Silica hollow sphere with a MOF composite layer and method for preparing the same

Abstract

The present invention relates to the technical field of surface modification of silica, and more particularly to silica hollow sphere with a MOF composite layer and method for preparing the same. The silica hollow sphere comprises: a hollow spherical structure made of silica, an amino-silane layer covalently grafted onto the surface of the core layer, a ZIF-8 crystal layer grown in situ with amino groups serving as nucleation sites, and thiol groups distributed on the outer surface of the MOF crystals. The purpose of the present invention is to provide silica hollow sphere with a MOF composite layer and method for preparing the same. The silica hollow sphere with the composite layer exhibits good compatibility and stability in an LCP matrix, while maintaining low dielectric loss, thereby effectively improving the performance of high-frequency FPCs.

Claims

1. A method for preparing a silica hollow sphere with a MOF composite layer, comprising the steps of: S100: pretreating a silica hollow spheres to activate surface hydroxyl groups and obtain a hollow spherical structure made of silica, having an average particle size of 200-800 nm, a wall thickness of 20-50 nm, and a specific surface area of 350 m2/g; S200: aminofunctionalizing the activated silica hollow spheres using a 3-8 wt % y-aminopropyltriethoxysilane solution at pH 4-6, 60-80 C. for 4-8 hours to obtain an amino-silane layer with an amino group density of 2.5-3.2 groups per nm.sup.2 and a layer thickness of 1-3 nm; S300: growing a ZIF-8 crystal layer in situ, with the amino groups serving as nucleation sites, in a methanol solution containing 0.05-0.2 M zinc nitrate and 0.3-0.6 M 2-methylimidazole at 30-40 C. for 8-24 hours to obtain a ZIF-8 crystal layer having a thickness of 50-150 nm, a crystal size of 20-50 nm, and a pore size of 3.3-3.5{acute over ()}; S400: reacting the product obtained in step S300 with a 0.3-0.8 M thioacetamide solution at 50-70 C. for 2-6 hours to perform surface thiolation modification, producing a thiol-modified product with a sulfur content of 0.5-2 wt %.

2. A silica hollow sphere with a MOF composite layer prepared by the method of claim 1, wherein the following structural relationship is satisfied: $\frac{N_{\mathrm{SH}}}{N_{{\mathrm{NH}}_2}}=a,0.3a0.6$ Where: N.sub.SH represents the thiol group density, N.sub.NH2 represents the amino group density.

3. The silica hollow sphere with a MOF composite layer according to claim 2, wherein a cavity volume of the core layer accounts for 40% of the total volume of the core layer, and the hydroxyl group density of surface hydroxyl groups in the silica hollow spheres after pretreatment activation is 4.8-5.2 groups per nm.sup.2; the silane molecules of the amino-silane layer are oriented upright, with a molecular tilt angle of 15 as measured by ellipsometry; the thiol groups are located at the edge positions of the MOF crystals.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present invention will be further described with reference to the accompanying drawing. However, the embodiments shown in the drawing shall not be construed as limiting the scope of the present invention. For those skilled in the art, other drawings may also be derived from the following drawing without the exercise of inventive effort.

(2) The FIGURE illustrates a method for preparing the silica hollow spheres according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(3) To make the above-mentioned objectives, features, and advantages of the present invention more apparent and easier to understand, a detailed description of the embodiments of the present invention is provided below with reference to the drawing. Numerous specific details are outlined in the following description to provide a thorough understanding of the present invention. However, the present invention may be embodied in many different ways other than those described herein, and those skilled in the art may make similar modifications without departing from the spirit of the present invention. Accordingly, the present invention is not limited to the specific embodiments disclosed below.

(4) Currently, liquid crystal polymer (LCP) composites in high-frequency flexible printed circuits (FPCs) face several critical issues, including poor dielectric properties before and after humid-heat aging, low adhesion strength with copper foil, high coating technique requirements, and insufficient technical data for substrate-integrated waveguide (SIW) design.

(5) Silica hollow spheres have attracted increasing attention in recent years due to their excellent physicochemical properties and broad application prospects. They have demonstrated good performance in fields such as catalysis, drug delivery, and adsorption materials. Research has shown that they can be applied in high-frequency FPC structures to enhance performance. However, prior silica hollow spheres still exhibit certain limitations in practical applications, mainly due to the singularity and insufficient functionalization of their surface properties.

(6) Conventional silica hollow spheres typically possess only hydroxyl groups on their surfaces, lacking other functional groups. This results in a poor dispersion state and instability of silica hollow spheres in the LCP matrix, thereby adversely affecting the performance of high-frequency FPCs.

(7) To address the above-mentioned problems, this embodiment discloses silica hollow sphere with a MOF composite layer, comprising: Core layer: a hollow spherical structure made of silica, having an average particle size of 200-800 nm, a wall thickness of 20-50 nm, and a specific surface area of 350 m.sup.2/g; Bridging layer: an amino-silane layer covalently grafted onto the surface of the core layer, with an amino group density of 2.5-3.2 groups per nm.sup.2, a layer thickness of 1-3 nm; MOF crystal layer: a ZIF-8 crystal layer grown in situ with the amino groups serving as nucleation sites, having a thickness of 50-150 nm, a crystal size of 20-50 nm, and a pore size of 3.3-3.5 ; Functionalized surface layer: thiol groups distributed on the outer surface of the MOF crystals, wherein the sulfur content is 0.5-2 wt %, and the following structural relationship is satisfied:

(8) 0$\frac{N_{\mathrm{SH}}}{N_{{\mathrm{NH}}_2}}=a,0.3a0.6$ where: N.sub.SH represents the thiol group density, N.sub.NH.sub.2 represents the amino group density.

(9) Furthermore, the cavity volume of the core layer accounts for 40% and serves as a nano-reactor for catalytic reactions; the hydroxyl group density is 4.8-5.2 groups per nm.sup.2, ensuring effective grafting of silane coupling agents; the silane molecules of the amino-silane layer are oriented upright, with a molecular tilt angle of 15 as measured by ellipsometry; the thiol groups are located at the edge positions of the MOF crystals, with a sulfur retention rate of 85% within the pH range of 3-10.

(10) In one embodiment, property testing was conducted on the silica hollow sphere:

(11) 1. Adsorption Properties

(12) Test conditions: mixed solution of Pb.sup.2+/Cd.sup.2+/Cu.sup.2+ (each at 100 ppm, pH=6)

Results

(13) TABLE-US-00001 Embodiment of the Conventional Amino- Index Present Invention functionalizedSiO.sub.2 Pb.sup.2+ adsorption capacity 832 mg/g 275 mg/g Selectivity coefficient 8.7:1 1.2:1 (Pb/Cd) Retention rate after 10 93% 67% cycles
2. Catalytic Properties

(14) Reaction system: 4-nitrophenol (0.1 mM)+NaBH.sub.4 (10 mM), loaded with 1.2 wt % Au nanoparticles

Results

(15) TABLE-US-00002 Embodiment of the Commercial Activated Index Present Invention Carbon Loaded with Au Reaction rate 0.49 min.sup.1 0.15 min.sup.1 constant k Conversion rate 99.7% 61.5% (10 min) Au cycle loss rate 0.45% 9.2%
3. Dielectric Properties

(16) Test frequency band: 28 GHz (5G millimeter wave communication band)

Results

(17) TABLE-US-00003 Embodiment of the Unmodified Parameter Present Invention LCP Permittivity 3.15 2.95 Loss factor tan 5.6 10.sup.1 8.2 10.sup.3 Temperature coefficient 4.8 ppm/ C. 18.5 ppm/ C. (40 to 125 C.)

(18) As illustrated in the FIGURE, furthermore, this embodiment discloses method for preparing silica hollow spheres, comprising the steps of: S100: pretreating the silica hollow spheres to activate surface hydroxyl groups; S200: aminofunctionalizing the activated silica hollow spheres using a -aminopropyltriethoxysilane solution to obtain aminofunctionalized silica; S300: growing a ZIF-8 crystal layer in situ, with the amino groups serving as nucleation sites, in a methanol solution containing zinc salt and 2-methylimidazole; S400: reacting the product obtained in step S300 with a thioacetamide solution to perform surface thiolation modification.

(19) In this embodiment, the amino group layer provides basic adsorption sites for capturing cations, resulting in at least a 2.1-fold improvement in adsorption property. The ZIF-8 crystal layer, with a pore size of 3.4 , enables sieving of small-sized ions, achieving a selectivity coefficient of up to 8.5:1. The SH of the thiol layer forms strong coordinate bonds with heavy metal ions, leading to an additional 45% increase in adsorption capacity.

(20) Furthermore, in step S200, the concentration of the -aminopropyltriethoxysilane solution is 3-8 wt %, ensuring a monolayer grafting coverage of 0.7-0.9, thereby avoiding multilayer stacking that could cause defects in the MOF layer. The reaction pH value is controlled between 4 and 6, maintaining the ratio of hydrolysis-to-condensation rates of KH550 in the range of 1.2-1.8, thereby preventing bulk phase gel formation. With a reaction time of 4-8 hours, the reaction temperature is set at 60-80 C., and enables the terminal amino groups of silane molecules to orient outward (contact angle <15), thereby enhancing the availability of nucleation sites for MOF.

(21) Furthermore, in step S300, the zinc salt is zinc nitrate, with a zinc ion concentration of 0.05-0.2 M to maintain the nucleation rate within 1.2-2.510.sup.3 s.sup.1. The concentration of 2-methylimidazole is 0.3-0.6 M, which regulates the crystal growth anisotropy index and promotes preferential growth of specific crystal planes. The reaction is conducted at a temperature of 30-40 C. for 8-24 hours to reduce grain boundary energy.

(22) In combination with the second aspect, furthermore, in step S400, the concentration of the thioacetamide solution is 0.3-0.8 M, the reaction temperature is 50-70 C., the reaction time is 2-6 hours, and the obtained product has a sulfur content of 0.5-2 wt %.

(23) In a third aspect, the present invention discloses method for predicting the dielectric loss of the silica hollow sphere, comprising the steps of: a. calculating the interfacial binding energy density E.sub.interface via molecular dynamics simulation:

(24) $E_{\mathrm{interface}}={}_{r_{\min }}^{r_{\mathrm{cut}}}\left[\frac{A}{r^{12}}-\frac{B}{r^6}+C.Math.e^{-\mathrm{Dr}}\right]\mathrm{dr}$ where: A represents the repulsive coefficient of the Lennard-Jones potential, determined by interatomic repulsion, having a dimension of eV\cdotp.sup.12, reflecting the short-range repulsion between Zn.sup.2+ in the MOF and the benzene rings of the LCP, thereby preventing excessive penetration of the molecular chain; B represents the attractive coefficient of the Lennard-Jones potential, determined by van der Waals forces, having a dimension of eV\cdotp.sup.6, characterizing the strength of - stacking interaction and directly affecting the interfacial binding energy; C represents the strength coefficient of the hydrogen-bond interaction, with a dimension of eV, quantifying the contribution of hydrogen bonding to the interfacial energy. A higher value indicates stronger interfacial chemical bonding; D represents the attenuation coefficient of the hydrogen-bond interaction, with a dimension of .sup.1, which governs the range of hydrogen-bond interaction. A larger D value indicates a more localized hydrogen-bond interaction; r.sub.min represents the minimum interatomic interaction distance, with a dimension of nm, introduced to avoid non-physical atomic overlap during calculation; r.sub.cut represents the cutoff radius, with a dimension of nm, balancing computational accuracy and efficiency, typically taken as the distance at which the interaction force decays to 1%.

(25) Specifically, the aforementioned step enables the quantification of the - stacking and hydrogen-bond synergy between the MOF layer and the LCP chain, achieving a technical effect with a computational error of 5%. b. establishing a three-dimensional electric field distribution equation based on the finite element method:

(26) $.Math.\left({}_r\right)=\frac{p_{\mathrm{interface}}}{{}_0}$$p_{\mathrm{interface}}=q.Math.n_{\mathrm{dipole}}.Math.\tanh \left(E_{\mathrm{interface}}\right)$$=\frac{1}{k_{B}T}$ where: .sub.r represents the position-dependent relative permittivity, determined by the filler distribution and interfacial properties. It characterizes the non-uniformity of the electric field distribution in the composite material and directly affects the local field strength; represents the electric potential distribution function, used for calculating the electric field strength, with a dimension of V. It is employed to evaluate the risk of dielectric breakdown; p.sub.interface represents the interfacial polarized charge density, arising from dipole orientation and interfacial bound charges, having a dimension of C/m.sup.3, quantifying the contribution of interfacial polarization to dielectric loss, and positively correlated with the density of functional groups on the filler surface; .sub.0 represents the vacuum permittivity, having a dimension of F/m, used for normalization in calculations; q represents the effective charge quantity, related to the polarity of interfacial chemical bonds, with a dimension of C. It reflects the degree of charge transfer between SH groups and LCP, where a higher value indicates a stronger interfacial dipole moment; n.sub.dipole represents the number of interfacial dipoles per unit volume, with a dimension of m.sup.3, jointly determined by the porosity of the MOF layer and the filling degree of LCP chain segments; represents the reciprocal of the Boltzmann factor, with a dimension of eV.sup.1, correlated with the influence of temperature on the distribution of polarized charges.

(27) Specifically, the aforementioned step elucidates the mechanism of electric field distortion at the filler-matrix interface and predicts the distribution of local field strength. c. constructing the frequency-domain response function of the complex permittivity:

(28) ${}^{*}\left(\right)={}_{}+\frac{}{1+{\left(j\right)}^{1-}}+\frac{}{j{}_0}$

(29) A fractional-order derivative was employed to describe interfacial polarization relaxation (j).sup.1-, thereby addressing the failure of the Cole-Cole model in the high-frequency band; where: .sub. represents the high-frequency baseline permittivity, reflecting the contribution of electronic polarization. It characterizes the polarization capability of the material in the optical frequency band and is related to the intrinsic polarity of LCP; represents the relaxation strength, corresponding to the permittivity variation contributed by interfacial polarization. A higher value indicates a more pronounced influence of interfacial polarization on dielectric properties; represents the angular frequency, with a dimension of rad/s, directly representing the operating frequency and determining the response rate of polarization mechanisms; represents the relaxation time, with a dimension of s, where a higher activation energy E.sub.a corresponds to slower relaxation; represents the Cole-Cole distribution parameter, where stronger interfacial bonding leads to more uniform relaxation; represents the direct current conductivity, characterizing ohmic losses caused by ion migration, with a dimension of S/m. It is influenced by proton conduction from SH groups on the filler surface, and excessively high values increase dielectric loss.

(30) Specifically, the aforementioned step accurately characterizes dielectric relaxation behavior in the 10-40 GHz frequency band, with a goodness of fit of R.sup.20.98. d. obtaining the total dielectric loss through vector superposition:

(31) $\tan {}_{\mathrm{total}}=\underset{i=1-1}{\overset{3}{.Math.}}{}_i.Math.\tan {}_i+.Math.{.Math.{}_r.Math.}^2$${}_1=0.5e^{-{\left(V_f/0.1\right)}^2}$${}_2=0.3S_{\mathrm{interface}}$${}_3=0.2\log \left(E_{\mathrm{interface}}\right)$$=0.05E^{1.2}$ where: .sub.1 represents the volume fraction weighting factor, .sub.2 represents the interfacial binding strength weighting factor, .sub.3 represents the activation energy correlation weighting factor, represents the dispersion correction term, and E represents the distribution uniformity index; .sub.r.sup.2 represents the gradient norm of the permittivity, characterizing the intensity of electric field distortion. Its dimension is V.sup.2/m.sup.2, and a larger value indicates more severe local field strength non-uniformity, which results in space charge accumulation losses.

(32) Specifically, the aforementioned step achieves a prediction error 7% at the 28 GHz frequency band, meeting the design requirements of 5G millimeter-wave materials.

(33) The dispersion correction term C.sup.1.2 accounts for the nonlinear influence of filler agglomeration on electric field distortion.

(34) Through four-step progressive modeling combined with a multiphysics coupling algorithm, the aforementioned step enables: Microscale controllability: nanoscale parameters such as interfacial binding energy and crystal orientation are directly correlated with macroscopic dielectric properties; High-frequency accuracy: achieving a prediction error <6% in the 28 GHz millimeter-wave frequency band, significantly outperforming the international state of the art; Strong universality: successfully applied to the development of six categories of high-frequency electronic devices, including 5G antenna radomes and radar-absorbing materials.

(35) By constructing a multiscale dielectric loss prediction model, the model overcomes three major technical bottlenecks of traditional empirical models in the high-frequency band (5G millimeter-wave frequency band): insufficient prediction accuracy, lack of microstructure correlation, and neglect of multiphysics coupling. This enables quantitative and precise prediction of macroscopic dielectric properties from microscopic interfacial properties.

(36) In combination with the third aspect, furthermore, the definition of the interfacial binding strength parameter E.sub.interface is expressed as:

(37) $S_{\mathrm{interface}}=\frac{1}{N_A}\underset{i=1}{\overset{N_A}{.Math.}}.Math.\frac{E_{\mathrm{interface}}}{r_i}.Math..Math.r_i$ where: N.sub.A represents the statistical number of atom pairs, counted within a 1 nm.sup.3 region at the MOF-LCP interface to ensure statistical significance; r.sub.i represents the interatomic distance variation, with a dimension of nm, reflecting the dynamic fluctuation at the interface, where a smaller value indicates more stable interfacial bonding;

(38) $\frac{E_{\mathrm{interface}}}{r_i}$
represents the partial derivative of interfacial energy with respect to interatomic distance, with a dimension of eV/nm, characterizing the gradient of interatomic interaction force, where a larger value indicates higher interfacial bonding stiffness.

(39) In combination with the third aspect, furthermore, in step c, the relaxation time at different temperatures is measured to fit the activation energy E.sub.a, wherein the extraction of the activation energy E.sub.a is expressed as:

(40) $\ln =\ln {}_0+\frac{E_a}{k_B}\left(\frac{1}{T}-\frac{1}{T_0}\right)$ where: .sub.0 represents the pre-exponential factor, with a dimension of s, related to the intrinsic polarization rate of the material; T.sub.0 represents the reference temperature, with a dimension of K, used to normalize the effect of temperature on relaxation time; k.sub.B represents the Boltzmann constant, with a dimension of eV/K, establishing the conversion relationship between thermal energy and activation energy.

(41) In combination with the third aspect, furthermore, in step d, the distribution uniformity index C is calculated as:

(42) $C=1-\frac{1}{N}\underset{k=1}{\overset{N}{.Math.}}{\left(\frac{.Math.x_k-.Math.}{}\right)}^3$ where: x.sub.k represents the position coordinate of the k-th filler particle, obtained by digitalized processing of SEM images with a resolution of 0.1 m/pixel; represents the mean value of the filler positions, and ideally should be close to the geometric center of the specimen; represents the positional standard deviation, which indicates improved dispersion uniformity; all the above-mentioned parameters have a dimension of m.

(43) By clearly defining the key parameters in the dielectric loss prediction model and their measurement methods, this addresses two major problems in conventional technologies, namely model unreliability and result incomparability caused by parameter ambiguity and inconsistent measurement standards, thereby providing a standardized and reproducible technical basis for dielectric property prediction.

(44) Finally, it should be noted that the above-mentioned embodiment is intended to illustrate the technical solutions of the present invention rather than limit its scope of protection. Although the present invention has been described in detail with reference to preferred embodiments, those ordinary technicians skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solution of the present invention without departing from its essence and scope.