Manufacturing process for a samarium monosulfide phase

Abstract

A process for producing samarium monosulfide crystals, comprising the steps: a) homogenisation of elemental sulphur with elemental samarium, in a ratio of 1 mol Sm to at least one mol S to form a homogenate in an open system under a protective gas atmosphere or vacuum for at least 6h, b) tempering the homogenate under a protective gas atmosphere or vacuum at a final temperature between 200°C and 800°C, producing an SmS powder after cooling to room temperature by means of a first temperature program in which the homogenate is heated from room temperature in a first step within 2 hours to a first temperature between 80°C and 150°C and then in a second step within at least 2 hours from this first temperature to a second temperature between 200°C and 800°C and this second temperature is maintained for at least 0.5 h, followed by a step of cooling to room temperature; b1) optionally performing a cold isostatic or uniaxial pressing of the powder to obtain a blank, c) Tempering the SmS powder or the blank under a protective gas atmosphere or vacuum to a maximum of 2,100°C, maintaining this temperature for a period of at least 18 hours.

Description

[0001] The present invention relates to a process for producing a samarium monosulfide phase (SmS) in either amorphous or crystalline form.

[0002] Samarium is one of the rare earth elements in which the 4f shell is successively filled without changing the occupancy of the previously occupied and overlying 5d and 6s shells. The probabilities of the electrons in the 4f states are highly localized and lie very close to the atomic core, and in the solid state, they do not contribute to bonding. The filling of this shell leads to the contraction of the atomic radii known as lanthanide contraction. Since the electrons closer to the nucleus have high velocities, which lead to a relativistic increase in the mass of the electrons and thus to a contraction of these inner shells, the nuclear charge is shielded and the 4f orbitals are relativistically destabilized. This behavior leads to interesting physical effects, particularly in samarium chalcogenide compounds such as samarium sulfide (SmS).

[0003] SmS is a black, semiconducting solid with a cubic NaCl crystal structure, with samarium in the +2 oxidation state surrounded by octahedral sulfur atoms. Samarium monosulfide has the property of changing its crystal structure upon application of mechanical stress, resulting, among other things, in a color change from black to golden yellow. This transition under mechanical stress does not alter the cubic NaCl structure of the lattice itself, but reduces the crystal volume. In particular, the lattice constant changes from 0.597 nm to 0.57 nm, the electrical resistance decreases abruptly, and the phase exhibits the conductivity of a poor metal. The reflectivity increases in the near-infrared range, exhibiting near-metallic values, and the magnetic susceptibility drops abruptly by 65%.After the mechanical load is removed, SmS returns to its original structure, so that it can be used in pressure sensors or storage media that can jump between a low-resistance and a high-resistance state by applying external pressure.

[0004] This presumably pressure-induced semiconductor-to-metal transition results from the delocalization of a 4f electron and thus from a valence transition of the samarium ions in the crystal. The reduced radius of the 4f shell leads to the isostructural volume collapse of the cubic unit cell. This is closely linked to a shift in the energy bands, particularly a broadening of the 4f and 5d bands, which increases the charge carrier density in the conduction band, leading to a transition of the localized 4f electrons into the conduction band.

[0005] It is currently known to produce thin SmS layers by sputtering or electron beam evaporation of the metal in an H2S2 atmosphere. However, a technical manufacturing process for SmS crystals, such as those required for the sensors mentioned, was not known until now.

[0006] DE 11 38 133 A discloses a process for producing thermocouples with an SmS leg material. In this process, a stoichiometric homogenate is sealed in tubes under vacuum, heated to 800°C, and then cooled to room temperature. The temperature is increased by 150°C to 250°C per hour, and the final temperature should be maintained for no more than 2 hours. The resulting reaction product is pressed into pellets, which are then heated to between 800°C and 1300°C in a protective gas atmosphere.

[0007] A similar process is disclosed in US 3 247 022 A, in which an equimolar mixture of Sm and S is melted in a thick-walled quartz tube and then heated to 1,000°C.

[0008] RU 2 031 382 C1 discloses a process in which S and Sm are melted at two spatially separated ends in a quartz ampoule under hydrogen and heated so that S initially enters the gas phase, after which the reaction continues at 700°C. After cooling and grinding, the pellets are pressed and sintered at 1,300°C to 1,400°C for 8 hours. The resulting product is homogenized in a mortar.

[0009] Strain gauges with an SmS film are known from US 2003 / 0 064 884 A1 and US 6 132 568 A.

[0010] Such processes may be suitable for laboratory use or scientific research, but not for economically viable production of SmS.

[0011] The present invention therefore has the object of providing a production process which is not limited to the laboratory scale and which avoids the disadvantages of the prior art.

[0012] This object is achieved by a process according to the invention for producing a samarium monosulfide phase, which comprises the following steps: a) homogenizing elemental sulfur with elemental samarium, in a ratio of 1 mol Sm to at least one mol S to form a homogenate in an open-circuit system under a protective gas atmosphere or vacuum for at least 6 hours, b) tempering the homogenate under a protective gas atmosphere or vacuum at a final temperature between 200°C and 800°C, producing an SmS powder after cooling to room temperature by means of a first temperature program, in which the homogenate, starting from room temperature, is heated in a first step within 2 hours to a first temperature between 80°C and 150°C and then, in a second step, is heated from this first temperature to a second temperature between 200°C and 800°C within at least 2 hours and this second temperature is maintained for at least 0.5 h,followed by a step of cooling to room temperature, optionally followed by a step b1) in which a cold isostatic or uniaxial pressing of the powder is carried out to obtain a blank, followed by a step c) of tempering the SmS powder or the blank under a protective gas atmosphere or vacuum to a maximum of 2,100°C, this temperature being maintained for a period of at least 18 hours.

[0013] Surprisingly, it has been shown that the stoichiometric reactant ratio of Sm to S with respect to S must not be substoichiometric when working in a non-closed system, i.e. at least one mole of S must be present for one mole of Sm in order to obtain the desired crystal in the desired composition at the end of the process according to the invention, which takes place in a non-closed system. Lower molar ratios lead to substoichiometric compositions of the subsequent crystal, down to Sm7S. It has been shown that the elemental sulfur used tends to volatilize from the reaction mixture. The molar ratio of Sm to S in the homogenate is preferably 1:3, very particularly preferably 1:5, in order to have a stoichiometric ratio of Sm to S after the first tempering step.The invention understands a non-closed system to be one in which an exchange of matter and also energy is possible, in particular one in which the reactants can escape into the environment or matter from the environment can enter the process space, for example through the outlet opening of a vacuum pump connected to a furnace or a homogenizer.

[0014] Surprisingly, it has also been shown that a phase according to the invention can only be obtained if two different tempering steps are carried out. In a first tempering step, the homogenate is heated to a first final temperature which is significantly below the melting point of SmS, namely to a final temperature between 200°C and 800°C, whereby these temperatures do not have to be maintained to the exact degree, although this is preferred. Due to the oxidation sensitivity of both Sm and S, this step must also be carried out under a protective gas atmosphere, in particular under an argon atmosphere. Alternatively, it can also be carried out with equal success under a vacuum, so that whenever a protective gas atmosphere is mentioned below, a vacuum, in particular a technical vacuum, is also meant and should be taken into account.Due to the lower technical complexity, however, process steps in a non-sealed system are preferred according to the invention, i.e. those which are in material and energetic contact with the environment. Accordingly, the three steps mentioned are each carried out individually in open systems under a protective gas, in closed systems under a protective gas and / or in vacuum systems with, if appropriate, residual components of a protective gas or air. After the powder produced in this way has cooled to room temperature, the second tempering step takes place according to the invention, which ends at a significantly higher second final temperature, namely at a second final temperature not exceeding 2,100°C, preferably this is below this value, in particular below 1,800°C, preferably below 1,500°C, very particularly preferably 1,200°C. Here too, the temperatures mentioned do not have to be maintained to the exact degree; this step too must be carried out under a protective gas atmosphere, ora vacuum.

[0015] Surprisingly, it has also been shown that a stoichiometric SmS phase is particularly well obtainable if tempering is carried out in two stages with intermediate cooling, whereby the first tempering, as will be described in more detail below, leads to a largely stable product.

[0016] In a further development of the process, step a) is carried out between 6 h and 72 h, preferably between 12 h and 48 h. Such slow homogenization significantly reduces S losses.

[0017] In a further development of the process, in the second step of step b), the temperature is increased from the first to the second temperature within 5 to 60 hours, preferably within 10 to 40 hours. Here, too, a slow process control improves the yield with regard to the temperature gradients.

[0018] In a further development of the method, it is provided that between step b) and step c) a step b1) of cold isostatic or uniaxial pressing of the powder, preferably under a protective gas atmosphere, is carried out in order to obtain a blank.

[0019] In a further development of the method, step b1) can be followed by step b2), in which the blank is machined, preferably under a protective gas atmosphere, to obtain a machined blank. Machining is understood to mean any machining, including manual machining, in particular cutting processes with geometrically defined or geometrically undefined cutting edges, such as grinding. It is very advantageous in this way to obtain a machinable or machined blank after the first tempering step, which is predominantly manufactured to its final size and which is advantageously further tempered for its later use. As already explained, this blank, like the SmS powder, can be in crystal form or in a more or less amorphous form.

[0020] In an advantageous development of the method, step a) is carried out in a mill under an argon atmosphere for 12 to 36 hours, in particular for 18 to 24 hours, in particular using zirconium oxide grinding balls. This enables a particularly high-quality homogenate in terms of grain size distribution and homogeneity. Other homogenization devices are also according to the invention, in particular other grinding media mills, rotor and disc vibrating mills, as well as other devices for comminution and / or mixing, such as in particular pressure, impact, friction, cutting and impact comminutors such as jaw crushers, impact crushers, material bed roller mills, cone and hammer crushers, single-shaft comminutors and others, as long as they are provided with a protective gas atmosphere or a vacuum can be applied to them, in particular a technical one.

[0021] According to the invention, it is further provided that step b) is carried out by means of a first temperature program in which the homogenate is heated from room temperature in a first step to a first temperature between 80°C and 150°C, then in a second step is heated from this first temperature to a second temperature between 200°C and 800°C, this second temperature is maintained for between 4h and 8h, followed by cooling to room temperature.

[0022] More specifically, it is provided that step b) is carried out by means of a first temperature program in which the homogenate is heated from room temperature to a first temperature of 115°C in two hours, then from this first temperature to the second temperature of 600°C in 40h and this second temperature is maintained for 6h, followed by cooling to room temperature.

[0023] In an embodiment of the method, it is further provided that step c) is carried out by means of a second temperature program in which the SmS powder of the blank is heated from room temperature to a first temperature between 1,000°C and 2,100°C, preferably to a temperature between 1,000°C and 1,500°C, and is held at this first temperature for a period of 18 h to 36 h, preferably for a period of between 20 h and 30 h.

[0024] More specifically, step c) is performed with a temperature program in which the SmS powder or blank is heated from room temperature at 80°C / h to a temperature of 1,000°C and held for a period of 24 hours. After this tempering treatment, no further changes occur in the crystal, in particular no volume shrinkage.

[0025] The process according to the invention uses an argon atmosphere as a protective gas atmosphere in the individual steps. Preferred embodiment

[0026] The process according to the invention begins with the preparation of a homogenate of elemental Sm and S. First, a sulfur powder of 99.5% purity is homogenized with a samarium powder of 99.99% purity, which has been kept under vacuum, in a ball mill with zirconium oxide grinding balls, with a molar ratio of samarium to sulfur of 1:5. Homogenization is carried out at low speeds for 24 hours under an argon atmosphere.

[0027] The resulting homogenate is then heated from room temperature to 115°C in two hours, followed by a temperature rise of 40 hours to 600°C, which is maintained for 6 hours. This step is carried out under an argon atmosphere. After cooling to room temperature, cold isostatic pressing followed by machining to final dimensions.

[0028] The resulting machined blank is a 45 mm diameter disk, which is heated from room temperature to 1,000°C under an argon atmosphere at a rate of 80°C per hour and held at this temperature for 24 hours. The invention also allows for a temperature control to 1,200°C under the same conditions.

[0029] The resulting products, which were bonded and used as sputtering targets, were stable, but not when the holding time was less than 18 hours.

[0030] Three SmS phases produced according to this embodiment had the following mass contents: Phase 1: Sm: 76.6 mass%, S: 19.8 mass%, rest: C, H and O, each less than 0.5 mass%, in total 100 mass%. Phase 2: Sm: 83.2 mass%, S: 14.3 mass%, rest: C, H and O, each less than 1.2 mass%, in total 100 mass%. Phase 3: Sm: 85.4 mass%, S: 13.3 mass%, rest: C, H and O, each less than 1 mass%, in total 100 mass%.

[0031] This phase according to the invention is characterized by the Fig. The X-ray diffraction spectrum shown in Figure 1 shows a sequence of strong, very strong, strong, medium, and two very weak peaks, all generated by SmS and marked with an x. The other peaks are caused by SmO and Sm2O3, which demonstrates the oxygen sensitivity of the process according to the invention. Further examples

[0032] Based on the process steps and the starting materials of the preferred embodiment, a molar ratio of samarium to sulfur of 1:3 was used in step a). Homogenization was carried out under vacuum for 24 hours. The homogenate thus produced was heated in the first tempering step from room temperature to 140°C in three hours and then to 750°C in 20 hours, and this temperature was held for 2 hours. This step was carried out under an argon atmosphere. After cooling to room temperature, a powder with a molar ratio of Sm to S of 1.6:1 was obtained. The SmS powder was heated under an argon atmosphere, starting from room temperature at a ramp of 100°C per hour to 1,500°C and held at this temperature for 30 hours.

[0033] Comparative Example: Based on the process steps and the starting materials of the preferred embodiment, a samarium to sulfur molar ratio of 1:1 was used in step a). Homogenization was carried out for 12 hours under an argon atmosphere. The homogenate thus produced was heated in the first tempering step from room temperature to 400°C in one hour and then to 500°C in 5 hours; this temperature was held for 8 hours. This step was carried out under an argon atmosphere. After cooling to room temperature, a powder with a molar ratio of Sm to S of 3:1 was obtained. The SmS powder was heated under an argon atmosphere, starting from room temperature at a ramp of 50°C per hour to 1,200°C and held at this temperature for 12 hours. Further tempering according to the first temperature program of the preferred embodiment resulted in an SmS powder with a stoichiometric molar ratio.

[0034] It was shown that SmS phases according to the invention, whether crystalline, semi-crystalline, or amorphous, with an approximately stoichiometric ratio of Sm to S, produce a stable product over a wide temperature range that hardly changes or does not change under operating conditions. A great advantage is that the process according to the invention produces a stable product, or rather a stable SmS phase, even at relatively low temperatures in the two tempering steps.

Claims

[1] A process for producing samarium monosulfide crystals, comprising the steps of: a) homogenisation of elemental sulphur with elemental samarium, in a ratio of 1 mol Sm to at least one mol S to form a homogenate in an open system under a protective gas atmosphere or vacuum for at least 6 hours, b) tempering the homogenate under a protective gas atmosphere or vacuum at a final temperature between 200°C and 800°C, producing an SmS powder after cooling to room temperature by means of a first temperature program in which the homogenate, starting from room temperature, is heated in a first step within 2 hours to a first temperature between 80°C and 150°C and then, in a second step, is heated within at least 2 hours from this first temperature to a second temperature between 200°C and 800°C and this second temperature is maintained for at least 0.5 h, followed by a step of cooling to room temperature; b1) optionally carrying out a cold isostatic or uniaxial pressing of the powder to obtain a blank, c) Tempering the SmS powder or the blank under a protective gas atmosphere or vacuum to a maximum of 2,100°C, maintaining this temperature for a period of at least 18 hours. [2] Process according to claim 1, wherein step a) is carried out between 6 h and 72 h, preferably between 12 h and 48 h. [3] Process according to claim 1 or 2, wherein step a) is carried out in a mill under an argon atmosphere for 12 h to 36 h, in particular for 18 h to 24 h, in particular using zirconium oxide grinding balls. [4] Process according to claim 1, 2 or 3, wherein in the second step of step b) the temperature is increased from the first to the second temperature within 5 h to 60 h, preferably within 10 h to 40 h. [5] Method according to one of the preceding claims, in which step b1) is carried out under a protective gas atmosphere. [6] Method according to one of the preceding claims, in which, after step b1), a step b2) of machining the obtained blank is carried out, preferably under a protective gas atmosphere, in order to obtain a machined blank. [7] Method according to one of the preceding claims, in which step b) is carried out by means of a first temperature program in which the homogenate is heated from room temperature in a first step to a first temperature between 80°C and 150°C, then in a second step is heated from this first temperature to a second temperature between 200°C and 800°C, this second temperature is maintained for between 4h and 8h, followed by cooling to room temperature. [8] A process according to any one of the preceding claims, wherein the temperature in step c) is maintained for a period of 18 h to 36 h. [9] Method according to one of the preceding claims, in which step b) is carried out by means of a first temperature program in which the homogenate is heated from room temperature to a first temperature of 115°C in two hours, then from this first temperature to the second temperature of 600°C in 40 hours and this second temperature is maintained for 6 hours, followed by cooling to room temperature. [10] Method according to one of the preceding claims, in which step c) is carried out by means of a second temperature program in which the SmS powder or the blank is heated from room temperature to a temperature between 1,000°C and 2,100°C, preferably to a temperature between 1,000°C and 1,500°C and is held at this first temperature for a period of 18 h to 36 h, preferably for a period of between 20 h and 30 h. [11] Method according to one of the preceding claims, in which step c) is carried out with a temperature program in which the SmS powder or the blank is heated from room temperature at 80°C / h to a temperature of 1,000°C and is held for a period of 24 hours. [12] Method according to one of the preceding claims, in which the protective gas atmosphere in one or more process steps is an argon atmosphere.

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