Preparation method of fly ash porous hollow microspheres with controllable aperture

Abstract

The application is a preparation method of fly ash porous hollow microspheres with controllable aperture, which can prepare fly ash porous hollow microspheres with controllable aperture, and the specific process is as follows: the fly ash hollow microsphere FAC raw material is sequentially passed through a 150-mesh screen, and the undersize part is the first particle size range; then it is continuously passed through a 60-mesh screen, and the undersize part is the second particle size range; then it is continuously passed through a 35-mesh screen, and the undersize part is the third particle size range, and the oversize part of the 35-mesh screen is discarded; the density of the fly ash hollow microsphere FAC in different particle size ranges is measured; the fly ash hollow microsphere FAC is processed in a nanoscale aperture by using a focused ion beam method, the ion source is a liquid gallium ion source, and the working distance is 5 mm; firstly, the acceleration voltage of the focused ion beam is determined according to the density of the fly ash hollow microsphere FAC in different particle size ranges; then the scanning speed of the focused ion beam is determined according to the particle size range. The aperture size of the microspheres obtained by the method is controllable and is in a nanoscale, and the requirements of low thermal conductivity and high strength can be met.

Description

Technical Field

[0001] The invention relates to a method for preparing fly ash porous hollow microbeads with controllable pore size. Background Art

[0002] Fly ash hollow microspheres (FAC), a type of fly ash, have a thick, hard shell. They not only have the advantages of low density, low thermal conductivity, excellent fluidity, good dispersibility and stability, but also possess excellent properties such as fire resistance, corrosion resistance, insulation, radiation protection, and non-toxicity. Fly ash hollow microspheres are composed of multiple components such as SiO2-Al2O3-Fe2O3-CaO-MgO-Na2O-K2O-TiO2, with a glassy phase content of over 80%, mainly quartz and mullite. However, the surface of FAC is coated with a glassy film, which hinders the chemical reaction between the active SiO2 in the FAC and alkaline substances such as Ca(OH)2, resulting in gaps between the FAC and the cementitious material, affecting the performance of concrete.

[0003] The three main ways of heat transfer are: heat conduction, heat convection and heat radiation. When the pore size of the porous material is less than 4mm, heat convection can be ignored. Since the outer shell of FAC is in glass phase, it has heat reflectivity. When the temperature is not high, heat radiation is constantly refracted in the FAC body, consuming heat energy and reducing heat transfer. Heat conduction is the main heat transfer method of FAC at room temperature. Heat is conducted from the outer wall of FAC to the inner wall. The gas molecules close to the inner wall obtain heat energy flow, and the gas molecules collide with each other to transfer heat. Since the flow rate and density of the gas in FAC are very small, the Reynolds number is also very small, and it is difficult for the gas in the sphere to turbulently form turbulence. Therefore, the heat transfer rate of the gas molecules is slow, which makes the FAC have a lower thermal conductivity. Therefore, the control of appropriate pore size is conducive to obtaining a FAC with low thermal conductivity and high strength.

[0004] At present, the commonly used methods for preparing fly ash porous hollow microspheres are high temperature calcination and chemical etching. The fly ash porous hollow microspheres prepared by these methods have uneven pore distribution, uncontrollable pore size, and damage to the original structure of the hollow microspheres. Excessive pore size will deteriorate the thermal insulation performance (i.e., increase in thermal conductivity) and mechanical properties (i.e., decrease in strength) of the fly ash porous hollow microspheres. For example:

[0005] (1) High-performance concrete and its preparation method (Application No.: 202111411861.5)

[0006] FAC is heated and stirred with one or more of citric acid, acetic acid, salicylic acid, and phthalic acid, followed by filtration, drying, and grinding to produce a fly ash porous hollow microsphere-based curing agent. This is then mixed with Portland cement, fly ash, sand, and pebbles to create a high-performance concrete mixture, improving the mechanical and durability properties of the concrete. This patent utilizes chemical etching technology, resulting in a random pore size distribution without strict control.

[0007] (2) A surface treatment process for fly ash hollow microspheres (201510894011.3)

[0008] The FAC surface was treated by alcohol washing, ultrasonic washing, acid and alkali treatment, and the density was 0.7-1g / cm 3 High-quality, lightweight microbeads suitable for coating. This patent uses sulfuric acid or hydrochloric acid for acid treatment, followed by an alkali treatment with a mixed solution of sodium carbonate and sodium hydroxide. The pore size of the fly ash porous hollow microbeads is uncontrollable and varies.

[0009] The surface of FAC, after acid treatment and high-temperature calcination, has irregular, large pores. When subjected to stress, this creates severe stress concentration, leading to cracking along the pores and causing pore collapse, significantly reducing the material's strength. Furthermore, after acid treatment and high-temperature calcination, FAC forms perforations of a certain diameter, often larger than several micrometers, which increases heat conduction within the FAC and leads to an increase in thermal conductivity. Furthermore, acid treatment and high-temperature calcination often destroy the original structure of the porous hollow fly ash microspheres, even breaking them into fragments, resulting in a reduction in the strength of concrete incorporating them. Summary of the Invention

[0010] In response to the shortcomings of the prior art, the present invention aims to provide a method for preparing porous hollow fly ash microspheres with controllable pore size. This method utilizes a focused ion beam method to produce porous hollow fly ash microspheres with controllable pore size, achieving both low thermal conductivity and high strength. The focused ion beam method penetrates the surface glassy film, introducing nanoscale pores. This method, while retaining the glassy film, reduces the thermal conductivity of the material while also maintaining its strength.

[0011] The technical solution adopted by the present invention to solve the technical problem is:

[0012] In a first aspect, the present invention provides a method for preparing fly ash porous hollow microspheres with controllable pore size, which can prepare fly ash porous hollow microspheres with controllable pore size. The preparation method comprises the following steps:

[0013] Step 1: The fly ash hollow microsphere FAC raw material is passed through a 150-mesh screen in sequence, and the undersize part is the first particle size range; then continue to pass through a 60-mesh screen, and the undersize part is the second particle size range; continue to pass through a 35-mesh screen, and the undersize part is the third particle size range, and the oversize part of the 35-mesh screen is discarded; measure the density of the fly ash hollow microsphere FAC in different particle size ranges;

[0014] Step 2: The fly ash hollow microsphere FAC is processed by a focused ion beam method, and the ion source is a liquid gallium ion source, and the working distance is 5mm:

[0015] Firstly, the acceleration voltage of the focused ion beam is determined according to the density of the fly ash hollow microsphere FAC in different particle size ranges, if the density of the fly ash hollow microsphere FAC is greater than 0.3g / cm 3 , the acceleration voltage is [15KV-35KV), otherwise the acceleration voltage is set to [5KV-15KV);

[0016] Then, the scanning speed of the focused ion beam is determined according to the particle size range, when the particle size of the fly ash hollow microsphere FAC is the first particle size range, the ion beam spot size is [10-15) nm, the scanning speed is selected as 5-10μm per second, and the processing pore size range is adjusted to 5-40nm;

[0017] When the particle size of the fly ash hollow microsphere FAC is the second particle size range, the ion beam spot size is [15-40) nm, and the scanning speed is selected as 15-30μm per second; adjust the processing pore size range to 20-80nm;

[0018] When the particle size of the fly ash hollow microsphere FAC is the third particle size range, the ion beam spot size is [40-60nm), and the scanning speed is selected as 30-50μm per second; adjust the processing pore size range to 50-140nm.

[0019] Further, the focused ion beam method is automatically processed, and the specific processing process is:

[0020] 1) Input the particle size of the target porous microsphere, the required processing pore size, and the density;

[0021] 2) Determine the processing parameter range, including: acceleration voltage of ion beam, scanning path, scanning speed, ion beam spot size:

[0022] The acceleration voltage of the focused ion beam is determined according to the density of the fly ash hollow microsphere FAC in different particle size ranges, if the density of the fly ash hollow microsphere FAC is greater than 0.3g / cm 3 , the acceleration voltage is adjusted in the range of [15KV-35KV), otherwise the acceleration voltage is set in the range of [5KV-15KV);

[0023] Then, the scanning speed of the focused ion beam is determined according to the particle size range. When the particle size of the fly ash hollow microbeads FAC is in the first particle size range, the ion beam spot size is adjusted within the range of [10-15) nm, and the scanning speed is adjusted within the range of 5-10 μm per second.

[0024] When the particle size of the fly ash hollow microbeads FAC is within the second particle size range, the ion beam spot size is adjusted within the range of [15-40) nm, and the scanning speed is adjusted within the range of 15-30 μm per second;

[0025] When the particle size of the fly ash hollow microbeads FAC is within the third particle size range, the ion beam spot size is adjusted within the range of [40-60nm], and the scanning speed is adjusted within the range of 30-50μm per second;

[0026] During the processing, the size of the processed hole is fed back in real time through SEM. If it does not match the target processing hole size, the FIB workstation will actively adjust the parameters within the parameter range to ensure that porous microbeads with the target hole size are processed, and the processing parameter values ​​that meet the target hole size are recorded;

[0027] 3) Aperture measurement of processed holes: Use SEM to take pictures and use the measurement tools in the image processing software to measure the pore size of the particles in the SEM image to obtain samples with the target pore size. At the same time, the pore size distribution after processing is obtained for subsequent strength and thermal insulation performance evaluation;

[0028] 4) Batch preparation: After obtaining the target pore size processing parameter values ​​and the strength and temperature performance evaluation is qualified, the obtained processing parameter values ​​are used for batch preparation. Through automated programming control, a large number of porous microbeads can be prepared at one time to improve preparation efficiency.

[0029] In a second aspect, the present invention provides fly ash porous hollow microspheres obtained by the preparation method, wherein the fly ash porous hollow microspheres have a pore size range of less than 100 nm, a regular pore size distribution, a thermal conductivity coefficient controlled within 0.05 W / m·K, and a crushing strength not less than 60% of the crushing strength of the fly ash hollow microspheres FAC raw material.

[0030] Furthermore, the pore size of the fly ash porous hollow microspheres is below 70 nm, and the thermal conductivity is less than that of air.

[0031] In a third aspect, the present invention provides an application of the fly ash porous hollow microspheres, wherein the fly ash porous hollow microspheres are used to prepare thermal insulation concrete, and the specific process is:

[0032] (1) Fly ash porous hollow microspheres and water were mixed in a vacuum cylinder at a volume ratio of 1:2, and the fly ash porous hollow microspheres were pre-absorbed with water under the action of a vacuum pump. The vacuum operating conditions were: vacuum pressure -0.03 MPa, and vacuuming time 24 h. After pre-absorption, excess water was filtered off, and the fly ash porous hollow microspheres were continuously wiped with absorbent paper towels until the absorbent paper towels no longer changed color.

[0033] (2) 486 kg of ordinary Portland cement and 2.5 kg of silica fume were added to a mixer and mixed for 3 minutes. 25.8 kg of pre-absorbed fly ash porous hollow microspheres were added and mixed for another 3 minutes. 9.2 kg of polycarboxylate superplasticizer was added to 170 kg of water and mechanically stirred at a speed of 300 r / min for 6 minutes. The mixture was then poured into a mixer at a stirring speed of 800 r / min for 6 minutes.

[0034] (3) After uniform mixing, the mixture is placed in a mold, compacted by vibration, and placed in a standard curing room (temperature 20-25°C, humidity 90-95%) for 24 hours before demolding. After demolding, the mixture is allowed to stand and cure for 28 days to obtain thermal insulation concrete.

[0035] Furthermore, the thermal conductivity of the thermal insulation concrete is controlled within 0.1 W / m·K, and the 28d compressive strength is greater than 30 MPa.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] Compared with fly ash hollow microbeads prepared by chemical etching and high-temperature sintering, the focused ion beam method will not excessively damage the glass phase film on the surface of the hollow microbeads. The glass phase shell has good toughness and strength, and can withstand a certain amount of pressure and impact when subjected to external forces, thereby protecting the interior of the microbeads from damage, avoiding the impurities and damage that may be present in the porous hollow microbeads of fly ash prepared by traditional mechanical processing. This high-strength microbead structure is more conducive to use as a thermal insulation material in construction.

[0038] (1) The present invention applies the focused ion beam method to the preparation of fly ash porous hollow microspheres with controllable particle size. During the preparation process of fly ash porous hollow microspheres, the parameters of the ion beam (such as acceleration voltage, ion spot size, scanning speed, etc.) can be automatically adjusted to precisely control the size, shape and distribution of the pores, thereby achieving a highly customized porous structure.

[0039] (2) The fly ash porous hollow microspheres prepared by the present invention have high purity, good surface quality, low thermal conductivity and high strength, which are beneficial to subsequent experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1This is the SEM image of fly ash porous hollow microspheres obtained under the processing parameters of serial number 1.

[0041] Figure 2 SEM image of the microbeads obtained under the processing parameters of Comparative Example 1. DETAILED DESCRIPTION

[0042] The present invention is further explained below with reference to the embodiments and drawings, but they are not intended to limit the scope of protection of the present application.

[0043] The present invention creatively applies the focused ion beam method to the preparation of fly ash porous hollow microbeads, can quickly prepare fly ash porous hollow microbeads with specific pore diameters, can be mass-produced, and improves perforation efficiency.

[0044] The porous fly ash microbeads prepared by the present invention have controllable pore size. The hollow fly ash microbeads themselves have a hollow structure, which allows for the presence of a large amount of air or gas within them. Since the thermal conductivity of air or gas is much lower than that of solid materials, this hollow structure inherently reduces the microbeads' thermal conductivity. Furthermore, nanoscale pore size regulation allows for precise control of the pore size and distribution within these hollow structures, further optimizing the microbeads' thermal conductivity. Adjusting the pore size and distribution can better isolate the air or gas within the microbeads from heat transfer, further reducing thermal conductivity.

[0045] The fly ash porous hollow microspheres prepared by the present invention have the following advantages as building thermal insulation fillers:

[0046] 1. Reduced thermal conductivity: When the pores of a material have a regular pore size, the gas flow inside the material is reduced. The movement paths of gas molecules in the regular pores are more orderly, reducing the direct conduction of heat through the gas molecules. This reduced gas flow increases the resistance to heat transfer, thereby reducing the thermal conductivity of the material.

[0047] 2. Reduced connectivity between pores: Regular pore arrangements reduce pore connectivity. Compared to random or irregular pores, regular pores are less likely to form direct pathways for heat transfer. This further reduces the efficiency of heat transfer through the pores, thereby enhancing the thermal insulation performance of the material.

[0048] 3. Improve the stability of the material: Regular pore size not only improves the thermal insulation performance of the material, but also enhances the stability and durability of the material. Due to the regularity of the pore structure, the material is less likely to be damaged or deformed when subjected to external forces or environmental changes, thereby maintaining its long-term thermal insulation effect. The present invention treats the FAC surface with a focused ion beam method, so that the pore size of the prepared fly ash porous hollow microbeads is below 100nm, which is about 100nm lower than the free path of gas molecules in the air. The pores are almost in a vacuum state, and the gas molecules are adsorbed on the pore walls. Adjacent gas molecules cannot collide, and heat transfer is terminated, so that the treated fly ash porous hollow microbeads still have a low thermal conductivity coefficient.

[0049] The present invention preferably controls the pore size to be below 70 nm, with a small stress concentration area, uniform pore size, regular pore shape, and low stress concentration in circular pores, thereby retaining the strength of FAC and opening pores in the glass phase film to enhance the volcanic ash effect of FAC, so that the active SiO2 in FAC reacts with the surrounding Ca(OH)2 to generate CSH, thereby improving the compatibility of FAC with cementitious materials and facilitating its mixing with concrete materials.

[0050] The present invention applies fly ash porous hollow microspheres to building insulation fillers, and the advantages of mixing with concrete are as follows:

[0051] 1. Continuous water supply capability: The porous hollow microspheres prepared by the present invention can absorb and store a large amount of water, which is gradually released during the concrete hardening process, providing the necessary water for cement hydration, ensuring that the concrete is fully hydrated, thereby improving its strength and durability.

[0052] 2. Improve the density and strength of concrete: As porous hollow microspheres can fill the pores in concrete, they reduce water seepage and cracks in the concrete, thereby improving the density and strength of the concrete. At the same time, the strength of the microspheres themselves can also increase the strength of the concrete.

[0053] 3. Improve the thermal insulation performance of concrete: Porous hollow microspheres have good thermal insulation performance, which can effectively reduce the heat transfer of concrete, thereby reducing the temperature difference between the inside and outside of the concrete and reducing the risk of concrete cracking.

[0054] The method for preparing fly ash porous hollow microspheres with controllable pore size of the present invention can prepare fly ash porous hollow microspheres with controllable pore size. The process of the preparation method is:

[0055] Step 1: The fly ash hollow microspheres (FAC) raw material is sequentially passed through a 150-mesh sieve to obtain the undersize fraction as the first particle size range; then passed through a 60-mesh sieve to obtain the undersize fraction as the second particle size range; then passed through a 35-mesh sieve to obtain the undersize fraction as the third particle size range, and the fraction above the 35-mesh sieve is discarded; the density of the fly ash hollow microspheres (FAC) in different particle size ranges is measured;

[0056] Step 2: Use the focused ion beam method to process the nano-scale aperture of the fly ash hollow microspheres FAC. The ion source is a liquid gallium ion source and the working distance is 5mm:

[0057] First, the accelerating voltage of the focused ion beam is determined according to the density of fly ash hollow microspheres FAC in different particle size ranges. If the density of fly ash hollow microspheres FAC is greater than 0.3 g / cm 3 , then the accelerating voltage is [15KV-35KV), otherwise the accelerating voltage is set to [5KV-15KV);

[0058] Then, the scanning speed of the focused ion beam is determined according to the particle size range. When the particle size of the fly ash hollow microbeads FAC is in the first particle size range, the ion beam spot size is [10-15) nm, the scanning speed is selected to be 5-10 μm per second, and the processing aperture range is adjusted to 5-40 nm.

[0059] When the particle size of the fly ash hollow microspheres FAC is in the second particle size range, the ion beam spot size is [15-40) nm, the scanning speed is selected to be 15-30 μm per second, and the processing aperture range is adjusted to be 20-80 nm;

[0060] When the particle size of the fly ash hollow microbeads FAC is within the third particle size range, the ion beam spot size is [40-60nm), the scanning speed is selected to be 30-50μm per second, and the processing aperture range is adjusted to be 50-140nm.

[0061] Furthermore, the focused ion beam method is used for automated processing, and the specific processing process is:

[0062] 1) Input the target porous microbead particle size, the desired processing hole size, and density;

[0063] 2) Determine the range of processing parameters, including: ion beam acceleration voltage, scanning path, scanning speed, and ion beam spot size:

[0064] The accelerating voltage of the focused ion beam is determined by the density of fly ash hollow microspheres FAC in different particle size ranges. If the density of fly ash hollow microspheres FAC is greater than 0.3 g / cm 3 , then the acceleration voltage is adjusted within the range of [15KV-35KV), otherwise the acceleration voltage is adjusted within the range of [5KV-15KV);

[0065] Then the scanning speed of the focused ion beam is determined according to the particle size range, when the particle size of the fly ash hollow microsphere FAC is in the first particle size range, the ion beam spot size is adjusted in the range of [10-15) nm, and the scanning speed is adjusted in the range of 5-10 mu m per second;

[0066] When the particle size of the fly ash hollow microsphere FAC is in the second particle size range, the ion beam spot size is adjusted in the range of [15-40) nm, and the scanning speed is adjusted in the range of 15-30 mu m per second;

[0067] When the particle size of the fly ash hollow microsphere FAC is in the third particle size range, the ion beam spot size is adjusted in the range of [40-60nm), and the scanning speed is adjusted in the range of 30-50 mu m per second;

[0068] During the processing, the size of the processing hole is fed back in real time through SEM, if it does not match the target processing hole size, the FIB workstation actively adjusts the parameters within the parameter range to ensure that the target hole size of the porous microsphere is processed, and the processing parameter value meeting the target hole size is recorded;

[0069] 3) Hole diameter measurement of the processed hole: SEM is used for shooting, and the measurement tool in the image processing software is used for hole diameter measurement of the particles in the SEM image, so as to obtain the sample with the target hole diameter, and the hole diameter distribution after processing is obtained, which is used for subsequent strength and heat preservation performance evaluation;

[0070] 4) Batch preparation: after obtaining the target hole size processing parameter value and the qualified strength and temperature performance evaluation, the obtained processing parameter value is used for batch preparation, a large number of porous microspheres are prepared at one time through automatic programming control, and the preparation efficiency is improved. The hole diameter of the fly ash porous hollow microsphere in the application is below 100 nm, the hole diameter distribution is regular, the thermal conductivity coefficient is controlled within 0.05 W / m·K, even lower than the thermal conductivity coefficient of air, and the breaking strength is not less than 60% of the breaking strength of the fly ash hollow microsphere FAC raw material.

[0071] The fly ash porous hollow microsphere is used for preparing thermal insulation concrete, and the specific process is as follows:

[0072] (1) The fly ash porous hollow microsphere: water = 1:2 (volume ratio) is mixed in a vacuum cylinder, and the fly ash porous hollow microsphere is pre-imbibed under the action of a vacuum pump, the vacuum operation condition is: vacuum pressure-0.03 MPa, vacuum time 24 h; after pre-imbibing, the excess water is filtered out, and the fly ash porous hollow microsphere is continuously wiped with a water absorption paper towel until the water absorption paper towel no longer changes color;

[0073] (2) Ordinary Portland cement 486 kg, silica fume 2.5 kg are added into the mixer in turn and stirred for 3 min, 25.8 kg of pre-hydrated fly ash porous hollow microsphere is added and stirred for 3 min; 9.2 kg of polycarboxylic acid superplasticizer is added into 170 kg of water, mechanically stirred at a speed of 300 r / min for 6 min, then poured into the mixer, and stirred at a speed of 800 r / min for 6 min;

[0074] (3) After stirring uniformly, the mold is filled, vibration compaction is carried out, and after being placed in a standard curing room (temperature 20-25℃, humidity 90-95%) for 24 hours, the mold is removed. After demolding, the thermal insulation concrete is obtained after being placed and cured for 28 days.

[0075] Further, the thermal conductivity of the thermal insulation concrete is controlled within 0.1 W / m·K, and the 28d compressive strength is greater than 30 MPa.

[0076] In the application, the controllable pore size can be understood as the pore size of the glass film penetrating the surface of the fly ash hollow microsphere can be controlled, and the pore size can be adjusted by changing the acceleration voltage, ion beam spot size and scanning speed.

[0077] Example 1

[0078] 1. Raw materials and properties

[0079] (1) Fly ash hollow microsphere (FAC): The product in this embodiment is a commercially available product, which is smooth in surface, silver-white in color, hollow spherical in shape, 125-250 μm in particle size, 162.5 μm in average particle size, 0.38 m 2 / g in specific surface area, 1600-1700℃ in refractoriness, 0.091 W / (m·k) in thermal conductivity, and 60.00%, 29.10%, 4.93%, 1.20%, 1.15%, 1.61%, 2.63%, 1.02%, 0.40% in contents of SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, TiO2 and SO3, respectively, 80% in glass phase, and mainly containing mullite and quartz and other mineral components.

[0080] It is assumed that the mass fraction of the main components of the FAC shell is calculated as 70% SiO2 and 30% Al2O3, and the theoretical density of the shell is 2.8 g / cm 3 , ignoring the mass of air, the shell thickness of the FAC is calculated as 2.93 μm-5.86 μm according to formulas (1) and (2), and the estimation of the shell thickness is completed.

[0081] [4π×(D / 2) 3 / 3-4π×(d / 2) 3 / 3]×1.6=ρ×4π×(D / 2)3 / 3 (1)

[0082] e = [1-(1-ρ / ρ0) 1 / 3 ]×D / 2 (2)

[0083] Wherein, ρ is the density of fly ash hollow microsphere, D is the outer diameter of fly ash hollow microsphere; ρ0 is the theoretical density of fly ash hollow microsphere.

[0084] According to the shell thickness, the acceleration voltage can be adjusted in the range of appropriate adjustment, using focused ion beam to process the FAC surface, break through the glass phase film, form uniform size nanoscale perforation on the surface, form fly ash porous hollow microsphere as thermal insulation filler. The role of fly ash porous hollow microsphere is: ① the surface glass phase film is broken, and has better compatibility with cementitious materials. ② Forming a perforation with a pore size of less than 100 nm, the perforation still has a low thermal conductivity; ③ The pore size is uniform, the pore shape is regular, the stress concentration area is small, and the stress concentration degree is low, and the perforation still has high strength. Especially when the formed pore size is less than 70 nm, the free path of gas molecules in air is about 70 nm, the hole is almost in vacuum state, the adjacent gas molecules cannot collide, and the heat transfer is terminated. Through calculation, the shell thickness of FAC is much larger than the pore size, and the heat radiation is refracted and weakened through the hole.

[0085] If the shell thickness is thick, a higher acceleration voltage can be selected, and if the shell is thin, a lower acceleration voltage can be selected.

[0086] (2) Ordinary Portland cement: strength grade PO·42.5, loss on ignition 2.65, chloride mass fraction 0.08%, SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, TiO2, SO3 content is 19.56%, 4.33%, 2.95%, 63.86%, 0.90%, 0.13%, 0.65%, 0.26%, 2.53%, respectively, fineness 322 m 2 / g, initial setting time 142 min, final setting time 210 min.

[0087] (3) Silica fume: average particle size 0.1-30 μm, density 750-800 kg / m 3 , specific surface area 20-30 m 2 / g. SiO2 in silica fume accounts for 87.6%.

[0088] The spherical morphology of silica fume can be filled between FAC and cementitious materials, and silica fume contains active Si monomer with high pozzolanic activity, which generates more C-S-H and C-A-S-H gel, and interweaves FAC and cementitious materials together to improve the compatibility of the two.

[0089] (4) Polycarboxylic acid high-efficiency water reducer: water reduction rate>25%, air content≤3%.

[0090] (5) Water, deionized water.

[0091] 2. Material ratio and preparation

[0092] Preparation of fly ash porous hollow microspheres with controllable pore size

[0093] (1) The particle size of the fly ash hollow microspheres (FAC) in the raw materials of this embodiment is in the range of 125 to 250 μm, all within the second particle size range, and no screening is required. The FAC is pretreated:

[0094] The surface of the fly ash hollow microspheres was cleaned by high-pressure water washing to reduce impurities adsorbed on the surface of the fly ash hollow microspheres, preventing impurities from affecting the reaction and the perforation effect. The fly ash hollow microspheres were mixed in a volume ratio of 1:2, gently stirred with a glass rod, and allowed to stand for 24 hours. The mixture was poured into a separatory funnel and allowed to stand for 1 hour. The fly ash hollow microspheres precipitated in the separatory funnel were discharged, and the fly ash hollow microspheres floating on the top were taken, filtered, and dried at 110°C for 24 hours. The purpose was to remove damaged fly ash hollow microspheres and retain fly ash hollow microspheres with smooth and intact surfaces. The density was measured to be 0.376g / cm 3 .

[0095] (2) The fly ash hollow microspheres FAC were processed with a focused ion beam method. The ion source was a liquid gallium ion source and the working distance was 5 mm. The focused ion beam method was based on the principles of ion beam imaging and ion beam etching. The specific process is as follows:

[0096] The density of the fly ash hollow microspheres FAC in this embodiment is 0.376 g / cm 3 , greater than 0.3g / cm 3 , then the acceleration voltage is adjusted within the range of [15KV-35KV);

[0097] The particle size of fly ash hollow microspheres (FAC) is 125-250 μm, which is within the second particle size range. The ion beam spot size is adjusted within the range of [15-40) nm, and the scanning speed is adjusted within the range of [15-30] μm per second.

[0098] The experimental parameters are adjusted according to the changes in pore size during the etching process. Taking fly ash hollow microspheres with the same average particle size of 125-200μm as an example, the shell thickness is 2.93-5.86μm. Within this range, if the shell thickness is thickened, the acceleration voltage required to prepare the same pore diameter perforation and the corresponding ion beam spot also need to be increased.

[0099] The specific operation process of perforation is:

[0100] (1) The fly ash hollow microbead sample was fixed on the sample stage by using sticky conductive tape to ensure that the microbeads would not move during ion beam treatment, and the conductivity of the tape helped to transfer charges during the experiment.

[0101] (2) Ion beam generation and focusing: A focused ion beam system generates an ion beam from a liquid metal ion source (such as gallium) and uses an electric lens to focus the ion beam to a very small size. This focused ion beam will be used to cut or perforate the sample.

[0102] (3) Experimental Parameter Settings: The acceleration voltage (starting at 15 kV) was set according to the above requirements for breakdown. A magnification of 10,000 and a resolution of 50 nm were selected. A liquid gallium ion source was used as the ion source. The working distance was 5 mm. The ion beam spot size and scanning speed were also selected from the smaller values ​​within the above range. The focused ion beam was applied to the surface of the fly ash hollow microbeads. By precisely controlling the ion beam's focus position, scanning speed, and ion beam spot, precise perforation of the microbeads was achieved.

[0103] (4) Perforation process monitoring and adjustment: During the perforation process, the perforation status can be observed in real time through the imaging function of the focused ion beam microscope. The aperture size can be observed through the imaging function of the ion beam microscope. If the expected aperture size is not reached, it can be corrected by adjusting the experimental parameters (increasing the current experimental parameters by 10% each time). The ion beam parameters are adjusted in real time to optimize the perforation effect and achieve the target perforation aperture.

[0104] (5) Perforation completion and post-processing: After the perforation is completed, the fly ash porous hollow microspheres are cleaned and dried to remove any remaining ion beam products or other impurities, and their compressive strength and thermal conductivity are tested.

[0105] 3. Preparation of fly ash porous hollow microsphere insulation concrete with controllable pore size

[0106] (1) 291 kg of porous hollow microspheres with controlled pore size fly ash were mixed with water in a ratio of 1:2 (volume ratio) in a vacuum cylinder. The vacuum pump was adjusted to -0.03 MPa to pre-absorb water to the hollow microspheres. The vacuum was applied for 24 hours to fully absorb water. After the pre-absorption, vacuum filtration was performed to remove excess water. The hollow microspheres were then wiped with absorbent paper towels until the absorbent paper towels no longer changed color.

[0107] (2) 486 kg of ordinary Portland cement and 2.5 kg of silica fume were added to a mixer and mixed for 3 minutes. 25.8 kg of pre-absorbed fly ash porous hollow microspheres with controlled pore size were added and mixed for another 3 minutes. 9.2 kg of polycarboxylate superplasticizer was added to 170 kg of water and mechanically stirred at 300 r / min for 6 minutes. The mixture was then poured into a mixer and stirred at 800 r / min for 6 minutes.

[0108] (3) After stirring, place in a mold, vibrate and compact, place in a standard curing room (temperature 20-25°C, humidity 90-95%) for 24 hours, then remove the mold and allow to stand for 28 days of curing.

[0109] (4) After curing, the material was cut into cubes for compressive strength test and thermal conductivity test.

[0110] Table 1 is a comparison of microbead performance under different processing parameters, among which:

[0111] The processing parameters of No. 1 are: acceleration voltage of 20 kV for breakdown, ion beam spot size of 20 nm, liquid gallium ion source, working distance of 5 mm, and scanning speed of 20 μm per second.

[0112] The processing parameters of No. 2 are: acceleration voltage of 15kV for breakdown, magnification of 10000, resolution of 50nm, ion beam spot of 15nm, liquid gallium ion source, working distance of 5mm, and scanning speed of 15μm per second.

[0113] The processing parameters of No. 3 are: acceleration voltage of 25kV for breakdown, magnification of 10000, resolution of 50nm, ion beam spot of 25nm, liquid gallium ion source, working distance of 5mm, and scanning speed of 25μm per second.

[0114] The processing parameters of No. 4 are: acceleration voltage of 30kV for breakdown, magnification of 10000, resolution of 50nm precision, ion beam spot size of 30nm, liquid gallium ion source, working distance of 5mm, and scanning speed of 30μm per second.

[0115] The processing parameters of comparative example 1 are: acceleration voltage of 5 kV for breakdown, magnification of 10,000, resolution of 50 nm, ion beam spot of 5 nm, liquid gallium ion source, working distance of 5 mm, and scanning speed of 5 μm per second.

[0116] The processing parameters of comparative example 2 are: acceleration voltage of 40 kV for breakdown, magnification of 10,000, resolution of 50 nm, ion beam spot of 50 nm, liquid gallium ion source, working distance of 5 mm, and scanning speed of 50 μm per second.

[0117] Table 1 Comparison of properties of fly ash porous hollow microspheres

[0118] Serial number Average pore size (nm) Crushing strength (Mpa) Thermal conductivity (W / m·K) 1 35 4.51 0.022 2 20 4.33 0.026 3 50 3.82 0.025 4 65 3.56 0.031 Comparative Example 1 10 5.54 0.150 Comparative Example 2 320 1.17 0.340 raw material 5 5.72 0.140

[0119] As can be seen from the above table, the average pore size in Comparative Example 1 is too small because the selected acceleration voltage and ion beam spot are too small, resulting in unclear perforation. Since the selected acceleration voltage and ion beam spot are too large, the hollow microbead structure is destroyed, making the average pore size in Comparative Example 2 too large.

[0120] Table 1 shows variations in crushing strength and thermal conductivity under different processing parameters. By adjusting the processing parameters, particles 1-4 achieve relatively high crushing strength and thermal conductivity, with average pore sizes ranging from 20 to 65 nm. In particular, particles 1-3 exhibit thermal conductivity significantly lower than that of air, demonstrating excellent insulation performance and high crushing strength. Table 2 compares the mechanical and thermal insulation properties of insulating concrete produced using microbeads obtained under these different processing parameters.

[0121] Table 2 Comparison of performance of micro-bead insulation concrete

[0122] Serial number 28d compressive strength (MPa) Thermal conductivity (W / m·K) 1 39.23 0.046 2 33.74 0.055 3 35.43 0.057 4 30.22 0.068 Comparative Example 1 18.59 0.330 Comparative Example 2 15.38 0.551 raw material 17.58 0.353

[0123] The results in Table 2 demonstrate that the fly ash hollow microspheres with controllable pore size prepared in this application, when used as an internal curing material, significantly improved the compressive strength of concrete while simultaneously reducing its thermal conductivity. In Comparative Example 1, no porous fly ash hollow microspheres were prepared, resulting in no increase in strength or reduction in thermal conductivity. In Comparative Example 2, the hollow microspheres were structurally destroyed, further reducing their strength and increasing their thermal conductivity.

[0124] Figure 1 The pore size diagram of fly ash porous hollow microspheres prepared by focused ion beam method under the processing parameters of No. 1 is shown. Regular pores can be observed. Figure 2 This is a picture of the microbeads under the processing parameters of Comparative Example 1. It can be observed that due to the small acceleration voltage and ion beam spot, the focused ion beam method does not form holes on the surface of the microbeads.

[0125] Example 2

[0126] The particle size range of the fly ash hollow microspheres FAC purchased in this embodiment is 5-300 μm. The fly ash hollow microspheres FAC raw material is sequentially passed through a 150-mesh sieve to obtain a first particle size range of the undersieve portion; then passed through a 60-mesh sieve to obtain a second particle size range of the undersieve portion; and then passed through a 35-mesh sieve to obtain a third particle size range of the undersieve portion, and the portion above the 35-mesh sieve is discarded.

[0127] Fly ash hollow microbeads (FAC) within three particle size ranges were pretreated to obtain fly ash hollow microbeads with smooth and complete surfaces within each particle size range. The density and porosity of the fly ash hollow microbeads (FAC) within different particle size ranges were measured.

[0128] When the particle size of the fly ash hollow microbeads FAC is within the second particle size range, the ion beam spot size is adjusted within the range of [15-40) nm, and the scanning speed is adjusted within the range of 15-30 μm per second;

[0129] When the particle size of the fly ash hollow microbeads FAC is within the third particle size range, the ion beam spot size is adjusted within the range of [40-60nm], and the scanning speed is adjusted within the range of 30-50μm per second;

[0130] During the processing, the size of the processed hole is fed back in real time through SEM. If it does not match the target processing hole size, the FIB workstation will actively adjust the parameters within the parameter range to ensure that porous microbeads with the target hole size are processed, and the processing parameter values ​​that meet the target hole size are recorded;

[0131] Aperture measurement of processed holes: Use SEM to take pictures and use the measurement tools in the image processing software to measure the pore size of the particles in the SEM image to obtain samples of the target pore size. At the same time, the pore size distribution after processing is obtained. The porosity can be obtained from the pore size distribution. By comparing it with the porosity before processing, the change in porosity due to perforation can be obtained, which is used for subsequent strength and thermal insulation performance evaluation;

[0132] Batch preparation: After obtaining the target pore size processing parameter values ​​and the strength and temperature performance evaluation is qualified, the obtained processing parameter values ​​are used for batch preparation. Through automated programming control, a large number of porous microbeads can be prepared at one time to improve preparation efficiency.

[0133] The pore size of fly ash porous hollow microspheres is below 100nm, the pore size distribution is regular, the thermal conductivity is controlled within 0.05W / m·K, and the crushing strength is not less than 60% of the crushing strength of fly ash hollow microspheres FAC raw materials

[0134] Any matters not described in the present invention are applicable to the prior art.

Claims

1. A method for preparing fly ash porous hollow microspheres with controllable pore size, characterized in that: The porous hollow microspheres of fly ash with controllable pore size can be prepared. The preparation process is as follows: Step 1: The fly ash hollow microspheres (FAC) raw material is sequentially passed through a 150-mesh sieve to obtain the undersize fraction as the first particle size range; then passed through a 60-mesh sieve to obtain the undersize fraction as the second particle size range; then passed through a 35-mesh sieve to obtain the undersize fraction as the third particle size range, and the fraction above the 35-mesh sieve is discarded; the density of the fly ash hollow microspheres (FAC) in different particle size ranges is measured; Step 2: Use the focused ion beam method to process the nano-scale aperture of the fly ash hollow microspheres FAC. The ion source is a liquid gallium ion source and the working distance is 5mm: First, the accelerating voltage of the focused ion beam is determined according to the density of fly ash hollow microspheres FAC in different particle size ranges. If the density of fly ash hollow microspheres FAC is greater than 0.3 g / cm 3 , then the accelerating voltage is [15KV, 35KV), otherwise the accelerating voltage is set to [5KV, 15KV); Then, the scanning speed of the focused ion beam is determined according to the particle size range. When the particle size of the fly ash hollow microbeads FAC is in the first particle size range, the ion beam spot size is [10, 15) nm, the scanning speed is selected to be 5-10 μm per second, and the processing aperture range is adjusted to 5-40 nm. When the particle size of the fly ash hollow microspheres FAC is in the second particle size range, the ion beam spot size is [15, 40) nm, the scanning speed is selected to be 15-30 μm per second, and the processing aperture range is adjusted to be 20-80 nm; When the particle size of the fly ash hollow microbeads FAC is in the third particle size range, the ion beam spot size is [40, 60 nm), the scanning speed is selected to be 30-50 μm per second, and the processing aperture range is adjusted to be 50-140 nm.

2. The preparation method according to claim 1, characterized in that The focused ion beam method is used for automated processing, and the specific processing process is: 1) Input the target porous microbead particle size, the desired processing hole size, and density; 2) Determine the range of processing parameters, including: ion beam acceleration voltage, scanning path, scanning speed, and ion beam spot size: The accelerating voltage of the focused ion beam is determined by the density of fly ash hollow microspheres FAC in different particle size ranges. If the density of fly ash hollow microspheres FAC is greater than 0.3 g / cm 3 , then the acceleration voltage is adjusted within the range of [15KV, 35KV), otherwise the acceleration voltage is set within the range of [5KV, 15KV); Then, the scanning speed of the focused ion beam is determined according to the particle size range. When the particle size of the fly ash hollow microbeads (FAC) is in the first particle size range, the ion beam spot size is adjusted within the range of [10, 15) nm, and the scanning speed is adjusted within the range of 5-10 μm per second. When the particle size of the fly ash hollow microbeads FAC is in the second particle size range, the ion beam spot size is adjusted within the range of [15, 40) nm, and the scanning speed is adjusted within the range of 15-30 μm per second; When the particle size of the fly ash hollow microbeads FAC is within the third particle size range, the ion beam spot size is adjusted within the range of [40, 60 nm), and the scanning speed is adjusted within the range of 30-50 μm per second; During the processing, the size of the processed hole is fed back in real time through SEM. If it does not match the target processing hole size, the FIB workstation will actively adjust the parameters within the parameter range to ensure that porous microbeads with the target hole size are processed, and the processing parameter values ​​that meet the target hole size are recorded; 3) Aperture measurement of processed holes: Use SEM to take pictures and use the measurement tools in the image processing software to measure the pore size of the particles in the SEM image to obtain samples with the target pore size. At the same time, the pore size distribution after processing is obtained for subsequent strength and thermal insulation performance evaluation; 4) Batch preparation: After obtaining the target pore size processing parameter values ​​and the strength and temperature performance evaluation is qualified, the obtained processing parameter values ​​are used for batch preparation. Through automated programming control, a large number of porous microbeads can be prepared at one time to improve preparation efficiency.

3. A fly ash porous hollow microsphere obtained by the preparation method according to claim 1 or 2, characterized in that: The fly ash porous hollow microspheres have a pore size range of less than 100 nm, a regular pore size distribution, a thermal conductivity controlled within 0.05 W / m·K, and a crushing strength not less than 60% of the crushing strength of fly ash hollow microspheres (FAC) raw materials.

4. The fly ash porous hollow microspheres according to claim 3, characterized in that: The fly ash porous hollow microspheres have a pore size range of less than 70 nm and a thermal conductivity coefficient less than that of air.

5. An application of the fly ash porous hollow microspheres according to claim 3 or 4, characterized in that: The fly ash porous hollow microspheres are used to prepare thermal insulation concrete, and the specific process is: (1) Fly ash porous hollow microspheres were mixed with water in a volume ratio of 1:2 in a vacuum cylinder, and the fly ash porous hollow microspheres were pre-absorbed with water under the action of a vacuum pump. The vacuum operating conditions were: vacuum pressure -0.03 MPa, vacuuming time 24 h; after pre-absorption, excess water was filtered off, and the fly ash porous hollow microspheres were continuously wiped with absorbent paper towels until the absorbent paper towels no longer changed color; (2) 486 kg of ordinary Portland cement and 2.5 kg of silica fume were added to a mixer and mixed for 3 minutes. 25.8 kg of pre-absorbed fly ash porous hollow microspheres were added and mixed for another 3 minutes. 9.2 kg of polycarboxylate superplasticizer was added to 170 kg of water and mechanically stirred at a speed of 300 r / min for 6 minutes. The mixture was then poured into a mixer at a stirring speed of 800 r / min for 6 minutes. (3) After uniform mixing, the concrete is placed in a mold, subjected to vibration compaction, and placed in a standard curing room for 24 hours before demolding; after demolding, the concrete is allowed to stand and cure for 28 days to obtain thermal insulation concrete; the temperature of the standard curing room is 20-25°C and the humidity is 90-95%.

6. The use according to claim 5, characterized in that The thermal conductivity of the thermal insulation concrete is controlled within 0.1 W / m·K, and the 28d compressive strength is greater than 30 MPa.

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