LITHIUM-BASED ALLOY AND METHOD OF PRODUCING THE SAME

Abstract

A lithium (Li)-based alloy and a preparation method thereof are disclosed, in which the lithium metal is wrapped by a metal foil with a higher melting point, followed by subjecting to multi-stage thermal treatment to cast alloy, thereby obtaining the Li-based alloy with high purity-Li.

Description

RELATED APPLICATIONS

This application claims priority to Taiwan Application Serial Number 099141471, filed Nov. 30, 2010, which is herein incorporated by reference.

BACKGROUND

1. Field of Invention

The present invention relates to a lithium (Li)-based alloy and a method of producing the same. More particularly, the present invention relates to a lithium (Li)-based alloy with high purity-Li and a method of producing the same.

2. Description of Related Art

Lithium (Li)-based alloy is generally referred to a lightweight alloy material including Li. Due to Li with extremely low density (0.534 g/cm³), the Li-based alloy has lower density as compared to other kinds of alloys. Recently, the Li-based alloy becomes the prime candidate material for designing lightweight structural devices.

Typically, vacuum induction melting (VIM) process is commonly applied to cast the Li-based alloy, and it is generally divided into forward feed casing process and reverse food casting process. During the forward feed casing process, low-melting-point Li is molten in a highly vacuumed chamber of the VIM furnace with nitrogen gas introduced therein, and then the molten Li is heated to a higher temperature and added with other higher-melting-point metal, so as to cast the Li-based alloy. On the contrary, during the reverse feed casing process, other higher-melting-point metal is molten in a highly vacuumed chamber of the VIM furnace with nitrogen gas introduced therein, and then it is added with low-melting-point Li, so as to cast the Li-based alloy.

In addition, mechanical alloying process is also applied to cast the Li-based alloy, in which Li and Al metals are ball milled for a long period, so as to form β-phase Li—Al alloy (or β-Li—Al alloy).

However, it is hardly to avoid the overheating conditions when using the aforementioned processes. Li is an active element and has lower melting point and density. During casting alloy in the conventional processes, the molten Li is easily floated on the molten Al, so that it hardly prevents the molten Li from being evaporated in the overheating condition and generating undesired Li oxides or impurities. Thus, it is more difficult to cast Li—Al alloy during the conventional processes, and the resulted Li—Al alloy has low purity Li, oxides and impurities.

Hence, it is necessary to provide a Li-based alloy and a method of making the same, thereby overcoming the disadvantages of evaporation, oxides or impurities of lithium during the conventional processes such as forward feed casing process, reverse food casting process and mechanical alloying process.

SUMMARY

According to an aspect of the invention, a method of producing lithium (Li)-based alloy is provided. The method of producing lithium (Li)-based alloy comprises the steps of wrapping lithium metal by using a metal foil with a higher melting point, followed by subjecting to multi-stage thermal treatment to cast alloy, thereby obtaining the Li-based alloy with more uniformly dispersed and high purity-Li. Since the method can save the total process time and reduce the occurrence of evaporation, oxides or impurities of lithium. Therefore, the present method overcomes the troubles of evaporation, oxides or impurities of lithium during the conventional processes such as forward feed casing process, reverse food casting process or mechanical alloying process.

Moreover, a Li—Al alloy is further provided, which has more uniformly dispersed and high purity-Li is casted by the aforementioned method. Therefore, the Li—Al alloy can be applied on the lightweight structural devices (e.g. materials of sports equipments or cases of military weapon cases) or composite hydrogen-storage materials.

Accordingly, the invention provides a method of producing Li-based alloy. In an embodiment, a lithium metal is firstly wrapped by using a metal foil for forming a wrapped lithium metal. In an example, the metal foil includes a material of a high-melting-point metal having a higher melting point than a melting point of the lithium metal.

Next, a raw material and the wrapped lithium metal are heated respectively to a first temperature. In an example, the raw material includes the same material of the high-melting-point metal as the metal foil, and the first temperature may be between the melting points of the metal foil and the lithium metal.

And then, the wrapped lithium metal is mixed with the raw material at the first temperature, so as to form a metal mixture, followed by heating the metal mixture to a second temperature, in which the second temperature is higher than the melting point of the high-melting-point metal, so as to form an molten alloy. Afterward, the molten alloy is mixed at the second temperature.

Subsequently, the alloy is cooled down for forming a lithium-based alloy, in which the lithium-based alloy is a β-phase lithium-based alloy, and the lithium metal has a weight loss rate of equal to or less than 1 percent.

According to an embodiment of the present invention, the high-melting-point metal may include but not be limited to aluminum, magnesium, manganese, zirconium, zinc, titanium, scandium, yttrium, copper, silver or any combination thereof.

According to an embodiment of the present invention, the raw material may further comprise a non-metal material including silicon.

According to an embodiment of the present invention, the raw material may further comprise another metal that includes a different material from the high-melting-point metal. In an example, the first temperature may be between two lower melting points of the lithium metal, the high-melting-point metal and the another metal. In another example, the second temperature may be higher than the highest melting point of the lithium metal, the high-melting-point metal and the another metal.

According to another aspect of the present invention, a β-phase Li—Al alloy (or β-Li—Al alloy) is further provided. In an embodiment, the β-Li—Al alloy may be casted by any one of the aforementioned methods. In another embodiment, the β-Li—Al alloy may be applied on the lightweight structural devices or composite hydrogen storage materials.

With application of the Li-based alloy and the method of making the same, the lithium metal is wrapped by using the metal foil with a higher melting point, followed by subjecting to multi-stage thermal treatment to cast alloy, thereby obtaining the Li-based alloy with more uniformly dispersed and high purity-Li. Since the present method can save the total process time and reduce the occurrence of evaporation, oxides or impurities of lithium, it successfully overcomes the troubles of evaporation, oxides or impurities of lithium during the conventional processes such as forward feed casing process, reverse food casting process or mechanical alloying process, thereby being applied on the lightweight structural devices (e.g. materials of sports equipments or cases of military weapons) or composite hydrogen storage materials.

It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows:

FIG. 1 depicts a flow chart diagram of the method of producing a Li-based alloy according to an embodiment of the invention; and

FIG. 2 depicts a diagram of a reacting system according to an embodiment of the invention.

DETAILED DESCRIPTION

As aforementioned, the present invention provides a Li-based alloy and a method of producing the same, in which the low-melting-point lithium metal is wrapped by using metal foil that is made from a high-melting-point metal having a higher melting point than lithium (or called as a high-melting-point metal foil). Following, the wrapped lithium metal is subjected to a multi-stage thermal treatment to cast alloy, thereby obtaining the Li-based alloy with more uniformly dispersed and high purity-Li. It should be clarified that the “Li-based alloy” is referred to a lightweight alloy material including the lithium metal. The “metal foil” herein may include but not be limited to a metal film, a metal sheet and other structural equivalents or alternatives.

Composition of Li-Based Alloy

In an embodiment, the Li-based alloy may be consisted of two metal materials such as lithium metal and a high-melting-point metal. The high-melting-point metal has a higher melting point than the lithium metal, which may be exemplified as but not limited to aluminum, magnesium, manganese, zirconium, zinc, titanium, scandium, yttrium, copper or silver, thereby forming Li—Al, Li—Mg, Li—Mn, Li—Zr, Li—Zn, Li—Ti, LiSc, Li—Y, Li—Cu or Li—Ag alloys. In an example, the Li-based may be Li—Al alloy, for example. In another example, the Li-based alloy may be also a β-phase Li—Al alloy (or n-Li—Al alloy).

In another embodiment, at least a third material may be alternatively added into the Li-based alloy that includes the aforementioned two metal materials, so as to form a Li-based alloy that includes at least three components. The third material may be a metal or non-metal material. When the third material is a metal material, it may be another high-melting-point metal that is different from the aforementioned two metal materials or other metal material. When the third material is a non-metal material, it may be silicon, for example.

Method of Making Li-Based Alloy

Since the lithium metal is easily oxidized and evaporated due to its lower melting point (approximately 180.54° C.), the present invention is directed to wrap the lithium metal by using a high-melting-point metal foil, thereby reducing the period of lithium oxidization and evaporation.

Reference is made to FIG. 1, which depicts a flow chart diagram of the method of producing a Li-based alloy according to an embodiment of the invention. The method 100 may include but not be limited to a multi-stage thermal treatment to cast alloy. Hereinafter, the Li-based alloy produced by two components is firstly exemplified as follows.

At first, in the step 101, the lithium metal is wrapped by using a metal foil for forming a wrapped lithium metal. In an example, a material of the metal foil may be a high-melting-point metal having a higher melting point than the lithium metal as exemplified as aforementioned.

Next, in the step 103 (or a first heating step), a raw material and the wrapped lithium metal are heated respectively to a first temperature. In an example, the raw material includes the same material of the high-melting-point metal as the metal foil, and the first temperature may be between the melting points of the metal foil and the lithium metal. Moreover, at the first temperature, the lithium metal must be molten but the high-melting-point metal is heated uniformly and close to a being-molten state, so that the high-melting-point metal can protect the molten lithium metal from being evaporated and oxidized. Thus, in an example, the first temperature is higher than the melting point of the lithium metal but lower than the one of the high-melting-point metal.

In another example, when the high-melting-point metal is aluminum metal that has a melting point of approximately 660° C., the first temperature is about 200° C. to less than 660° C., or about 630° C. to about 650° C., or alternatively 640° C.

For the purpose of decreasing the probability of the Li oxidation, in the step 103, the reacting chamber can be filled with a protection gas or under vacuum. In an example, when the reacting chamber is filled with a protection gas, the protection gas may be nitrogen gas or other inert gases, and an internal pressure may be more than one atmosphere (atm), for example. In other examples, when the reaction chamber is under vacuum, the internal pressure may be less than approximately 10⁻² Torr (i.e. approximately 1.33 Pa), for example.

In order to reduce the subsequent time of Li oxidation and evaporation, the step 103 can be carried out in the aforementioned conditions for about one hour.

And then, in the step 105, the wrapped lithium metal is mixed with the raw material at the first temperature, so as to form a metal mixture. In an example, the pre-heated high-melting-point metal may be added into the pre-heated wrapped Li metal, for forming the metal mixture. Following, in the step 107 (or a second heating step), the metal mixture is heated to a second temperature, in which the second temperature is higher than the melting point of the high-melting-point metal, so as to form a molten alloy. For completely melting the high-melting-point metal and the Li metal to form the molten alloy, the second temperature is necessarily higher than the melting point of the high-melting-point metal. According to an embodiment, the second temperature may be 80° C. to 100° C. higher than the melting point of the high-melting-point metal.

Afterward, in the step 109, the molten alloy is mixed well at the second temperature. Subsequently, in the step 111, the molten alloy is cooled down for forming a lithium-based alloy. In an example, the molten alloy may be cooled down to 0° C. to 50° C. approximately.

Due to the low-melting-point Li metal wrapped with the high-melting-point metal foil, followed by the multi-stage thermal treatment to cast alloy, the preset method can produce the Li-based alloy with more uniformly dispersed and high purity-Li in mass in a shorter process time (4 to hours approximately), in which the Li-based alloy is a β-phase lithium-based alloy, and the lithium metal has a weight loss rate of equal to or less than 1 percent.

It is worth mentioning that, instead of directly heating to the second temperature in the prior process, the present method reduces the difference between the first and second temperatures in the step 107 because the Li metal contacts with the being-molten high-melting-point metal prior to contacting with the protecting gas. Hence, it drastically decreases the probability of the impurities since the Li metal is hardly to contact with the impurities in the protecting gas. Moreover, it also reduces the evaporation time of the Li metal due to the high thermal treatment.

Furthermore, the Li-based alloy may be produced by at least three components, and in this case, the at least three components may include but be not limited to the Li metal, the high-melting-point metal and a third material. The third material may be another metal or a non-metal material.

In addition, the Li-based alloy produced by the at least three components may be obtained by using the aforementioned method. For example, in the step 101, the lithium metal is wrapped by a metal foil for forming wrapped lithium metal, in which the metal foil includes a material of the high-melting-point metal.

Next, in the step 103, the wrapped lithium metal and a raw material consisting of the high-melting-point metal and the third material are heated respectively to a first temperature. In an example, the first temperature may be the first temperature is between two lower melting points of the lithium metal, the high-melting-point metal and the third material.

Following, in the step 107, the second temperature is higher than the highest melting point of the lithium metal, the high-melting-point metal and the third material, or the second temperature may be 80° C. to 100° C. higher than the highest melting point of the lithium metal, the high-melting-point metal and the third material.

It should be supplemented that, during producing the Li-based alloy with the at least three components, only the Li metal is wrapped by the high-melting-point metal, but the third material doesn't be wrapped.

Reacting System for Producing Li-Based Alloy

Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

In an embodiment, the method of making the Li-based alloy can be carried out in conventionally reacting system or the reacting system 200 of FIG. 2. Hereinafter, the reacting system 200 of FIG. 2 is exemplified to clarify this disclosure. Reference is made to FIG. 2, which depicts a diagram of a reacting system according to an embodiment of the invention. In an embodiment, the reacting system 200 of FIG. 2 may be a step-controlled casting furnace, for example. The aforementioned step 101 may be carried out in two reacting chambers such as a pre-heat crucible 211 and a main crucible 212 of the reacting system 200 of FIG. 2, in which the main crucible 212 is connected to a side of the pre-heat crucible 211.

The pre-heat crucible 211 is used for receiving the high-melting-point metal 218 (e.g. aluminum or at least two different high-melting-point metals). Heating devices 223 may be disposed on an outside of the pre-heat crucible 211 for performing the aforementioned first heating step (e.g. the step 103), for pre-heating the high-melting-point metal 218 uniformly in the pre-heat crucible 211 to the first temperature. The main crucible 212 is used for receiving the Li metal 220 wrapped by the high-melting-point metal foil 219. There are also heating devices 203 disposed on an outside of the main crucible 212 for performing the multi-stage thermal treatments such as the first heating step (e.g. the step 103), the second heating step (e.g. the step 107), the mixing step at the second temperature (e.g. the step 109), the cooling step (e.g. the step 111) and so on.

The Li metal has a very lower melting point than the high-melting-point metal 218, that is to say, it is longer for heating the high-melting-point metal 218 to the first temperature. However, during the first heating step, the high-melting-point metal 218 heated in the pre-heat crucible 211 may generate radiant heat for melting the Li metal 220 probably. Thus, in an embodiment, a heat-insulation material, for example, the heat-insulation plate 215 of FIG. 2, may be disposed between the pre-heat crucible 211 and the main crucible 212 of the reacting system 200. The heat-insulation plate 215 may be made of refractory fibers, so that it can prevent the heat of the pre-heat crucible 211 from radiating to the main crucible 212, thereby avoiding the Li metal 220 to melt too early. Besides, an opening 215a optionally disposed on the heat-insulation plate 215 may be corresponded to a lowering position of a mixing bar 214, so that the mixing bar 214 can pass through the opening 215a and extend into the main crucible 212 for stirring and mixing the molten alloy.

When the Li metal 220 wrapped by the high-melting-point metal foil 219 in the main crucible 212 is heated to the first temperature by using the heating devices 203, the high-melting-point metal 218 is also heated uniformly and close to a being-molten state. In the meanwhile, by using a pushing bar 213 along a direction of an arrow 231, the pre-heated high-melting-point metal 218 is pushed into the Li metal 220 wrapped by the high-melting-point metal foil 219 in the main crucible 212, followed by heating to the second temperature by using the heating devices 203, and allowing both of the high-melting-point metal 218 and the Li metal 220 wrapped by the high-melting-point metal foil 219 to form a molten alloy.

Later, the mixing bar 214 can pass through the opening 215a, extend into the main crucible 212, stir and mix the molten alloy in the main crucible 212 along a direction of an arrow 233, so as to make all components more uniformly. In an example, the mixing bar 214 can be made of a heat-resistant material such as stainless, and a refractory material can be coated on a surface of the mixing bar 214. In another example, the mixing bar 214 can be disposed above the main crucible 212 and fixed on a side of the reacting system 200 by using an 0-ring. The mixing bar 214 can be operated along upward, downward, clockwise or counterclockwise direction for stirring and mixing the molten alloy uniformly.

During the cooling step (e.g. the step 111), the molten alloy can be cooled down naturally for forming the Li-based alloy.

Reference is made to FIG. 2 again. In an embodiment, during performing the aforementioned multi-stage thermal treatment, at least one gas inlet 216 and a gas outlet 217 can be disposed on a backside of the reacting system 200, for providing a higher vacuum environment or an environment filled with the protection gas. In an example, the gas inlet 216 can be disposed on any position freely depending on the casting requirements, and it can be connected with a gas bottle for introducing the protection gas from an opening 205 into the reacting system 200. In this example, a ball valve (unshown) can be disposed at a connection site of the gas inlet 216 connecting with the reacting system 200, for controlling the transfer of the protection gas. In another example, the gas outlet 217 can be connected with a mechanical pump (unshown) to vacuum the inner chamber (e.g. the pre-heated crucible 211 and the main crucible 212) of the reacting system 200 from an opening 207 for achieving a vacuum or highly vacuum state. In this example, another ball valve (unshown) can be also disposed at a connection site of the gas outlet 217 connecting with the reacting system 200, for controlling the vacuum degree.

Since the metal foil with the higher melting point than the Li metal is used to wrap the Li metal in the present method, followed by subjecting to the multi-stage thermal treatment to cast alloy, thereby saving the total process time and drastically reduce the occurrence of evaporation, oxides or impurities of lithium. Therefore, the present method successfully overcomes the troubles of evaporation, oxides or impurities of lithium during the conventional processes such as forward feed casing process, reverse food casting process or mechanical alloying process. Moreover, the Li-based alloy has more uniformly dispersed and high purity-Li, thereby being applied on the lightweight structural devices (e.g. materials of sports equipments or cases of military weapons) or composite hydrogen storage materials.

Thereinafter, various applications of the Li-based alloy and the method of making the same will be described in more details referring to several exemplary embodiments below, while not intended to be limiting. Thus, one skilled in the art can easily ascertain the essential characteristics of the present invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

EXAMPLE

Preparation of β-Phase Li—Al Alloy

In this example, the β-phase Li—Al alloy was prepared by the reacting system 200 of FIG. 2. At first, the aluminum metal and the lithium metal with the weight ratio of 5:1 were weighted. The aluminum metal was pre-heated uniformly to the first temperature, and the first temperature was about 200° C. to about 660° C., or about 630° C. to about 650° C., or about 640° C. Meanwhile, the Li metal 220 wrapped by the high-melting-point metal foil 219 in the main crucible 212 was also heated uniformly to the first temperature.

Next, the pre-heated aluminum metal was added into the main crucible 212, followed by performing the second heating step to the second temperature. The second temperature was about 740° C. to about 760° C., or 750° C., for completely melting the aluminum metal and the Li metal to form the molten alloy.

Before contacting with the protecting gas, the Li metal contacted with the being-molten aluminum metal firstly. Hence, it drastically decreased the probability of the impurities since the Li metal was hardly to contact with the impurities in the protecting gas. Moreover, since the aluminum was pre-heated before being molten, it reduced the heating time of the main crucible 212 and the evaporation time of the Li metal under the high thermal treatment.

Afterward, the molten alloy was mixed well at the second temperature for approximately 10 minutes.

Subsequently, the molten alloy was cooled down to 0° C. to 50° C. approximately for forming the β-phase Li—Al alloy. The aforementioned process time was spent only about 4 to 5 hours.

The content of the resulted β-phase Li—Al alloy was further analyzed by a commercially available equipment, for example, inductively coupled plasma—atomic emission spectrometer (ICP-AES), for obtaining the result as shown in TAB. 1. According to the result of TAB. 1, the content of the cast β-phase Li—Al alloy was very close to the theoretical ratios, and the total casting process was spent only about 4 to 5 hours.

TABLE 1
Content of Elements in Li—Al alloy (wt. %)
	Li	Al	Fe	Mn	Cr	O
	19.8	80.01	0.10	0.02	0.03	0.04

It is worth mentioning that, the aluminum foil with the higher melting point is used to wrap the Li metal in the present method, followed by subjecting to the multi-stage thermal treatment to cast alloy, thereby saving the total process time (about 4 to 5 hours) and producing the Li-based alloy with more uniformly dispersed and high purity-Li in mass. Additionally, the lithium metal has a weight loss rate of equal to or less than 1 percent. Therefore, the present method successfully overcomes the troubles of evaporation, oxides or impurities of lithium during the conventional processes such as forward feed casing process, reverse food casting process or mechanical alloying process.

The resulted Li-based alloy can be applied on any type of the lightweight structural devices or composite hydrogen storage materials, which include but not are limited to materials of sports equipments, cases of military weapons or composite hydrogen storage materials, rather than describing in detail herein. However, it is necessarily supplemented that, some technical details such as specific kinds or ratio of metals, specific processing conditions, specific equipments and specific analyzing methods are employed as exemplary embodiments in the present invention, for obtaining and evaluating the applications of the resulted Li-based alloy. However, it is not necessary to use all aforementioned details in all embodiments. As is understood by a person skilled in the art, the Li-based alloy of the present invention can include other kinds or ratio of metals, other processing conditions, other equipments and other analyzing methods rather than limiting to the aforementioned examples. Moreover, all known structure or devices familiarly to the person skilled in the art are merely depicted schematically in the accompanying drawings for illustration purposes only.

According to the embodiments of the present invention, the Li-based alloy and the method of making the same advantageously include to wrap the lithium metal by using the metal foil with a higher melting point, followed by subjecting to multi-stage thermal treatment to cast alloy, thereby obtaining the Li-based alloy with more uniformly dispersed and high purity-Li. Since the present method can save the total process time and reduce the occurrence of evaporation, oxides or impurities of lithium, it can be applied on the lightweight structural devices (e.g. materials of sports equipments or cases of military weapons) or composite hydrogen storage materials.

As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrated of the present invention rather than limiting of the present invention. In view of the foregoing, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. Therefore, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.

Claims

1. A method of producing lithium-based alloy, comprising:

wrapping lithium metal by using a metal foil for forming a wrapped lithium metal, wherein the metal foil includes a material of a high-melting-point metal having a higher melting point than a melting point of the lithium metal;

heating a raw material and the wrapped lithium metal respectively to a first temperature, wherein the raw material includes the material of the high-melting-point metal, and the first temperature is between the melting points of the metal foil and the lithium metal;

mixing the wrapped lithium metal with the raw material at the first temperature, so as to form a metal mixture;

heating the metal mixture to a second temperature, wherein the second temperature is higher than the melting point of the high-melting-point metal, so as to form an molten alloy;

mixing the molten alloy at the second temperature; and

cooling down the alloy for forming a lithium-based alloy, wherein the lithium-based alloy is a β-phase lithium-based alloy, and the lithium metal has a weight loss rate of equal to or less than 1 percent.

2. The method of producing lithium-based alloy of claim 1, wherein the high-melting-point metal comprises at least two different metals of aluminum, magnesium, manganese, zirconium, zinc, titanium, scandium, yttrium, copper, silver or any combination thereof.

3. The method of producing lithium-based alloy of claim 1, wherein the second temperature is 80° C. to 100° C. higher than the melting point of the high-melting-point metal.

4. The method of producing lithium-based alloy of claim 1, wherein the raw material further comprises another metal, and the another metal includes a material different from the high-melting-point metal.

5. The method of producing lithium-based alloy of claim 4, wherein the first temperature is between two lower melting points of the lithium metal, the high-melting-point metal and the another metal.

6. The method of producing lithium-based alloy of claim 4, wherein the second temperature is higher than the highest melting point of the lithium metal, the high-melting-point metal and the another metal.

7. The method of producing lithium-based alloy of claim 4, wherein the second temperature is 80° C. to 100° C. higher than the highest melting point of the lithium metal, the high-melting-point metal and the another metal.

8. The method of producing lithium-based alloy of claim 1, wherein the raw material further comprises a non-metal material.

9. The method of producing lithium-based alloy of claim 8, wherein the non-metal material comprises silicon.