BIODEGRADABLE Zn-Mg-Bi ZINC ALLOY AND PREPARATION METHOD THEREOF

Abstract

A biodegradable Zn—Mg—Bi zinc alloy and a preparation method thereof. The method including: melting magnesium under an inert atmosphere to obtain a magnesium melt; adding bismuth particles to the magnesium melt followed by reaction under stirring and heat preservation treatment to obtain a Mg—Bi alloy melt; allowing the Mg—Bi alloy melt to stand in a furnace; subjecting the Mg—Bi alloy melt to refining, slagging-off, casting and demoulding to obtain Mg-50 wt. % Bi alloy ingot; melting zinc to obtain a zinc melt; adding the Mg-50 wt. % Bi alloy ingot and pure magnesium or pure bismuth followed by heating to a preset temperature, stirring and heat preservation to obtain a Zn—Mg—Bi alloy melt; allowing the Zn—Mg—Bi alloy melt to stand in a furnace followed by refining, slagging-off, casting and demoulding to obtain the biodegradable Zn—Mg—Bi zinc alloy.

Claims

1. A method for preparing a biodegradable Zn—Mg—Bi zinc alloy, comprising: (1) melting magnesium under an inert atmosphere to obtain a magnesium melt; adding bismuth particles to the magnesium melt followed by reaction under stirring and heat preservation treatment to obtain a Mg—Bi alloy melt; allowing the Mg—Bi alloy melt to stand in a furnace; and subjecting the Mg—Bi alloy melt to refining, slagging-off, casting and demoulding to obtain a Mg-50 wt. % Bi alloy ingot; and (2) melting zinc to obtain a zinc melt; adding the Mg-50 wt. % Bi alloy ingot and pure magnesium or pure bismuth to the zinc melt, followed by heating under stirring, and heat preservation to obtain a Zn—Mg—Bi alloy melt; allowing the Zn—Mg—Bi alloy melt to stand in the furnace; and subjecting the Zn—Mg—Bi alloy melt to refining, slagging-off, casting and demoulding to obtain the biodegradable Zn—Mg—Bi zinc alloy.

2. The method of claim 1, wherein the Mg—Bi alloy melt is prepared through steps of: melting the magnesium at 650-700° C. in argon gas in the presence of a covering agent followed by keeping at 650-700° C. for 60-90 min to obtain the magnesium melt; and adding the bismuth particles to the magnesium melt followed by reaction under stirring at 50-80 rpm and 640-660° C. for 20-30 min and keeping at 640-660° C. for 12-15 min to obtain the Mg—Bi alloy melt.

3. The method of claim 1, wherein in step (1), the casting is performed at 580-610° C.; and the Mg—Bi alloy melt is allowed to stand in the furnace for 3-5 min.

4. The method of claim 1, wherein the Zn—Mg—Bi alloy melt is prepared through steps of: melting the zinc at 420-500° C. in argon gas for 60-90 min to obtain the zinc melt; adding the Mg-50 wt. % Bi alloy ingot and pure magnesium or pure bismuth to the zinc melt followed by stirring at 50-80 rpm and 640-660° C. for 20-30 min and keeping at 640-660° C. for 12-15 min to obtain the Zn—Mg—Bi alloy melt.

5. The method of claim 4, wherein the Zn—Mg—Bi alloy melt comprises 6-20% by volume of Mg.sub.3Bi.sub.2 as main strengthening phase; the Mg.sub.3Bi.sub.2 is produced by reaction of magnesium atoms and bismuth atoms; and a weight ratio of bismuth to magnesium in the Zn—Mg—Bi alloy melt is less than or equal to 2.2.

6. The method of claim 1, wherein in step (2), the casting is performed at 510-550° C., and a mold used in the demoulding is preheated to 180-200° C.

7. A biodegradable Zn—Mg—Bi zinc alloy prepared by the method of claim 1, consisting of: 1.10%-1.20% by weight of Mg, 0.50%-2.50% by weight of Bi and Zn.

8. The biodegradable Zn—Mg—Bi zinc alloy of claim 7, wherein a matrix structure of the biodegradable Zn—Mg—Bi zinc alloy is a zinc dendrite; there is a Mg.sub.2Zn.sub.11 strengthening phase and a Mg.sub.3Bi.sub.2 strengthening phase in the zinc dendrite; the Mg.sub.2Zn.sub.11 strengthening phase is a rod-shaped Mg.sub.2Zn.sub.11 eutectic phase; and the Mg.sub.3Bi.sub.2 strengthening phase consists of a rod-shaped Mg.sub.3Bi.sub.2 phase and a granular Mg.sub.3Bi.sub.2 phase.

9. The biodegradable Zn—Mg—Bi zinc alloy of claim 8, wherein a grain size of the matrix structure of the biodegradable Zn—Mg—Bi zinc alloy is 15-40 μm; the rod-shaped Mg.sub.2Zn.sub.11 eutectic phase has a length of 3-8 μm and a thickness of 0.5-1.2 μm; the short rod-shaped Mg.sub.3Bi.sub.2 phase has a thickness of 1-1.5 μm; and the granular Mg.sub.3Bi.sub.2 phase has a diameter of 1-5 μm.

10. The biodegradable Zn—Mg—Bi zinc alloy of claim 7, wherein the biodegradable Zn—Mg—Bi zinc alloy has a Brinell hardness of 45-77 HBS.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 illustrates a microstructure of a Mg-50 wt. % Bi intermediate alloy; and

[0040] FIG. 2 depicts a microstructure of a biodegradable Zn-1.2 wt. % Mg-0.5 wt. % Bi zinc alloy.

DETAILED DESCRIPTION OF EMBODIMENTS

[0041] The technical solutions of this disclosure will be described clearly and completely below. Obviously, described below are merely some embodiments of this disclosure, which are not intended to limit the disclosure. It should be noted that other embodiments obtained by those of ordinary skill in the art based on the embodiments of this disclosure without paying creative efforts shall fall within the scope of this disclosure.

[0042] Unless otherwise specified, all the embodiments and preferred implementation methods mentioned herein can be combined with each other to form new technical solutions.

[0043] Unless otherwise specified, all the technical features and preferred features mentioned herein can be combined with each other to form a new technical solution.

[0044] In this disclosure, unless otherwise specified, percentage (%) or part refers to the weight percentage or weight part of the composition.

[0045] In this disclosure, unless otherwise specified, the involved components or the preferred components thereto can be combined with each other to form a new technical solution.

[0046] In this disclosure, unless otherwise specified, the numerical range “a-b” represents any combination of real numbers between a and b, where both a and b are real numbers. For example, the numerical range “6-22” indicates that all real numbers between “6-22” have been listed in this disclosure, and “6-22” is just a simplified representation of the combination of these numerical values.

[0047] In this disclosure, a “range” disclosed herein is in the form of a lower limit and an upper limit, and there may be one or more lower limits, and one or more upper limits, respectively.

[0048] In this disclosure, the term “and/or” used herein refers to any combinations of one or more of associated listed items and includes any combinations.

[0049] In this disclosure, unless otherwise specified, each reaction or operation step can be performed in sequence. Preferably, the reaction methods herein are performed in sequence.

[0050] Unless otherwise specified, professional and scientific terms used herein have the same meanings as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to those described herein can also be used in this disclosure.

[0051] Provided herein is a biodegradable Zn—Mg—Bi zinc alloy and a preparation method thereof. A Mg-50 wt. % Bi intermediate alloy is added to a melt-down zinc melt followed by reaction under stirring under the protection of high-purity argon gas atmosphere at 640-660° C. for 20-30 min to obtain a melt. The melt is kept at 640-660° C. for 12-15 min. The melt is cast into a preheated cylindrical graphite mold at 510-550° C. to obtain a Zn—Mg—Bi biodegradable zinc alloy. The Zn—Mg—Bi biodegradable zinc alloys with different bismuth contents are provided. The structure and microstructure of zinc alloy are observed and the strengthening effect is analyzed to guide the study of the influence of bismuth on the structure and properties of zinc alloy.

[0052] The disclosure provides a biodegradable Zn—Mg—Bi zinc alloy, consisting of 1.10%-1.20% by weight of Mg, 0.50%-2.50% by weight of Bi, and Zn.

[0053] The matrix structure of the biodegradable Zn—Mg—Bi zinc alloy is a zinc dendrite, and the dual-strengthening-phase generated in the metallic zinc dendrite is a rod-shaped eutectic Mg.sub.2Zn.sub.11 phase, short rod-shaped Mg.sub.3Bi.sub.2 phase and granular Mg.sub.3Bi.sub.2 phase.

[0054] A grain size of the matrix structure of the biodegradable Zn—Mg—Bi zinc alloy is 15-40 μm. The rod-shaped eutectic Mg.sub.2Zn.sub.11 phase has a length of 3-8 μm and a thickness of 0.5-1.2 μm. The short rod-shaped Mg.sub.3Bi.sub.2 phase has a thickness of 1-1.5 μm. The granular Mg.sub.3Bi.sub.2 phase has a diameter of 1-5 μm.

[0055] The biodegradable Zn—Mg—Bi zinc alloy has a Brinell hardness of 45-77 HBS.

[0056] This disclosure provides a preparation method of the biodegradable Zn—Mg—Bi zinc alloy. The method is performed using high-purity raw materials by the high-purity smelting technology. In the raw material, the purity of the Zn is no less than 99.995%, the purity of the Mg no less than 99.999%, and the purity of the Bi no less than 99.999%. The preparation method is performed as follows.

[0057] (S1) Mg-50 wt. % Bi (mass fraction, wt. %, the same below) intermediate alloy is is specifically prepared as follows.

[0058] (S101) Outer oxide films of the raw materials of (5N grade for the purity) pure magnesium ingot and (5N grade for the purity) pure bismuth particle are removed, and the raw materials and a silicon carbide crucible are preheated to 180-200° C. in a vacuum heat treatment furnace followed by drying. Considering a burning loss of the raw materials, the raw materials are weighed according to a weight ratio using an electronic balance.

[0059] The alloy density ρ.sub.alloy is calculated as follows:

ρ.sub.alloy=M/V,in g/cm.sup.3;

[0060] where M is an alloy mass, and V is an alloy volume.

[0061] (S102) High-purity argon is introduced into a well-sealed pit type resistance furnace. A special covering agent used for magnesium alloy smelting is sprinkled at a bottom of the silicon carbide crucible. The magnesium ingots are put on the bottom of the silicon carbide crucible one by one using crucible tongs. And then, the covering agent is sprinkled on the magnesium ingots followed by heating to 650-700° C. for 60-90 min to completely melt the magnesium.

[0062] The melting point of pure magnesium is 648.9° C., such that the magnesium is melted at 650-700° C. for 60-90 min to obtain the magnesium melt.

[0063] (S103) Pure bismuth particles are slowly added into the magnesium melt after melting down, followed by stirring under a stirring speed of 50-80 rpm at 640-660° C. for 20-30 min, using a stirring device. After complete reacting the magnesium with bismuth, the temperature is kept at 640-660° C. in the furnace for 12-15 min to obtain a Mg—Bi alloy melt.

[0064] (S104) The Mg—Bi alloy melt is allowed to stand in the furnace for 3 to 5 min to make harmful inclusions in the melt fully floated and aggregated followed by refinement and slagging-off. Th casting temperature is measured by a temperature sensor. After that, the Mg—Bi alloy melt is cast in the preheated cylindrical graphite mold, and then demolded to obtain a Mg-50 wt. % Bi alloy ingot.

[0065] (S2) The Mg-50 wt. % Bi alloy ingot prepared in step (S1) is used to prepare a biodegradable Zn—Mg—Bi zinc alloy.

[0066] (S201) The silicon carbide crucible and the raw materials are preheated to 180-200° C. followed by drying. Each raw material is weighed with the consideration of the burning loss of each raw material (Zinc is cut into small pieces followed by cleaning at the surface and drying to obtain zinc pieces. The Mg-50 wt. % Bi intermediate alloy ingot is polished at the surface, followed by cleaning and drying).

[0067] (S202) The well-sealed pit type resistance furnace is heated to 420-500° C., and the high-purity argon gas is introduced. The zinc pieces are added to the silicon carbide crucible one by one in the well-sealed pit type resistance furnace followed by keeping at 420-500° C. for 60-90 min to completely melt the zinc pieces to obtain a zinc melt. Pure magnesium and/or pure bismuth are added to the zinc melt, and then Mg-50 wt. % Bi intermediate alloy ingot is added, followed by stirring at 50-80 rpm and 640-660° C. for 20-30 min, and keeping at 640-660° C. for 12-15 min to obtain a Zn—Mg—Bi alloy melt. The Zn—Mg—Bi alloy melt includes 6-20% by volume of Mg.sub.3Bi.sub.2 as main strengthening phase, The Mg.sub.3Bi.sub.2 is produced by reaction of magnesium atoms and bismuth atoms. A weight ratio of bismuth to magnesium in the Zn—Mg—Bi alloy melt is less than or equal to 2.2.

[0068] Under the protection of the high-purity argon, the smelling process is performed in a special silicon carbide crucible, and a covering agent and refining agent are added to purify. The smelting temperature is 50° C. above the melting point to avoid overburning. The Zn—Mg—Bi biodegradable zinc alloy is cast at 510-550° C., and a forming mold is a graphite mold preheated at 180-200° C.

[0069] (S203) The Zn—Mg—Bi alloy melt are subjected to standing in the furnace for 3-5 min to make the harmful inclusions and gases in the Zn—Mg—Bi alloy melt fully floated and aggregated followed by refinement and slagging-off. After that, the Zn—Mg—Bi alloy melt is cast into the preheated cylindrical graphite at 510-550° C., and then demoulded to obtain a biodegradable Zn—Mg—Bi zinc alloy.

[0070] The biodegradable Zn—Mg—Bi zinc alloy is subjected to structural observation, structural characterization and hardness characterization.

[0071] The specific morphology of the biodegradable Zn—Mg—Bi zinc alloy and the composition of the biodegradable Zn—Mg—Bi zinc alloy are analyzed using a field emission scanning electron microscope (Zeiss, GeminiSEM 500), a supporting energy-dispersive spectrometer (EDS), an electron probe (EPMA, JEOL JXA-8230), X-ray diffraction (XRD range of 20 is 10-90, current is 200 mA, voltage is 40 KV and the scanning rate is 2°/min). The hardness of the biodegradable Zn—Mg—Bi zinc alloy is tested by a Brinell hardness tester.

[0072] In order to clearly illustrate the objectives, technical solutions and advantages of this disclosure, the technical solutions of this disclosure will be clearly and completely described below with reference to the accompanying drawings in the embodiments of this disclosure. It is obvious that described below are merely some embodiments of this disclosure, which are not intended to limit the disclosure. The components described in the embodiments of this disclosure and illustrated in the accompanying drawings herein may be arranged and designed in various configurations. Thus, the following detailed description with reference to the accompanying drawings is not intended to limit the scope of this disclosure as, but is merely illustrative of selected examples of this disclosure. It should be noted that other examples obtained by those of ordinary skill in the art based on the examples of this disclosure without making creative efforts shall fall within the scope of the disclosure defined by the appended claims.

Example 1

[0073] Provided herein was a preparation of a biodegradable Zn—Mg—Bi zinc alloy, which was specifically performed through the following steps.

[0074] (S1) A magnesium ingot, a zinc ingot and a Mg-50 wt. % Bi intermediate alloy were cut into small pieces with a sawing machine, ground using a grinder, ultrasonically cleaned with alcohol and acetone and dried at 180° C. under vacuum to remove surface oxide films and oil from the sawing machine.

[0075] (S2) A Zn-1.1 wt. % Mg-0.5 wt. % Bi zinc alloy was prepared, where a weight ratio of Mg to Bi to Zn was 1.1:0.5:98.4.

[0076] (S3) The well-sealed pit type resistance furnace was heated to 420° C., and the high-purity argon gas was introduced. The zinc pieces were added to the crucible one by one and kept at 420° C. for 60 min, so as to completely melt the zinc pieces. After that, the Mg-50 wt. % Bi intermediate alloy and pure magnesium were added, fully melted, stirred at 50 rpm and 640° C. for 20 min, and then kept at 640° C. for 12 min to obtain a Zn-1.1 wt. % Mg-0.5 wt. % Bi alloy melt.

[0077] (S4) The Zn-1.1 wt. % Mg-0.5 wt. % Bi alloy melt was subjected to standing in the well-sealed pit type resistance furnace, refinement and slagging-off, and cast into a preheated mold at 510° C., and demolded to obtain a Zn-1.1 wt. % Mg-0.5 wt. % Bi zinc alloy ingot.

[0078] In Example 1, the Zn-1.1 wt. % Mg-0.5 wt. % Bi biodegradable zinc alloy was prepared by introducing the Mg—Bi intermediate alloy and pure magnesium into the zinc melt. The microstructure of the Zn-1.1 wt. % Mg-0.5 wt. % Bi biodegradable zinc alloy was analyzed and identified, and the results revealed that after the addition of 0.50 wt. % Bi, the rod-shaped Mg.sub.3Bi.sub.2 strengthening phase was precipitated in the microstructure. The formation of the rod-shaped Mg.sub.3Bi.sub.2 strengthening phase can significantly improve the alloy hardness, and the continuous addition of Bi is beneficial to the refinement of the metal zinc matrix and the strengthening phase.

Example 2

[0079] Provided herein was a preparation of a biodegradable Zn—Mg—Bi zinc alloy, which was specifically performed through the following steps.

[0080] (S1) A magnesium ingot, a zinc ingot and a Mg-50 wt. % Bi intermediate alloy were cut into small pieces with a sawing machine, ground using a grinder, ultrasonically cleaned with alcohol and acetone and dried at 190° C. under vacuum to remove surface oxide films and oil from the sawing machine.

[0081] (S2) A Zn-1.2 wt. % Mg-1.0 wt. % Bi zinc alloy was prepared, where a weight ratio of Mg to Bi to Zn was 1.2:1.0:97.8.

[0082] (S3) The well-sealed pit type resistance furnace was heated to 440° C., and the high-purity argon gas was introduced. The zinc pieces were added to the silicon carbide crucible one by one, and kept at 440° C. for 70 min, so as to completely melt the zinc pieces. After that, the Mg-50 wt. % Bi intermediate alloy and pure magnesium were added, fully melted, stirred at 55 rpm and 640° C. for 22 min, and then kept at 640° C. for 12 min to obtain a Zn-1.2 wt. % Mg-1.0 wt. % Bi zinc alloy melt.

[0083] (S4) The Zn-1.2 wt. % Mg-1.0 wt. % Bi zinc alloy melt was subjected to standing in the well-sealed pit type resistance furnace, refinement and slagging-off, and cast into a preheated mold at 520° C., and demolded to obtain a Zn-1.2 wt. % Mg-1.0 wt. % Bi zinc alloy ingot.

Example 3

[0084] Provided herein was a preparation of a biodegradable Zn—Mg—Bi zinc alloy, which was specifically performed through the following steps.

[0085] (S1) A magnesium ingot, a zinc ingot and a Mg-50 wt. % Bi intermediate alloy were cut into small pieces with a sawing machine, ground using a grinder, ultrasonically cleaned with alcohol and acetone and dried at 200° C. under vacuum to remove surface oxide films and oil from the sawing machine.

[0086] (S2) A Zn-1.2 wt. % Mg-1.5 wt. % Bi zinc alloy was prepared, where a weight ratio of Mg to Bi to Zn was 1.2:1.5:97.3.

[0087] (S3) The well-sealed pit type resistance furnace was heated to 460° C., and the high-purity argon gas was introduced. The zinc pieces were added to the silicon carbide crucible one by one, and kept at 460° C. for 75 min to completely melt the zinc pieces. After that, the Mg-50 wt. % Bi intermediate alloy and pure bismuth were added, fully melted, stirred at 60 rpm and at 650° C. for 26 min, and then kept at 650° C. for 13 min to obtain a Zn-1.2 wt. % Mg-1.5 wt. % Bi zinc alloy melt.

[0088] (S4) The Zn-1.2 wt. % Mg-1.5 wt. % Bi zinc alloy melt was subjected to standing in the well-sealed pit type resistance furnace, refinement and slagging-off, and cast into a preheated mold at 530° C., and demolded to obtain a Zn-1.2 wt. % Mg-1.5 wt. % Bi zinc alloy ingot.

Example 4

[0089] Provided herein was a preparation of a biodegradable Zn—Mg—Bi zinc alloy, which was specifically performed through the following steps.

[0090] (S1) A magnesium ingos, a zinc ingot and a Mg-50 wt. % Bi intermediate alloy were cut into small pieces with a sawing machine, ground using a grinder, ultrasonically cleaned with alcohol and acetone and dried at 180° C. under vacuum to remove surface oxide films and oil on the sawing machine.

[0091] (S2) A Zn-1.2 wt. % Mg-2.0 wt. % Bi zinc alloy was prepared, where a weight ratio of Mg to Bi to Zn was 1.2:2.0:96.8.

[0092] (S3) The well-sealed pit type resistance furnace was heated to 480° C., and the high-purity argon gas was introduced. The zinc pieces were added to the silicon carbide crucible one by one, and kept at 480° C. for 80 min to completely melt the zinc pieces. After that, the Mg-50 wt. % Bi intermediate alloy and pure bismuth were added, fully melted, stirred under at 70 rpm and 650° C. for 28 min, and then kept at 650° C. for 14 min to obtain a Zn-1.2 wt. % Mg-2.0 wt. % Bi zinc alloy melt.

[0093] (S4) The Zn-1.2 wt. % Mg-2.0 wt. % Bi zinc alloy melt was subjected to standing in the well-sealed pit type resistance furnace, refinement and slagging-off, and cast into a preheated mold at 540° C., and demolded to obtain a Zn-1.2 wt. % Mg-2.0 wt. % Bi zinc alloy ingot.

Example 5

[0094] Provided herein was a preparation of a biodegradable Zn—Mg—Bi zinc alloy, which was specifically performed through the following steps.

[0095] (S1) A magnesium ingot, a zinc ingot and a Mg-50 wt. % Bi intermediate alloy were cut into small pieces with a sawing machine, ground using a grinder, ultrasonically cleaned with alcohol and acetone and dried at 200° C. under vacuum to remove surface oxide films and oil on the sawing machine.

[0096] (S2) A Zn-1.2 wt. % Mg-2.5 wt. % Bi zinc alloy was prepared, where a weight ratio of Mg to Bi to Zn was 1.2:2.5:96.3.

[0097] (S3) The well-sealed pit type resistance furnace was heated to 500° C., and the high-purity argon gas was introduced. The zinc pieces were added to the silicon carbide crucible one by one, and kept at 500° C. for 90 min to completely melt the zinc pieces. After that, the Mg-50 wt. % Bi intermediate alloy and pure bismuth were added, fully melted, stirred at 80 rpm and 660° C. for 30 min, and then kept at 660° C. for 15 min to obtain a Zn-1.2 wt. % Mg-2.5 wt. % Bi zinc alloy melt.

[0098] (S4) The Zn-1.2 wt. % Mg-2.5 wt. % Bi zinc alloy melt was subjected to standing in the well-sealed pit type resistance furnace, refinement and slagging-off, and cast into a preheated mold at 550° C., and demolded to obtain a Zn-1.2 wt. % Mg-2.5 wt. % Bi zinc alloy ingot.

[0099] Referring to an embodiment shown in FIG. 1, a metallographic microstructure of the Mg-50 wt. % Bi alloy is composed of a-Mg dendrite structure and α-Mg+Mg.sub.3Bi.sub.2 eutectic structure.

[0100] Referring to an embodiment shown in FIG. 2, a metallographic microstructure of the Zn-1.2 wt. % Mg-0.5 wt. % Bi biodegradable zinc alloy is composed of Zn matrix, α-Zn+Mg.sub.2Zn.sub.11 eutectic structure, rod-shaped Mg.sub.2Zn.sub.11 eutectic phase, and granular Mg.sub.3Bi.sub.2 phase.

[0101] With regard to the method provided herein, the Mg—Bi intermediate alloy is added to introduce the bismuth element into the zinc alloy under the protection of high-purity argon gas and a covering agent, so as to obtain the bismuth-containing biodegradable zinc alloy, which was rarely studied. According to the microstructure of biodegradable Zn—Mg—Bi zinc alloy, it can be found out that the mechanical properties of the alloy are improved by strengthening in two direction of grain boundary and the intracrystalline.

[0102] It should be noted that the above embodiments are merely illustrative of the technical solutions of this disclosure, but not intended to limit this disclosure. Although this disclosure has been described in detail with reference to the above-mentioned examples, it should be understood that those skilled in the art can still make some modifications, and replacements to the technical features described in the above-mentioned examples. Those modifications or replacements made without departing from the spirit of the disclosure shall fall within the scope of the disclosure defined by the appended claims.