-100 μηι, such as for example 50-200 μηι, such as for example 100-500 μηι, it is possible to convert essentially all of the iron material into iron nitride. In such cases the porous iron thus produced may be characterised as sponge-like, because of its pore structure, or fractal, i.e. with a multimodal pore size distribution. This sponge-like sheet or plate can be further processed by crushing the sheet or plate into powder. A skilled person would know how to perform such a crushing process.

In an embodiment of the invention, a porous iron material having a bimodal or a multimodal pore size distribution, is produced by the processes described.

It is an aspect of the present invention that the porous iron material or de-nitrided porous material obtained by any of the processes herein has a pore wall thickness of less than 200 nm, such as less than 150 nm, such as less than 100 nm, such as less than 50 nm.

By the term "pore wall thickness" is meant the pore-to-pore distance, such as the thickness of the material between two pores.

The nitriding steps, e.g. any one or more of steps (b) and optional steps (c), (d), (e) and (f), may be performed with ammonia containing gas or any gas or other fluid that has a high nitrogen activity, i.e. which brings about the formation of metastable iron nitrides. A person of skill in the art knows the nitriding and/or denitriding potential of a material at given temperatures, pressures and activities with the aid of a Lehrer diagram [1 1 ]. In an embodiment of the invention, the high nitrogen activity fluid is an ammonia containing gas.

In a further embodiment, the fluid with a high nitrogen activity of step (b) and optional steps (c), (d), (e) and (f) is a gas, such as ammonia gas, such as ammonia gas comprising 50-100% ammonia, such as ammonia gas comprising 70-100% ammonia. Urea is an example of a material, which is not a gas and is applicable in the present process: urea has a high nitrogen activity when heated to above 130°C and thus may be used in any of the above mentioned nitriding steps. Preferably the material is a gas. More preferably the gas comprises 50 to 100% ammonia.

The gas of step (g) used for de-nitriding may be any gas with no ammonia or a lower ammonia content than the gas used for the final nitriding process, i.e. the gas may be a gas with a very low nitrogen activity. The gas may be a hydrogen containing gas.

Preferably, the gas of step (g) comprises 80-100% hydrogen.

In an embodiment of the invention, the gas of step (g) is a hydrogen containing gas, such as a gas preferably comprising 80-100% hydrogen. In one specific embodiment, the present invention relates to the process for preparing a porous sponge-like iron material by using the novel process mentioned above comprising steps (a) to (g) with at least three of the steps (b) to (f).

The present invention also relates to a porous nitrided iron material produced by the process comprising at least the two steps (i) and (ii). Depending on the thickness of the starting iron material and depending on the period of time that the nitriding steps last, the content of Fe16N2 (a") in the final porous nitrided iron material may vary. In one embodiment the final material comprises 10-20% Fe16N2 (a") and 80-90% Fe (a). In another embodiment the final material comprises 10-50% Fe16N2 (a"), such as 20- 40%. In another embodiment the final material comprises 20 to 80% Fe16N2 (a"), such as 30-50% such as 30 to 60%. In another embodiment the final material comprises 50- 100% Fe16N2 (a") and 0-50% Fe (a). In another embodiment the final material comprises 50-100% Fe16N2 (a") and less than 50% Fe (a). In another embodiment the final material comprises 95-100% Fe16N2 (a") and 0-5% Fe (a). In a preferred embodiment the final material is essentially made of Fe16N2 (a").

The two steps (i) and (ii) of the present invention, may be further repeated

independently and any multiple of times to obtain a certain content of Fe16N2 (a") in the final porous nitrided iron material. In an embodiment of the invention, step (i) and/or (ii) is repeated one or more times where the temperature is lowered compared to the preceding step (i) and/or (ii) for each repetition.

In a preferred embodiment of the invention, the porous material of step (i) is provided by the process of preparing a porous iron material described previously, according to the second aspect of the present invention. In a further embodiment, the iron material provided is a material selected from the group consisting of iron foil, iron powder, iron granulate, iron plate, iron sheet, iron wire, iron slab, or iron rod. In a further embodiment of the invention, the porous iron material or porous nitrided iron material obtained in step (i) and/or (ii) has a pore wall thickness of less than 200 nm, preferably less than 100 nm.

The present invention also relates to a porous sponge-like nitrided iron material having a multimodal pore size distribution. This material is typically formed when the size of the starting iron material is in the range 10-100 μηι so the porosity network can develop essentially all the way through the material. In one embodiment the final sponge-like material comprises 10-50% Fe16N2 (a"), such as 20-40%. In another embodiment the final material comprises 20 to 80% Fe16N2 (a"), such as 30-50% such as 30 to 60%. In another embodiment the final material comprises 95-100% Fe16N2 (a") and 0-5% Fe (a). In another embodiment the final sponge-like material comprises 99-100% Fe16N2 (a") and 0-1 % Fe (a). In a preferred embodiment the final sponge-like material is essentially made of Fe16N2 (a"). This porous (sponge-like) nitrided iron material is obtained by employing the process comprising steps (a) to (g) with at least three of the steps (b) to (f) and then step (ii) of the nitriding process. It is an object of the present invention that the porous nitrided iron material produced by the processes mentioned herein is magnetic. In another embodiment, the de-nitrided porous iron material obtained by any of the process herein can be used as a magnetic material. The present invention is also directed to different uses of the porous nitrided iron material, which may be produced by the process according to the present invention. In particular the present invention is directed to any use connected to the magnetic properties of the porous nitrided iron material. In one embodiment the porous nitrided iron material is used for the production of magnets. Examples of such magnets include permanent magnets and/or magnetic bearings. In other embodiments the porous nitrided iron material is used as a starting material for producing a catalyst. In yet an embodiment the nitrided iron material is used as a precursor for the synthesis of nitrides or carbides. It is also an aspect of the present invention to use the porous nitrided or de-nitrided iron material obtained by any of the processes herein disclosed as substrate material for deposition of other materials.

The present invention is now disclosed in further details in the Examples below.

Examples

The preparation of porous iron foils and Fe16N2 formation in porous iron foils are disclosed below.

Example 1 - Synthesis of porous iron.

25 μηι iron foils were used as starting material. The foils were nitrided in a balance applying an atmosphere of pure NH₃. One of the following time-temperature programs was applied: a) 650 ^<€ for 2 hours; or

b) 600 ^<€ for 12 hours, 550 ^<€ for 12 hours, 500 °C for 8 hours; or

c) 650 ^<€ for 2 hours, 580^<€ for 4 hours, 490°C for 7 hours; or

d) 700 ^<€ for 2 hours, 650^<€ for 2 hours, 590°C for 4 hours and 490°C for 6 hours.

For all experiments denitriding of the nitrided samples was performed at 420 °C for 2 hours in hydrogen.

The surface area of the fully converted porous foils was measured with BET analysis. For the 4 different experiments the following results were obtained:

a) 0.0088 m²/g

b) 0.2637 m²/g

c) 0.5273 m²/g

d) 0.5856 m²/g

The degree of porosity increases with the number of nitriding steps and successively lower temperatures. The presence of a multimodal pore size distribution throughout the foil (experiment d) was confirmed with SEM (see figure 1 .). Figure 1 shows scanning electron microscopy images and illustrates the multimodal porosity in nitrided 25 μηι iron foil, at low magnification showing the entire cross-section of the through-nitrided foil (fig. 1 A) and at high magnification showing porosities in detail (fig. 1 B). The converted foil has a stack-like appearance with a remaining "wall- thickness" of iron of less than 100-200 nm.

Example 2 - Fe16N2

25 μηι iron foils were heated to 590 °C in flowing NH₃ in a thermo-balance and held at this temperature for sufficient time to attain full through-nitriding and a stationary state in uptake of nitrogen (figure 2). The temperature was then changed to 450 °C. This resulted in a bi-modal pore size distribution. Thereafter the temperature was lowered to 420 ^ and the gas was changed to H₂. The foil was held at this temperature for sufficient time to completely denitride the foil (retraction of all dissolved nitrogen by development of NH₃). Thereafter the temperature was lowered to 170 °C and the gas was changed to pure NH₃. The foil was held at this temperature for a time of 10 hours. The nitriding temperature was then lowered to Ι ΘΟ 'Ό and held there for 10 hours and subsequently lowered to Ι δΟ 'Ό and held there for 70 hours. This resulted in a mixture of Fe16N2 (a") and ferrite (a). Figure 2 shows the therm ogravimetry data for this experiment, in which the mass change and time-temperature profile were recorded.

Example 3 - Fe16N2

25 μηι iron foils were heated to 650 °C in flowing NH₃ in a thermobalance and held at this temperature for sufficient time to attain full through-nitriding and a stationary state in uptake of nitrogen (figure 3). Two more nitriding steps were applied for porosity formation - at 580 °C and 480 'Ό. This resulted in a multi-modal pore size distribution. Thereafter the temperature was lowered to 420 °C and the gas was changed to H₂. The foil was held at this temperature for sufficient time to completely denitride the foil (retraction of all dissolved nitrogen by development of NH₃). Thereafter the temperature was lowered to 170 °C and the gas was changed to pure NH₃. The foil was held at this temperature for a time of 10 hours. The nitriding temperature was then lowered to Ι θδ'Ό and held at this temperature for 59 hours. This resulted in a mixture of Fe16N2 (a") and ferrite (a). Figure 3A shows the thermo-gravimetry data for this experiment, in which the mass change and time-temperature profile were recorded.

Figure 3B shows an X-ray diffractogram proving the presence of Fe16N2 and ferrite (iron).

Example 4 - Fe16N2

25 μηι iron foils were heated to 590 °C in flowing NH₃ in a thermo-balance and held at this temperature for sufficient time to attain full through-nitriding and a stationary state in uptake of nitrogen. Two more nitriding steps were applied for porosity formation - at 520 ^ and 460^qC. This resulted in a multi-modal pore size distribution. Thereafter the temperature was lowered to 420 °C and the gas was changed to H₂. The foil was held at this temperature for sufficient time to completely denitride the foil (retraction of all dissolved nitrogen by development of NH3). Thereafter the temperature was lowered to 140 ^ and the gas was changed to pure NH3. The foil was held at this temperature for a time of 48 hours. This resulted in a mixture of Fe16N2 (a") and ferrite (a).

Example 5 - Fe16N2

25 μηι iron foils were heated to 650 °C in flowing NH₃ in a thermobalance and held at this temperature for sufficient time to attain full through-nitriding and a stationary state in uptake of nitrogen. Three more nitriding steps were applied for porosity formation - at 580 °C, 520 °C and 450^qC. This resulted in a multi-modal pore size distribution.

Thereafter the temperature was lowered to 420 °C and the gas was changed to H2. The foil was held at this temperature for sufficient time to completely denitride the foil (retraction of all dissolved nitrogen by development of NH₃). Thereafter the temperature was lowered to 175°C and the gas was changed to pure NH₃. The foil was held at this temperature for a time of 4 hours. The nitriding temperature was then lowered to 170 °C and held there for 8 hours and subsequently lowered to 160°C and held there for 17 hours. This resulted in a mixture of Fe16N2 (a") and ferrite (a).

Example 6 - Fe16N2

3-6 μηι iron powder was heated to 500 'Ό in flowing NH₃ in a thermobalance and held at this temperature for sufficient time to attain full through-nitriding and a stationary state in the uptake of nitrogen. During nitriding N₂ gas porosity develops throughout the powder. In the second step the temperature was lowered to 380 °C and the gas was changed to H₂. The powder was held at this temperature for sufficient time to completely de-nitride the powder (retraction of all dissolved nitrogen by development of NH₃). In the third step the temperature was lowered to Ι ΟΟ 'Ό and the gas was changed to pure NH₃. The powder was heated from Ι ΟΟ'Ό to Ι δδ'Ό at a constant heating rate of 0.2 K/min. This resulted in mixture of Fe16N2 (a") and ferrite (a).

Figure 4 shows an X-ray diffractogram proving the presence of Fe16N2 and ferrite (iron). Example 7 - Fe16N2

25 μηι iron foils were heated to 700 °C in flowing NH₃ in a thermo-balance and held at this temperature for sufficient time to attain full through-nitriding and a stationary state in uptake of nitrogen. Four more nitriding steps were applied for porosity formation - at 650 ^<€, 580 °C, 520 ^<€ and 480 ^qC. This resulted in a multi-modal pore size distribution. Thereafter the temperature was lowered to 420 °C and the gas was changed to H₂. The foil was held at this temperature for sufficient time to completely denitride the foil (retraction of all dissolved nitrogen by development of NH₃). The temperature was then lowered to 130°C and the gas was changed to pure NH₃. The foil was held at this temperature for a time of 99 hours. The nitriding temperature was then lowered to 120 ^ and held this temperature for 99 hours. This resulted in a mixture of Fe16N2 (a") and ferrite (a).

Fig. 5 shows the thermo-gravimetry data for this experiment, in which the mass change and time-temperature profile were recorded.

Example 8 - Fe16N2

6-8 μηι iron powder was placed in a ceramic crucible and slightly compacted and then sintered together at 1 100 'Ό for two hours in a tube furnace in an atmosphere of flowing hydrogen. As known to the skilled person within the art, by sintering, the powder becomes consolidated into a bulk geometry, where the geometrical dimensions of the bulk may be any shape. Depending on the degree of consolidation during sintering, the bulk geometry may be dense or porous. In principle, any given shape can be produced by this method. The resulting sintered iron component was penetrable for gas, and thus analogous to conventional powder metallurgical (PM) components. The sintered iron component was placed in a thermo-balance and the following time- temperature-gas program was applied for production of porosity and Fe16N2 formation:

After this treatment the component maintained its original shape (coherency) and could be handled as such. The resulting nitrogen content after step (10+1 1 ) was 2.4 wt% as determined by thermogravimetry. The treatment resulted in a mixture of Fe16N2 (a") and ferrite (a).

References

[I ] K.H. Jack, Proceedings of the Royal Society of London, Series A, Math. & Phys. Sci. 208(1093) (1951 ) 216-224.

[2] T.K. Kim, M. Takahashi, Applied Physics Letters 20 (1972) 492-494.

[3] M.Q: Huang, W.E. Wallace, S. Simizu, AT. Pedziwiatr, R.T. Obermyer, Journal of

Applied Physics 75 (1994), 6574-6576.

[4] K.H. Jack, Journal of Alloys and Compounds 222 (1995) 160-166

[5] S. Malinov, A.J. Bottger, E.J. Mittemeijer, M.I. Pekelharing, M.A.J. Somers

Metallurgical and Materials Transactions A (32A) (2001 ) 59-73.

[6] A. Nagatomi, S. Kikkawa, T. Hinomura, S. Nasu, F. Kanamaru, J. Jpn. Soc. Powder

Powder Metal. 46 (1999)151 .

[7] K. Nakajima & S. Okamoto, Applied Phys. Letters 56 (1990) 92.

[8] M. Komuro, Y. Kozono, M Hanazono Y. Sugita, J. Applied Phys. 67(9) (1990) 5126.

[9] C. Middendorf, W. Mader, Z. Metallkd. 94 (2003) 333-340.

[10] L. Maldzinski, Z.Przylecki, and J. Kunze: Steel Res., 1986, vol. 57, pp.645-49.

[I I ] E. Lehrer, Zeitschrift fur Elektrochemie 36 (1930) 383-392].

Claims

1 . A process for providing porous iron material, comprising the steps of:

(a) providing an iron material,

2. The process according to claim 1 , wherein at least one of step(s) (c) and/or (d) is mandatory.

3. The process according to any of the preceding claims, wherein step (b) and/or step (c) is carried out for each temperature for a period of below 33 hours, more preferably below 24 hours, and most preferably equal to or below 12 hours, such as 8, 6, 4 or 2 hours.

4. The process according to any of the preceding claims, wherein the iron material provided in step (a) may assume any geometrical shape.

5. The process according to any of the preceding claims, wherein the iron material provided in step (a) is selected from the group consisting of iron foil, iron plate, iron sheet, iron powder, iron granulate, iron wire, iron slab, and iron rod.

6. The process according to claim 5, wherein the iron material provided in step (a) is a powder or granulate, wherein the particles are in the micron size range, such as equal to or above 1 μηι, more preferably equal to and above 2 μηι or 5 μηι or 30 μηι, and most preferably between 3-6 μηι.

7. The process according to any of the preceding claims, wherein the iron material provided in step (a) has been fabricated from sintered powder comprising iron.

8. A process for preparing a porous nitrided iron material, said method comprising the steps of:

(i) providing a porous iron material obtained by the process according to any of claim 1 - 7, and

9. The process of claim 8, wherein step (i) comprises the process for providing porous iron material comprising the steps of:

(a) providing an iron material,

(e) treating said nitrided porous iron material obtained in step (b) or one or more optional steps (c) and/or one or more optional steps (d), with a fluid with a nitrogen activity that is lower than necessary to stabilise iron nitrides and at a temperature lower than that of the final nitriding step in order to obtain a de-nitrided porous iron material.

10. The process according to any of the preceding claims, wherein the high nitrogen activity fluid is an ammonia containing gas.

1 1 . The process according to any of the preceding claims, wherein the temperatures of steps (c) and (d) lie in the range of 350 to 800°C.

12. The process according to claim 8, wherein step (i) and/or (ii) is repeated one or more times where the temperature is lowered compared to the preceding step (i) and/or (ii) for each repetition.

13. The process according to any of claims 8-12, wherein the porous iron material or porous nitrided iron material obtained in step (i) and/or (ii) has a pore wall thickness of less than 200 nm, preferably less than 100 nm.

14. The process for providing porous iron material according to claim 1 or 9, wherein steps (c), (d) (e) are replaced with the following steps:

(d) optionally further treating said nitrided porous iron material obtained in step (c) with a high nitrogen activity fluid at a temperature lying in the range of 500-650°C, and

(e) optionally further treating said nitrided porous iron material obtained in step (d) with a high nitrogen activity fluid at a temperature lying in the range of 350-580°C, and

(f) optionally further treating said nitrided porous iron material obtained in step (e) with a high nitrogen activity fluid at a temperature lying in the range of 400-500°C, and (g) treating said nitrided porous iron material obtained in step (b) or optional steps (c), (d), (e) or (f) with a fluid having a nitrogen activity that is lower than necessary to stabilise iron nitrides and at a temperature lower than that of the final nitriding step in order to obtain a de-nitrided porous iron material and wherein the temperature of each steps (c) to (g) is lower than that of the preceding step and/or wherein the step before step (g) is performed at a temperature higher than step (g) , and wherein each of the optional steps (c), (d), (e), (f) and/or (g) preferably are performed for a duration of time such that a steady-state is achieved.

15. The process for providing porous iron material according to claim 14, wherein step (g) is replaced as follows: (g) placing said nitrided porous iron material in a vacuum at a temperature of at least 400 °C in order to obtain a de-nitrided porous iron material.

16. The process for preparing porous nitrided iron material according to any of claims 8-12, wherein the porous material of step (i) is provided by the method of any of claims 1 -7.

17. The process according to any of claims 1 -7 or 14-15, wherein the obtained de- nitrided porous iron material has a pore wall thickness of less than 200 nm, preferably less than 100 nm.

18. The process according to any of claims 1 -7, 9, or 14-17, wherein the iron material is iron or an iron alloy.

19. The process according to any of claims 1 -7, 9, or 14-18, wherein the iron material provided in step (a) is a material selected from the group consisting of iron foil, iron powder, iron granulate, iron plate, iron sheet, iron wire, iron slab, or iron rod.

20. The process according to any of claims 1 -7, 9, or 14-19, wherein the fluid with a high nitrogen activity of step (b) and optional steps (c), (d), (e) and (f) is a gas, such as ammonia gas, such as ammonia gas comprising 50-100% ammonia, such as ammonia gas comprising 70-100% ammonia.

21 . The process according to any of claims 14-20, wherein the gas of step (g) is a hydrogen containing gas, such as a gas preferably comprising 80-100% hydrogen.

22. A porous nitrided iron material produced by the process according to any of claims 8-13 or 16-21 .

23. The porous nitrided iron material according to claim 22, wherein the product comprises 50-100% Fe16N2 (a") and less than 50% Fe (a).

24. A porous iron material having a bimodal or a multimodal pore size distribution and produced by the process according to any of claims 1 -7 or 14-21 .

25. Use of the porous nitrided or de-nitrided iron material according to any of claims 22- 24 or as obtained by any of claims 1 -21 as a magnetic material.

26. Use of the porous nitrided or de-nitrided iron material of any of claims 22-24 or as obtained by any of claims 1 -21 as a starting material for a catalyst.

27. Use of the porous nitrided or de-nitrided iron material of claim 24 or as obtained by any of claims 1 -7 or 14-21 as a substrate material for deposition of other materials.