阅读说明：本技术 碳化硼结构可控的碳化硼/碳复合材料及其制备方法和应用 (Boron carbide/carbon composite material with controllable boron carbide structure and preparation method and application thereof ) 是由 于迎涛 黄佳瑩 张学翰 韩皓璇 于 2021-10-25 设计创作，主要内容包括：本发明涉及碳化硼结构调控及碳化硼/碳复合材料制备方法,属于陶瓷材料技术领域。所述制备方法包括以下步骤：将碳化硼粉体与助剂粉体充分混合、加压成型后,进行烧结,烧结温度为1000-1600℃,得到碳化硼/碳复合材料；通过控制加入的助剂的种类,以调控碳化硼三原子链与二十面体结构的相对比例,助剂为Y-(2)O-(3)、BaTiO-(3)、MgO、Fe-(3)O-(4)、Al-(2)O-(3)中一种或几种。本发明制备碳化硼-碳复合材料,并且调控碳的D峰与G峰的强度,使之具有更好的材料硬度和韧性性能,并将碳良好的抗热冲击性能于碳化硼良好的耐高温等离子体冲刷性能结合,大大加快了材料制备的生产周期,降低了生产成本,拓展碳化硼复合材料的应用前景。(The invention relates to a boron carbide structure regulation and control method and a boron carbide/carbon composite material preparation method, and belongs to the technical field of ceramic materials. The preparation method comprises the following steps: fully mixing boron carbide powder and auxiliary agent powder, pressing and molding, and sintering at the sintering temperature of 1000-1600 ℃ to obtain a boron carbide/carbon composite material; the relative proportion of the boron carbide triatomic chain and the icosahedral structure is regulated and controlled by controlling the types of the added additives, and the additive is Y 2 O 3 、BaTiO 3 、MgO、Fe 3 O 4 、Al 2 O 3 One or more of them. The invention prepares the boron carbide-carbon composite material and regulates and controls the intensity of the D peak and the G peak of the carbonThe carbon-boron carbide composite material has better material hardness and toughness, and the good thermal shock resistance of the carbon is combined with the good high-temperature plasma scouring resistance of the boron carbide, so that the production period of material preparation is greatly shortened, the production cost is reduced, and the application prospect of the boron carbide composite material is expanded.)

1. A preparation method of boron carbide/carbon composite material with controllable boron carbide structure comprises the following steps:

(1) fully mixing boron carbide powder and auxiliary agent powder, pressing and molding, packaging in a graphite tank, and sintering in a muffle furnace at the sintering temperature of 1000-1600 ℃ to obtain a boron carbide/carbon composite material;

(2) the relative proportion of the boron carbide triatomic chain and the icosahedral structure is regulated and controlled by controlling the type of the added auxiliary agent, namely Y₂O₃、BaTiO₃、MgO、Fe₃O₄、Al₂O₃One or more of them.

2. The method according to claim 1, wherein the weight ratio of the boron carbide to the auxiliary agent is (1-99): 1.

3. the method of claim 1, wherein the muffle ramp rate is 3 ℃/min.

4. The method of claim 1, wherein the sintering time is 2-4 hours.

5. A boron carbide/carbon composite material produced by the production method according to any one of claims 1 to 4.

6. Use of a boron carbide/carbon composite material prepared by the method of any one of claims 1 to 4.

Technical Field

The invention relates to a boron carbide structure regulation and control and a boron carbide/carbon composite material preparation technology, and belongs to the technical field of ceramic materials.

Background

Boron carbide has the characteristics of high hardness, low density, high temperature resistance, corrosion resistance, neutron radiation absorption and the like, is a high-temperature semiconductor material with excellent performance, and is widely applied in the fields of aerospace, nuclear industry, light armor, electronics and the like.

Boron carbide ceramics generally have the problems of high brittleness, easy fracture and low toughness. The boron carbide/carbon composite material is prepared, and the boron carbide crystal grains are prevented from growing too large on a microscopic level, so that the toughness of the material is improved, the conductivity is improved, the boron carbide-carbon composite material has the double characteristics of wear resistance and conductivity, and the boron carbide/carbon composite material has wide application prospects in the fields of high-speed electric locomotive pantograph sliding plate materials and the like. On the other hand, the boron carbide-carbon composite material also has potential application in the field of unmanned aerial vehicle shells.

Boron carbide (B)₄C) Of rhombohedral structure comprising B₁₁The structure of the C icosahedron and the C-B-C triatomic chain is that the two microstructure units are important factors directly influencing the mechanics, the electrical characteristics, the temperature coefficient and the crystal form change of boron carbide. The relative proportion of the triatomic chain and the icosahedron structure is regulated, and the method has very important significance for researching and developing a new boron carbide-carbon composite material.

Disclosure of Invention

According to the invention, the addition of the auxiliary agent is combined with the high-temperature heat treatment process, and the microstructure of an icosahedron chain and a triatomic chain in boron carbide is regulated and controlled, so that the boron carbide/carbon composite material with a controllable boron carbide structure is prepared, and the boron carbide/carbon composite material has an important significance for popularization and application of boron carbide functional materials.

In order to achieve the purpose, the technical scheme of the invention is as follows:

the invention provides a preparation method of a boron carbide/carbon composite material with a controllable boron carbide structure, which comprises the following steps:

(1) fully mixing boron carbide powder and auxiliary agent powder, pressing and molding, packaging in a graphite tank, and sintering in a muffle furnace at the sintering temperature of 1000-1600 ℃ to obtain a boron carbide/carbon composite material;

(2) the relative proportion of the boron carbide triatomic chain and the icosahedral structure is regulated and controlled by controlling the type of the added auxiliary agent, namely Y₂O₃、BaTiO₃、MgO、Fe₃O₄、Al₂O₃One or more of them.

In the above technical solution, further, the weight ratio of the boron carbide to the auxiliary agent is (1-99): 1.

in the above technical scheme, further, the muffle furnace has a heating rate of 3 ℃/min.

In the above technical solution, further, the sintering time is 2-4 hours.

The invention also provides the boron carbide/carbon composite material prepared by the preparation method.

The invention further provides application of the boron carbide/carbon composite material prepared by the preparation method.

The invention utilizes nano metal oxide additive and combines high temperature treatment process to regulate and control the triatomic chain and icosahedron structure in boron carbide, and characterizes through the Raman peak intensity ratio of the extension vibration of the triatomic chain and the icosahedron respiration vibration, and can also convert part of boron carbide into free carbon to generate boron carbide/carbon composite material, and characterizes and regulates the intensity of D peak and G peak by utilizing Raman spectrum.

The micro confocal Raman spectrum of the pure boron carbide powder and the boron carbide/carbon composite material prepared as above is measured by using 532nm exciting light, and the respiration of the icosahedron is vibrated (1068 cm)^-1Near) the intensity of the raman characteristic peak was normalized with reference (1).

Boron carbide/carbon prepared as above, in contrast to pure boron carbide powderComposite material at 482cm^-1The relative peak intensity of the near chain stretching vibration characteristic Raman peak can be increased, unchanged and reduced.

When using Y, as compared with pure boron carbide powder₂O₃When the compound is used as an auxiliary agent, the heat is preserved for 2 hours at 1550 ℃, and the relative peak intensity of the Raman peak of the stretching vibration characteristic of the three-atom chain is kept nearly unchanged; at 1341cm^-1In the vicinity, the relative peak intensity of the D peak of the generated free carbon is increased by 16 times, and the relative peak intensity of the G peak is increased by 10-13 times; the intensity ratio of the D peak to the G peak is close to 1.2.

Compared with pure boron carbide powder, when BaTiO is adopted₃When the compound is used as an auxiliary agent, the heat preservation is carried out for 2h at 1550 ℃, so that the relative peak strength of the Raman peak of the stretching vibration characteristic of the three-atom chain is almost unchanged; at 1341cm^-1In the vicinity, the relative peak intensity of the D peak and the relative peak intensity of the G peak of the generated free carbon are increased by 12 times and 7 times respectively; the intensity ratio of the D peak to the G peak is close to 1.4.

Compared with pure boron carbide powder, when MgO (5% by weight) is used as an auxiliary agent, the heat is preserved for 2 hours at 1550 ℃, and the relative peak strength of the Raman peak of the stretching vibration characteristic of the three-atom chain can be increased by 30-40%; at 1341cm^-1Nearby, the relative peak intensity of the D peak of the generated free carbon is increased by 20-30 times, and the relative peak intensity of the G peak is increased by 10-15 times; the intensity ratio of the D peak to the G peak is close to 2, and a large amount of disordered carbon is produced.

When using BaTiO, compared with pure boron carbide powder₃(10% by weight) and Fe₃O₄(10% by weight), MgO (1.67% by weight), Al₂O₃(weight ratio: 1.67%) and Y₂O₃When the components (the weight ratio is 1.67%) are jointly used as an auxiliary agent, the relative peak strength of a Raman peak of the stretching vibration characteristic of the three-atom chain can be reduced by about 25% when the components are kept at 1200 ℃ for 4 hours; at 1341cm^-1In the vicinity, the relative peak intensity of the D peak and the relative peak intensity of the G peak of the generated free carbon are increased by 10 times and 7 times respectively; the intensity ratio of the D peak to the G peak is close to 1.3.

The invention has the beneficial effects that:

according to the invention, the metal oxide is used as an auxiliary agent, the relative ratio of the triatomic chain and the icosahedron structure in the boron carbide is regulated, the boron carbide-carbon composite material is prepared, the strength of the D peak and the G peak of carbon can be regulated, the characteristic of poor toughness of a single boron carbide material can be effectively overcome, the boron carbide-carbon composite material has better material hardness and toughness, the good thermal shock resistance of carbon is combined with the good high-temperature plasma scouring resistance of boron carbide, the production cycle of material preparation is greatly accelerated, the production cost is reduced, and the application prospect of the boron carbide composite material is expanded.

Drawings

FIG. 1: a schematic structural diagram of pure boron carbide;

FIG. 2: raman spectroscopy before pure boron carbide sintering;

FIG. 3: XRD spectrum before sintering of pure boron carbide;

FIG. 4: the Raman spectrum of pure boron carbide in comparative example 1 after being treated at 1550 ℃ for 2 h;

FIG. 5: raman spectrum of boron carbide/carbon composite material prepared in example 1;

FIG. 6: XRD spectrum of the boron carbide/carbon composite material prepared in example 1;

FIG. 7: raman spectrum of boron carbide/carbon composite material prepared in example 2;

FIG. 8: raman spectrum of boron carbide/carbon composite material prepared in example 3;

FIG. 9: raman spectrum of boron carbide/carbon composite prepared in example 4;

FIG. 10: raman spectrum of boron carbide/carbon composite prepared in example 5.

Detailed Description

The following examples are presented to enable one of ordinary skill in the art to more fully understand the present invention and are not intended to limit the invention in any way.

Boron carbide used in the following examples was purchased from Shanghai Neihou nanotechnology, Inc., having a particle size of 800 nm; the barium titanate and ferroferric oxide powder is purchased from Andi metal materials Co., Ltd, and the particle size is 1 μm. The particle sizes of the magnesium oxide, the aluminum oxide and the yttrium oxide are all 30nm and are all purchased from Zhongmeixin shield company.

In the following examples, high temperature treatment was carried out using a KSL-1700X muffle furnace, a Mesoco Federation materials technology Co.

The boron carbide/carbon composites prepared in the following examples were tested according to the following method:

measuring a Raman spectrum by using a HORIBA JY-XploRA microscopic confocal Raman spectrometer, wherein the laser wavelength is 532 nm; measuring an XRD spectrum and a Co target of the sample by using an X-ray diffraction spectrometer of Rigaku-D/MAX-3A; pdf card search alignment was performed using the JADE 5.0 software.

Comparative example 1

Commercially available boron carbide powder was purchased and designated as comparative sample 1.

FIG. 1 shows boron carbide (B)₄C) The structure is schematically shown in FIG. 1, and the structure of the boron carbide comprises C-B-C triatomic chains and B₁₁A C icosahedron structure. FIG. 2 shows boron carbide (B)₄C) The microscopic confocal Raman spectrum of (1) is 473cm, as shown in FIG. 2^-1、524cm^-1、1068cm^-1The characteristic peaks of the three-atom chain respectively correspond to the stretching vibration, the icosahedron swinging vibration and the icosahedron breathing vibration of the three-atom chain; 1341 and 1587cm^-1Corresponding to the D peak and the G peak of residual carbon in boron carbide; the D peak of carbon corresponds to sp²Respiratory vibration of hybridized carbon aromatic ring, G peak of carbon corresponding to sp²Stretching vibration of hybridized C ═ C bond and aromatic ring. Based on the peak intensity of the icosahedron respiration vibration (1.00), the relative ratio of the intensities of the D peak and the G peak of the triatomic chain stretching, icosahedron rocking, icosahedron respiration, and carbon was 0.60: 0.34: 1.00: 0.13: 0.16, indicating a relatively low carbon residue content; wherein the intensity ratio of the D peak to the G peak is 0.83.

FIG. 3 is an XRD spectrum of boron carbide powder, JADE software search shows, and B₄XRD cards for C (35-0978) correspond to space group R-3m (166).

Comparative example 2

And putting the boron carbide powder into a stainless steel mold for cold press molding, wherein the pressure is 20MPa, and the pressure maintaining time is 1 min. And packaging the cold-pressed block in a graphite groove, packaging the graphite groove in a corundum square boat, heating to 1550 ℃ at the speed of 3 ℃/min by using a muffle furnace, and preserving heat for 2h to obtain a product which is marked as a comparison sample 2. And (3) after the temperature is reduced to the normal temperature, determining the microscopic confocal Raman spectrum of the contrast sample 2 in the graphite groove.

As shown in fig. 4, the relative ratio of raman peak intensities of the triatomic chain stretching vibration, icosahedron rocking vibration, icosahedron respiratory vibration, D peak and G peak was 0.66: 0.30: 1.00: 0.37: 0.25.

compared with the boron carbide powder in the figure 2, the relative peak intensity ratio of the stretching of the triatomic chain and the swinging of the icosahedron in the figure 4 is not obviously changed, and the relative peak intensities of the D peak and the G peak are increased to 0.37 and 0.25 respectively. The ratio of the intensity of the D peak to that of the G peak was 1.46, indicating that sp is present in the produced carbon material²The proportion of hybridized carbon aromatic rings is increased. The half-value width of the G peak is 65cm^-1Greater than 50cm^-1Indicating sp produced²The average size of the hybrid carbon structure is less than 1 nm.

Example 1

Fully mixing boron carbide powder and an yttrium oxide auxiliary agent according to a mass ratio of 95:5, and then putting the mixture into a stainless steel mold for cold press molding, wherein the pressure is 20MPa, and the pressure maintaining time is 1 min. And packaging the cold-pressed block in a graphite groove, packaging the graphite groove in a corundum square boat, heating to 1550 ℃ at the speed of 3 ℃/min by using a muffle furnace, and then preserving heat for 2 hours. And (5) after the temperature is reduced to the normal temperature, determining the microscopic confocal Raman spectrum of the bulk material in the graphite groove.

As shown in fig. 5, the relative raman peak intensity ratios of the trion chain stretching vibration, the icosahedron rocking vibration, the icosahedron respiratory vibration, the D peak and the G peak were 0.61: 0.38: 1.00: 2.16: 1.75. compared with the boron carbide powder which is not subjected to high-temperature treatment in the figure 2, the relative peak intensity ratio of stretching of the triatomic chain and swinging of the icosahedron in the figure 5 is not obviously changed, and the relative peak intensities of the D peak and the G peak are obviously enhanced and respectively increased by 15.4 times and 10.0 times. Compared with the results of the high-temperature treatment without the addition of the auxiliary in FIG. 4, the relative peak intensities of the D peak and the G peak were increased by 4.9 times and 6.0 times, respectively.

The result shows that the adoption of 5 percent of yttrium oxide auxiliary agent is beneficial to generating more carbon structures in the high-temperature treatment process, and simultaneously, the relative peak strength of the triatomic chain and the icosahedron is kept not to be obviously changed. The intensity ratio of the D peak to the G peak in FIG. 5 was 1.23, indicating that sp was generated during the high temperature treatment with 5% yttria added²Hybrid carbon aromaThe ring structure content is higher than the generated sp²Hybrid C ═ C bond structures; the half-value width of the G peak is 63cm^-1Greater than 50cm^-1Indicating sp produced²The average size of the hybrid carbon structure is less than 1 nm.

The above materials were ground into powder and XRD spectrum was measured. As shown in FIG. 6, a relatively obvious strong peak appears near 31 degrees, and the JADE search result shows that the new peak is close to the characteristic of the XRD card (35-0978) of 3R-graphite and the space group is R3 (146).

Example 2

Fully mixing boron carbide powder and a barium titanate auxiliary agent according to a mass ratio of 95:5, and then putting the mixture into a stainless steel mold for cold press molding, wherein the pressure is 20MPa, and the pressure maintaining time is 1 min. And packaging the cold-pressed block in a graphite groove, packaging the graphite groove in a corundum square boat, heating to 1550 ℃ at the speed of 3 ℃/min by using a muffle furnace, and then preserving heat for 2 hours. And (5) after the temperature is reduced to the normal temperature, determining the microscopic confocal Raman spectrum of the bulk material in the graphite groove.

As shown in fig. 7, the relative raman peak intensity ratios of the trion chain stretching vibration, the icosahedron rocking vibration, the icosahedron respiratory vibration, the D peak and the G peak were 0.58: 0.35: 1.00: 1.62: 1.14. compared with the boron carbide powder in the figure 2, the relative peak strength ratio of the stretching of the triatomic chain and the icosahedron swing in the figure 7 is not obvious, the relative peak strength of the D peak and the G peak is obviously enhanced and is respectively increased by 11.3 times and 7.1 times, and compared with the result of high-temperature treatment under the condition of not adding the auxiliary agent in the figure 4, the relative peak strength of the D peak and the G peak is respectively increased by 3.4 times and 3.5 times.

The result shows that the adoption of 5 percent of barium titanate auxiliary agent is beneficial to generating more carbon structures in the high-temperature treatment process, and simultaneously, the relative peak strength of the triatomic chain and the icosahedron is kept not to be obviously changed. In FIG. 6, the intensity ratio of the D peak to the G peak was 1.43, and the half-value width of the G peak was 63cm^-1Greater than 50cm^-1Indicating sp produced²The average size of the hybrid carbon structure is less than 1 nm.

Example 3

Fully mixing boron carbide powder and a magnesium oxide auxiliary agent according to a mass ratio of 95:5, and then putting the mixture into a stainless steel mold for cold press molding, wherein the pressure is 20MPa, and the pressure maintaining time is 1 min. And packaging the cold-pressed block in a graphite groove, packaging the graphite groove in a corundum square boat, heating to 1550 ℃ at the speed of 3 ℃/min by using a muffle furnace, and then preserving heat for 2 hours. And (5) after the temperature is reduced to the normal temperature, determining the microscopic confocal Raman spectrum of the bulk material in the graphite groove.

As shown in fig. 8, the relative raman peak intensity ratios of the trion chain stretching vibration, the icosahedron rocking vibration, the icosahedron respiratory vibration, the D peak and the G peak were 0.82: 0.41: 1.00: 3.74: 2.02. compared with the boron carbide powder in fig. 2, the relative peak strength of the three-atom chain stretching and the icosahedron swinging in fig. 8 is obviously increased by 37% and 20% respectively, which indicates that the proportion of the three-atom chain structure can be increased in the high-temperature treatment process using 5% magnesium oxide as an auxiliary agent.

In fig. 8, the relative peak intensities of the D peak and the G peak were increased by 27 times and 12 times, respectively, as compared with the boron carbide powder of fig. 2. Compared with the result of high-temperature treatment without the addition of the auxiliary agent in fig. 3, the relative peak intensities of the D peak and the G peak are respectively increased by 9.2 times and 7 times, which indicates that the magnesium oxide auxiliary agent is beneficial to generating more carbon structures in the high-temperature treatment process.

The intensity ratio of the D peak to the G peak in FIG. 8 is 1.85, which shows that sp is generated during the high temperature treatment with the addition of the magnesium oxide auxiliary agent²The content of hybridized carbon aromatic ring structure is higher than that of generated sp²Hybrid C ═ C bond structures; the half-value width of the G peak is 63cm^-1Greater than 50cm^-1Indicating sp produced²The average size of the hybrid carbon structure is less than 1 nm.

Comparing fig. 5 (5% added yttrium oxide) and fig. 7 (5% added barium titanate), the intensity ratios of the D peak and the G peak in fig. 8 (5% added magnesium oxide) are 1.23, 1.43, and 1.85, respectively, which shows that the addition of the magnesium oxide auxiliary agent is more advantageous for generating sp during the high temperature treatment than the addition of the barium titanate and the yttrium oxide auxiliary agent²A hybrid carbon aromatic ring structure.

Example 4

Mixing boron carbide powder, magnesium oxide, aluminum oxide and yttrium oxide auxiliaries according to a mass ratio of 95: 1.67: 1.67: 1.67, and placing the mixture into a stainless steel mold for cold press molding, wherein the pressure is 20MPa, and the pressure maintaining time is 1 min. And packaging the cold-pressed block in a graphite groove, packaging the graphite groove in a corundum square boat, heating to 1550 ℃ at the speed of 3 ℃/min by using a muffle furnace, and then preserving heat for 2 hours. And (5) after the temperature is reduced to the normal temperature, determining the microscopic confocal Raman spectrum of the bulk material in the graphite groove.

As shown in fig. 9, the relative raman peak intensity ratios of the triatomic chain stretching vibration, icosahedron rocking vibration, icosahedron respiratory vibration, D peak and G peak were 0.79: 0.33: 1.00: 3.98: 2.44. the relative peak strength of elongation and contraction of the triatomic chain in fig. 9 was increased by 32% as compared with the boron carbide powder in fig. 2.

The relative peak intensities of the D peak and the G peak in fig. 8 were increased by 29.2 times and 14.4 times, respectively, as compared with the boron carbide powder of fig. 2. Compared with the result of high-temperature treatment without the addition of the auxiliary agent in fig. 3, the relative peak intensities of the D peak and the G peak are respectively increased by 9.8 times and 8.7 times, which indicates that the magnesium oxide auxiliary agent is beneficial to generating more carbon structures in the high-temperature treatment process.

The intensity ratio of the D peak to the G peak in FIG. 9 is 1.63, which shows that sp is generated in the high temperature treatment process of boron carbide added with the mixed auxiliary agent of magnesium oxide, aluminum oxide and yttrium oxide²The content of hybridized carbon aromatic ring structure is higher than that of generated sp²Hybrid C ═ C bond structures; the half-value width of the G peak is 57cm^-1Greater than 50cm^-1Indicating sp produced²The average size of the hybrid carbon structure is less than 1 nm.

Example 5

Mixing boron carbide powder with barium titanate, ferroferric oxide, magnesium oxide, aluminum oxide and yttrium oxide auxiliaries according to the mass ratio of 75: 10: 10: 1.67: 1.67: 1.67, and placing the mixture into a stainless steel mold for cold press molding, wherein the pressure is 20MPa, and the pressure maintaining time is 1 min. And packaging the block formed by cold pressing in a graphite groove, packaging the graphite groove in a corundum ark, heating to 1200 ℃ at the speed of 3 ℃/min by using a muffle furnace, and then preserving heat for 4 hours. And (5) after the temperature is reduced to the normal temperature, determining the microscopic confocal Raman spectrum of the bulk material in the graphite groove.

As shown in fig. 10, the relative raman peak intensity ratios of the triatomic chain stretching vibration, icosahedron rocking vibration, icosahedron respiratory vibration, D peak and G peak were 0.45: 0.20: 1.00: 1.34: 1.07. the relative peak strength of the three-atom chain expansion and contraction in fig. 10 was reduced by 24% as compared with the boron carbide powder in fig. 2.

In fig. 10, the relative peak intensities of the D peak and the G peak were increased by 9.2 times and 5.7 times, respectively, as compared with the boron carbide powder of fig. 2. Compared with the results of the high-temperature treatment without the addition of the auxiliary in FIG. 3, the relative peak intensities of the D peak and the G peak were increased by 2.7 times and 4.3 times, respectively. The intensity ratio of the D peak to the G peak in FIG. 10 was 1.25, indicating that the content of the sp 2-hybridized carbon aromatic ring structure was higher than that of the sp-hybridized carbon aromatic ring structure²Hybrid C ═ C bond structures; the half-value width of the G peak was 61cm^-1Greater than 50cm^-1Indicating sp produced²The average size of the hybrid carbon structure is less than 1 nm.

Table 1 shows the preparation conditions of the boron carbide/carbon composite material and the raman spectrum test results.

TABLE 1

The above examples are merely preferred examples of the present invention, and are not intended to limit the embodiments. The protection scope of the present invention shall be subject to the scope defined by the claims. Other variations and modifications may be made on the basis of the above description. Obvious variations or modifications of this invention are within the scope of the invention.