Influence of partial charge on the material removal rate during chemical polishing

A partial charge-based chemical polishing model has been developed, which can serve as metric for describing the relative polishing material removal rate for different combinations of slurries and workpieces. A series of controlled polishing experiments utilizing a variety of colloidal polishing slurries (SiO₂, CeO₂, ZrO_2, MgO, Sb₂O₅) and optical materials [single crystals of Al₂O₃ (sapphire), SiC, Y₃Al₅O₁₂ (YAG), CaF₂, and LiB₃O₅ (LBO); a SiO₂-Al₂O₃-P₂O₅-Li₂O glass ceramic (Zerodur); and glasses of SiO₂:TiO₂ (ULE), SiO₂ (fused silica), and P₂O₅-Al₂O₃-K₂O-BaO (Phosphate)] was performed and its material removal rate was measured. As previously proposed by Cook (J Non-Cryst Solids. 1990;120:152), for many polishing systems, the removal rate is governed by a series of chemical reactions which include the formation of a surface hydroxide, followed by condensation of that hydroxyl moiety with the polishing particle, and a subsequent hydrolysis reaction. The rate of condensation can often be the rate limiting step, thus it can determine the polishing material removal rate. By largely keeping the numerous other factors that influence material removal rate fixed (such as due to particle size distributions, interface interactions, pad topography, kinematics, and applied pressure), the material removal rate is shown to scale exponentially with the partial charge difference (δ_wp-s) between the workpiece and polishing slurry particle for many of the slurry-workpiece combinations indicating that condensation rate is the rate limiting step. The partial charge (δ) describes the equilibrium distribution of electron density between chemically bonded atoms and is related to the electronegativity of the atoms chemically bonded to one another. This partial charge model also explains the age-old experimental finding of why cerium oxide is the most effective polishing slurry for chemical removal of many workpieces. Some of the slurry-workpiece combinations that did not follow the partial charge dependence offer insight to other removal mechanisms or rate limiting reaction pathways.     

Synthesis and characterization of nanocomposite Fe3O4/SiO2 core–shell abrasives for high-efficiency ultrasound-assisted magneto-rheological polishing of sapphire

A functional Fe₃O₄/SiO₂ core–shell abrasive was synthesized via hydrolysis of tetraethyl orthosilicate. A silica shell was successfully coated on a Fe₃O₄ core, resulting in a core-shell particle with an average diameter of 140 nm. The prepared core–shell abrasives was utilized for ultrasound-assisted magneto-rheological polishing (UAMP) of sapphire substrate. The experimental results showed that the Fe₃O₄/SiO₂ core–shell abrasives exhibited a remarkable polishing performance for the sapphire material, resulting in smooth and detect-free surfaces with a high material removal rate (MRR) compared to mixed abrasives (Fe₃O₄ and SiO₂) and pure Fe₃O₄ particles. The application of ultrasonic vibration to the sapphire wafer further improved the MRR, which was approximately 3.4 times higher than that of traditional magneto-rheological polishing. The largest MRR (1.974 μm/h) and comparatively low surface roughness (0.442 nm) of the polished sapphire wafer were achieved by UAMP with the Fe₃O₄/SiO₂ core–shell abrasives. The polishing mechanism of the sapphire wafer is discussed in terms of chemical reactions and mechanical polishing.     

Chemical-mechanical polishing of low dielectric constant poly(silsesquioxane): HSQ

In this study, the chemical-mechanical polishing (CMP) characteristics of the low dielectric constant poly(silsesquioxane) (HSQ) were investigated. CMP behavior was studied using different kinds of slurries, additives, and pads. The slurriesused included SiO₂ based slurry (SS-25), ZrO₂ based slurry (A-1), and Al₂O₃ based slurry (WA400). The additives used to change the surface interaction behavior were Triton X-100 and HNO₃. The role of the polishing pad was investigated by a hard pad (IC 1400) and a soft pad (Politex). The experimental results suggested that the hardness of the abrasives and pads and the electrostatic interaction between the abrasive and polymer surface were responsible for the polishing results.     

Competitive effects of free volume,rigidity, and self-adaptivity on indentation response of silicoaluminoborate glasses

Lithium aluminoborate glasses have recently been found to feature high resistance to crack initiation during indentation, but suffer from relatively low hardness and chemical durability. To further understand the mechanical properties of this glass family and their correlation with the network structure, we here study the effect of adding SiO₂ to a 25Li₂O–20Al₂O₃–55B₂O₃ glass on the structure and mechanical properties. Addition of silica increases the average network rigidity, but meanwhile its open tetrahedral structure decreases the atomic packing density. Consequently, we only observe a minor increase in hardness and glass transition temperature, and a decrease in Poisson's ratio. The addition of SiO₂, and thus removal of Al₂O₃ and/or B₂O₃, also makes the network less structurally adaptive to applied stress, since Al and B easily increase their coordination number under pressure, while this is not the case for Si under modest pressures. As such, although the silica-containing networks have more free volume, they cannot densify more during indentation, which in turn leads to an overall decrease in crack resistance upon SiO₂ addition. Our work shows that, although pure silica glass has very high glass transition temperature and relatively high hardness, its addition in oxide glasses does not necessarily lead to significant increase in these properties due to the complex structural interactions in mixed network former glasses and the competitive effects of free volume and network rigidity.     

Effect of mixed-shaped silica sol abrasives on surface roughness and material removal rate of zirconia ceramic cover

Zirconia ceramic, as mobile phone body-materials, will become increasingly important with the coming of 5G communication technology. Surface quality and material removal rate of zirconia ceramic cover are vital factors to determine its wide application. Therefore, mixed-shaped silica sol abrasives were prepared by ion connecting-inducting method and applied to achieve a good surface quality and a high material removal rate on zirconia ceramic cover by using chemical mechanical polishing (CMP). Mixed-shaped silica sol abrasives contained spherical and beaded shapes were measured by scanning electron microscopy (SEM). Si–O–Al bonds were formed in the mixed-shaped silica sol abrasives and were proved by X-ray photoelectron spectroscopy (XPS). Results of CMP tests showed that zirconia ceramic cover obtained a low surface roughness of 1.824 nm and an efficient material removal rate of 0.33 μm/h. Compared with traditional spherical silica sol abrasives, the polishing rate of mixed-shaped silica sol abrasives increased by 242%. Additionally, solid-phase chemical reactions happened to formed ZrSiO₄, ZrAl₂Si₂O₉ in the CMP process. Moreover, friction coefficient was tested and polishing mechanism had been explored by a contact-friction model in this work.     

Microscopic Removal Function and the Relationship Between Slurry Particle Size Distribution and Workpiece Roughness During Pad Polishing

Various ceria and colloidal silica polishing slurries were used to polish fused silica glass workpieces on a polyurethane pad. Characterization of the slurries' particle size distribution (PSD) (using both ensemble light scattering and single particle counting techniques) and of the polished workpiece surface (using atomic force microscopy) was performed. The results show the final workpiece surface roughness is quantitatively correlated with the logarithmic slope of the distribution function for the largest particles at the exponential tail end of the PSD. Using the measured PSD, fraction of pad area making contact, and mechanical properties of the workpiece, slurry, and pad as input parameters, an Ensemble Hertzian Gap (EHG) polishing model was formulated to estimate each particle's penetration, load, and contact zone. The model is based on multiple Hertzian contact of slurry particles at the workpiece–pad interface in which the effective interface gap is determined through an elastic load balance. Separately, ceria particle static contact and single pass sliding experiments were performed showing ~1‐nm depth removal per pass (i.e., a plastic type removal). Also, nanoindentation measurements on fused silica were made to estimate the critical load at which plastic type removal starts to occur (P_crit~5 × 10^?5 N). Next the EHG model was extended to create simulated polished surfaces using the Monte Carlo method where each particle (with the calculated characteristics described above) slides and removes material from the silica surface in random directions. The polishing simulation utilized a constant depth removal mechanism (i.e., not scaling with particle size) of the elastic deformation zone cross section between the particle and silica surface, which was either 0.04 nm (for chemical removal) at low loads (<P_crit) or 1.0 nm (for plastic removal) at intermediate loads (>P_crit). The simulated surfaces quantitatively compare well with the measured rms roughness, power spectra, surface texture, absolute thickness material removal rate, and load dependence of removal rate.     

Effect of kyanite on rheological properties of dense aqueous alumina suspensions

Abstract

The present work examines the effect of pH on aqueous suspensions of alumina and alumina–kyanite mixtures to assess the influence of kyanite additions on suspension properties. This system is of interest because on heating to temperatures above 1400°C the mineral kyanite (Al₂O₃ . SiO₂ ) reacts to form silica and mullite (3Al₂O₃ . 2SiO₂ ) and as the latter mineral has attractive elevated temperature properties, its incorporation into a common refractory material such as alumina is of importance in the field of technical ceramics. A popular method to produce such materials is to prepare a stable suspension of the components of interest and cast a green compact from the slurry; this compact is subsequently sintered at an appropriate temperature to densify the product. As the solids content of the original suspension influences the final product density, rheological properties are of critical importance. Both microelectrophoresis and rheological techniques suggested that a pH of 3–4 provided optimum stability. The flow behaviour of the binary mixture could be predicted by the Casson model and it is suggested that the surface characteristics of the kyanite were primarily responsible for the resulting stability regime.     

Effect of hydroxy carboxylates as complexing agent on improving chemical mechanical polishing performance of M-plane sapphire and action mechanism analysis

As sapphire device performance continues to improve, greater challenges are posed to the chemical mechanical polishing (CMP) of sapphire, with its high degree of hardness and brittleness. M-plane sapphire substrates are not widely used because they are more difficult to process, despite having higher luminous efficiency than C-plane substrates. In this study, the effect of three hydroxyl carboxylates, namely potassium tartrate (PT), potassium citrate (Cit) and sodium gluconate (Gluc), as complexing agents on the CMP of M-plane sapphire was investigated to obtain a high material removal rate (MRR) and low root mean square surface roughness (Sq). First, the chemical reactivities of the three complexing agents were predicted with Material Studio (MS) software. The predicted results showed that the complexing ability of the three complexing agents was greatest for Gluc, followed by Cit, with PT having the least complexing ability. Experimental results confirmed that Gluc was the optimal complexing agent for the M-plane sapphire CMP. The mechanism of action during CMP was revealed by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR). The results showed that the Al(OH)₄^? ions produced by the sapphire were complexed by Gluc to form the soluble complex Al(OH)₄^?/Gluc^?. At the same time, a solid phase reaction also occurred between the M-plane sapphire, SiO₂, and water during CMP, and Al₂Si₂O₇?2H₂O was generated. After polishing with the optimized slurry, the M-plane MRR was improved to 5.358 μm/h, a 50% improvement compared with the reference slurry, and the Sq decreased from 0.345 nm to 0.172 nm. These findings provide important guidance for the development of high-performance sapphire devices.     

Synthesis of a mesoporous material from two natural sources

Mesoporous materials were obtained from two natural silica sources, diatomite and pumicite, under hydrothermal conditions, autogenic pressure and in presence of the surfactant cetyltrimethylammonium bromide (CTAB) as the template. Using diatomite, a temperature of 383 K and the following molar ratios in the initial reaction gel: SiO₂/Al₂O₃ = 8.86; CTAB/SiO₂ = 0.1; Na₂O/SiO₂ = 0.10–0.25 and H₂O/Na₂O = 250–300, the mesoporous material MCM‐41 was obtained in a reaction time of 48 h. When pumicite was used, a mesoporous material was obtained in a reaction time of 96 h, a reaction temperature of 423 K and an initial reaction gel with the following molar ratios: SiO₂/Al₂O₃ = 8.86; CTAB/SiO₂ = 0.1; Na₂O/SiO₂ = 0.25 and H₂O/Na₂O = 330. Copyright © 2006 Society of Chemical Industry     

Relationship between surface μ‐roughness and interface slurry particle spatial distribution during glass polishing

During optical glass polishing, a number of interactions between the workpiece (i.e., glass), polishing slurry, and pad can influence the resulting workpiece roughness at different spatial scale lengths. In our previous studies, the particle size distribution of the slurry, the pad topography, and the amount of material removed by a single particle on the workpiece were shown to strongly correlate with roughness at AFM scale lengths (nm‐μm) and weakly at μ‐roughness scale lengths (μm‐mm). In this study, the polishing slurry pH and the generation of glass removal products are shown to influence the slurry particle spatial and height distribution at the polishing interface and the resulting μ‐roughness of the glass workpiece. A series of fused silica and phosphate glass samples were polished with various ceria and colloidal silica slurries over a range of slurry pH, and the resulting AFM roughness and μ‐roughness were measured. The AFM roughness was largely invariant with pH, suggesting that the removal function of a single particle is unchanged with pH. However, the μ‐roughness changed significantly, increasing linearly with pH for phosphate glass and having a maximum at an intermediate pH for fused silica. In addition, the spatial and height distribution of slurry particles on the pad (as measured by laser confocal microscopy) was determined to be distinctly different at low and high pH during phosphate glass polishing. Also, the zeta potential as a function of pH was measured for the workpiece, slurry, and pad with and without surrogate glass products (K₃PO₄ for phosphate glass and Si(OH)₄ for silica) to assess the role of interfacial charge during polishing. The addition of K₃PO₄ significantly raised the zeta potential, whereas addition of Si(OH)₄ had little effect on the zeta potential. An electrostatic DLVO three‐body force model, using the measured zeta potentials, was used to calculate the particle–particle, particle–workpiece, and particle–pad attractive and repulsive forces as a function of pH and the incorporation of glass products at the interface. The model predicted an increase in particle–pad attraction with an increase in pH and phosphate glass products consistent with the measured slurry distribution on the pads during phosphate glass polishing. Finally, a slurry “island” distribution gap (IDG) model has been formulated which utilizes the measured interface slurry distributions and a load balance to determine the interface gap, the contact area fraction, and the load on each slurry “island”. The IDG model was then used to simulate the workpiece surface topography and μ‐roughness; the results show an increase in roughness with pH similar to that observed experimentally.     

Role of MgO in lowering glass transition temperature and increasing hardness of lithium silicate glass and glass-ceramics

The effect of variation of MgO (1.5, 4.5 and 7.5 mol%) content on glass structure, crystallization behavior, microstructure and mechanical properties in a Li₂O–K₂O–Na₂O–CaO–MgO–ZrO₂–Al₂O₃–P₂O₅–SiO₂ glass system has been reported here. Increased amount of MgO enhanced the participation of Al₂O₃ as a glass network former along with [SiO₄] tetrahedra, reducing the amount of non-bridging oxygen (NBO) and increasing bridging oxygen (BO) amount in glass. The increased BO in glass resulted in a polymerized glass structure which suppressed the crystallization and subsequently increased the crystallization temperature, bulk density, nano hardness, elastic modulus in the glasses as well as the corresponding glass-ceramics. MgO addition caused phase separation in higher MgO (7.5 mol%) containing glass system which resulted in larger crystals. The nano hardness (～10 GPa) and elastic modulus (～127 GPa) values were found to be on a much higher side in 7.5 mol% MgO containing glass-ceramics as compared to lower MgO containing glass-ceramics.     

Effect of TiO2 on crystallization and mechanical properties of MgO–Al2O3–SiO2 glasses containing P2O5

MgO–Al₂O₃–SiO₂ glass-ceramics have been widely used in military, industrial, and construction applications. The nucleating agent is one of the most important factors in the production of glass-ceramics as it can control the crystallization temperature or the grain size. In this study, we investigated the effect of replacing P₂O₅ with different amounts of TiO₂ on the crystallization, structure, and mechanical properties of an MgO–Al₂O₃–SiO₂ system. The crystallization and microstructure were investigated by differential scanning calorimetry, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The mechanical properties were investigated by measuring the Vickers hardness, Young's modulus, and fracture toughness. The results showed that adding TiO₂ favored the precipitation of fine grains and significantly increased the Vickers hardness, Young's modulus, and fracture toughness of the glasses. Introducing an appropriate amount of TiO₂ can make a glass structure more compact, promote crystallization, and improve the mechanical properties of MgO–Al₂O₃–SiO₂ glass-ceramics.     

The effect of alumina-based sintering aid on the microstructure,selected mechanical properties,and coefficient of friction of Cf/SiC composite prepared via spark plasma sintering (SPS) method

C_f-SiC air brake discs are being developed due to their high-temperature oxidation resistance compared to conventional C_f/C discs. The C_f-SiC air brake discs should have a coefficient of friction (COF) close to 0.4, a low wear rate, a density higher than 95% of the theoretical density, and flexural strength of more than 200 MPa. To reach the properties of C_f-SiC composite to the required characteristics of the air brake disc, different amounts of alumina-based sintering aid were used. For this purpose, first silicon carbide nanoparticles, sintering aids Al₂O₃–MgO, MgAl₂O₄, Al₂O₃–Y₂O₃, Al₂O₃–SiO₂–MgO, and carbon fiber (20 wt%) with a 5-mm length were prepared. Next, the final composite bulk was created via the SPS method at 1900 °C under a pressure of 50 MPa. The density of the sample sintered with the Al₂O₃–SiO₂–MgO sintering aid was higher than that of other sintering aids. The density value was obtained at 98% and 100% at 8 wt% and 4 wt% respectively. It was also found that the use of 4 wt% of Al₂O₃–SiO₂–MgO offered better mechanical properties compared to 8 wt%, due to the absence of Al₈Si₄O₂₀ phase at 4 wt%. The examination of mechanical properties showed that the hardness (3564 Vickers) and flexural strength (479 MPa) of the sample with the Al₂O₃–SiO₂–MgO sintering aid were higher than those of other sintering aids. The samples with the Al₂O₃–SiO₂–MgO sintering aid with 4 wt% revealed a COF of 0.41, showing the closest feature to the desired indices of aircraft brake discs.     

Integrated experimental phase equilibria study and thermodynamic modelling of the binary ZnO–Al2O3, ZnO–SiO2, Al2O3–SiO2 and ternary ZnO–Al2O3–SiO2 systems

Phase equilibria of the ZnO–SiO₂, Al₂O₃–SiO₂ and ZnO–Al₂O₃–SiO₂ systems at liquidus were characterized at 1340–1740 °C in air. The ZnO–Al₂O₃ subsolidus phase equilibria were derived from the experiments with the SiO₂- and CaO + SiO₂-containing slags. High-temperature equilibration on silica or platinum substrates, followed by quenching and direct measurement of Zn, Al, Si and Ca concentrations in the phases with the electron probe X-ray microanalysis (EPMA) was used to accurately characterize the system. Special attention was given to zincite phase that was shown to consist of two separate ranges of compositions: round-shaped low-Al zincite (<2 mol.% AlO_1.5) and platy high-Al zincite (4–11 mol.% AlO_1.5). A technique was developed for more accurate measurement of the ZnO solubility in the low-ZnO phases (corundum, mullite, tridymite and cristobalite) surrounded by the ZnO-containing slag, using l-line for Zn instead of K-line, avoiding the interference of secondary X-ray fluorescence. Solubility of ZnO was found to be below 0.03 mol.% in corundum and cristobalite, and below 0.3 mol.% in mullite. Present experimental data were used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in this system using FactSage computer package. The modified quasichemical model with two sublattices (Zn²⁺, Al³⁺, Si⁴⁺) (O^2?) was used for the liquid slag phase; the compound energy formalism was used for the spinel (Zn²⁺,Al³⁺)[Zn²⁺,Al³⁺,Va]₂O^2-₄ and mullite Al³⁺₂(Al³⁺,Si⁴⁺) (O^2?,Va)₅ phases; the Bragg-Williams formalism was used for the zincite (ZnO, Al₂O₃); other solid phases (tridymite and cristobalite SiO₂, corundum Al₂O₃, and willemite Zn₂SiO₄) were described as stoichiometric. Present study is a part of the research program on the characterization of the multicomponent Pb–Zn–Cu–Fe–Ca–Si–O–S–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni system.     

Mg2SiO4-MgAl2O4 directionally solidified eutectics: Hardness dependence modelled through an array of screw dislocations

Mg₂SiO₄-MgAl₂O₄ eutectic ceramics have been fabricated by means of the laser floating zone (LFZ) technique. The microstructure has revealed as an unusual one at lower growth rate, composed of broken lamellae of MgAl₂O₄ distributed randomly along one matrix, composed of Mg₂SiO₄. At higher growth rates, a cell structure with intra-cell lamella structure is dominant. Contrary to most eutectic systems, hardness is not dependent upon the inter-spacing, but it does depend on one characteristic length of lamellae: their perimeter. One simple model based upon the dislocation is proposed, which successfully accounts for such extraordinary hardness law. Accordingly, Mg₂SiO₄-MgAl₂O₄ eutectic ceramics fabricated at 50 mm/h growth rate with the smallest MgAl₂O₄ lamella perimeter favorably showed more elevated hardness (13.4 GPa from Vickers indentation and 15.3 GPa from nanoindentation) and strength (∼430 MPa) than those found in the monolithic Mg₂SiO₄ matrix.     

Microscopic Origins of Compositional Trends in Aluminosilicate Glass Properties

Revealing and understanding the microscopic origins of the macroscopic properties of aluminosilicate glasses is important for the design of new glasses with optimized properties. In this work, we study the composition‐structure‐property relationships in 20 MgO/CaO sodium aluminosilicate glasses upon Al₂O₃‐for‐SiO₂ and MgO‐for‐CaO substitutions. We find that some properties (density, molar volume, Young's modulus, and shear modulus) are linear through the investigated range of Al₂O₃ compositions, while others (refractive index, coefficient of thermal expansion, Vickers hardness, isokom temperatures, and liquid fragility index) exhibit a change in the slope around the composition with [Al₂O₃] = [Na₂O], which is especially pronounced for the glasses containing MgO. We discuss these phenomena based on structural information obtained by NMR spectroscopy and topological considerations.     

Mixture Design and Response Surface Analysis of Densification of Silicon Carbide Ceramics with (SiO2–Dy2O3–Al2O3) Additives

Statistical mixture designs are used to systematically study the densification properties of silicon carbide (SiC) ceramics sintered with SiO₂, Dy₂O₃, and Al₂O₃. Mixture models for percentage theoretical density and SiC weight loss as a function of the SiO₂, Dy₂O₃, and Al₂O₃ oxide proportions have been determined and validated by analysis of variance. The results indicate a region confined by about 0–20 mol% silica, 50–65 mol% dysprosia, and 40–65 mol% alumina, with all samples containing 10% by volume of additives, and simultaneously maximization of density values and minimization of weight loss during SiC-based ceramic sintering.     

Synergetic strengthening effects on copper matrix induced by Al2O3 particle revealed from micro-scale mechanical deformation and microstructure evolutions

The micro-scale effect of Al₂O₃ particle on the deformation behaviors of the copper matrix was investigated using nanoindentation. Moderate strengthening effects were produced by the Al₂O₃ as indicated from the mechanical deformation evolutions. Specifically, the displacement recovery ratio and elastic work ratio is 6% and 9% higher for the Cu-5 wt% Al₂O₃ (C5A) composite material compared with that of the pure copper (PC) material, respectively. While for the indentation hardness and indentation modulus, the increment is 36% and 75%, respectively. Notably, the moderate strengthening effects were quantitatively illuminated from the power law index m for the C5A indent (1.2) and PC indent (1.1–1.3). In addition, synergetic strengthening effects were proposed from the microstructure evolutions in the C5A composite material. Specifically, the increment in yield strength deduced from the grain refinement is 120 MPa, which is 67% higher than that of the Al₂O₃ particle dispersion strengthening. The synergetic strengthening effects revealed from the microstructure evolutions are expected to provide new strengthening approaches for the ceramic particle reinforced metal matrix composite materials.     

Fabrication of SAPO-34 Crystals with Different Morphologies by Microwave Heating

The crystal size and shape of the silicoaluminophosphate molecular sieve SAPO-34 have been effectively controlled in the reaction system of Al₂O₃–P₂O₅–SiO₂–TEAOH–H₂O under microwave radiations. Nano sheet-like SAPO-34 crystals are obtained when using colloidal silica as the silica source. When TEOS is used as the source of silica, homogeneous SAPO-34 crystals with a particle size of about 100 nm are formed, and the morphology of the crystals changes from uniform nano particles (~100 nm) to microspheres (~1.5 μm) by varying the H₂O/Al₂O₃ molar ratio. To further investigate the morphology control of SAPO-34, the synthetic factors, such as the silica source, water content, crystallization time and aging time have been studied in detail.     

Effect of sodium polyacrylate on mechanical properties and microstructure of metakaolin-based geopolymer with different SiO2/Al2O3 ratio

The mechanical properties and microstructure of geopolymer are affected by the molar ratio of SiO₂/Al₂O₃. Meanwhile, organic polymer has the effect of improving the toughness of geopolymer, which depends on the SiO₂/Al₂O₃ ratio of geopolymer inevitably. Therefore, it is important to investigate the effect of the organic polymer on the mechanical properties and microstructure of geopolymer with varying SiO₂/Al₂O₃ ratio for using organic polymer to modify geopolymer. In this work, the SiO₂/Al₂O₃ ratios of metakaolin-based geopolymers are adjusted to 2.0, 2.5, 3.0, 3.5 and 4.0 by adding silica fume and β-Al₂O₃, with Na₂O/SiO₂, H₂O/SiO₂ being maintained at 0.2, 4.0, respectively. The geopolymers with each SiO₂/Al₂O₃ ratios are modified by addition of 0, 0.4, 0.8, 1.2 and 1.6?wt% of sodium polyacrylate (PAAS).The mechanical properties of these samples are measured and the rate of change is used to characterize the effect of PAAS on the metakalin-based geopolymers. The mechanism is also shown by 29Si NMR, XPS and FTIR. The results show that the effects of polymer on the mechanical properties of metakaolin-based geopolymer are affected by SiO₂/Al₂O₃ ratio and the effect becomes less obvious with SiO₂/Al₂O₃ ratio increasing from 2.0 to 4.0. Incorporation of PAAS can reduce the degree of polymerization of [SiO]₄ or [AlO]₄ in geopolymer and form the Si?O?C bond, which are two main reasons for polymer improving the toughness of geopolymer. But these effects decrease when the SiO₂/Al₂O₃ ratio of geopolymer increases from 2.0 to 4.0, which is corresponding to the effect on the mechanical properties. The toughening effect of organic polymer on geopolymer depends on the SiO₂/Al₂O₃ ratio of geopolymer, and only the geopolymer with lower SiO₂/Al₂O₃ ratio (no more than 2.5 in this work) can be significantly toughening modified by organic polymer. Therefore, it is necessary to consider the SiO₂/Al₂O₃ ratio of the geopolymer when geopolymer modified by organic polymer is designed.