CaTiO3-BASED OXIDE THERMOELECTRIC MATERIAL AND PREPARATION METHOD THEREOF

Abstract

A CaTiO.sub.3-based oxide thermoelectric material and a preparation method thereof are disclosed. The CaTiO.sub.3-based oxide thermoelectric material has a chemical formula of Ca.sub.1-xLa.sub.xTiO.sub.3, where 0<x≤0.4. The present disclosure makes it possible to prepare a CaTiO.sub.3-based thermoelectric material with properties comparable to n-type ZnO, CaTiO.sub.3, SrTiO.sub.3 and other oxide thermoelectric materials. Among them, the La15 sample has a power factor reaching up to 8.2 μWcm.sup.−1K.sup.−2 (at about 1000 K), and a power factor reaching up to 9.2 μWcm.sup.−1K.sup.−2 at room temperature (about 300 K); and a conductivity reaching up to 2015 Scm.sup.−1 (at 300 K). The CaTiO.sub.3-based oxide thermoelectric material exhibits the best thermoelectric performance among calcium titanate ceramics. The method for preparing the CaTiO.sub.3-based oxide thermoelectric material of the present disclosure is simple in process, convenient in operation, low in cost, and makes it possible to prepare a CaTiO.sub.3-based ceramic sheet with high thermoelectric performance.

Claims

1. A CaTiO.sub.3-based oxide thermoelectric material, having a chemical formula of Ca.sub.1-xLa.sub.xTiO.sub.3, where 0<x≤0.4.

2. The CaTiO.sub.3-based oxide thermoelectric material of claim 1, wherein 0.05≤x≤0.3.

3. A method for preparing the CaTiO.sub.3-based oxide thermoelectric material of claim 1 or 2, comprising: (1) dissolving La(NO.sub.3).sub.3.Math.6H.sub.2O in distilled water and stirring for 5-10 minutes, to obtain an aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution; (2) dissolving CaCl.sub.2 in distilled water and stirring for 5-10 minutes, to obtain an aqueous CaCl.sub.2 solution; (3) dissolving NaOH in distilled water and stirring for 5-10 minutes, to obtain an aqueous NaOH solution; (4) dissolving tetrabutyl titanate in ethylene glycol and stirring for 5-10 minutes, to obtain a solution of tetrabutyl titanate in ethylene glycol; (5) adding distilled water to the solution of tetrabutyl titanate in ethylene glycol, stirring to obtain a suspension, and adding the aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution, the aqueous CaCl.sub.2 solution, and the aqueous NaOH solution in sequence, and stirring for 10-15 minutes, to obtain a precursor solution, wherein a molar ratio of the La(NO.sub.3).sub.3.Math.6H.sub.2O, the CaCl.sub.2, the tetrabutyl titanate, and the NaOH is in a ranger of x:(1-x):1:10, with the proviso that 0<x≤0.4; (6) placing the precursor solution into an autoclave, moving the autoclave into a drying box, and keeping at 160-200° C. for 6-24 hours, to obtain a solid product; (7) mixing glacial acetic acid and distilled water in a volume ratio of 1:(5-15), and stirring for 3-5 minutes, to obtain a mixed solution of glacial acetic acid and distilled water; (8) adding the solid product into the mixed solution of glacial acetic acid and distilled water, wherein a ratio of the solid product to the mixed solution is in a range of 2 to 4 g:100 mL; stifling, and filtering, to obtain a filter cake, washing the filter cake with distilled water for 3 to 5 times, and drying the washed filter cake, to obtain a La-doped CaTiO.sub.3 powder; and (9) sintering the La-doped CaTiO.sub.3 powder in a vacuum hot-pressing sintering furnace at 1300 to 1600° C. for 1-3 hours, with a vacuum degree of not more than 0.1 Pa, and a press of 10 to 40 MPa, to obtain a CaTiO.sub.3-based oxide thermoelectric material; wherein (1) to (4) are performed in any order; and there is no time sequence limitation between (7) and any one of (1) to (6).

4. The method of claim 3, wherein the aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution has a La(NO.sub.3).sub.3.Math.6H.sub.2O concentration of not more than 0.27 mol/L.

5. The method of claim 3, wherein the CaCl.sub.2 aqueous solution has a CaCl.sub.2 concentration of 0.4-0.67 mol/L.

6. The method of claim 3, wherein the aqueous NaOH solution has a NaOH concentration of 4.5-7 mol/L.

7. The method of claim 3, wherein in step (4) the tetrabutyl titanate solution in ethylene glycol has a tetrabutyl titanate concentration of 0.1-1.5 mol/L.

8. The method of claim 3, wherein a volume ratio of ethylene glycol to distilled water in the precursor solution in step (5) is in a range of (1-3):7.

9. The method of claim 3, wherein a volume ratio of distilled water to ethylene glycol in the suspension in step (5) is larger than or equal to 1:1.

10. The method of claim 3, wherein, in step (9), sintering the La-doped CaTiO.sub.3 powder in a vacuum hot-pressing sintering furnace is performed at 1400-1500° C. for 1.5-3 hours, with a vacuum degree of not more than 0.1 Pa, and a press of 20-40 MPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 shows scanning electron microscope (SEM) images of CaTiO.sub.3 powder as prepared in Example 1 and Ca.sub.0.8La.sub.0.2TiO.sub.3 as prepared in Example 2, in which, (a) shows a first SEM image of pure CaTiO.sub.3 powder, (b) shows a first SEM image of Ca.sub.0.8La.sub.0.2TiO.sub.3, (c) shows a second SEM image of pure CaTiO.sub.3 powder, and (d) shows a second SEM image of Ca.sub.0.8La.sub.0.2TiO.sub.3 powder.

[0031] FIG. 2 shows X-ray diffraction (XRD) patterns of powder materials as prepared in Examples 1 and 2.

[0032] FIG. 3 shows XRD patterns of bulk materials as prepared in Examples 1 and 2.

[0033] FIG. 4 shows a temperature dependent electronic conductivity of ceramic sheets as prepared in Examples 3, 4, and 5.

[0034] FIG. 5 shows a temperature dependent Seebeck coefficient of ceramic sheets as prepared in Examples 3, 4, and 5.

[0035] FIG. 6 shows a temperature dependent power factor of ceramic sheets as prepared in Examples 3, 4, and 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The present disclosure will be described in conjunction with the examples below.

EXAMPLE 1

[0037] This example discloses a CaTiO.sub.3-based oxide thermoelectric material having a chemical formula of CaTiO.sub.3 prepared by a method comprising:

[0038] (1) weighing 2.21 g of CaCl.sub.2 (with a purity of ≥99.99%), dissolving the CaCL.sub.2 in 30 mL of distilled water, and stirring for 5 minutes to obtain a uniform aqueous CaCl.sub.2 solution;

[0039] (2) weighing 7.68 g of NaOH (with a purity of ≥98%), dissolving the NaOH in 30 mL of distilled water, and stirring for 5 minutes to obtain a uniform aqueous NaOH solution;

[0040] (3) weighing 6.68 mL of tetrabutyl titanate (with a purity of ≥99%), dissolving the tetrabutyl titanate in 15 mL of ethylene glycol, and stirring for 5-10 minutes to be uniform to obtain a solution of tetrabutyl titanate in ethylene glycol;

[0041] (4) adding 15+30 mL of distilled water to the solution of tetrabutyl titanate in ethylene glycol and stirring to obtain a suspension, and adding the aqueous CaCl.sub.2 solution and the aqueous NaOH solution in sequence, the resulting mixture being stirred for 10-15 minutes to obtaining a uniform precursor solution;

[0042] (5) placing the precursor solution into an autoclave, and moving the autoclave into a drying box kept at 180° C. for 24 hours to obtain a solid product;

[0043] (6) adding the solid product into 100 mL of a mixed solution of glacial acetic acid and distilled water in a volume ratio of 1:10, and stirring, and filtering to obtain a filter cake; the filter cake being washed with distilled water for 3 to 5 times, and dried to obtain a CaTiO.sub.3 powder having a chemical composition of CaTiO.sub.3; and

[0044] (7) sintering the CaTiO.sub.3 powder in a vacuum hot-pressing sintering furnace at 1500° C. for 2 hours, with a vacuum degree of not more than 0.1 Pa, a press of 20 MPa to obtain a CaTiO.sub.3-based thermoelectric ceramic sheet, i.e. a ceramic sheet with a chemical formula of CaTiO.sub.3 (also referred to as CaTiO.sub.3 ceramic sheet), wherein the vacuum degree reaches ≤0.1 Pa at a high temperature.

EXAMPLE 2

[0045] In this example, a CaTiO.sub.3-based oxide thermoelectric material having a nominal chemical formula of Ca.sub.0.8La.sub.0.2TiO.sub.3, is prepared by a method comprising:

[0046] (1) weighing 1.71 g of La(NO.sub.3).sub.3.Math.6H.sub.2O (with a purity of ≥99.99%) and dissolving the La(NO.sub.3).sub.3.Math.6H.sub.2O in 30 mL of distilled water, and stirring for 5 minutes to obtain an aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution;

[0047] (2) weighing 1.77 g of CaCl.sub.2 (with a purity of ≥99.99%) and dissolving the CaCl.sub.2 in 30 mL of distilled water, and stirring for 5 minutes to obtain a uniform aqueous CaCl.sub.2 solution;

[0048] (3) weighing 7.68 g of NaOH and dissolving the NaOH in 30 mL of distilled water (with a purity of ≥98%), and stirring for 5 minutes to obtain a uniform aqueous NaOH solution;

[0049] (4) dissolving 6.68 mL of tetrabutyl titanate (with a purity of ≥99%) in 15 mL of ethylene glycol, and stirring for 5-10 minutes to be uniform to obtain a solution of tetrabutyl titanate in ethylene glycol;

[0050] (5) adding 15 mL of distilled water to the solution of tetrabutyl titanate in ethylene glycol, and stirring to obtain a suspension, and adding the aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution, the aqueous CaCl.sub.2 solution, and the aqueous NaOH solution in sequence, and stirring for 10-15 minutes to obtain a uniform precursor solution;

[0051] (6) placing the precursor solution in an autoclave, and moving the autoclave into a dry box kept at 180° C. for 24 hours to obtain a solid product;

[0052] (7) adding the solid product into 100 mL of a mixed solution of glacial acetic acid and distilled water in a volume ratio of 1:10, and stirring and filtering to obtain a filter cake; the filter cake being washed with distilled water for 3 to 5 times and dried to obtain a La-doped CaTiO.sub.3 powder or a CaTiO.sub.3 powder with a small amount of La(OH).sub.3, i.e. a powder having a nominal chemical formula of Ca.sub.0.8La.sub.0.2TiO.sub.3 (nominal composition), also referred to as Ca.sub.0.8La.sub.0.2TiO.sub.3 powder; and

[0053] (8) sintering the Ca.sub.0.8La.sub.0.2TiO.sub.3 powder in a vacuum hot-pressing sintering furnace at 1500° C. for 2 hours, with a vacuum degree of ≤0.1 Pa, a press of 20 MPa to obtain a CaTiO.sub.3-based thermoelectric ceramic sheet, i.e. a ceramic sheet having a nominal chemical formula of Ca.sub.0.8La.sub.0.2TiO.sub.3 (also referred to as Ca.sub.0.8La.sub.0.2TiO.sub.3 ceramic sheet), wherein the degree of vacuum reached ≤0.1 Pa at a high temperature.

[0054] The CaTiO.sub.3 material in Example 1 and Ca.sub.0.8La.sub.0.2TiO.sub.3 material in Example 2 have been characterized, and the results are shown in FIG. 1 to FIG. 3.

[0055] FIG. 2 shows X-ray diffraction (XRD) patterns of the pure CaTiO.sub.3 powder as prepared in Example 1 and the Ca.sub.0.8La.sub.0.2TiO.sub.3 powder as prepared in Example 2. The XRD patterns of the pure CaTiO.sub.3 powder were in good agreement with the standard PDF card and no impurity peak can be identified. When the La-doping concentration was 20%, a small amount of La(OH).sub.3 impurity could be detected in the powder, but the main peaks still were indexed as CaTiO.sub.3. The impurity La(OH).sub.3 disappeared and the La element entered the lattice of CaTiO.sub.3 after being hot-pressing sintered at 1500° C. under vacuum, so the XRD patterns of bulk Ca.sub.0.8La.sub.0.2TiO.sub.3 in FIG. 3 were indexed exhibited as a single CaTiO.sub.3. FIG. 1 shows scanning electron microscope (SEM) images of the two powders, in which pure CaTiO.sub.3 powder particles display a shape of laths or cross-shaped laths, with a size of about 10 μm, while La-doped CaTiO.sub.3 powder particles have a smaller particle size, about 5 μm. Thus, the aforementioned results show that La doping greatly affects the size and shape of the particles, allowing them to be more easily sintered into a bulk with a high power factor.

EXAMPLE 3

[0056] In this example, a CaTiO.sub.3-based oxide thermoelectric material having a nominal chemical formula of Ca.sub.0.85La.sub.0.15TiO.sub.3 is prepared by a method comprising:

[0057] (1) weighing 1.31 g of La(NO.sub.3).sub.3.Math.6H.sub.2O (with a purity of ≥99.99%) and dissolving the La(NO.sub.3).sub.3.Math.6H.sub.2O in 30 mL of distilled water, and stirring for 5 minutes to obtain an aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution;

[0058] (2) weighing 1.88 g of CaCl.sub.2 (with a purity of ≥99.99%) and dissolving the CaCl.sub.2 in 30 mL of distilled water, and stirring for 5 minutes to obtain a uniform aqueous CaCl.sub.2 solution;

[0059] (3) weighing 7.68 g of NaOH and dissolving the NaOH in 30 mL of distilled water (with a purity of ≥98%), and stirring for 5 minutes to obtain a uniform aqueous NaOH solution;

[0060] (4) dissolving 6.68 mL of tetrabutyl titanate (with a purity of ≥99%) in 15 mL of ethylene glycol, and stirring for 5-10 minutes to be uniform to obtain a solution of tetrabutyl titanate in ethylene glycol.

[0061] (5) adding 15 mL of distilled water to the solution of tetrabutyl titanate in ethylene glycol, and stirring to obtain a suspension, and adding the aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution, the aqueous CaCl.sub.2 solution, and the aqueous NaOH solution in sequence, and stirring for 10-15 minutes to obtain a uniform precursor solution;

[0062] (6) placing the precursor solution in an autoclave, and moving the autoclave into a dry box kept at 180° C. for 24 hours to obtain a solid product;

[0063] (7) adding the solid product into 100 mL of a mixed solution of glacial acetic acid and distilled water in a volume ratio of 1:10, and stirring, and filtering to obtain a filter cake; the filter cake being washed with distilled water for 3 to 5 times, and dried to obtain a La-doped CaTiO.sub.3 powder or a CaTiO.sub.3 powder with a small amount of La(OH).sub.3, i.e. a powder having a nominal chemical formula of Ca.sub.0.85La.sub.0.15TiO.sub.3 (nominal composition), also referred to as Ca.sub.0.85La.sub.0.15TiO.sub.3 powder; and

[0064] (8) sintering the Ca.sub.0.85La.sub.0.15TiO.sub.3 powder in a vacuum hot-pressing sintering furnace at 1500° C. for 2 hours, with a vacuum degree of ≤0.1 Pa, a press of 20 MPa, obtaining a CaTiO.sub.3-based thermoelectric ceramic sheet, i.e. a ceramic sheet having a nominal chemical formula of Ca.sub.0.85La.sub.0.15TiO.sub.3 (also referred to as Ca.sub.0.85La.sub.0.15TiO.sub.3 ceramic sheet), wherein the degree of vacuum reached ≤0.1 Pa at a high temperature.

EXAMPLE 4

[0065] In this example, a CaTiO.sub.3-based oxide thermoelectric material having a nominal chemical formula of Ca.sub.0.9La.sub.0.1TiO.sub.3 is prepared by a method comprising:

[0066] (1) weighing 0.866 g of La(NO.sub.3).sub.3.Math.6H.sub.2O (with a purity of ≥99.99%) and dissolving the La(NO.sub.3).sub.3.Math.6H.sub.2O in 30 mL of distilled water, and stifling for 5 minutes to obtain an aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution;

[0067] (2) weighing 1.99 g of CaCl.sub.2 (with a purity of ≥99.99%) was weighed and dissolving the CaCl.sub.2 in 30 mL of distilled water, and stifling for 5 minutes to obtain a uniform aqueous CaCl.sub.2 solution;

[0068] (3) weighing 7.68 g of NaOH was weighed and dissolving NaOH the in 30 mL of distilled water (with a purity of ≥98%), and stirring for 5 minutes to obtain a uniform aqueous NaOH solution;

[0069] (4) dissolving 6.68 mL of tetrabutyl titanate (with a purity of ≥99%) in 15 mL of ethylene glycol, and stifling for 5-10 minutes to be uniform, and obtaining a solution of tetrabutyl titanate in ethylene glycol;

[0070] (5) adding 15 mL of distilled water to the solution of tetrabutyl titanate in ethylene glycol, and stifling to obtain a suspension, and adding the aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution, the aqueous CaCl.sub.2 solution, and the aqueous NaOH solution in sequence, and stifling for 10-15 minutes to obtain a uniform precursor solution;

[0071] (6) placing the precursor solution in an autoclave, and moving the autoclave into a dry box kept at 180° C. for 24 hours to obtain a solid product;

[0072] (7) the solid product being added into 100 mL of a mixed solution of glacial acetic acid and distilled water in a volume ratio of 1:10, and stirring, and filtered to obtain a filter cake; the filter cake being washed with distilled water for 3 to 5 times, and dried to obtain a La-doped CaTiO.sub.3 powder or a CaTiO.sub.3 powder with a small amount of La(OH).sub.3, i.e. a powder having a nominal chemical formula of Ca.sub.0.9La.sub.0.1TiO.sub.3 (nominal composition), also referred to as Ca.sub.0.9La.sub.0.1TiO.sub.3 powder; and

[0073] (8) sintering he Ca.sub.0.9La.sub.0.1TiO.sub.3 powder in a vacuum hot-pressing sintering furnace at 1500° C. for 2 hours, with a vacuum degree of ≤0.1 Pa, a press of 20 MPa to obtain a CaTiO.sub.3-based thermoelectric ceramic sheet, i.e. a ceramic sheet having a nominal chemical formula of Ca.sub.0.9La.sub.0.1TiO.sub.3 (also referred to as Ca.sub.0.9La.sub.0.1TiO.sub.3 ceramic sheet), wherein the degree of vacuum reached ≤0.1 Pa at a high temperature.

EXAMPLE 5

[0074] In this example, a CaTiO.sub.3-based oxide thermoelectric material having a nominal chemical formula of Ca.sub.0.95La.sub.0.05TiO.sub.3, is prepared by a method comprising;

[0075] (1) weighing 0.433 g of La(NO.sub.3).sub.3.Math.6H.sub.2O (with a purity of ≥99.99%) and dissolving the La(NO.sub.3).sub.3.Math.6H.sub.2O in 30 mL of distilled water, and stirring for 5 minutes to obtain an aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution;

[0076] (2) weighing 2.1 g of CaCl.sub.2 (with a purity of ≥99.99%) and dissolving the CaCl.sub.2 in 30 mL of distilled water, and stirring for 5 minutes to obtain a uniform aqueous CaCl.sub.2 solution;

[0077] (3) weighing 7.68 g of NaOH dissolving the NaOH in 30 mL of distilled water (with a purity of ≥98%), and stirring for 5 minutes to obtain a uniform aqueous NaOH solution;

[0078] (4) dissolving 6.68 mL of tetrabutyl titanate (with a purity of ≥99%) in 15 mL of ethylene glycol, and stirring for 5-10 minutes to be uniform to obtain a solution of tetrabutyl titanate in ethylene glycol;

[0079] (5) adding 15 mL of distilled water to the solution of tetrabutyl titanate in ethylene glycol, and stirring to obtain a suspension, and adding the aqueous La(NO.sub.3).sub.3.Math.6H.sub.2O solution, the aqueous CaCl.sub.2 solution, and the aqueous NaOH solution in sequence, and stirring for 10-15 minutes to obtain a uniform precursor solution;

[0080] (6) placing the precursor solution in an autoclave, and moving the autoclave into a dry box kept at 180° C. for 24 hours to obtain a solid product;

[0081] (7) adding the solid product into 100 mL of a mixed solution of glacial acetic acid and distilled water in a volume ratio of 1:10, and stirring, and filtering to obtain a filter cake; the filter cake being washed with distilled water for 3 to 5 times, and dried to obtain a La-doped CaTiO.sub.3 powder or a CaTiO.sub.3 powder with a small amount of La(OH).sub.3, i.e. a powder having a nominal chemical formula of Ca.sub.0.95La.sub.0.05TiO.sub.3 (nominal composition), also referred to as Ca.sub.0.95La.sub.0.05TiO.sub.3 powder; and

[0082] (8) sintering the Ca.sub.0.95La.sub.0.05TiO.sub.3 powder in a vacuum hot-pressing sintering furnace at 1500° C. for 2 hours, with a vacuum degree of ≤0.1 Pa, a press of 20 MPa, obtaining a CaTiO.sub.3-based thermoelectric ceramic sheet, i.e., a ceramic sheet having a nominal chemical formula of Ca.sub.0.95La.sub.0.05TiO.sub.3 (also referred to as Ca.sub.0.85La.sub.0.15TiO.sub.3 ceramic sheet), wherein the degree of vacuum reached ≤0.1 Pa at a high temperature.

[0083] The Ca.sub.0.85La.sub.0.15TiO.sub.3 ceramic sheet as prepared in Example 3, the Ca.sub.0.9La.sub.0.1TiO.sub.3 ceramic sheet as prepared in Example 4 and the Ca.sub.0.95La.sub.0.05TiO.sub.3 ceramic sheet as prepared in Example 5 are characterized, and the results shown in FIG. 4 to FIG. 6.

[0084] FIG. 4 shows a temperature dependent electrical conductivity of: (1) a Ca.sub.0.85La.sub.0.15TiO.sub.3 ceramic sheet (referred as La15 for short) as prepared in Example 3; (2) a Ca.sub.0.9La.sub.0.1TiO.sub.3 ceramic sheet (referred as La10 for short) as prepared in Example 4; and (3) a Ca.sub.0.95La.sub.0.05TiO.sub.3 ceramic sheet (referred as La5 for short) as prepared in Example 5, showing that the conductivity decreases with the increase of temperature, in which La15 sample shows a high conductivity.

[0085] FIG. 5 shows a temperature dependent Seebeck coefficient of: (1) a Ca.sub.0.85La.sub.0.15TiO.sub.3 ceramic sheet (referred as La15 for short) as prepared in Example 3; (2) a Ca.sub.0.9La.sub.0.1TiO.sub.3 ceramic sheet (referred as La10 for short) as prepared in Example 4; and (3) a Ca.sub.0.95La.sub.0.05TiO.sub.3 ceramic sheet (referred as La5 for short) as prepared in Example 5, in which all samples exhibit a negative Seebeck coefficient, indicating that the CaTiO.sub.3 ceramic sheets as prepared by the method of the present disclosure show a typical n-type thermoelectric material.

[0086] FIG. 6 shows a temperature dependent power factor of: (1) a Ca.sub.0.85La.sub.0.15TiO.sub.3 ceramic sheet (referred as La15 for short) as prepared in Example 3; (2) a Ca.sub.0.9La.sub.0.1TiO.sub.3 ceramic sheet (referred as La10 for short) as prepared in Example 4; and (3) a Ca.sub.0.95La.sub.0.05TiO.sub.3 ceramic sheet (referred as La5 for short) as prepared in Example 5, in which La15 sample shows a fairly high power factor in the whole temperature range, and a power factor of 8.17 μWcm.sup.−1K.sup.−2 at 1000 K, showing that a high performance n-type thermoelectric material can be successfully prepared by the method of the present disclosure from CaTiO.sub.3 oxide.

[0087] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present disclosure, but not to limit them; although the present disclosure has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that it is still possible to modify the technical solutions described in the foregoing embodiments, or equivalently replace some or all of the technical features; and that inclusion of these modifications or replacements and any resulting embodiments nonetheless fall within the scope of technical solutions of the present disclosure.