Preparation method of high entropy ceramic thermal barrier coating for small-scale non-premixed low heat dissipation combustion chamber

Abstract

The present invention discloses a method for preparing a small-scale non-premixed low-heat dissipation combustion chamber high-entropy ceramic thermal barrier coating, which belongs to the technical field of coatings. The method for preparing a small-scale combustion chamber high-entropy thermal barrier coating comprises: S1, drying the raw materials; S2, ball milling, drying, and sieving the raw materials; S3, sintering the sieved powder, and ball milling, drying, and sieving after sintering; S4, spraying a bonding layer on the inner wall of the combustion chamber; S5, vacuum heat treatment of the combustion chamber sprayed with the bonding layer; S6, spraying a high-entropy ceramic layer on the bonding layer. The chemical formula of the high-entropy ceramic layer is La 2( Z r0.3 Hf 0.2 Ce 0.2 Sn 0.2 Ti 0.1 )2O7. The bonding layer is NiCo 23 Cr 17 Al 12 Y 0.5 The present invention adopts the above-mentioned preparation method of the small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating. The high entropy ceramic layer has a higher thermal expansion coefficient and lower thermal conductivity, which is beneficial to alleviate the heat load on the channel combustion chamber, increase the combustion chamber gas boundary contact temperature, adjust the thermal flow field in the cylinder, promote the complete combustion of the fuel, and effectively slow down the flame quenching phenomenon.

Description

Technical Field

[0001] The invention relates to the technical field of coatings, and in particular to a method for preparing a small-scale non-premixed low-heat dissipation combustion chamber high-entropy ceramic thermal barrier coating. Background Art

[0002] Micro-scale combustion fueled by hydrocarbons and nitrogen-hydrogen compounds has an extremely high energy density and has become a strong competitor for the power and energy systems of future micro-sized devices. Micro-scale burners with greatly reduced size will bring various problems. For example, the sharp increase in the surface-to-volume ratio (generally more than dozens of times that of conventional burners) will cause a large amount of surface heat dissipation loss, the short residence time of the gas mixture in the burner (about 0.5ms) will lead to a reduction in heat release, and the collision and destruction of free radicals in the chemical reaction at the wall will lead to a longer reaction time and wall quenching. These factors will seriously affect the performance of the burner, making the combustion unstable, and in some cases, the flame may even go out.

[0003] For micro-scale burners made of special materials, the wall limit temperature needs to be strictly controlled during the combustion process. For example, for a burner made of silicon as the main material, when the burner wall temperature exceeds the limit temperature of 1300°C, the silicon begins to soften, thereby destroying the integrity of the structure. However, due to the high thermal conductivity and high surface heat transfer performance of silicon materials, the burner wall temperature can be effectively reduced by strengthening external heat dissipation, but the reduction in wall temperature will bring about heat loss and thermal flameout problems. In addition, for micro internal combustion engines with rotating parts, a lower wall temperature (less than 730°C) must be maintained to prevent material creep. These problems seriously restrict the development of micro power devices.

[0004] Thermal barrier coatings (TBCs) are used on high-temperature components in many fields to protect the substrate from heat. The material used in traditional thermal barrier coatings is yttria-stabilized zirconia (YSZ), which has the advantages of high thermal expansion coefficient, low thermal conductivity and low price. However, YSZ coatings will undergo phase change and sintering in an environment above 1200°C, which will affect the life of the coating. 2 Zr 2 O 7 ) The material has the characteristics of good high temperature thermal stability, sintering resistance, low thermal conductivity, but low thermal expansion coefficient. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a small-scale non-premixed low-heat dissipation combustion chamber high-entropy ceramic thermal barrier coating. The high-entropy ceramic layer has a higher thermal expansion coefficient and a lower thermal conductivity, which is beneficial to alleviating the heat load on the channel combustion chamber and increasing the gas boundary contact temperature of the combustion chamber. It can also regulate the thermal flow field in the cylinder, promote the complete combustion of the fuel, and effectively slow down the flame quenching phenomenon.

[0006] To achieve the above object, the present invention provides a method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating, comprising the following steps:

[0007] S1, the raw material CeO 2 ,La 2 O 3 、TiO 2 , HfO 2 SnO 2 and ZrO 2 The powder is placed in a crucible and dried;

[0008] S2, ball-milling the mixed powder obtained by mixing the raw materials; drying and sieving after ball-milling to obtain sieved powder;

[0009] S3, sintering the sieved powder, ball milling, drying, and sieving after sintering to obtain a high entropy ceramic layer powder;

[0010] S4. Clean the inner wall of the combustion chamber and spray a bonding layer on the inner wall of the combustion chamber using an atmospheric plasma spraying device;

[0011] S5, performing vacuum heat treatment on the combustion chamber sprayed with the bonding layer;

[0012] S6. In an atmospheric environment, a high entropy ceramic layer is sprayed on the bonding layer using a plasma spraying device to obtain a thermal barrier coating.

[0013] Preferably, in S1, CeO 2 、ZrO 2 、TiO 2 , HfO 2 SnO 2 and La 2 O 3 The molar ratio is 2:3:1:2:2:5, the drying temperature is 400°C, and the drying time is 2 hours.

[0014] Preferably, in S2, the ball milling time is 20 hours, the grinding balls are zirconia balls, and the volume ratio of the zirconia balls to the mixed powder is 0.4-0.6:1.

[0015] Preferably, in S2, the drying temperature is 70° C., the drying time is 20 h, and a 70-mesh standard sieve is used for sieving.

[0016] Preferably, in S3, the sintering temperature is 1200° C., the sintering time is 10 hours, and the sintered product passes through a 70-mesh standard sieve.

[0017] Preferably, in S4, the bonding layer is NiCo 23 Cr 17 Al 12 Y 0.5 The thickness of the bonding layer is 0.3-0.8mm. The surface roughness of the inner wall of the combustion chamber after surface cleaning is 25-60. The bonding layer NiCo 23 Cr 17 Al 12 Y 0.5 The molar ratio of Ni, Co, Cr, Al and Y in the powder is 1:23:17:12:0.5.

[0018] Preferably, in S5, the temperature of the vacuum heat treatment is 800K and the pressure is 10 -3 Pa.

[0019] Preferably, in S6, the chemical formula of the high entropy ceramic layer is La 2( Z r0.3 Hf 0.2 Ce 0.2 Sn 0.2 Ti 0.1 ) 2 O 7 .

[0020] Preferably, the high entropy ceramic layer is a uniform thickness coating, and the thickness of the high entropy ceramic layer is 1-4 mm.

[0021] Preferably, the high entropy ceramic layer is a non-uniform thickness coating, the thickness of the high entropy ceramic layer gradually decreases from the combustion chamber inlet to the combustion chamber outlet, and the thickness of the high entropy ceramic layer is 1-4 mm.

[0022] The advantages and positive effects of the method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating described in the present invention are:

[0023] 1. The use of thermal barrier coatings increases the reaction temperature in the burner, promotes the complete combustion of the fuel, and changes the flow field and temperature changes in the cylinder, making the fuel combustion in the combustion chamber closer to the stratified complete combustion state. On the other hand, the increase in temperature will also improve the combustion efficiency of the entire combustion system.

[0024] 2. At the fluid-solid interface, the surface temperature of the thermal barrier coating is higher than the temperature of the inner wall of the uncoated combustion chamber, which slows down the occurrence of flame quenching.

[0025] 3. The thermal barrier coating reduces the surface temperature of the inner wall of the combustion chamber, avoiding the softening of the burner made of silicon as the main material, thereby maintaining the integrity of the structure. At the same time, the lower operating temperature increases the service life of the combustion chamber.

[0026] 4. By designing the thickness of the thermal barrier coating, the shape of the internal combustion chamber can be changed. By slowing down the flow rate of the fuel gas, the residence time of the fuel in the combustion chamber is increased, which is conducive to further improving the fuel combustion efficiency.

[0027] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A combustion chamber temperature field diagram of an embodiment of a method for preparing a high entropy ceramic thermal barrier coating for a small-scale non-premixed low heat dissipation combustion chamber of the present invention;

[0029] Figure 2 This is the temperature field diagram of the combustion chamber without coating;

[0030] Figure 3 It is a combustion chamber isotherm diagram of an embodiment of a method for preparing a high entropy ceramic thermal barrier coating for a small-scale non-premixed low heat dissipation combustion chamber of the present invention;

[0031] Figure 4 This is the isotherm diagram of the uncoated combustion chamber;

[0032] Figure 5 A combustion chamber flow field diagram of an embodiment of a method for preparing a high entropy ceramic thermal barrier coating for a small-scale non-premixed low heat dissipation combustion chamber of the present invention;

[0033] Figure 6 This is the flow field diagram of the uncoated combustion chamber;

[0034] Figure 7 This is a temperature distribution diagram of line 11 at the end of a combustion chamber of an embodiment of a method for preparing a high entropy ceramic thermal barrier coating for a small-scale non-premixed low heat dissipation combustion chamber of the present invention;

[0035] Figure 8 This is the temperature distribution diagram of line 8 at the end of the uncoated combustion chamber;

[0036] Fig. 9 A schematic diagram of the combustion chamber end surface structure of an embodiment of a method for preparing a high entropy ceramic thermal barrier coating for a small-scale non-premixed low heat dissipation combustion chamber of the present invention;

[0037] Fig.10 A schematic diagram of the combustion chamber structure of an embodiment of a method for preparing a high entropy ceramic thermal barrier coating for a small-scale non-premixed low heat dissipation combustion chamber of the present invention;

[0038] Fig.11A half-section structural diagram of a combustion chamber of a gradually expanding thermal barrier coating of a method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to an embodiment of the present invention;

[0039] Fig.12 This is a combustion chamber temperature field diagram of a gradually expanding thermal barrier coating of a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to an embodiment of the present invention;

[0040] Fig.13 This is a combustion chamber flow field diagram of a gradually expanding thermal barrier coating in an embodiment of a method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating of the present invention. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0042] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0043] Example 1

[0044] A method for preparing a small-scale non-premixed low-heat dissipation combustion chamber high-entropy ceramic thermal barrier coating comprises the following steps:

[0045] S1, the raw material CeO 2 、ZrO 2 、TiO 2 , HfO 2 SnO 2 and La 2 O 3 The powder was placed in a crucible and dried in a box-type resistance furnace.

[0046] CeO 2 、ZrO 2 、TiO 2 , HfO 2 SnO 2 and La 2 O3 The molar ratio is 2:3:1:2:2:5. The drying temperature is 400°C and the drying time is 2 hours.

[0047] S2. Put the mixed powder after mixing the raw materials into a ball mill for ball milling. The grinding balls are zirconium oxide balls. The volume ratio of zirconium oxide balls to mixed powder is 0.4-0.6:1, preferably 0.5:1. Add alcohol to the ball mill. Use a planetary ball mill for ball milling for 20 hours.

[0048] The ball-milled powder was placed in a drying oven for drying at a temperature of 70°C and a drying time of 20 hours.

[0049] The dried powder was sieved using a 70-mesh standard sieve.

[0050] S3. The sieved powder is placed in a molybdenum crucible and sintered in a box-type resistance furnace at a sintering temperature of 1200° C. for 10 hours.

[0051] After sintering, the high entropy ceramic layer powder is obtained by repeated ball milling, drying and sieving. The sieving adopts a 70-mesh standard sieve.

[0052] S4. Clean the inner wall of the combustion chamber. Use acetone to ultrasonically clean the inner wall for 10 minutes, and use 20# brown corundum sand for sandblasting to roughen the surface of the inner wall to a surface roughness R of 25-60. Increasing the surface roughness of the inner wall is conducive to improving the bonding strength between the coating and the inner wall.

[0053] The atmospheric plasma spraying equipment (Praxair 5500-2000, USA) is used to spray the bonding layer on the inner wall of the combustion chamber. The combustion chamber is an aluminum alloy substrate. For square and cylindrical small-sized combustion chambers, the combustion chamber needs to be cut into flat plates or semi-cylinders using a laser cutting machine, and then welded from the outside after the spraying is completed.

[0054] The bonding layer is NiCo 23 Cr 17 Al 12 Y 0.5 , bonding layer NiCo 23 Cr 17 Al 12 Y 0.5 The molar ratio of Ni, Co, Cr, Al and Y in the powder is 1:23:17:12:0.5. The thickness of the bonding layer is 0.3-0.8 mm.

[0055] S5. The combustion chamber sprayed with the bonding layer is subjected to vacuum heat treatment in an electric furnace. The temperature of the vacuum heat treatment is 800K and the pressure is 10 -3 Pa. Heat treatment is beneficial to eliminate the residual stress of the bonding layer.

[0056] S6. In an atmospheric environment, a high entropy ceramic layer is sprayed on the bonding layer using a plasma spraying device to obtain a thermal barrier coating. The chemical formula of the high entropy ceramic layer is La 2( Z r0.3 Hf 0.2 Ce 0.2 Sn 0.2 Ti 0.1 ) 2 O 7 The high entropy ceramic layer is a uniform thickness coating with a thickness of 1-4 mm.

[0057] La 2( Z r0.3 Hf 0.2 Ce 0.2 Sn 0.2 Ti 0.1 ) 2 O 7 The high-entropy ceramic layer has good high-temperature thermal stability and is still stable at 1200°C without phase change or decomposition. Compared with the general pyrochlore structure of lanthanum zirconate, its thermal expansion coefficient is larger, which helps to alleviate the impact of high-temperature thermal expansion of the substrate. The bonding layer uses NiCo 23 Cr 17 Al 12 Y 0.5 The material has a chemical composition close to that of the substrate, and a thermal expansion coefficient between the ceramic layer and the substrate, which can reduce the thermal stress caused by the temperature gradient, and also has the functions of anti-oxidation, anti-corrosion and protection of the substrate. The substrate material is a high-temperature material aluminum alloy.

[0058] During spraying, the powder feeding gas used is mainly argon, and the auxiliary gas is hydrogen. The spraying parameters of the high entropy ceramic layer and the bonding layer are shown in Table 1.

[0059] Table 1 Spraying parameters of high entropy ceramic layer and bonding layer

[0060]

[0061] The performance of the high entropy ceramic layer and the bonding layer was tested, and the results are shown in Table 2.

[0062] Table 2 Performance of high entropy ceramic layer, bonding layer and burner substrate

[0063]

[0064] Thermal simulation technology is used to perform combustion simulation on a cylindrical combustion chamber (100×φ32mm) model, with the temperature field and flow field inside the uncoated combustion chamber and the coated combustion chamber, as well as the temperature at the end of the combustion chamber as comparisons. The ambient temperature is set to 300k, and the pressure is the default atmospheric pressure of 101325Pa. The fuel methane (initial temperature 300k) enters the combustion chamber from the left center inlet of the combustion chamber (diameter 5mm) at a speed of 5m / s. The radial 11mm annular area on the left side close to the combustion chamber wall is the air inlet (inner diameter 10mm, outer diameter 32mm), and the air flow rate is 1m / s. The structure of the combustion chamber is as follows Fig. 9 , Fig.10 shown.

[0065] The temperature field of the combustion chamber sprayed with the thermal barrier coating described in this embodiment is as follows: Figure 1 As shown, the temperature field of the combustion chamber without coating is as follows Figure 2 It can be seen that the maximum temperature of methane combustion in the combustion chamber increased by about 70K after coating.

[0066] The isotherm diagram of the combustion chamber sprayed with the thermal barrier coating described in this embodiment is as follows: Figure 3 As shown, the isotherm diagram of the uncoated combustion chamber is as follows Figure 4 As shown. It can be seen that the high temperature area in the combustion chamber with coating is larger and the flame front temperature is greater than that of the combustion chamber without coating. The increase in the reaction temperature in the burner makes the flammable limit gradually wider and the chemical reaction time gradually shorter, which improves the flame stability and promotes the complete combustion of the fuel. On the other hand, it will also improve the combustion efficiency of the entire combustion system, effectively reduce the wall heat loss, and the thermal flameout phenomenon will gradually disappear because the combustion is easier to maintain.

[0067] The flow field diagram of the combustion chamber sprayed with the thermal barrier coating described in this embodiment is as follows: Figure 5 As shown, the flow field diagram of the combustion chamber without coating is as follows Figure 6 It can be seen that after adding the thermal barrier coating, the gas velocity in the combustion chamber near the center of the combustion chamber end increases, which is beneficial to enhance the flame combustion speed, thereby promoting the stability of the flame.

[0068] The temperature distribution of line 11 at the end of the combustion chamber sprayed with the thermal barrier coating described in this embodiment is as follows: Figure 7 As shown, the temperature distribution of line 8 at the end of the combustion chamber without coating is as follows Figure 8 It can be seen that the temperature of the combustion chamber is reduced at the radial position of 16 mm at the end, which is located on the inner wall of the combustion chamber, indicating that the low thermal conductivity of the thermal barrier coating of this embodiment plays a heat-insulating role, reducing the heat flow of the fuel combustion in the combustion chamber to other areas while reducing the temperature of the inner wall of the channel combustion chamber.

[0069] The active groups in the flame diffuse to the wall due to the increase of concentration gradient. After the flame diffuses to the low-temperature wall, it can be adsorbed with the active sites on the surface of the wall, and after inertization reaction, it becomes the final combustion product and desorbs and re-enters the gas phase. The flame in the center area is chemically quenched by the wall, which weakens the gas phase reaction in the flame, further reduces the flame temperature, and finally quenches the flame. However, it can be seen from the simulation test that at the fluid-solid interface, the surface temperature of the coating is higher than the temperature of the inner wall of the combustion chamber without the coating, thus slowing down the occurrence of flame quenching.

[0070] Example 2

[0071] The difference between this embodiment and embodiment 1 is that the high entropy ceramic layer in this embodiment is a non-uniform thickness coating, and the thickness of the high entropy ceramic layer gradually decreases from the combustion chamber inlet to the combustion chamber outlet, and the thickness of the high entropy ceramic layer is 1-4 mm. The thickness of the high entropy ceramic layer is 4 mm at the left combustion chamber inlet, and the thickness decreases linearly along the axial direction of the combustion chamber, and the thickness is 1 mm at the end of the combustion chamber. The semi-sectional structure of the microchannel combustion chamber of the gradually expanding thermal barrier coating is shown in FIG. Fig.11 As shown. By applying a coating with a linearly varying thickness, the internal space of the original cylindrical combustion chamber can be changed. The temperature field and flow field of the gradually expanding thermal barrier coating are shown as follows: Fig.12 , 13 As shown. It can be seen that compared with the combustion chamber uniformly coated with thermal barrier coating, the temperature field inside the gradually expanding combustion chamber increases due to the reduction of fuel flow rate, and the high temperature area increases, indicating that the fuel combustion efficiency is further improved. From the combustion chamber flow field, it can be seen that the end gas flow rate of the gradually expanding combustion chamber is lower, which can effectively avoid the occurrence of backflow.

[0072] Therefore, the present invention adopts the above-mentioned preparation method of small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating. The higher thermal expansion coefficient and lower thermal conductivity of the high entropy ceramic layer are beneficial to alleviate the heat load on the channel combustion chamber, increase the gas boundary contact temperature of the combustion chamber, and can regulate the thermal flow field in the cylinder, promote the complete combustion of the fuel, and effectively slow down the flame quenching phenomenon.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating, characterized in that: The following steps are involved: S1, placing raw materials CeO2, La2O3, TiO2, HfO2, SnO2 and ZrO2 powders in a crucible and drying them; S2, ball-milling the mixed powder obtained by mixing the raw materials; drying and sieving after ball-milling to obtain sieved powder; S3, sintering the sieved powder, ball milling, drying, and sieving after sintering to obtain a high entropy ceramic layer powder; S4. Clean the inner wall of the combustion chamber and spray a bonding layer on the inner wall of the combustion chamber using an atmospheric plasma spraying device; S5, performing vacuum heat treatment on the combustion chamber sprayed with the bonding layer; S6. In an atmospheric environment, a high entropy ceramic layer is sprayed on the bonding layer using a plasma spraying device to obtain a thermal barrier coating; In the S1, the molar ratio of CeO2, ZrO2, TiO2, HfO2, SnO2 and La2O3 is 2:3:1:2:2:5, the drying temperature is 400°C, and the drying time is 2 hours.

2. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 1, characterized in that: In S2, the ball milling time is 20 hours, the grinding balls are zirconia balls, and the volume ratio of the zirconia balls to the mixed powder is 0.4-0.6:

1.

3. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 1, characterized in that: In S2, the drying temperature is 70° C. and the drying time is 20 h; a standard sieve of 70 mesh is used for sieving.

4. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 1, characterized in that: In S3, the sintering temperature is 1200° C., the sintering time is 10 hours, and the sintered product passes through a 70-mesh standard sieve.

5. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 1, characterized in that: In the S4, the bonding layer is NiCo 23 Cr 17 Al 12 Y 0.5 The thickness of the bonding layer is 0.3-0.8mm. The surface roughness of the inner wall of the combustion chamber after surface cleaning is 25-60. The bonding layer NiCo 23 Cr 17 Al 12 Y 0.5 The molar ratio of Ni, Co, Cr, Al and Y in the powder is 1:23:17:12:0.

5.

6. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 1, characterized in that: In S5, the vacuum heat treatment temperature is 800K and the pressure is 10 -3 Pa.

7. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 1, characterized in that: In S6, the chemical formula of the high entropy ceramic layer is La 2( Z r0.3 Hf 0.2 Ce 0.2 Sn 0.2 Ti 0.1 )2O7.

8. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 7, characterized in that: The high entropy ceramic layer is a uniform thickness coating, and the thickness of the high entropy ceramic layer is 1-4 mm.

9. The method for preparing a small-scale non-premixed low heat dissipation combustion chamber high entropy ceramic thermal barrier coating according to claim 7, characterized in that: The high entropy ceramic layer is a non-uniform thickness coating, the thickness of the high entropy ceramic layer gradually decreases from the combustion chamber inlet to the combustion chamber outlet, and the thickness of the high entropy ceramic layer is 1-4 mm.

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